UNIT VII REVISION

Landfills \\

Regardless of how much reduction, reese:- recycling, and ener y IIIe111,1 "

achieved, some fraction lof the MSW must be returned to the nvh 11I11t1 This chapter discusses how residues that have no value can b I H I f

aged and disposed of. Although the majority of the text is d v 1,.,1 Iii disposal of raw, untreated MSW, it is equally applicable to any 11.1111111

MSW that remains after the recoverable materials have been r I (IV' II

Discounting outer space (although the US Environment I PI'" II

Agency (EPA) did a feasibility study of sending waste into s dl II III

1970s-it was not economical) and air (from whence it eventu Ily ,,"

back to the earth's surface), the only two locations for the ultimate- III / I

of wastes are (1) in the oceans and other large bodies of water 1 ( ') "'

in land. With rare exceptions, most solid waste may no longer Ill' I. I

dumped into oceans. Most developed countries have enacted str 1111'11

dumping legislation, and the once ubiquitous refuse-or sludqe-loach ,01 I es have all but disappeared. New York City no longer uses barges 10 II Ifl

waste to Staten Island since the Fresh Kills Landfill has closed. The 11'111ill

ocean disposal problems appear to result from the discharge of r 111'11 II

ships and debris washing from the land into the ocean. The total allllil/ll

refuse deposited on the sea bottom in some of the more frequently II IV

shipping lanes (such as the North Atlantic) and areas such as the NOIIII I'

Gyro is impressive as well as depressing. Philosophically, theref II', II

ocean disposal is merely a storage process and not a method of trl'.!111I

In

o tion of I . liul i In hi

d disposal (as our nly

h o 1/ nd only th

) is discussed here.

n engineered method for land disposal of solid or hazard-

manner that protects the environment. Within the landfill

1/ ch mical, and physical processes occur that promote the deg-

F wastes and result in the production of leachate (polluted water from the base of the landfill) and gases. In the United States,

W I mdfills are regulated under Subtitle D of the Resource Conservation very Act (Public Law 94-580) passed in 1976. Specific landfill design

'I 'PI r tiona I criteria were issued under 40 CFR Part 258 in 1991. I II European Union (EU) Council Directives on Landfilling of Wast

I 0 identified the need to optimize final waste disposal methods and II nsure uniform high standards of landfill operation and regulation

1111!llout the European Union. These standards require a strategy that,

(II/limits the quantity of biodegradable wastes entering the landfill

,,' I f the total amount by weight of biodegradable waste produced in

, I ut many European countries have moved more aggressively to end

I'I ictice of landfilling biodegradable waste. For example, Denmark has

.Iy reached the last reduction target by banning the landfilling of all t • uitable for incineration. German landfills may accept only municipal

t that has been incinerated or that has undergone mechanical-biological

1111 nt (MBT). Consequently, most waste in Europe is incinerated or

Ii j prior to landfilling. In the United States, some states (such as

Ii" I nia) have adopted a goal of 75% reduction of solid waste being land- III y the year 2020.

I me communities in the United States have taken a different approach

II rating a landfill as a bioreactor. The bioreactor landfill provides con- I Old process optimization, primarily through the addition of leachate

,,11 r liquid amendments, if necessary. Beyond that, bioreactor landfill lollion may involve the addition of wastewater sludge and other amend-

111',temperature control, and nutrient supplementation. The bioreactor

11111attempts to control, monitor, and optimize the waste stabilization

" s rather than contain the wastes as prescribed by most regulations.

II I 0 'sible, in-place density (the density once the refuse has been compact d II Il00lnd) should be estimated. If an existing landfill is available, density can I II d't rmined by routinely conducting aerial surveys of the landfill and then

111.11Il! the volumes. This method includes the volume of cover material in I 1I1111.Hin. If dirt is used as daily and final cover, 20 to 50% of the volume of Iliid II may be cover material. An in-place density in the range of 1200 Ib/y 1\ II It/Iii \) is typical.

VI! -n some materials are recovered from solid waste, the compaction char- II II may change, and it may be necessary to estimate the compaction of the II II it dividual refuse components. For example, if yard waste is diverted from I III 11111,the landfill will have a more uniform compaction. Bulk densities of .11111nents can be used in such calculations, even though the actual compac- 1110,b quite different. Table 8-1 lists some bulk densities that can be used for 1'111\ e, and Example 8-1 illustrates the procedure. A more complete listing

I III d nsities is found in Appendix B. I lurne, mass, and density calculations for mixed materials can be simplified

1111It! ring a container that holds a mixture of materials, each of which has 'II hulk density. Knowing the volume of each material, the mass is calculated 1111ontributing material, added, and then divided by the total volume. In

1111)11form,

(PA X VA) + (PI! X VJ!) = P(A+B)

VA + VB

PA = bulk density of material A PB = bulk density of material B VA = volume of material A VB = volume of material B

II II re are more than two different materials, this equation is extended.

8-1 PLANNING, SITIN , AN OF LANDFILLS

MI IN

A solid waste engineer, when asked to take on the job of rn na in{ lilt' Idll\ system for a major city, had only one question: "Do the existing 1.111111111 enough remaining capacity to last until I retire?"

No essential public facility, with the possible exception f c 11,iiipi III I difficult to plan, site, and permit than a landfill. In the last 2" y ars. Ilw 11111111. operating landfills has decreased from about 8000 to under 1700, prhu.u tl I stringent requirements under the RCRA(40 CFR Part 258). How v 1", Iandllill II' is greater now than it was a decade ago due to the expansion of mallY .~II. III advantage of economies of scale achieved in operating the large III 1'1111.11111111

Commonly, it now takes over 10 years to go through the pro 'ss c.1III I a new landfill. During the 1O-yearprocess, many of the rules will ha 111\1',1111Itl l regulations, permits, and approval requirements. In addition, publk Ill!)"! \I and even lawsuits during the process are more than likely. The d i(fin III Y Itl I landfills makes disposal in outer space appear to be a more attra Iivc .ilil II Clearly, one quality that is needed when siting a landfill is persev raiu t'

8-1-1 Planning In planning for solid waste disposal, communities must look many Y<"II 11111 future. A 10-year time frame is considered short-term planning. Thirty c.u to be an appropriate time frame, because after 30 years, it becomes IIII/It I anticipate solid waste generation and new disposal technology.

The first step in planning for a new landfill is to establish th I"('qlllill for the landfill site. The site must provide sufficient landfill capacity fOI I11I I design period and support any ancillary solid waste functions, SLl h.1 It I treatment, landfill gas management, and special waste services (i.e., 1111' I items, household hazardous wastes). Some sites also house facilities 10111111I recyclable materials (materials recovery facilities) and composting. To iii It II landfill capacity, the disposal requirements for the community or WIIIIIIIII must be estimated. A landfill that is too small will not have an ad qu.iu life and will not justify the expense of building it. On the other hand, ,I I I that is too large may eliminate many potential sites and will result in h igll I" I capital costs that would preclude the community from constructing 011'1"II public facilities.

Records from the past several years provide a historical guide cIi ii' I quantities. In some cases, this information can be very accurate if scales WI I at existing landfills. In other cases, the information is suspect because 1111'1111 ties of solid waste delivered to the landfills were only estimated. Finally. II, If the historical population, a per capita disposal rate can be calculated. Slt"l I U.S. per capita solid waste generation rates have remained at about 4.51'111111I person per day. On the other hand, per capita disposal has decreased 1111111 pounds per person per day to 2.43 pounds per person per day. Finally, II" III amount of waste land filled each year has remained fairly constant SIIIII I ranging from 134 to 142 million tons per year.

Uncomp d

g/cm3 Iblft3

6.36 2.36

18.45 3.81 6.19 2.37 1.87

23.04 4.45

14.9

0.100 0.038 0.295 0.061 0.099 0.037 0.030 0.368 0.071 0.238

If the twO 11at 'l'ialH lit 111'1"1' 'Ill d -n II' I'" PI''' l'\ 1\ t'lll1 III w 'ighl Ira ti n, ih 11lh IL\(ti n f I' nl "daling Ill' ov .rull bull I'll I I

MA + MB

M = mass of material A M

A = mass of material B

p B = bulk density of material A P: = bulk density of material B

If more than two materials are involved, this equation is extended. The volume reduction achieved in refuse baling or landfill co1111.11 t\1111I

important design and operational variable. If the original volume .01' a :",lIllpl solid waste is denoted by V

o ' and the final volume, after compactl 11,I', \

the calculation of the volume reduction is

V ~=F V

o

where F = fraction remaining of initial volume as a result of compa titIII

V = initial volume o

V = compacted volume c

Because the mass is constant (the same sample is compacred so there i~ I\I, I' I loss of mass) and volume = mass/density, volume reducuon also can be (,III ItI "

Po = F P,

where P = initial bulk density po = compacted bulk density .F = fraction remaining of initial volume as a result of compaCI\(III

For illustrative purposes only, assume that refuse has the followln I components and bulk densities.

Component

Miscellaneous paper Garden waste Glass

Percentage (by weight)

50 25 25

Uncompacted I\I~I Density (Ib/i' ')

3.81 4.45

18.45

Assume that the compaction in the landfill is 1200 Ib/yd 3 . (44.4 1\ 1/11 'I

Estimate the fraction remaining of the initial volume achieved (hllilll,

un

[0 ,81

II bulk density prior to compaction is

i- 25 + 25 = 4 98 lb/ft? 25 25 . --+-- 4.45 18.45

I)v r the life of a landfill, the volume of waste may change due to 111(' \IIIV 11 :

"" Uigutations. Many states have adopted waste diversion/recycling goals. If fully IIlpl .rn nted, diversion rates of 70% and beyond may be achieved. IfIII/'I'Ling Facilities. Other landfills may exist or be planned that would fee ivc 11111'f the planned waste, or other existing landfills may close, resulting in the

"11"utation of solid waste. "fI,'" 'I1t Cover Options. Dirt used as daily and final cover at a landfill may consu me III) 0% of the available landfill volume, depending on the size of the Ian Ifill,

11\1'n w landfill may rely on foam, tarps, or mulched green waste for cover and lill significantly increase the volume available for the solid waste. 1I,II'eiidentia! Waste Changes. A per capita generation rate takes into account ;iI I 11111).idential waste currently going into the landfill. In some cases, the IlOII

Itl ntial waste can account for more than 50% ofthe landfill volume ne d xl. 1111\;examples are large military facilities, agriculture operation, manufacturing .Illties, and cities with a large percentage of communities and tourists. EiIIH.:1'

I" losing or opening of these facilities can significantly affect future wast« III 1 tions.

111 fr iction remaining of the initial volume which has been achieved I hllll19 compaction is

4,98=0.11 114.4

I I 1 required landfill volume is approximately 11% of the volume 1 lulr d without compaction. If the mixed paper is removed, the 1111 mpacted density is

25 + 25 25 25-+--4.45 18.45

= 7.1 8 Ib/ft '

Itlng ,lilt' II',' flh landfill h sb n Ictcrmlncd.ft ls n s ry t find n appro- II I', Whil th n ept is sirnpl r lh uti n i far from easy. A buu n

1'1 N l n Plan t Earth) seen at one landfill siting public hearing can best lilt' 111' hall ng s faced when siting a landfill. This attitude is referred to as III : not in my back yard. Politicians are more likely to embrace the principles 1M'!' -not in my term of office. III tori ally, for a local community or regional agency, an appropriate land-

I III \ within the geographic boundary of the agency. The relationships between IIlh p,dili s are such that few would allow a neighboring community to site a 11111 In lts jurisdiction if the host community was not a participant. The real- h,l I' me that few local communities can site new landfills. The barriers I \11e ,I I planning and engineering staff face when trying to site a new landfill 1111111'I' us and include lack of political support, high development costs, and

III di losure laws. PI I' < private company, the geographic location is not as critical. Recently,

I ,III mpanies have been siting large remote landfills in rural areas. Host fees IIII'll p id to the local community. For example, both Seattle, Washington, and 11,11\ 1/ regan ship waste to a private regional landfill in eastern Oregon. New It (, ty also exports waste to other states.

t )n the geographical boundary of the potential site has been determined, II III I locations should be identified. This process is a pass/fail test referred

,I [uta! flaw analysis. Some fatal flaws are established in regulations such as I I t I' of the Resource Conservation and Recovery Act. For example, no landfill II III sued near an active seismic fault or an airport. Other criteria are subjective I II Illy be established by the local community, such as no landfill will be sited

III II ne mile of a school. Because of local regulations and criteria, it is not pos- III. II provide a complete list of potential fatal flaws, but the following should

1111 idered as fatal flaws:

It is too small. Il is on a flood plain. ite includes wetlands.

,I mic zone is within 200 ft of the site. u ,'ndangered species habitat is on the site.

1111 site is too close to an airport (not within 5000 ft for propeller aircraft or !l1,1l 0 ft ifturbine engine aircraft). lit. ite is in an area with high population density. 1111 site includes sacred lands. 1111 site includes a groundwater recharge area. IIII uitable soil conditions (e.g., peat bogs) exist on the site.

II of the areas exhibiting one or more of the above fatal flaws should be I 1111 hted on the search map. If no unhighlighted areas remain within the geo-

pill boundary, then it will not be possible to site a new landfill. However, if

Calculate t~e requ!red 20-year landfill capacity for a community w iii the population projection. per capita waste generation rat , IIvl I sion rate shown in the following table. Note that the waste n fdlllill is expected to .increase at approximately 3% per year through Y( \I II and then remain constant. Note also that the community is ex (II II to increase Its rate of waste diversion to 35% in year 4 throu 11 1\11 aggressive recycling and yard waste composting program. As UI1lI soil daily cover is used that accounts for 25% of the landfill vol mlC

Per Capita Generation

Year Population

(000)

Rate, lb/cap/

day Diversion, Fraction

Waste to Landfill,

tons

Wast III Landfill,

ydl

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20

Total

105.4 108.6 109.8 112.2 115.2 117.7 121.1 124.7 128.4 133.4 139.1 144.5 150.7 155.6 163.1 169.4 175.3 181.4 187.7 194.3

5.6 5.8 6.0 6.2 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4 6.4

0.25 0.28 0.30 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35

8.08E+04 8.28E+04 8.42E+04 8.25E+04 8.75E+04 8.94E+04 9.19E+04 9.47E+04 9.75E+04 1.01 E+05 1.06E+05 1.10E+05 1.14E+05 1.18E+05 1.24E+05 1.29E+05 1.33E+05 1.38E+05 1.43E+05 1.48E+05 2.15E+06

1.35 1.38 1.40 1.38 1.46 1.49 1.53 1.58 1.62 1.69 1.76 1.83 1.91 1.97 2.06 1 Oil 2.14_ I Oil 2.22 I Oil 2.30 I nil 2.38 1011 2.46 I Oil 3.59 I (lfl

A typical calculation for one year's Volume is (population) X (percapita generation rate) X (1 - diversion) X (36 d"y

1200lb/yd3

Total landfill waste volume is 3.59 X 106 yd3• To account for the voltIII 11 requirement for the cover soil,

0.25(T) + 3.59 X 106 = T T = 4.79 X 106 yd3

8·1 Effect on property values of siting a new landfill at a distance of three ource: Vesilind, P. A., and E. I. Pas. 1998. "Discussion of A. C. Nelson, J.

III J UX, and M. M. Genereux, 'A Price Effect of Landfills on Different House Value 1111,1,''' Journal of Urban Planning and Development, ASCE, 123, n. 3: 59-68. With

'"11' ion from ASCE.

After extensive hearings across the country, the EPA decided to use a combi- tit 11 f performance standards and design standards. A design standard specifies Ii" 'lfic design-for example, a requirement for a composite liner. On the other

uul, the requirement for no off-site migration of gas is a performance standard. I" v this standard is met is up to the applicant. In addition to Subtitle D require- lilt, landfills are usually subject to permitting for land use conformance, air

III ins, groundwater and surface water discharge, operations, extraction for , 'I I' material, and closure.

arcasd rm.l.n,th·,llIgpr c,sgo'.lOth'II')(I,I('I,wh(hl'lVOlv· <11./1'11 a ranking system. In addition to th f, I~I flaws, [11('1'(' Ill" still' IiV(' 1'\1\1 I'I 1111 I which are also used. In many cases, th publi is ask 'd t I' I'li Ip,lIl' II &I •• 1 ing the ranking criteria. For example, a site that h s g d ss nllgllt I,',. I points, while a site with poor access might receive only on pint. (lillil 'I" tative features might include relative population density, land use dl'/ 1\11111 groundwater quality, visual and noise impacts, site topography, sit(' OWII.I I soil conditions, and proximity to the centroid of solid waste g nerntiou

Once the criteria are developed, they are applied to any r rnr iniol'. ,III I do not have fatal flaws. Potential sites are developed and rank d. Tilt' Illp II sites are then designated for more detailed investigation that includes Oil I \I ysis of habitat, groundwater, and soil conditions plus physical surv 'ys ,11111 III' environmental assessment. Once this information is available, addiiion.rl 1.11 hearings are held, and the elected officials select a site or sites for th III t'I" II II engineering design, permitting application, and detailed environmcut.rl 111'1 report. Typically, the public is opposed to landfills, even though th y IlIoIV III distances from their point of interest (such as their home), but thei r OPIII •• IIe 'I most intense when the landfill is to be constructed close to their horne 1\111I11 this opposition can be attributed to such undesirable developments (\s add 1111 traffic, noise, odor, and litter. Most important is the belief that the siting of ,\ III fill will reduce property values and decrease residents' quality of life.

There is little question that siting a landfill near residential areas will II hi property values. In one study, a typical town was configured with variou- II I borhoods having different property values, and a hypotheticallandfi II w.i- pili in this town. Real estate assessors were asked to estimate the effect )f Ihl II landfill on the property values of existing homes. As expected, as the valur- III I property increased, so did the percentage drop in the property value." Mil I " matic was the far-reaching effect of the landfill. For the more expensive lilt 1\ the effect on property values exceeded a three-mile radius from the plfll'" landfill site. These findings were confirmed in a study using actual horne v,lIl! Figure 8-1 shows that the largest impact on private property values is on 1111 1111 expensive homes that are closest to the landfills.'

Because there is no compensation for the loss in the value of hOIIlI', I any wonder that there is such strong opposition to the siting of landfills/ :.1111111 society pay property owners some fraction of their loss as a fair compens,u IIIII the siting of undesirable public facilities close to their property? However. I III owners are compensated, what about the owners near other locally ulld,"IIIII land uses (LULUs), such as an airport or a wastewater treatment plant?

8-1-3 Permitting On October 9, 1991, the EPA issued 40 CFR Part 258, regulations peruuu II landfills under Subtitle D of the Resource Conservation and RecoveryAct; II,,' I lations were implemented two years later. States were required to incorpor.u: II, federal standards into their regulations. In addition, the states had the nt'~It III of adding more stringent requirements. Subtitle D addressed such issues ,I' III tion, design requirements, operating conditions, groundwater monitoring, 1.111111 closure and post-closure, and financial assurance.

0,3

Original value of house

•.... VI.8 (l.)

~0.2 > (l.) en ::s o....c:

'-'-'o ~ .90.1 '-'u (1j

J::

123 Distance from landfill (miles)

LANDFILL PROCESSES

-1 Biological Degradation hll is approximately 75 to 80% organic matter composed mainly of proteins,

phIs,carbohydrates (cellulose and hemicellulose), and lignins. Approximately two- hit Is of this material is biodegradable, while one-third is recalcitrant (Figure 8-2). " biodegradable portion can be further divided into a readily biodegradable utl n (food and garden wastes) and a moderately biodegradable fraction (paper, I I s, and wood). Figure 8-3 summarizes the predominant biodegradation thwaysfor the decomposition of major organic classes in solid waste.

thcr 2%

P:.1p 'I' nnd pal crhourd

27'1.,

Food 14%

Rubber, leather, and textiles

9% Plastics 13%

morgaruc material

2%

Figure 8-2 Total MSW generation by category in 2012. Source: Municipal ," dill Waste Generation,Recyciing, and Disposal in the United States: Detailed 1"I iii and Figures for 2012, US Environmental Protection Agency Office of Res I111 I Conservation and Recovery.

Proteins Carbohydrates Lipids

t t l Amino acids Simple sugars Glycerol/ long-chain

] I volatile acids

; Hydrogen! Acetate Short-chain

carbon dioxide

I volatile acids

t l Methane Methane + - Acetatecarbon dioxide

Figure 8-3 Predominant decomposition pathways for common organic waste: constituents.

n MI r bl I roup Prom In An robl W t D

Substr t

Starches Proteins Cellulose Hemicellulose Hydrogen Acetic acid Sulfate

rticle was published in Sanitary Landfilfing: Process, Technology and Environmental I , R., C. Harries, I. Viney, and J. F. Rees, "Activities and Distribution of Key Microbial ndfills," Copyright Academic Press 1989.

Ill) landfill ecosystem is quite diverse due to the heterogeneous nature or II ind landfill operating characteristics. The diversity of the ecosystem pro-

III . ( bility; however, the system is strongly influenced by environmental tit I I ns (such as temperature, pH, the presence of toxins, moisture content, I fli • oxidation-reduction potential). The landfill environment tends to be ri h III (I' n donors, primarily organic matter. The dominant electron receptors ar

1111111 Ii xide and sulfate. Seven key physiological microbial groups that partici- III II r l -limiting stabilization steps of fermentation and methanogenesis ar It Ii 11 Table 8-2.

number oflandfill investigation studies" have suggested that the stabiliza- oil I II w ste proceeds in five sequential and distinct phases. During these phases,

" ',Ill' and characteristics of leachate produced and gas generated from a landf II I It I( nly dissimilar, but also reflect the microbially mediated processes taking I ••I 11 ide the landfill. The phases experienced by degrading wastes are described

I I,', hate characteristics during the waste degradation phases are summarized I Iill 8-3.

Landfill Constituent Concentration Ranges as a Function of the Degree of Landfill Stabilization

Methane Transition Acid Formation Fermentation M

I mlcal oxyqen 480-18,000 1500-71,000 580-9760 I In nd, mg/I I 11 volatile acids, 100-3000 3000-18,800 250-4000

1111)/1 as acetic acid I 1111 nia, mg/I-N 120-125 2-1030 6-430 II 6.7 4.7-7.7 6.3-8.8 uuluctivity, /-LS/cm 2450-3310 1600-17,100 2900-7700

Ph sa I-Inlt! I Adjustment Ph This phas is ss i l d with initi I pi ·11 III f s Ii I wast' and n lIlIllIl,I"11 moisture within landfills. An acclimation period ( L' in ili I Iag Iill1\') I III I I until sufficient moisture develops and supports an a tiv rni roblal ('11111111111I Preliminary changes in environmental components occur in I'd '1' to 11\'1111I I able conditions for biochemical decomposition.

Phase II-Transition Phase In the transition phase, the field capacity is often exceeded, an I H 11,11111111 tion from an aerobic to an anaerobic environment occurs, as vidru: I'd II depletion of oxygen trapped within the landfill media. A trend row.u d II It ing conditions is established in accordance with shifting of el Iro II ,II I I I' from oxygen to nitrates and sulfates and the displacement of oxygen IIv I II I dioxide. By the end of this phase, measurable concentrations 01 ,1111,,1 oxygen demand (COD) and volatile organic acids (VOAs) can be dcllll II the leachate.

Phase III-Acid Formation Phase The continuous hydrolysis (solubilization) of solid waste, followed hy (III I comitant with) the microbial conversion of biodegradable organic c 111('111,I It in the production of intermediate volatile organic acids at high c 11(1'1111III throughout this phase. A decrease in pH values is often observed, ac OIllI'.lIlh d metal species mobilization. Viable biomass growth associated with the ,II IIII11 ers (acidogenic bacteria), and rapid consumption of substrate and nu II'il'III' 1111II predominant features of this phase.

Phase IV-Methane Fermentation Phase During Phase IV, intermediate acids are consumed by methane-formi ng 111111111 (methanogenic bacteria) and converted into methane and carbon dioxide ',1111·, and nitrate are reduced to sulfides and ammonia, respectively. The p II \ .dll elevated, being controlled by the bicarbonate buffering system and, consl'qlllllll supports the growth of methanogenic bacteria. Heavy metals are removed I1IIIII 1 leachate by complexation and precipitation.

Phase V-Maturation Phase During the final state of landfill stabilization, nutrients and available """ II become limiting, and the biological activity shifts to relative dormancy. ("I duction dramatically drops, and leachate strength stays steady at much II)WI'I I I centrations. Reappearance of oxygen and oxidized species may be observcd sl II I However, the slow degradation of resistant organic fractions may coruiruu the production of humic-like substances.

The progress toward final stabilization of landfill solid waste is subjc. I 1111 physical, chemical, and biological factors within the landfill environment, 1111 and characteristics of landfilled waste; the operational and management IIIIIHI applied; and the site-specific external conditions.

r ductlon I

III 101II [uarulty PI' du d an b stimat d ith r by usins 1111iri al dnln I I Viii 'I' I nl, II 1 hniqu that s ts up a mass balan am ng pr ipitation,

1j1l111,lIISl'il'(li n, surfac runoff, and soil moisture storage.t= Landfills (1)('1 It d 1\ Ill' N rth asi have been designed using leachate generation rat s ori: ()() \I II ()() H' II ns per a re per day (gpad) (11,200 to 14,000 liters/he tap/day) Ii 1III (t l lv I has s, 500 gpad (4,700 L/ha/day) following temporary losun-, 1111IO() I P< I.( a L/ha/day) following final closure. A 1993 study found landfills

iii II I I 'gi ns generate only 1 to 7 gpad (9 to 60 L'ha/day}.'? Clearly, lirnat« 111111lOllS ignificantly affect leachate generation rates. .

wn t r balance uses site-specific data to track water volumes, as shown s h ' " II I illy in Figure 8-4. Prior to closure, which generally involves use of an iIn 1'\'1' III lilll' ap. the water balance can be described. Some fraction of precipiiatlon .111'1II I 'Ill n runoff characteristics and soil type and conditions) will p rcolnu- 11111111h th cover soil, and a fraction of this water is returned to the atrnosph 'I('

Surface water run-on-

I Leachate t escape to

environment

II ure 8-4 Schematic of components of water balance within a landfill.

through .vap transptratton. II Ill' I "'("OIIlIOli 'X "Is ·V,IIH(\'i\Il.pl,.l11111 I a sum iently long tirn , th m unt r w, 1'1' the il Is nil' 10 hold (II , capacity) will be exceeded, The field capa ity is th m ximurn 111 ISIIIIt' III (or any other material, such as refuse) can retain with ut a ntinuc l\, do VII percolation due to gravity,

Without plants, the soil on top of a landfill eventually rea h sits Il\'ld I II I ity so that any excess moisture will displace the moisture in th s iI. Willi Itllt cover, the plants may extract water from the soil and release it by vnpol',11I11'1t tion, thus drying the soil to below field capacity, In some commun ilies wlIIi wet rainy seasons, temporary plastic liners are placed on the unfinished sidl 141"1 of active landfills to minimize water infiltration,

All layers of a mixture of soil and refuse in the compacted Ian I(j II .11'11 II I a field capacity (or the ability to retain moisture), If the field capacity 01111 III ture is exceeded, the liquid (leachate) will drop to the next lowest soil/n-Iu:« II A soil/refuse mixture that does not attain field capacity discharges SSVllt1,t11 I water to the deeper layers.

In the finished landfill, if the field capacity of the soil covering W,I I exceeded, the water percolates through the soil and into the buried solid W.I II in turn, the field capacity of the waste is exceeded, leachate flows into Iltl II ate collection system. The water balance method is a means of calculathu; II by how much field capacities are exceeded and, thus, is a way of cal ul.u II production of leachate by landfills.

Some rough estimates can be used to develop the necessary calculatioi I~ 111111 runoff coefficients, for example, can be estimated for different soils and Slllill shown in Table 8-4. Precipitation data are available through the Weathc 111111 I Evapotranspiration rates are also available or can be calculated using the nnIlu ul Thornthwaite, II which takes into account the fact that evapotranspiration is 11·,11 \ as the soil moisture drops. In other words, plants use less water in dry wcatlu I

The lysimeter can also be used to estimate evapotranspiration. I , III III" cases, however, yearly average figures are sufficiently accurate for design. l:il\llI' shows average yearly evapotranspiration in the United States.

The field capacities of various soils and wastes are listed in Table H I, II field capacity of compacted waste has been estimated as 20 to 35% by VI diU! (about 30%), which translates to 300 mm of water/rn of MSW. This value 11111II corrected for moisture already contained in the refuse, which is about 151' II ,\ II field capacity of 150 mm/rn of refuse (1.8 in/foot) is therefore not unrcasou.tld

Table 8-4 Runoff Coefficients for Grass-Covered Soils

Surface Runoff Coefficient

Sandy soil, flat to 2% slope Sandy soil, 2% to 7% slope Sandy soil, over 7% slope Heavy soil, flat to 2% slope Heavy soil, 2% to 7% slope Heavy soil, over 7% slope

0.05-0.10 0.Q1-O.15 0.15-0.20 0.13-0.17 0.18-0.22 0.25-0.35

Source: 112]

It I f V rlou Porou

FI Id Capacity. as mm wat

120 200 300 375 450 200-350

11I1,: III

0 < 24 0 24-36 Average

36-48 annual

•48-60 potentialevapotranspiration• > 60 (inches) I lure 8-5 Average annual potential evapotranspiration in the continental United St l s. 1111 Il)t ntial evapotranspiration will occur if the soil is completely saturated; hence these fi<)III! I

/I highest probable values, in inches of water. The actual evapotranspiration will alw yn 1111 I. w r. Source: [12J

The actual calculations of leachate production involve a one-dimensional uulysis of water movement through soil and the compacted refuse, as shown in I /-lure8-6. The figure also defines the symbols used in the calculation, which are 11.1. d on the following mass balance equation:

C = pel - R) - S - E

E RJ

(1 - R)P

s

Figure 8-6 Mass balance of moisture in a landfill.

where C = total percolation into the top soil layer, mm/yr P = precipitation, mm/yr R = runoff coefficient S = storage within the sailor waste, mm/yr E = evapotranspiration, mm/yr

The net percolation calculated for three areas of the United Stall S I It" in Table 8-6. Note that the net percolation for a Los Angeles landfill is :II III explains the absence of leachate in Southern California landfills.

Using the data from Table 8-6, it is possible to estimate the number I" before leachate is produced, since the soil/waste mixture will absorb 1111' I' I lated water until its field capacity is reached. In Los Angeles, there seldom \ III1 leachate generated by a landfill because of the lack of percolation. In 01'1.1111111 I a landfill 7.5 m deep, the first leachate should appear in 15 years. In Cilulllll' for a landfill depth of 20 m, the first leachate should appear in 11 YC.II', lit calculations are based on the assumption that moisture moves as a welling 111'11 Often there are channels and cracks in the waste that permit preferential I1II I

Table 8-6 Percolation in Three Landfills

Precipitation Runoff Evapotranspiration P<'IIIIi III (mm/yr) Coefficient (mm/yr) (1l\1I1/~11

p R E t ----

Cincinnati 1025 0.15 658 ill! Orlando 1342 0.07 1173 III Los Angeles 378 0.12 334 II

Source: [7]

r I I' I" uuton 11111111111 OV( I' 11l( hn: In. y b (and [rcqucruly is) produ cd long I 'Ie I"

tlrn t the percolation of water through a landfill 10m deep, with 1 m cover of sandy loam soil. Assume that this landfill is in southern hio, and that

p = 1025 mm/yr R = 0.15 E = 660 mm/yr oil field capacity, Fs = 200 mm/m

Refuse field capacity, Fr = 300 mm/m, as packed

A sume further that the soil is at field capacity when applied, and th t the incoming refuse has a moisture content of 150 mm/m and ch refore has a net absorptive capacity of 150 mm/m. Percolation hrough the soil cover is

C = P(1 - R) - S - E = 1025(1 - 0.15) - 0 - 660 = 211 mm/y

211 mm/y 150mm/m = 1.4m/y

or it will take 10m

1 .4 m/y = 7.1 Y

to produce a leachate that will be collected at a rate of (211 mm X area of landfill) per year.

nce a final cap is placed on the landfill, the water balance must include a 11111 [deration of the permeability of the cap and the reduced ability of the water III p rcolate into the waste. Obviously, leachate volume will be reduced following l.uulfill closure.

The model for leachate generation most frequently used today, the Hydrologic I '1lluation of Landfill Performance (HELP), was developed by the U.S. Army

II,! of Engineers." This model requires detailed on-site morphology and It nsive hydrologic data to perform the water balance. The HELP computer pro-

i.un, which is currently in its third version (3.07), is a quasi-two-dimensional h drologic model of water movement across, into, through, and out of a landfill.

'It '-81 ' I I informatioll is 11" 1'1 for I I" lplt.ulou, 'V,I!>O!I'I"',jll'tll 1l1I, II III P rature, win I sp d, infiltrau n rat s. and W;\[ rshc I I art 111't .rs (,II( II II' .111I imperviousness, slope, and depression storag ). Th 111 I I I ts wc.uhn, II/I and design data and uses solution techniques that ac ount f r th ffc ts of 111111II storage, snowmelt, runoff, infiltration, evapotranspiration, veg tativ growth, 1111 moisture storage, lateral subsurface drainage, leachate recirculati n, '")M"IIII III d vertical drainage, and leakage through soil, geomembrane, OJ con I osuc 11111I variety oflandfill systems can be modeled, including various combinations III \I etation, cover soils, waste cells, lateral drain layers, low-permeability barrier '1.111 and synthetic geomembrane liners. The HELP model is most useful for 10111'.11111 prediction of leachate quantity and comparison of various design altcllI.11I I however, it is not suitable for prediction of daily leachate production.

Leachate Quality Within a landfill, a complex sequence of physically, chemically, and bi )IOn" III mediated events occurs. As a consequence of these processes, refuse is I 'gI,1CII III transformed. As water percolates through the landfill, contaminants arc 11'.1111 from the solid waste. Mechanisms of contaminant removal include Ic,l(" 11/1I inherently soluble materials, leaching of soluble biodegradation products (II 111111 plex organic molecules, leaching of soluble products of chemical react iOIl', ,III washout of fines and colloids. The characteristics of the leachate produ. .'d II highly variable, depending on the composition of the solid waste, prccipu.uu« rates, site hydrology, compaction, cover design, waste age, sampling prcx n 1111 interaction of leachate with the environment, and landfill design and OIH'I,IIIIIII Some of the major factors that directly affect leachate composition in Ilidl III degree of compaction and composition of the solid waste, climate, site hyd 111111111 ogy, season, and age of the landfill. Table 8-7 shows the wide variation or k,11II I quality as determined by various researchers.

'1 Ranges of Various Parameters in Leachate

Florida Landfills Nation .1 Oasirn and Grosh, 1996 Datab.i

Ehrig 1989 Chiang 1994 (mean value) (mean v,lltl

II) 20-40,000 80-28,000 0.3-4660 (149) 0-100,000 ( 1/ il) 500-60,000 400-40,000 7-9300 (912) 11-84,000 ( I 11 I) 3-2100 0.6-325 4-27()() (mg/I) 30-3000 56-482 BDL-5020 (257) 0.01-290() ( I

:mg/l) 100-5000 70-1330 BDL-5480 (732) 6.2-67,000 ( II I) 0.03-120 0.1-30 BDL-3.02 (0.158) 0.005-8t1b (II 19/1) 0.1-30 8-35 0.02-7 (I )

4.5-9 5.2-6.4 3.93-9.6 6.7 HI.II) 0.008-1.020 0.5-1.0 (29.2±114) 0.00-2.5!1 (0 I (mg/I) < 0.05-0.140 < 0.05 (7.52 ± 23.9) 0.0-0.564 (()(l

~w detection limit inhart, D. R., and C. J. Grosh. 1997. "Analysis of Florida MSW Landfill Leachate Quality Data." Rei ,,,,1 " Iter for Solid and Hazardous Waste Management (March).

p )lIuling ,(I' ',]Ill. :llld Ink '/I Is ), wh] 11 istirnatcs 111' I

I 1"11,1101'the s, I· of mpari n, lh B lor W S We IIIIII\( I 111, ns ~ r xp sing p Iluti nal p L ntial i

I IlIhlll I. , [) is always gr at r than BOD. 1,\.1 hat ten I to contain a large variety of organic and inorgani ornpounds

II u-lntlvcty low n entration that can be of concern if groundwater and surf c II1II conic minati n occurs. These compounds are often constituents of gas line

11111 ill 'I ils (aromatic hydrocarbons such as benzene, xylene, and toluen ,), 1111111I gradation by-products (phenolic compounds), chlorinated solvents (SL! h I 1\ 'I ill city cleaning), and pesticides. Inorganic compounds of conc rn arc "III ind adrnium, which come from batteries, plastics, packaging, ele tro n i '1'1111111 ,and light bulbs.

'I'h quality of leachate directly affects viable leachate treatment alternativ 's. 1111illS' I achate quality varies from site to site and over time, neither biologi ;)1 IIIIUIIl 'I1t nor physical/chemical treatment processes separately are able to achi vc itlflll (I' trnent efficiencies. A combination of both types of treatment is one or the 111111I'·m ctive process trains for the treatment of leachate. Physical/chemical pro- II ',HI" needed for the pretreatment of young leachate to make it amenable to I IIII( gi al treatment and to hydrolyze some refractory organic compounds found IiI It 11 hate from older landfills. Biological treatment is primarily used to stabi Iizc d. I.idable organic matter found in young and middle-aged leachates.

·3 Gas Production Quantity

1,11\lflll operators, energy recovery project owners, and energy users need to be "ril' L project the volume of gas produced and recovered overtime from a landfill. lit I ov ry and energy equipment sizing, project economics, and potential en rgy II I depend on the peak and cumulative landfill-gas yield. The composition of tltl gas (percent methane, moisture content) is also important to energy produ ers 11111u ers. Proper landfill management can enhance both yield and quality of gas.

Mathematical and computer models for predicting gas yields are based on I'lljlltiation, per capita generation, waste composition and moisture content, percent It III Ily landfilled, and expected methane or landfill-gas yield per unit dry weight III biodegradable waste." Mathematical models also can be used to model extra - 111111ystems, including layout, equipment selection, operation optimization, failure

mulation, and problem determination and location within existing extraction ( ms." Four parameters must be known if gas production is to be estimated with

III accuracy: gas yield per unit weight of waste, the lag time prior to gas production, 1111hape of the lifetime gas production curve, and the duration of gas producti n.

In theory, the biological decomposition of one ton of MSW produ cs I I, 00 ft3 (442 m") of landfill gas containing 55% methane (CH4) and a h ,I .tlue of 530 Btu/ft" (19,730 kT/m3). Since only part of the waste converts to

(II due to moisture limitation, inaccessible waste (plastic bags), and nonbio- 4

lit radable fractions, the actual average methane yield is closer to 3900 ft3/l011 (100 11)3/tonne) of MSW. Theoretical landfill gas production potential for the

Table 8-7 Ranges of Various Parameters in Leachate

Parameter Ehrig 1989

20-40,000 500-60,000

3-2100 30-3000 100-5000 0.03-120 0.1-30 4.5-9

0.008-1.020 < 0.05-0.140

Oasim and Chiang 1994

80-28,000 400-40,000

0.6-325 56-482

70-1330 0.1-30 8-35

5.2-6.4 0.5-1.0 < 0.05

Florida Landfills Grosh,1996 (mean value)

Sit i-spc i I inf rrnailon is 11" 1'1 1,1' I I'l'(IIlIIilIIOII, 'v.I( )111111 I II III perature, wind spe d, infiltrati n rat ,< 11c.J WaL'rsh 'eJ p<11" 111('\('1 (Ill I imperviousness, slope, and depression torag ). Th 111 IeI .) ('pi , I and design data and uses solution techniques that aunt 1'(r 111(' I'll, II I storage, snowmelt, runoff, infiltration, evapotranspirati I, V '!-\~'loIllvl 1111 moisture storage, lateral subsurface drainage, leachat r ir ul.u 011 1111 vertical drainage, and leakage through soil, geomembran , or ('0111(111 II I variety of landfill systems can be modeled, including various routl II11,1111111 etation, cover soils, waste cells, lateral drain layers, lOW-I rrn 'ahlllV 1111 I and synthetic geomembrane liners. The HELP model is III 81 IIsl'flilllll h t prediction of leachate quantity and comparison of various dc, II111ill 1 however, it is not suitable for prediction of daily leachate pro Ilil t 1111

Leachate Quality Within a landfill, a complex: sequence of physically, cherni ally, ,11111 I I I mediated events occurs. As a consequence of these processes, I' 'fllSt' 1'.11 I transformed. As water percolates through the landfill, contarnin.uu II I from the solid waste. Mechanisms of contaminant removal inclurl. I I I inherently soluble materials, leaching of soluble biodegradati n (1111111 II I plex: organic molecules, leaching of soluble products of chem i ,II 11',11' I'll washout of fines and colloids. The characteristics of the lea hall' pili III highly variable, depending on the composition of the solid wnsu-, I'H I rates, site hydrology, compaction, cover design, waste age, sarnpl i "1\ (lit interaction of leachate with the environment, and landfill design ,11111 III Some of the major factors that directly affect leachate composition lur l degree of compaction and composition of the solid waste, climate, Hilt " ogy, season, and age of the landfill. Table 8-7 shows the wide varinrlou 1111 quality as determined by various researchers.

uantity 1111 operators, energy recovery project own rs, oI1HI' III pl' ject the volume of gas produced and r ovcrrt]

1'1'1 and energy equipment sizing, project CCOII!)I!! dl'p nd on the peak and cumulative landfill-g.is

(percent methane, moisture content) is also iIllP( III I rs. Proper landfill management can enhan c h011

Inthematical and computer models for pI' litlil1f ul.ul n, per capita generation, waste composition ,1I1d

1111 landfilled, and expected methane or landfill-g,!. IllId gradable waste." Mathematical models also Coli!

sterns, including layout, equipment selection, 01h'1, ul.uion, and problem determination and location

I 1118.17 Four parameters must be known if gas product i \I uracy: gas yield per unit weight of waste, the I;]g 1111 lupe of the lifetime gas production curve, and the du In theory, the biological decomposition or on«

00 ft3 (442 rn") of landfill gas containing 550,11 111(' I f 530 Btu/ft" (19,730 kJ/m3). Since only p.iu i

due to moisture limitation, inaccessible waste (pi II lable fractions, the actual average methane yield

II m3/tonne) of MSW. Theoretical landfill gas prod

0.3-4660 (149) 7-9300 (912)

BDL = below detection limit Source: Reinhart, D. R., and C. J. Grosh. 1997. "Analysis of Florida MSW Landfill Leachate Quality Dill.. I. I Florida Center for Solid and Hazardous Waste Management (March).

BOD (mg/I) COD (mg/I) Iron (mg/I) Ammonia (mg/I) Chloride (mg/I) Zinc (mg/I) Total P (mg/I) pH Lead (mg/I) Cadmium (mg/I)

BDL-5020 (257) BDL-5480 (732) BDL-3.02 (0.158)

3.93-9.6 (29.2 ± 114) (7.52 ± 23.9)

Unit I Su l 'S is 'SI lrnn I It t 1,11 u+ll] 11111/ (\ I 101 m'/ ). Ilow 'VI'I I II conservativ stirnat w uld put th n LII" at roughly ;':00 bill! 11It'/ (11111 m3/y) with an oil equivalent energy pot ntial ro:o milli n l ns/y.11I

Gas production data collected at landfills acros th Unit d SLlt(' 1111 there is significant variation in the data due to differences in nvir 11111\'111,111'I tions and landfill management. The methane generation usually is hl'1 WI' II , 2 ft3/lb (0.06 to 0.12 m3/kg) of waste on a dry basis-over c period III II 40 years. The recoverable fraction for energy use is between 50 and 9()i}11 I'

Once the expected yield is determined, a model is select d lO iii' I1III pattern of gas production over time. As mentioned above, many modi I I been proposed, ranging from single valued, to linear increas /Iillc;ll' tll'lllIl exponential decline. The EPA has published a model called LandCI·:tvl1',1 I the following equation:

n Q = "" 2kL Me-Ill,T.L.J 01

j= 1

where QT = total gas emission rate from a landfill, volume/time

n = total time periods of waste placement k = landfill gas emission constant, time:"

Lo = methane generation potential, volume/mass of waste tj = age of the ith section of waste, time

Mi = mass of wet waste, placed at time i

This model can be downloaded at http.z/www.epa.gov/ttn/ .111/1'11111 .html#software.

The following example shows how this model is applied for a ill 'I " case. A landfill cell is open for three years, receiving 165,700 t Illlt! of waste per year (recall that 1 tonne = 1000 kg). Calculate th III I gas production if the landfill-gas emission constant is 0.0307 yr I 011111 the methane generation potential is 140 m3/tonne.

0t = 2 (0.0307) (140) (165,700)(e-00307(1)) = 1,381,000 m3 For the second year, this waste produces less gas, but the next III W layer produces more, and the two are added to yield the tot I II I production for the second year.

These results are plotted as Figure 8-7. The annual peak 11.1 production is found from this figure as about 4,000,000 m3/yr.

7~ / ~

>I< ~

/ .~ /

IX

I I I I I I I I I I I I I I I I I I I 7 9 11 13 15 17 19

Year since opened 1 53

" 6·7 Landfill gas generated in a landfill. [SeeExample8-4.)

,'h lag period prior to methane generation may range from a few weeks to " \ ars, depending on landfill conditions. The duration of gas production is I II IIOU need by environmental conditions within the landfill. For the LandGEM 1,"11 I, no lag period is assumed, and the duration is controlled by the magnitude IIIII' It ndfill-gas emission constant.

A practical guide to forecasting of yield-based on waste generation, the It III f wastes to decompose, and the efficiency of collecting gases from that IIII1111position-uses the following assumptions."

, IIIX) f the MSW generated is landfilled. II'VtI f the organic material will actually decompose. III~hof the landfill gas generated is recoverable. 11%of the landfills are operating within a favorable pH range.

II Ing these percentages, the actual methane yield turns out to be one-tenth of III Ih oretical yield. Unfortunately, estimates and rules of thumb are oflittle use " lzing equipment, determining project life, and analyzing project econom-

I lor a specific landfill. Gas generation then may be more closely estimated I 111 use of models and/or by laboratory tests of waste samples, small- and I 'HI'· cale pilot tests, and test cells incorporated in the landfill itself. Sources of uur include the inability to select representative landfill samples or well sites, III Inability to accurately determine the volume from which a well is actually

t I,\ ting, and the variation in gas generation across the landfill and over time I .rsonally}. Of course, once a recovery system has been installed and is func-

1IIIIIngat a steady state, reasonably precise recovery data can be collected to II diet future results.

T ible 8-8 Typic Org nl Compound Found In Florid

Component % by Volum (dry)

45-60 40-60

2-5 0.1-1.0 0.1-1.0

0-0.2

240 247 161 76

858 241 229

5559 8872

94 2872

372 449 131

6313 19 20

333 1328 573 268

1293

Concentration in ppb Methane Carbon dioxide Nitrogen Oxygen Ammonia Hydrogen

Average Range

BDL-470 BDL-1978 BDL-796 BDL-379 BDL-1760 BDL-1113 BDL-544

2304-7835 3918-12675 BDL-749

1244-4378 BDL-699 BDL-963 BDL-561

2443-12215 BDL-149 BDL-160 BDL-1448 244-2239 BDL-1140 BDL-1137 265-2646

Source: Based on by Tchobanoglous, G., H. Theisen, and S. Vigil, Integrate solid waste management: engineering principles and management issues, McGraw-Hili, 1993.

Gas Quality Because of the prevailing anaerobic conditions within a biologi ;111 ,II II I fill, these sites produce large quantities of gas composed of 111<:111.11\1I dioxide, water, and various trace components such as ammonia, sulfldv, ,lilt! methane volatile organic carbon compounds (VOCs). Tables 8-8 and II '\ I \I typical composition data for MSW landfill gas. Landfill gas is general] 11111I by installing vertical or horizontal wells within the landfill. Thes well: •• 11 vented to the atmosphere (if gas migration control is the primary inn III system) or connected to a central blower system that pulls gas to (1 fl.llI' I I I ment process.

The gas can pose an environmental threat, because methane I, ,I It I greenhouse gas, and many of the VOCs are odorous and/or toxic. /\111".III the U.S. EPA, methane is 23 times more potent as a greenhouse gas 111.111 dioxide.

Over the past two centuries, methane concentrations in th .1111111p' have more than doubled, largely due to human-related activities. MI'IIIIII accounts for 16% of global greenhouse gas emissions from human ,1111 Landfills are the second largest source of human-related methane 1'11\1I in the United States, accounting for approximately 23% of these tll1l IIiH 2007. Thus, it is important that landfill gas is not released to the a11111'II rather, it should be collected and the methane converted to carbon dl(l II water.

Given the large impact that landfilling can have on the generation ." house gas, the USEPA developed a computer model, the Waste Redu. lilili (WARM), to measure greenhouse gas impacts oflandfilling compared In 11'1\ composting, or source reduction. .

However, the gas has a high energy content and can be captured .1111111111 for power, steam, or heat generation. Treatment of landfill gas is requ i11,,11111 its beneficial use. Treatment may be limited to condensation of water .uul of the organic acids or may include removal of sulfide, particulates, 111',\\ I als, VOCs, and carbon dioxide. Some of the more potential innovative II gas include power generation using fuel cells, vehicle fuel (compressed Iii Iill natural gas), and methanol production.

I "low detection limit

,',' linhart, D. R., C. D. Cooper, and N. E. Ruiz. 1994. Estimation of Landfill Gas Emissions at , 1/ Illge County Landfill. Orlando: Civil and Environmental Engineering Department, University 1111I Florida.

LANDFILL DESIGN

III lundfill design and construction must include elements that permit control , I. ,I hate and gas. The major design components of a landfill, as shown in I III 1 8-8, include the liner, the leachate collection and management system, gas 111,1ment facilities, stormwater management, and the final cap.

I••,IIner system is required to prevent migration of leachate from the landfill and to ,III[ te removal of leachate. It generally consists of multiple layers of natural mate- I .ind/or geomembranes selected for their low permeability. Soil liners usually are

III tructed of natural clays or clayey soils. If natural clay materials are not readily tllable, commercial clays (bentonite) can be mixed with sands to produce a suit-

hll' liner material. Geomembranes are impermeable (unless perforated) thin sheets lilt! ' fr~m syn~etic resins, such as polyethylene, polyvinyl chloride, r ther I oly- '" , High-density polyethylene (HOPE) tends to be used in MSW landfill lin rs II t commonly, because it is resistant to most chemicals found in IfllldnJi Ie, hillel',

Final cover-, C0111'01 (; rnuu I willi I11l0ldl (i1'11I1\Methanemonitoring-:

Liner (synthetic ----..,... or natural)

Waste

Lea hall' \'~~o~~o~~o~~o~~o~~o~~I-- collection

system

Groundwater

Figure 8-8 Design components in a Subtitle 0 landfill.

Landfills may be designed with single, composite, or double liners, iii II 1\ ing on the applicable local, state, or federal regulations (see Figure B-')) A III liner is constructed of clay or a geomembrane. A composite liner, whi: II I 11 minimum liner required by RCRASubtitle D, consists of two layers: The 111111"1" a clay material and the top layer is a geomembrane. The two layers of a « 1I111111 ! liner are in intimate contact to minimize leakage. A double liner may be cif lu I single liners or two composite liners (or even one of each). Figure B- I () ·.It" synthetic liner on a side slope, ready for the earth cover. The clay layer alII',,," " been installed under this synthetic liner.

Each liner is provided with a leachate collection system. The (0111 III system separating the two liners is a leak detection system-a series of pip,,·, 1'1 \I between the liners to collect and monitor any leachate that leaks through 1111 1 liner. Recently, the geosynthetic clay liner has been introduced for use ,\:-,1111 1 component in the double liner system. This liner is composed of a thin, I,IV I (usually sodium bentonite) supported by geotextiles (a geosynthetic 11111I! geomembranes. The geosynthetic clay liner is easily placed in the field an: I II I less volume, allowing for more volume to be used for waste deposition.

Clearly, the more layers that are included, the more protective 11,1' 1111 system will be. The costs, however, increase dramatically. A composite IiIIII I cost as much as $250,000 per acre. Because the liner is so critical to gronurh II protection, an exhaustive quality control/quality assurance program is 1('1\1111 during liner installation.

8-3-2 Leachate Collection, Treatment, and Disposal Leachate is directed to low points at the bottom of the landfill through lilt' II an efficient drainage layer composed of sand, gravel, or a geosynthetic 111" IIII

"

Sill I i-Iin 'I' systems

eomembrane single- liner system

II'I'{

(b) Low-permeability soil single- liner system

(e) Single composite liner system

I ouble-lin 'I' yst 'II1S

I IIr 8-9 Examples of liner systems in municipal solid waste landfills. lCS = leachate IIIction system, GCl = geosynthetic clay liner, and LOS = leachate detection system.

1'1 IIorated pipes are placed at low points to collect leachate and are sloped to allow III moisture to move out of the landfill.

chate Collection and Storage IIII' primary purpose of lining a landfill cell is to minimize the potential for «urndwater contamination. The liner serves as a barrier between the buried ,I I and the groundwater and forms a catch basin for leachate produced by the

IlIIdfill. The leachate that is collected within the cell must be removed from above III' liner as quickly as possible, since the RCRA Subtitle D regulations restrict III' head of leachate (free liquid depth) on a liner system to 30 cm. Leachate

typically removed by two means: gravity flow or pumping. The various

Liner or GCL

(d) Double (geomernbran .) liner system

or GCL (e) Double-liner system with bottom composite liner

Figure 8-10 Synthetic liner on slope, ready for earth cover. (Courtesy Vvilliam A. Worrell)

components of a leachate collection system for an MSW landfill typically IIH 11111 the following:

• Protective and drainage layers • Perforated collection lateral and header pipes • Pump station sump • Leachate pumps • Pump controls • Pump station appurtenances • Force main or gravity sewer line

Table 8-10 provides general guidelines for leachate collection system (1111'1 III nents based on a survey oflandfill design engineers."

Leachate removed from the landfill cell(s) is temporarily stored on sill' 111111 it can be treated, recirculated, or transported off site for final treatment ;1I1d III posal. Storage of leachate is also important for equalization of flow quanti Ii," IHI I constituent quality to protect downstream treatment facilities. The typical 1('.11 11111 storage alternatives are surface impoundments and tanks.

Leachate Collection System Design Equations and Techniqu Because of federal regulations." that restrict leachate head to 12 in (30 cm) till iii of the liner, much attention has been devoted to predicting this value. The d 111111 age length, drainage slope, permeability of the drainage materials, and the il'.11 It II impingement rate control this depth on the liner.

I n all 'tl n y t m Comp

R ng

600-1000 9-12

60-400 6-8

rvc or HDPE 0.5-2 0.2-2

750 11 180

8 HDPE

1 1

'R . h t D Rand T Townsend. 1998. "Assessment of Leachate Collection System till' , In art. .., . I ·d C f 5 Id

1'1 ) ,1119 t Florida Municipal Solid Waste Landfills." Report to the F on a enter or 0 I II. Illnz rdous Waste Management (April).

I arcy's law (in conjunction with the law of continuity) can be used t? .devel- "I' If) quation to predict the leachate dq~t~ on. the liner based on. ant~Clpat cI lit! Ill', tion rates, drainage material permeability. distance from the dram pipe, an I

11· t 25,26It II} f the co ectron sys em. For R < 1/4,

[ (1 - A - 2R)(l + A - 2RS)]1/2.4

Ymax = (R - Rs + R2S2)1/2 (1 + A - 2R)(1 - A - 2RS)

For R = 1/4,

Y max

= (R - Rs + R2S2)1/2exp[~tan-l(2RSB- 1) - ~tan-leR B- 1)]

For R > 1/4, R(1 - 2RS) [ 2R(S - 1) ]

Ymax = 1 - 2R exp (1 - 2RS)(l - 2R)

II 'J" R = q/(K sin- a), unitless A = (1 - 4R)2, unitless B = (4R - 1)2, unitless S = tan a, slope of liner, unitless

Y = maximum head on liner, ft fir: = horizontal drainage distance, ft a = inclination of liner from horizontaL degrees q = vertical inflow (infiltration) per ~nit of horizontal area, ft/day K = hydraulic conductivity of the dramage layer, ft/ day

These equations are obviously cumbersome to use, and a more conservative hilt far easier to use equation has been proposed:"

P(Q)[Ktan2a Ktan a ( 2 + 1)1/2]Y =-- +1- tarr « K rnax 2 K q q

whcr Y:,I;" = maximum Sf lure led d I th v I' I h ' II n 'I', f'L

P = distance b tw n coll tion pip s. ft q = vertical inflow (infiltration), defin d in this 'llIllt\OIl I [I

25-year, 24-hour storm, ft/day

This equation can be used to calculate the maximum allowable pil'I II' based on the maximum allowable design head, anticipated I a hnrc illipli I I rate, slope of the liner, and permeability of the drainage mal ri: IH, '1'111' IIII ~ equation suggests that, holding all other parameters constant, the ('Iwi I III the pipes are placed (at greater construction cost), the lower th hr.u] \ III I reduced head on the liner results in a lower hydraulic driving Iorr« llillililli liner, and the consequence of a puncture in the liner is likewis rcdut I'd

Determine the spacing between pipes in a leachate c II I 111111 system using granular drainage material and the following pr pI III Assume that in the most conservative design all stormwat r 1111111 I 25-year, 24-hour storm enters the leachate collection system.

Design storm (25 years, 24 hours) = 8.2 in = 0.00024 cm/ Hydraulic conductivity = 10-2 cm/s Drainage slope = 2% Maximum design depth on liner = 15.2 cm

2Y

P=(Q)[Ktan2a Km;:na( q)1/2]- + 1 - tan2a + - K q Q K

P= 2(15.2)

( 0.00024)[0.01 (0.02)2 + 1 _ 0.01 (0.02)( 2 0.00024)1121

0.01 0.00024 0.00024 0.02) + 0.01

Geosynthetic drainage materials (geonets) have been introduced H I III the efficiency of the leachate collection systems over such natural material- It~ or gravel. A geonet consists of a layer of ribs superimposed over each 01111'1, IIIII I ing highly efficient in-plane flow capacity (or transmissivity). If a geolll'l I II between the liner and drainage gravel, the spacing between the collet 11<111 II' can be estimated from"

qp2e = ---------- 4Y + 2Psin amax

where e = transmissivity of the geonet, ft2/day

umm

R moval Objective Commentstlcn

BOD/COD Best used on "young" leach t Flexible, shock resistant, prov 11, Ill" II SRT increases with increasing r Ul ( strength, > 90% BOD removal II Good application to small flow, . 1)( BOD removal possible Aerobic polishing necessary to A hi! V high-quality effluent > 95% COD removal, > 99% BOD r<

I II u I goons BOD/COD II Obi BOD/COD

BOD/COD

I III I Ie I oxidation COD

Useful as polishing step or for tr of "old" leachate High removal of Fe, Zn; mod rt'l Cr. Cu. or Mn; little removal of Raw leachate treatment requir hi h dosages, better used as polishin t I 10-70% COD removal, slight m tnl : ( 30-70% COD removal after bioi 91 1 or chemical treatment 90-96% TDS removal

III Jul tion/precipitation Heavy metals

1,," xchange I' rption

COD BOD/COD

Total dissolved solids (TDSs)

~ V r e osmosis

11111: Reinhart, D. R., and C. J. Grosh. 1997. "Analysis of Florida MSW Landfill Leachate Quality D, ta." I~'I I lid I Center for Solid and Hazardous Waste Management (March).

hate Treatment and Disposal III wet areas, leachate treatment, use, and disposal represents one of the major

1ruses of landfill operations-not only during the active life of the landfill bUI I II ~ r a significant period of time after closure. Due to the cost implications, con- hit ", ble attention must be given to selecting the most environmentally responsibl ,I' ! effective alternative for leachate treatment and disposal. The optimal treatment 111m ion may change over time as new technologies are developed, new regulations'I I romulgated, and/or leachate quality varies as a function oflandfill age.

Leachate-treatment needs depend upon the final disposition of the leachate. l uml disposal of leachate may be accomplished through co-disposal at a wastewa- , I treatment plant or through direct discharge, both of which could be preceded Itv n-site treatment. Leachate treatment is often difficult because of high organ i umgth. irregular production rates and composition, variation in biodegradabil-

IIV, nd low phosphorous content (if biological treatment is considered). Several uthors have discussed leachate treatment options.P<" which are briefly summa- 11.( d in Table 8-11. Generally, where on-site treatment and discharge is selected,

• V ral unit processes are required to address the range of contaminants present. 1m example, a leachate treatment facility at the Al Turi Landfill in Orange County, N('w York uses polymer coagulation, flocculation, and sedimentation, followed Iy anaerobic biological treatment, two-stage aerobic biological treatment, and

lill'd I 'lilt OV 'I'llii' lln ,'I', II 1,1'11 011 tion I II'S, n

,.v'(inOllrati n), cI (in I in Ihis cqutu 011 I IllrSLOon, ft/day

clLD calculate the maximum II w,lhl\' 1'1111 '11)ledesign head, anti ipat die, (\1111i'II\jdl.

""~rmeabilityof the drainag mat 'rial., '11iI I" P g all other parameters constant. 111' In II I I

11 r constmction cost), the low r the Iw,ld III iet,Jlts in a lower hydraulic drivin (()I'i(' 11111111I : a puncture in the liner is lik wis rcdiu ,'d

between pipes in a leachate 11111111111 ~ainage material and the following pr I H III ~t conservative design all stormwat I 1111111 :> ",ters the leachate collection syst I , 91'

Co 24 hours) = 8,2 in = 0,00024 crn/sr,-' ~ 10-2 cm/s

i

,t[1 on liner = 15,2 cm

~( q)1/2] ~ tan2a + K =1

2(15,2)

~ 0,01 (0.02)(( )2 + 0,00024)1// I _1 0,00024 0,02 0.01

erials (geonets) have been introduced III 1111I '~ection systems over such natural rnatcri.il- ,I ,i:Jyerof ribs superimposed over each otlu-: I'lt -vol capacity (or transmissivity). If a gcoru t \I : ~ravel, the spacing between the collc(IIIIIIIIII

eonet, ft2/day ~

umm

Removal Objectiveptlon Comments

ludg BOD/COD Best used on "young" leachate Flexible, shock resistant, proven, minimum SRT increases with increasing organic strength, > 90% BOD removal possible Good application to small flows, > 90% BOD removal possible Aerobic polishing necessary to achieve high-quality effluent > 95% COD removal, > 99% BOD removal

BOD/COD

BOD/COD

BOD/COD

Heavy metals

Useful as polishing step or for treatment of "old" leachate High removal of Fe, Zn; moderate removal of Cr, Cu, or Mn; little removal of Cd, Pb, or Ni Raw leachate treatment requires high chemical dosages, better used as polishing step 10-70% COD removal, slight metal removal 30-70% COD removal after biological or chemical treatment 90-96% TDS removal

h'II11 1oxidation COD

11/\ I change II I rption

COD BOD/COD

Total dissolved solids (TOSs)

II r osmosis

"" : Reinhart, D. R., and C. J. Grosh. 1997. "Analysis of Florida MSW Landfill Leachate Quality Data." Report to the I/o IN Center for Solid and Hazardous Waste Management (March).

hate Treatment and Disposal I ( areas, leachate treatment, use, and disposal represents one of the major I'C'I1Ss of landfill operations-not only during the active life of the landfill but

I II r r a significant period of time after closure. Due to the cost implications, con- I!II r.ible attention must be given to selecting the most environmentally responsible, I I effective alternative for leachate treatment and disposal. The optimal treatment dtlll n may change over time as new technologies are developed, new regulations

1\ pr mulgated, and/or leachate quality varies as a function of landfill age. Leachate-treatment needs depend upon the final disposition of the leachate.

IlI,ddisposal of leachate may be accomplished through co-disposal at a wastewa- I I II' atment plant or through direct discharge, both of which could be preceded

on-site treatment. Leachate treatment is often difficult because of high organic "'"gth, irregular production rates and composition, variation in biodegradabil- IV,.ind low phosphorous content (if biological treatment is considered). Several 1111rs have discussed leachate treatment options.">" which are briefly summa-

I/I'd in Table 8-11. Generally, where on-site treatment and discharge is selected, vrral unit processes are required to address the range of contaminants present. II xample, a leachate treatment facility at the Al Turi Landfill in Orange County, C'W York uses polymer coagulation, flocculation, and sedimentation, followed

naerobic biological treatment, two-stage aerobic biological treatment, and

filtrati n prior t fis he rgc t the wallklll Rlv 'I, I I 1'1' 'II' -atrn 'Ill I' • 11!iI'\'IIII"1I II address only specific c ntaminants that m y r '( tc I r bl .ms tit ll1 ' W,I II if treatment plant. High lime treatment has b 11 pr ti d at the t\I, rillltl t 1111I11 Florida, Southwest Landfill to ensure low heavy-metal loadin n till' 11'11 It treatment facility.

The most economical alternative for leachate treatment i n -n In 11,111 the wastewater off-site to a publicly owned treatment works (P 'IW) ()I I 11111111 cial wastewater treatment facility. This option allows landfi II ow n '1,/()IlI'l .\1 11\ focus on their primary solid waste management charge while I uing W,I' II I experts handle the treatment of contaminated liquids. There are al 'OIlIJIIIIt scale for larger treatment plants that cannot be realized for small plants III I II to treat only the flow from a single landfill, Off-site treatment of I 'i"\( h'lll II alleviates some of the permitting, testing, monitoring, and reporting 1" Iitlil 1111111 for the landfill owner. Disposal to POTWs sometimes generates opp Slllllli II , the plant owner, typically a municipal entity or a private operator I'll11IIiII, I plant on a contract basis for a municipal owner. While the acceptan 'of II tli " certainly merits careful consideration, the waste stream should be eval utlt('d II II other industrial wastewater stream. Leachate is generally more corn] r('ltl'll 1\ characterized than other industrial waste streams. Modern Subtitle D land/lll III rigorous waste acceptance criteria to keep hazardous, toxic, radioactive, .uul I III noncompatible waste streams out of the landfill. Control of waste in))111Itl I' effect of improving the quality of the leachate.

The Clean Water Act (CWA) mandates that sources directly diM 11.11I into surface waters must comply with effluent limitations listed in the N,IIIIII Pollution Discharge Elimination System (NPDES) permit(s). The CW,\ II requires the EPA to promulgate nationally applicable pretreatment stand.url II restrict pollutant discharges for those who discharge wastewater indirectly 1II1IIii sewers flowing to wastewater treatment plants. National pretreatment st.uul \I are established for those pollutants in wastewater from indirect disch.uju II may pass through or interfere with wastewater treatment plant operations, III III tion, wastewater treatment plants are required to implement local treatment 11111 applicable to their industrial indirect discharges to satisfy any local requiu iu I (40 CFR 403.5).

Emerging Technologies for the Treatment of Leachate Because the treatment of leachate is both difficult and expensive, new In Iilllllil are being developed for managing this potential pollutant."

Reverse osmosis has nothing to do with osmotic pressure or osmosis, 11111 the common name for forcing a fluid through a semipermeable rnernbr.uu-, lit separating soluble components on the basis of molecular size and shape. II j 1111 of the treatment systems capable of removing dissolved solids. A high )111' \I pump forces the leachate through a membrane, overcoming the natural 11'1111111 pressure, and dividing the leachate into two parts: a water stream (pernn-,u product) and a concentrated (brine) stream. Molecules of water pass thnlllf,h III membrane, while contaminants are flushed along the surface of the rucuilu 1\ and exit as brine. Typical recovery rates (percentage of the feed that h.'11I1I1 permeate or clean water) range from 75 to 90%.

Il/I'I~/,( OS'''I'S/.~ 1'1I111'('/11/'111/011 Is ,I )kl·1 'Ill)) .rnturc-m nuhrnnc !,IO liI,11 II,II,'I! w: HI' SlI' '< ms In n I W·f'll' 'SSLII').nvironm nt, '1'11 'ySI m ( p 'I't IC'S I Y I In '- I 1 mil 'I'm t bl m 11brs n b tw I th I a h t nd an sm ti ( I t, tYI i- II", alt brin with c n ntration of approxirnat ly 10%, Th s mip I'm (I I

1IIIIIIIIlil all W th passag of water from the leachate (without outsid PI' SSLII') 11111II's, It brin but rejects the contaminants found in the leachate. As th PI' SS 111111111II, th brine becomes dilute. This process is called osmosis, and it ontinucs

11111lib w< l r oncentrations on both sides of the membrane are equivalent. IlllllfJoraLion will actually dispose of the water component of water-bas I

I 1\ str ms, uch as leachate. This technology can reduce the total volume f I. II h,1I l be managed to less than 5% of the original volume. Other technol -

It (I' v r osmosis, ultrajmicrofiltration. and conventional treatment systems) I )l11.H th waste stream into two components, but the water produced by th s

111111'1U' atrnent technologies must still be disposed of. Using landfill gas as the fuel Itl I'V,'r> ration is a technology that effectively integrates the control oflandfill gas lid I. ndfill leachate. Evaporative systems typically require an air permit for th '

11.111find, usually, a modification to the landfill's solid waste permit to address th ' " ,IIh, L management practices.

1\ vapor compression distillation process differs from an evaporation proc SI) III Ihnt a dean effluent is produced. Leachate is introduced into the VCD system tluuu h a recirculation loop. This loop constantly pumps leachate and conc n- II,lll8 t a high rate from the bottom of a steam disengagement vessel through a 1" 11l'"y heat exchanger and back again. The leachate and concentrate enter thc "I I"n agement vessel through a tangential nozzle at a velocity sufficient to creal'

I I nic separation of steam from the liquid. As the leachate and concentrat ' \I I' pidly recirculated, active boiling occurs on the inside of the primary h a I

lilt nger and within the cyclonic pool formed inside the disengagement vessel. The mechanical vapor recompression process uses the falling film principl

III ,I vacuum. The core of the process is a polymeric evaporation surface (heal u.uisfer element) on which water can boil at a temperature of 50 to 60°C (120 III 140°F). The process functions similarly to a heat pump. The raw leachate I purnped through two parallel heat exchangers and into the bottom of the

VIIJ oration vessel. From there, a circulation pump transfers a small volume or IIIi I achate into the top of the vessel, where it is evenly distributed on the heat II.msfer element,

Several different types of land treatment systems are available for treating 1,\1\ fill leachates. However, these systems are usually sized for small leachat tlows. since a significant amount ofland could be required to handle larger flows, III ' three most common forms of land treatment systems found at MSW landfills \I(' onstructed wetlands, windrow composting, and growing poplar trees.

achate Recirculation I .mdfills can be used as readily available biological reactors for the treatment or ",Icilate. The landfill can be essentially transformed into an engineered react r

stem by providing containment using liners and covers, sorting and discrete dis- JIll al of waste materials into dedicated cells, collection and recirculation of leach- III" and management of gas. Such landfills are capable of accelerated biochemical II nversion of wastes and effective treatment of leachate.

M sr sanitary landfllls arc tradiu 11aIIY('OIISlllI(l'IS)lh'I',Hlldl' Iltll and removed. TI1 rat of st bilizau n in II lry" 1,11 IIdls may I' 'qll 11'1111111 thereby extending the acid formation and III th n ~ rill ntati 11pll,I I' III stabilization over long periods of time. Under these circurnst n 'S, d '( I lilli' I I of biodegradable fractions of solid waste will be impeded and ill 01111'''II preventing commercial recovery of methane gas and delayi n losur ' urul I II future reuse of the landfill site. Furthermore, during singl -pass Opt'I"111I1I leachate must be drained, collected, and treated prior to final dis haq~I'

In contrast, leachate recirculation may be used as a rnanag '1IlI'II1 ,tli I tive that requires the containment, collection, and recirculation r 1(''\111,111I through the landfilled waste. This option offers more rapid dev 10pIlll'III I II I anaerobic microbial populations and increases reaction rates (an I prvd IIIdll1i of these organisms. The time required for stabilization of the radii .1 11111 organic constituents can be compressed to as little as two to three y t rs 1,111111II the usual 15- to 20-year period. This accelerated stabilization is enhnu« d It I routine and uniform exposure of microorganisms to constituents in IllI' II 111 thereby providing the necessary contact time, nutrients, and substrat 'S 1011/1 I conversion and degradation. Hence, leachate recirculation esseruinll I III I the landfill into a dynamic anaerobic bioreactor that accelerates the ouvr« hll organic materials to intermediates and end products.

The advantages of leachate recirculation have been well do. 111111II Application of leachate recirculation to full-scale landfills has 0 (11111tI increasing frequency in recent years. Leachate is returned to the landfill II II variety of techniques, including wetting of waste as it is placed, spraying (II " III over the landfill surface, and injecting of leachate into vertical 011111111 horizontal trenches installed within the landfill. It is important to ll\', I'll operate other landfill components-such as gas management systems, II hi collection, and final and intermediate cover-so that they are compauhh bioreactor operation.

To optimize bioreactor operations, the waste moisture levels must lit I trolled by the rate of leachate recirculation, which is a function of waste "vii "III conductivity and the efficiency of the leachate introduction technique. To I I11I UI the effect ofleachate, the recirculation operations should be moved fro III 11111I to another, pumping at a relatively intense rate for a short period of lillll' lit moving to another area. Many factors impact the rate of moisture input, 'ltl" the site specific capacity and the design of the recirculation system.

The quantity of liquid supplied is a function of such waste characu-n-ulr moisture content and field capacity. In some cases, the infiltration 01 11111II resulting from rainfall is insufficient to meet the desired waste moisture 01111III , optimal decomposition, and supplemental liquids (i.e., leachate from o11H'1ill water, wastewater, or biosolids) may be required. Sufficient liquid supply 11\11I , ensured to support project goals. For example, the goal of moisture disu rhnu might be to bring all waste to field capacity. The most efficient approach III II I field capacity is to increase moisture content through wetting of the wasu- ,II II working face and then uniformly to reach field capacity through liquid ',11111 application or injection.

I'll' id lltl: 1101\ uppl 'III '11([\111[ul Is III tll . 1.1<{' II )W ol le.u h.u ' 1'1011\ 1,1111111.'rhts ad lill 11<11flow I11USt ' nsl I I' I lurin I 'sign, 'sp'dull 1'01

, 111\1.\11 vents when Itl"J am unts fl t hat III y n r tcd. Sufi lent u luu • st r( 'must b provi led to nSLlI that p ak I a hat g n r ti n v nts

III 1 ( ac rnrn I. t d. While a properly designed and operated Ian lfill will IIIIt I"I~' tr In flu tuations of leachate generation with rainfall vents, in w I tllthlt '~, I hat g neration will at times exceed the amount needed for recircu- I I 1111, Lit r fa tors-such as construction, maintenance, and regulations-may I II I HIt that leachate not be recirculated from time to time. Therefore, it is y ry

1!11j1l1l1.1I1lt have contingency plans in place for off-site leachate management for 111111"1wh n leachate generation exceeds on-site storage capacity.

I, -n hate recirculation should be controlled to minimize outbreaks and l 1'1III I': the biological processes. Grading the cover to direct leachate movement

.1 II' In side slopes, providing adequate distance between slopes and leacha tc IIII1ton, liminating perforations in recirculation piping near slopes, and avoid- II uv 'r that has hydraulic conductivity significantly different from the waste III ntrol seeps. In addition, it may be desirable to reduce initial compaction I IIId I to facilitate leachate movement through the waste. A routine monitoring

1"1111,111.designed to detect early evidence of outbreaks should accompany th 11"1,\ II n of any leachate recirculation system. Alternate design procedures-such

1,II1y apping of side slopes and installation of subsurface drains-also may be 1111 I red to minimize problems with side seepage.

'I'll depth ofleachate on the liner is a primary regulation in the United States " pHIL ct groundwater and is a major concern for regulators approving bioreactor

1"llItit . Control of head on the liner requires the ability to maintain a properly Ii 11111d leachate collection system, monitor head on the liner, store or dispose of

I. u hut outside of the landfill, and remove leachate at rates two to three times the lit· o normal leachate generation. Several techniques are used to measure head III Ih Iiner, including sump measurements, piezometers, bubbler tubes, and pres- 1111transducers. Measuring the head with currently available technology provides

1111,II information regarding leakage potential; however, for a more realistic evalu- 11111I,a more complete measurement may be required.

The construction, operation, and monitoring of leachate recirculation sys- It III will affect daily landfill operations. If a leachate recirculation system is to be II I rl, it should be viewed as an integral part of landfill operations. Installation of II I ulation systems must be coordinated with waste placement and should be 1111idered during planning of the fill sequence.

..3 Landfill Gas Collection and Use nerated within a landfill will move by pressure gradient, following paths

II I a t resistance. Uncontrolled migrating gas can collect in sewers, sumps, and III cments, leading to tragic consequences if explosions occur. To prevent gas III I" tion, gas vents or wells must be provided. There are two basic systems for I missions control: passive collection and active extraction. Passive collection

I' IIrns collect landfill gas using vent collectors and release the gas to the atmo- I'lwre without treatment or conveyance to a common point. Passive vents are III n provided using natural convective forces within the landfill to direct gas

to th aun sph I' '. In t IdlU n. passiv' vents ml] hI he 11,\ \' I :11 .ll1' \lltlll II boundary of an old landfill that is PI' du ing rnlnirns I am IIlIS 01 1.\11111 II These vents would prevent the gas from moving off- iL . Passive v '11111 11I,\\' I I I only a few feet below the cap or may reach uP. to 75% r 1I: landl II II \,11 designed in a similar manner to the active extracnon well des rib d IH'XI I I" spacing for a passive vent is one per 9000 yd3 (7500.m3) ...

Active collection systems link collection wells With plpmg nd .xu.u I 1111 under vacuum created by a central blower. Active extraction w lis rn: Y IWI II" or horizontal wells, although vertical wells are more frequently 1111 10y('\1 V. lilt wells are installed in landfills using auger or rotary drills. A typi al eXI I',l( I It III is shown in Figure 8-11. Wellheads provide a means of controlling t1H' ,II 1111 applied at the well as well as monitoring of gas flow rate, temp raturv, ,lIld I

r24 "-36" (tYP.II

muld IIn

R omm nd tl n

ln 'I 'I n (on center)

75% of depth or to water tabl ,whichever com s fir t

Bottom 1/3 to 2/3 Minimum 25 ft below surface 3-8 in PVC or HDPE, telescoping well joint Interior collection system 200-500 ft Perimeter collection system 100-250 ft One well per 1/2 to 2 acres 3%

12 to 36 in is standard (24, 30, 36 in most common)

III't: [351

nes" " § § ~

I 1

'111111 ly. pacing of wells is a function of gas flow. Table 8-12 provides general 11111:111 on well construction.

Landfill gas is extracted by central blowers that create negative pressure in the I' II \ n twork (Figure 8-12). These blowers are sized according to the volume or I,I Ih y must move. The greater the required flow, the greater must be the nega- II I PI' ssure that must be created, and therefore, the more energy that is required. 1111' !lection system should be designed to minimize head loss by providi ng 1111 i ntly large pipes and by minimizing the number of valves and bends in the

I' I " However, large pipes can be costly, and the design must balance the cost or IIII'I ipes and valves against the energy requirements of the blowers. The velocity or 1111' n w through the piping system can be estimated using the continuity equation II I ornpressibility is neglected .

Q= vA

Cover sod 10-6(01' IWII\ I)

Native backfill ~~--- 10-5 (or better)

Well bore seal

25'-0'

(min')J _--+-~~

. ,',

", ',;

+ Well bar' 2' 0"___ ---'-f- seal zone

--------- --------- II 'I'

Q = landfill-gas flow rate, ft3/sec v = landfill-gas velocity, ft/sec A = cross-sectional interior area of the pipe, ft2

The head loss through the pipe can be estimated using the Oarcy-Weisbach l'i(l lion, usually stated as

pfLv2 6.P= --

2gD

--------- ._-------- oo.~ frl n 0 0 0 cb Cd

0> 0:> o 0 0 0 Cb

CY Q, 0 00 o.~ o ~o GI n 0

o 0 u 0 0 0;00 • o ,00 0 0. 00 0 1" I I

C;O 0 k- ~1-'2 Crus lee rock (typ.)o

1"Wide slots 4J,i.-.....QL,...+----(-;-;:-6" -8 "), 12"ClC 6.P = pressure drop, ftf = Oarcy-Weisbach friction factor, function of pipe roughness

and diameter L = length of pipe, ft v = velocity, ft/sec g = gravitational constant, ftjsec2

D = diameter, ft p = gas density, lb/ft3

o

o 0<0

r qur 8·11 Typical vertical gas well.

Figure 8-12 Landfill gas well and piping network. (Courtesy William A. WI III' II,

The Darcy-Weisbach friction factor can be read from a Moody iii I which is found in all fluid mechanics textbooks. The diagram is lIsI'd II calculating E/D, where E is the pipe roughness-a function of the pip" 11\ I' The rougher the pipe, the greater is the value of E.

This equation gives the pressure drop in feet of water if all length 111111 feet and the fluid is water. If the fluid is a gas, the equation can be mod ill 1'1 I (I into account the density ofthe fluid) as

(O.0096)pjLv2 tl.P=-----

Dg where

tl.P = pressure drop when a gas is flowing in the pipe, ft of watci p = density of the gas, Ib/ft3

/1

ol'll!' ilH S calculutc IUHlllg Ih i unlv '1\:11 gas law: Mfl

Wf'

(I c.l nsity of th gas, lb/ft" !vI I I ular weight of the gas, lb/rnole II pres ure, Iblft2 U iniv rsal gas constant, lb-ft/rnole-T; 't' - absolute temperature, OR (OF + 460)

1'11,111 I (\1 pressure inside the collection pipes is dose to atmospheric, about 111111 II '. (water (13.6 lb/in? or about 1958 lb/ft"). The molecular weight of th

11111 I stimated as 28 lb/rnole, with the universal gas constant then having 1111 11/ Ib·fttR.

Iculate the pressure drop in 800 ft of 4 in diameter PVC pipe rrying 500 ft3/min of landfill gas at 120°F. Assume the pressure

11'1 the pipeline is close to atmospheric (about 13.6 lb/in-) and the molecular weight of the landfill gas is 28 Ib/mole. Assume the gas is Incompressible.

The area of a 4 in diameter PVC Schedule 40 pipe (4.026 in ID) 0.0884 ft2.

v = (500/60)/0.0884 = 94 ft/sec The roughness, E, for PVC pipe is 0.000005 ft. Thus, E/D =

0.000005/(4/12) = 0.000015. From the modified Moody diagram, the friction factor is read as f = 0.016.

The gas density is estimated assuming the pressure at 1958 lb/ft", the molecular weight at 28 Ib/mole, the universal gas constant at 1543 Ib-ftFR, and the absolute temperature at 120°F + 460. Thus,

_ (28)(1958) _ 3 p - (1543)(120 + 460) - 0.0613 lb/ft

The pressure drop is then calculated as

tl.P = (0.0096)(0.0613)(0.016)(800)(94)2 (4.02/12)(32.2)

= 6.2 ft of water

Repeating th calculatio s for 6 nd 8 i ii' h I I III drops are 0.8 ft and less than 0.1 ft. respectively, of w t r col 11lJ1,

Assuming head loss in the valves and fittings at 20% f th I I II pressure drop and adding an additional 10 in of water colu 11I I maintain negative pressure at the wellhead, the 4 in pipe re uir 'I 0\ negative blower pressure at the suction side of the blower t all III water column. The 6 in pipe requires about a 20 in water lumn, and the 8 in pipe requires less than a 12 in water column.

In designing the collection system it is important to recogniz the \111IIIII of the collection. A gas collection system can be designed and operate I 10 ,I(ItII maximum gas collection so as to minimize the release of landfill gas or il 1,1111\ designed and operated to collect landfill gas with the highest energy value. II 1111 former is the objective, a high vacuum would be applied so that all gas is Ii1.11II to the collection wells. This could result in air being pulled through th OVl'I IIII into the landfill. Since methane generation is an anaerobic process, pull hu IIII with oxygen would have a negative impact on methane generation. Landfills \ 'III active collection systems routinely monitor oxygen levels at the well head ,IS ,I 'I of adjusting the vacuum. If, on the other hand, the purpose is to produc tll\' 1I11\1t est quality gas for subsequent use, then a low pressure system is required !lLII III minimize the seepage of oxygen into the gas wells.

Technical Issues in landfill Gas Use Some of the technical issues regarding the use of landfill gas include gas ((111'1111 sition, the effects of corrosives and particulates on equipment, potential I IIII losses, and gas extraction and cleanup. Energy users are concerned with tlu: pllIl. lems and solutions associated with the use of landfill gas as a fuel sour I.'. ~IIIII information can be obtained from equipment manufacturers as to the fund,1I1I1II tals and site-specific applications of gas extraction and cleanup.

The physical. chemical. and combustion characteristics of landfill g,I" I II have significant impact on energy recovery equipment selection and OIWloi11tl11 Trace organics (such gases as hydrogen sulfide and others) and particul.ur-i I II cause corrosion and excessive wear. Carbon dioxide, nitrogen, and watci v,II"1 (with various inert materials) may reduce efficiency. Variability of gas COII,!"II tion and production rate over time exacerbates the problems associated wirh ] I cleanup operations and energy applications.

The primary source of trace gases is from discarded volatile rnateri.ils '"HI their transformation by-products. Although most trace gases, primarily hyd 1111111 bons, are harmless to energy use, halogenated hydrocarbons may cause proh], lit upon combustion. Volatile and nonvolatile acidic hydrocarbons-such ,1'1II organic acids found in untreated landfill gas-are also highly corrosive. 11I11 constituents have been reported to cause corrosion, combustion chamber 1111'1iII and deposits on blades of turbine engines as well as internal combustion englill

Hydrogen sulfide and water vapor also can have corrosive effects. '1'111II of landfill gas as a vehicle fuel requires removal of hydrogen sulfide and W II I

''11111III'W 01"'0, Oil ploll'lll wh .n rhey (Oil I .nsc lu1'111['!-Ins )1111"'sNI0I1,111I IIhllll1!, II II' gcn 1Iind 'In n .ntrations a I was 10 pprn m,y I'nd I) ,', Iti 1111In \11 ing, st I' g l nks, and ngin s." Lan Ifill a with hydr g n sulfide 111I111\1\ I' n ntrati ns, hi fly chlorid and fluorid , as Jaw as 21 pprn an I I PI 111, r sp tiv ly, r quires pretreatment prior to application in fu J lis. ili II" IfI'lit J nu p t h nology is also applied to remove the trace constitu nts fr III

1111II II ~S wh n purifying it to pipeline quality natural gas." l.andfill ontain a large amount of soil and other particulate matt r.

I IhI 'I j n y t rns can dislodge and take up these particulates into the gas str aIll. I II posns in ngines and buildup in oil result in increased wear with a simultan - 1I11d r as in lubricant capacity and life. Particulates can be removed by gas I 111,11i n r gas refrigeration. Dimethyl siloxane (a gaseous silicon compound) ,II rnbust to produce silica deposits in internal combustion engine cylinders

III I ' s turbines, resulting in decreased combustion chamber volume, increased Il 1\11J' ssion ratio, a tendency to detonate, and abrasion of valve stems and guides 1111111hardened deposits.P 39

The purpose of treatment systems is to remove particulates, condensates, and II,!) mpounds and to upgrade landfill-gas quality for direct use or for energy/

nth tic fuel conversion. Filtration and condensate removal are the more com- 1111111leanup approaches, whereas refrigeration and desiccation are used less fre- '1\1\ntly or as a means to enhance cleanup efficiency."

. plications of landfill Gas Use l undfill gas can be flared (burned) on site, but this is not a beneficial application III this resource. Beneficial energy recovery systems include direct use, electricity II n ration, and conversion to chemicals or fuels. In 2010, the USEPAreported 519 iip 'rating projects generating 1597 MW and 306 mmscfd.

Boilers and other direct combustion applications are by far the cheapest and I ,I, i st options and represent about one-third of the current operating projects. I) r ct uses of landfill gas to replace or supplement coal, oil, propane, and natural l,oIShave been successfully demonstrated. Applications include boiler firing, space 111'<ting, sludge drying, and leachate drying and incineration. In most cases, gas (I anup consists of little more than condensate removal. There is little risk for the 'Ild user in terms of gas quality, use, and continuity of supply. The payback period

,,\11 be as little as a few months but is dependent on the negotiated price paid for III fuel replaced. The typical discounts of 10 to 20% are influenced by the pipeline tI , tance and the gas quantity, quality, and variations permitted. The ideal situation

ne where a user, located within a two-mile radius of the landfill, could accept .rll of the gas generated on a continuous basis."

The potential for use as vehicle fuel exists if the gas is upgraded to natural fiI,1Squality, vehicles are modified to operate on some form of natural gas, and lI,rueling stations are widely available and equipped for dispensing natural gas in III proper form. This technology is well established, and gas-powered, stationary lnternal combustion engines have been available for decades. Anaerobic digester fI,1 is processed into fleet vehicle and engine generator fuels in several locations III Florida, including Plantation and Tampa." In New Zealand, many vehicles can ,III'ady run on upgraded landfill gas, and the number is increasing." Worldwide, over 700,000 vehicles (many of them passenger cars) are fueled by natural gas."

II w v r, in m st untrl s, the LIS f 1.111 II gas 1111 (l vchk+c rill I 111111I landfill or oth r muni ipal fle ts with a limit d r< J1g r )P('I', rlon, \ 1111I" II I number of vehicles (modified for process d landfill gas us ) provkle tlu .111111 necessary to support a more diverse infrastructure, the g n r"lion or 10111111111 will outpace the fuel requirements of a landfill fleet. IS

The benefits of using landfill gas as a vehicle fuel incJud iInproved I I IIII!II benefits that extend from reduced flare emissions to lower emissions 1'1'0111I I It I that burn landfill gas as an alternative to diesel. In 2010, th Allflll)()111I 111111 near San Francisco installed a world's largest landfill gas (LFG) l Ii 1[I('I1I'd111111, gas (LNG) plant. This plant is designed to produce up to 13,000 gilllClI1'11>1I' per day or enough fuel for 300 garbage trucks.

Conversion of landfill gas to synthetic fuels and chemicals is a lso po 11,1 not economically feasible. The technologies include hydrocarbon I rocill' 1111"I the Fischer-Tropsch process and methanol synthesis by high-pressure I It. 11th catalysis and partial biological' oxidation. Gas-based chemical proc SS('S II" I thesizing acetic acid and other compounds are also available. Thes tcd IIIIIII'I I were developed for large-scale production of synfuels using coal-gas 1(,(," 11111 Production ventures were costly, and their products were expensive. 'I'lli'I "It I landfill can produce only 1 to 10% of the gas required for the size of plant I' ," I I ered for these technologies and processing techniques. 19

Electrical power generation (internal combustion engines and gas 111I111,,, II by far the most common landfill gas-to-energy application. The generation (II , I tricity from LFG makes up about two-thirds of the currently operational 1"1II•• I in the United States. Electricity for on-site use or sale to the grid can be ~('III'III I using a variety of different technologies, including internal combustion ('111\111 turbines, microturbines, and fuel cells. The vast majority of projects use illil II combustion (reciprocating) engines or turbines with microturbine t 11111"" being used at smaller landfills and in niche applications. Technologies Sill" I Stirling and organic Rankine cycle engines and fuel cells are still in developn« III

Projects are set up according to the perceived electrical power g<'II('I.111I1 capacity and the number of generating units. If landfill gas production is III III ficient to support at least one MW of power generation, it is generally ((\'\ 'II tI economically unsuitable. Internal combustion engines are typically used "I '.11 capable of producing less than three MW. Three to five engines are employed I' I project. Turbines, with higher horsepower and performance when operating ,I' 11111 load, are used at landfills where gas quantity can support greater than three M requiring only one or two turbine units for generating electricity.'?

Electrical power generation (fuel cells) is another technology that is sultllli to unfavorable economics when using landfill gas. Fuel cells are basically ch'. 1111 chemical batteries utilizing molten carbonate or phosphoric acid, fueled by .111,1 petroleum, natural gas, or other such hydrocarbon feedstocks. Hydrogen from IIII converted fuel combines with oxygen to produce electricity. The advantages 01 '11I1 cells over other use options include higher energy efficiency, availability to sm.ilh : landfills, minimal by-product emissions, minimal labor and maintenance, .llItl minimal noise impact.w Economic analysis reveals the benefits of fuel cells COIIIIII rable to their environmental impacts. Locations with higher population denslth generally have greater air emission problems. Likewise, commercial and indusn II power rates are greater for these highly industrialized, highly urbanized areas,

III 11/JI:l/lllll'I,I"~I/II:~,I~' ,111/IIII/III"I""I/,)! g(/.~ N ,11() .111aura '1Iv' ,HI( 11 /( I' rhc lI~' )(' H·, HI ,11 111i1)I'dl("I"11 'S I I " I ,nil '

1\ "1111111"1M In Ill/ slli n nd 11 rnv W II I n I gas : nd pi, lin '. lunlil It III ombllSl9 1 I. g~ III I l. L< ndfill <s has I w I' Btu 011

I , • (t Wit rnp rallll· IS rn r 0 .. .. d coruai • I II II en II' ti n f d .. bl ' . UOSIV, an C ntarns mu h 01'" I·

f un sua gas s ( 0 0 N) d 0 Ilhlll ! 1/ lin -quality natural gas. Dili ent ~ra2' . 2 an h~.rmfl.ll hal (rbOl1s 1111'1('/(rc 11 ssa,y to render landfill :s virtu ction ~nd stungent I anup arc 11111111111''1'1, 1· q .. dIg ally deVOid of all compon nts S;1V', lilt gas c eanup an ex . d ' • II I I,ll 'rn( Iives, includes nearly co~plete cf~n:lve an 1 ~omplex ~ro~ess for th 'I'

111101'I ndfJll gas to natural gas that only la: ee~o~~il 0 expen~Ive ISth convcr- It/ (,11 n ssary to support operatio A1~ an I scan attam the econ mics I I, 'W r than 50 landfills in the Un~:d Stat~~gp~o~~ceer'galeSs;ela?orl~te pro; , Is

lor pipe me LIS . ntechnical Issues related to landfill Ga U

111\1I ineficial use of landfill gas has safe . 5 se 11111" ,/,I~re also exist economic (as well as~~ e~vlIon)~ental, and .energy impl: n· IIiDlu lion, or co-generation. For an land~latory Iss~es for direct use, en rg I II(Il must be identified and th·:' gas project, these nontech n i a I /.11I ir d into an overall c~st-bene~~ra~a~;~~s~valuated, quantified, and ultimalely

/I ,~~:~·~~ec~::~}or aof~site use generally involves investor capital for d fray- II ¥11rs/opera~ors. Ge:~ra~ngu::v~~~ee~~o~s~::nd gene~ates ~even~e for landfill 11011mic benefits from landfill-gas-to-ener rd~ll gas ~s ~asler said than don . (01/1I in some cases, state) tax incentives and~t ejects exist I.nthe form of federal dl,'I',Yof stat: and local governments to enact l::i:~~~: ~~~l::gu.~~:eryver, ~~ert n~ II I at~r stnngency than those of federal standar . gui e! ncs I I nornically available for the recove d ds may stifle the use of opn 118 Ilhstacles, a landfill-gas project must ~t~? h:: ~ landfill ?as. Despite al.l o~ Lh'S ' ,.'Iurn on investment from operations (without the :otentlal to ama~s signif ani 111'11fits) to attract investment M . e enefitoftaxcreditsand other Ililly through tax credits and fa~ora~~~ pro~cts, h~wever, are. justified finanCially

Consider tw . . pur ase pnce as required by regulation. o competmg scenarios Some st t 1 f,

111 nerate a percentage of their elect .. ty f a es set goa s or power utilities hutld new solar and wind plants t IlC! rom1 gr~e.npower. These utilities could

o generate e ectncity 0 .f th T I I d plant, they can switch to landfill gas. The utility wo~l;'~ ~ ut~:ty has a gas- K' n electricity from its existing facility. en e a e to general

-3-4 Ge.~te.chnicalAspects of Landfill Design Lindfill stability IS an important as ect f d . . I(~mplex, m.ultilayer construction of ~ode~ la~~~is.P;a~I~~~:r1y in light of lh~ /,lIlures during the construction of a landfill ft th 1 can occur as sl p( I'he critical point of failure is usuall ~ or .~/ er e an~fill has been clos d. .osynrhenc interfacial surfaces as weIias :a:~I geosynthetlC an~ geosyntheli /

I .itastrophj- results, including dama e of 1 ch slopes. Landfill .fallures can have 1.lmination of the surrounding envi; ea ate-and ga~-collectIOn systems, COil life. The stability of landfills must b n~ent,. anddeven-lO extreme cases-loss (l(

. . e mvestIgate under both I· d . , I nditions, as required by RCRASubtitle D. stau an S ISIllI(,

I., ndflll Hinbilily III normally nlwly:t. '{llIN liB I' ',1 II avull.ihl I (1111111111 war. Thi analysis I' q iires kn wi d O/'PIOIl '1[1 •• Oi'W[1SI', 111 [ll('1 .111II II I liners and caps (synthetic and natural), 11 I ~ un latlon s iils, The {' 1IIIIId well-known geotechnical properties as unit w ight, sh rstr ngth, 111'1111111 internal friction angle, cohesion, and internal par PI' ssurc. pili. I '" lid material properties such as tensile strength, surface rough n 'SIl, II(' II III surface wetness. Waste properties are often difficult to dei rrnh • hl'l dllil II/ heterogeneity, changes in properties with time, and th dim ull II 11111 samples necessary to evaluate these properties.

The operation of the landfill as a bioreactor can creat In iqur fJ,1'1111, I, conditions. Increased moisture content leading to waste saturation (,III II ,iI positive internal pore pressure and reduced angle of friction (a ngl . al wi ill II I failure occurs at an interface). The impact of waste decornpositi non sll '.11 II I is aiso of critical concern. Many studies report finding mud-Iik on It" III bottom of wet, deep landfills. Overaggressive leachate recirculaii n (('IlII,!1I111I1 by the use of impermeable cover soil) has been cited as a contributing 1.111111II catastrophic slope failure of a landfill in Colombia, South Ameri a.

To minimize the probability of landfill failure, it is recornrn nd('d dill slopes of completed and capped landfills be no greater than 1:3, witll I I I preferable. Shallow slopes mean reduced air space for waste disposal; 1)(IWI I I reduced risk of slope failure outweighs economic advantages of incrcasc.] III I' volume. Use of textured geomembranes can also reduce slippage al III III II Finally, proper drainage and gas-pressure relief in the cap will redu pOll 1'1 and reduce the probability of failure. Given the uncertainties of design, NIHIIIII factors of safety are highly recommended.

8-3-5 Stormwater Management Many operating and design controls are available to minimize leachau 1"11I tion, including control of the size of the working face, placement (II 1111I cover on the waste, and use of proper stormwater runoff and run-Oil 1IIIili Control of stormwater run-on and runoff is also required by Subtitle I) I" I I (Section 258.26). Run-on control prevents the introduction of stormw.u. I III active area of the landfill, thus minimizing the production of leachate I Ii I I and contamination of surface water. Reducing run-on also limits the prlldlllil of runoff from the landfill surface.

Run-on can be prevented by diverting stormwater from active a 1'('.1"III , landfill. Any facility constructed to control run-on must be capable 01 11,111111, peak volumes generated by a 24-hour, 25-year storm. Typical measures ((11111111 run-on include contouring the land surrounding the landfill cell or COII~"111111 ditches, dikes, or culverts to divert flow.

Runoff that is generated can be collected by swales (see Figure 8-D). dill" berms, dikes, or culverts that direct contaminated runoff from active areas II' 'dill I and treatment facilities, and uncontaminated runoff from closed areas to dl'll 1111 facilities. Runoff from active areas must be collected and at least the VOIUIlII'1\111 ated from a 24-hour, 25-year storm must be controlled. Local regulations wlll,1 tate the design of uncontaminated stormwater management facilities. For I'X,IIIII' in Florida, a detention pond must store the first inch of runoff for 14 days

Eroslon .ontro] matting

uti 25'- 40' 1

Vegetative Support: lay 'I'

Geocomposire drainage 111 .din ::::::::=J 1 3-4

'I III 8-13 Side slope swale in a landfill final cover.

-6 Landfill Cap 1\ Subtitle D landfills have to be capped, regardless of the potential for water

1111111ion. The purpose is to prevent the production of leachate that can contami- 11.11' groundwater. The effect of keeping water out of the landfill is to maintain .II nditions and hinder the process of biodegradation, making most landfills 1111rely storage facilities.

nee the landfill reaches design height, a final cap is placed to minimize 11111~l'ation of rainwate~, minimize dispersal of wastes, accommodate settling, and 'II lltate long-term. mamtenance of the landfill. The cap may consist (from top to \11111,m) of vegetation and supporting soil, a filler and drainage layer, a hydraul: ",111'lr, fo~ndatlOn for the hydraulic barrier, and a gas-control layer. Figure 8-14 is I h mauc of a recommended top slope cap. EPA regulations require that the top I ,Ip b less permeable than the bottom liner.

Slope stability and soil erosion are critical concerns for landfill caps. Typical II' lopes are 1:3 to 1:4, and the interface friction between adjacent layers must

" III seepage forces and may decrease the contact stresses between layers due to I II ldup of water and/or gas pressures. Consequently, on slide slopes, composite 1111(r caps (geomembrane placed directly on top of a low permeability soil) are not III rnmended.

An alternative to the conventional landfill cap design-the evapotranspiratioll 1/1111':,als~ known as ~e capillary barrier-has been introduced. This cover places I 111m(6 m) lay~r of silt over a 2.5 ft thick layer of uncompacted soil. The silt layer upports vegetauon for transpiration, while the soil layer provides moisture storage.

lit' ause the uncompacte~ soil layer remains unsaturated and at lower suction pres- III than the satura.ted ~Ilt layer, the flow tends to be driven upward by capillary

1111s'. an.d percolation mto the waste is prevented. Moisture is then removed by II.tnsplIatl?~ and evaporation. Studies have shown that the evapotranspiration cover I .tppropnate for most areas of the United States west of the Mississippi River. 43

Iflm1lt~m~I~III-Gravel layer (6") Daily cover and refuse

Clay cover

Strip drains Shear settling

- Rev zetat d topsoil (6")~~~--~~--~~~~

- Clay barrier (24")

- Revegetated topsoil (6")--~~--~~~~~~~ Protective material (18-. ()")~.".~~~~~f4.~~- LOPE barrier (60 mil)

:: : : : : : : : : : : : : : : : : : : : : : : : : : : =: : : : : : '--- Sand bedding (4") _- Gravel layer (6")

Daily cover and refuse LOPE cover

Figure 8-14 Typical caps used for closing landfills.

As landfills settle, caps can fail and allow storrnwater to penetrate into I,tlul fills. Figure 8-15 shows three modes of cap failure. Although data are S(.\14', I seems reasonable to assume that the caps are at best short-range covers, ;Jlld ill within some years, they will become ineffective. Caps are expensive, appro.u hili $200,000 per acre oflandfill area, and it is unlikely that they contribute 111111 It hi the protection of groundwater.

8-4 LANDFILL OPERATIONS

8-4-1 Landfill Equipment The movement, placement, and compaction of waste and cover in a 1.11111111 require a variety of large machines, including tractors, loaders (track and will' II and compactors. In addition, a variety of support equipment is needed, in(illIlllI motor graders, hydraulic excavators, water trucks, and service vehicles. .

Originally, track-type bulldozers were used in landfills. Not only did III spread and compact the refuse, they could be used for placing and spreading Iii 'I material as well as preparing the site, building roads, hauling trees, and rernovh

-Liner~--------------------

: .

Rotation settling

Ilgure 8-15 Three modes of cap failure.

lumps. They typically can achieve waste densities of 800 to 1000 lb/yd" (475 LO i) 0 kg/rn"). See Figure 8-16a.

Specially built landfill compactors (Figure 8-16b) are now almost universally ns d. They are effective in spreading and compacting large quantities of waste. .II" quite heavy (exceeding 25 tons), and are equipped with knobbed steel wheels rapable of tearing and compacting waste to densities of 1200 to 1600 lb/y 1\ (710 to 950 kg/rn").

Daily dirt cover is excavated and placed on the operating face of a Iandf II II ing pans or scrapers. Designed for the sale purpose of scraping up dirt an I

.,, .,. r:

Figure 8-16 (a) A landfill dozer. (b) Landfill compactor. (Courtesy William A. W Jill III

hauling it to another location, these machines bring the cover dirt to the 1.111I1i11l and then run down the compacted slope, discharging the dirt evenly over the 11'111

The type and quantity oflandfill vehicles are determined by the amount 111111 type of waste handled, amount and type of soil cover, distance of moving Wit I and cover, weather requirements, compaction requirements, landfill configui.u 111\ budget, expected growth, and supplemental tasks anticipated.

IIIIn9 qu nc 11111.1 tlv '1<111 I ill, I, ily 1.liv ri s f' wast ar pi d in lifts r lay .rs II ( P of

Illt,lln'l'tlnll, hat 011 ti n svstem ro depths of zo m tris ft) r greater.Typlcnl m nl m thods used for landfill construction are shown in Figur 8-17,

1111' ar '< r m und type of landfill construction is commonly used in ar as with I 111\11 1'0 indwat r table or subsurface conditions that prevent excavati n. Till' I 1,IVllli n r trench technique provides waste placement in cells or trench s dUI' lito 111' sub urface. The removed soil is often used as daily, intermediate, or final

Figure 8-17 Commonly used landfilling methods: (a) excavated cell/trench, (b) area, n I (e) canyon/depression. Source: [48)

Earth embankment

,Final cover (sloped)

(a)

Earth embankment

Solid waste cells Final cover ( loped)

(b)

Final cover (sloj 'd)

Original ground surface

cover. Where suitabl ,wast als an b pic d a alnst Hned anyon 01 I ,IV lit' III slopes. Slope stability and leachate and gas ernlssi I ntrol t r rili "I L III' 1111 this type of waste placement.

Because it is imperative that the leachate collection system b pr 1(,(('II dill ing landfill operations, waste in the first lift is selected to avoid h avy ncl 1111,111 objects. This layer is often called the operational layer. The wast also 11111'11 II placed in a manner to keep compactor wheels away from the leachai 0111'1111111 system. Filling then begins in subsequent lifts, generally in a corner and IIIIIV I"I outward to form the next layer. The filling sequence is established at the 11111' II landfill design and permitting. The working face must be large enough 10 ,H 111111 modate several vehicles unloading simultaneously, typically 12 to 20 ft (4 10 (, 1111 per vehicle. Some landfill operations will, for safety reasons, provide, 1'111111 dumping area in order to separate the public from the garbage trucks.

As waste is placed in the landfill, heavy equipment is used to cornp.u t tit waste to maximize use of airspace. Airspace is the volume of space on a land 1111 II permitted for the disposal of waste. At first, the space is occupied by air, Willi" replaced by waste as the landfill fills. The degree of compaction expected is a hUll tion of several factors, including refuse layer thickness (see Figure 8-18), numlu III/ passes made over the waste (see Figure 8-19), slope (flatter slopes camp" 1 hi III I by landfill compactors, steeper slopes (maximum 1:3) compact better by 11,1t type tractors), and moisture content (wetter waste compacts more effectively 111i1U dry waste).

8-4-3 Daily Cover Waste is covered at the end of each working day with soil (typically 6 inclu-r] III alternative daily cover (such as textiles, geomembrane, carpet, foam, greenwa: II " other proprietary materials). Daily cover is required to control disease veCIOI ,III I rodents; to minimize odor, litter, and air emissions; to reduce the risk of fire; ,1111111 minimize leachate production. The landfill sides are sloped to facilitate maintcu.uu. and to increase slope stability; generally, a maximum slope of 1:3 is maintaim-d

o

lb/yd-' kg/m3 1000.-----------------~ 750 Layer thickness

1500

1000 500 500 250

00 .5 1.0 1.5 2.0 2.5 3.0 Meters I I I I I I I I I I I o 1 2 3 4 5 6 7 8 9 10 Feet

Figure 8-18 Waste density of a landfillas a function of layer thickness.

1 Increasing

density

o 1 2 3 4 5 6 7 8 9 10 Number of passes made with each steel wheel, rubber tire, or track

l lqure 8-19 Waste density of a landfillas a function ofthe number of , I m actor passes.

4-4 Monitoring I .indfill monitoring is critical to the operation of a landfill. Most commonly, land- I II perators monitor the following: leachate head on the liner, leakage through till landfill liner, groundwater quality, ambient air quality (to ensure complianc

rh the Clean Air Act), gas in the surrounding soil, leachate quality and quantity, l.mdfill-gas quality and quantity, and stability of the final cover.

Control of head on the liner requires the ability to maintain a properly ill Igned leachate collection system, monitor head on the liner, store or dispose of 11,\ hate outside of the landfill, and remove leachate at appropriate rates. Leakag III 1'0 ugh a single liner is often detected using a lysimeter, such as that shown in I Igure 8-20. The locations and number of lysimeters are based on the landfill drsign: however, more than one should be provided to ensure redundancy. The I simeter should be located below the crest of the landfill liner where the maxi- Ilium head will be found at the point of maximum leakage potential.

Groundwater monitoring is generally accomplished through the construe- lion and sampling of monitoring wells in the vicinity of the landfill. Optimum .leaign would place a cluster of wells in each location to provide the means to rvaluate groundwater quality at multiple depths. These well clusters should be I laced up-gradient from the landfill to evaluate background groundwater quality, ,I well as immediately down-gradient from the landfill to determine the influ- I 11 e of the landfill on groundwater. Additional monitoring wells are also placed .11 the property boundary. These wells are sampled quarterly for a variety or III' anic and inorganic groundwater constituents as stipulated in RCRA Subtitle D II' ulations. If down-gradient constituents are found to have a statistically sig- nlficant increase in concentration as compared with up-gradient concentrations, III re complete monitoring must begin. If down-gradient levels continue to I' eed drinking water standards, more extensive monitoring and a correctiv ,\ tion plan may be required.

Sample b rrlc

vacuum In

3 . 16-m. copper

tube Backfill Discharge tube

6-in. diameter hole backfilled with silica s:l1Id

2-to 4-in. bentonite seal

Plastic pipe 24 in. long

Porous cup

Figure 8-20 Porous cup suction Iysimeter for the collection of liquid samples from the landfill. Source: Based on by Tchobanoglous, G., H. Theisen, and S Vigil, Integrated solid waste management: engineering principles and management iSSl1f"f, McGraw-Hill,1993.

The EPA permits a waiver of groundwater monitoring requirements if il 1.111 be demonstrated that there is no potential for the migration of hazardous const itIt ents from the landfill to the uppermost aquifer during the active life of the la rull II and the post-closure period. To accomplish this demonstration, computer model are generally used that simulate contaminant movement in the subsurface. '1111 Multimedia Exposure Assessment Model (MULTIMED) was developed by the 1'1' specifically for this purpose."

Landfill operators must ensure that the concentration of methane gas dlli'~ not exceed 25% of the lower explosive limit for methane (5% concentration) II facilities, nor does it exceed the lower explosive limit at the landfill boundary. (;,\

Concrete

t::::*-- Bentonite pellets ~±d-;:--l

.~~-- 2"0Threaded SCH 40 pve well screen

Washed pea gravel

Figure 8-21 Landfill-gas monitoring probe.

monitoring probes, such as the one shown in Figure 8-21, are generally placed C1( the property boundaries and various other locations around the landfill properly to allow the landfill owner to test for landfill gas. In addition, gas extraction wells, s shown in Figure 8-22, may be placed at the property boundaries and other 10 :t-

tions to collect any landfill gas that is migrating toward the property boundaries,

~igure 8-22 Gas extraction well in place. (Courtesy William A. Worrell)

The hydrogeologic properties of the site, soil conditions, and the placerncut II! landfill structures determine the locations of the probes.

8-5 POST-CLOSURE CARE AND USE OF OLD LANDFILLS

RCRA Subtitle D requires close attention to a landfill for 30 years follow "I closure. The following are required during the 30-year post-closure period:

• Maintenance of the integrity and effectiveness of the final cover • Operation of the leachate collection system • Groundwater monitoring • Gas-migration monitoring A detailed post-closure care report must be prepared and approved prior to W,I'I!I receipt at the landfill. The report must also include assurance that sufficient fillIII are available to meet the costs of closure, post-closure care, and corrective action IIII release of contaminants. These funds can be ensured using a variety of instrument such as surety bonds, cash deposits, irrevocable letters of credit, or private trust flllill

Once a landfill is closed, the land can be reused for other purposes. II 1\ important to consider the final use of a laridfill during planning and design ph,I'H'~

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"M unt Trashmore" in the otherwise relatively flat state of Indiana is a III pular ski location. The Harborside International Golf Complex near Chicago, III uoi -formerly the Chicago MSW landfill-hosts two world-class 18-hole golf I lillI'S and a 58-acre practice facility. A 50-acre landfill in urban Cambridg , tv!.!, s chusetts, now provides a variety of sports and recreational facilities, includ- I 1\ 1h r e soccer fields, three softball fields, two play areas, bocce courts, and j g-

I Il trails. The project increased the city's open space by 20%. The reuse of a landfill site is complicated by leachate generation, gas em is-

IOl1s, and the potential for large differential settlement. Differential settlement Illn be particularly damaging to overlying structures and embankments, as well as .tu I achate- and gas-control facilities in place. Settlement is the result of added III i ture, external loads such as final and daily cover, overlying wastes, and any ,II I I d structures such as buildings and roads. Settlement typically occurs quite 1,l['idly immediately after closure (first 1 to 12 months) as a result of the rapid Ills ipation of pore fluid and gas. This period of primary settlement is followed by I wer settlement over the next 15 to 20 years when secondary settlement occurs as

.t r ult of waste creep and biodegradation. This long-term settlement may result II ompaction of as much as 30% of the waste depth. The rate and magnitude of -ulernent are difficult to predict due to the heterogeneity of the waste and com-

pi degradation processes. Poor foundations can result in unfortunate situations, II h as the destruction of the motel shown in Figure 8-23.

Settlement must be considered when designing structures such as roads, park- lng lots, and structures supported on shallow foundations. Explosive gas can collect h tween the landfill surface and the supported floor of structures employing deep loundations built on top of closed landfills. To compensate for settlement, perma- 1\ nt structures may need to be equipped with costly foundations and articulated and ( .lescoping pipe connections to maintain proper drainage and integrity of facilities.

8-6 LANDFILL MINING

If significant biodegradation occurs in a landfill, it might be possible to dig up old landfills, separate the nonbiodegradable fraction, and use the dirt and organic

il as a cover material for present landfills. This seems like a reasonable option r communities seeking new landfills, and at one time it appeared that landfill

mining might be widely practiced. For example, a south Florida landfill mined an

Figure 8-23 An unfortunate motel built on a settling landfill. (Courtesy P Aarno VI illll"

old landfill and recovered some metals. However, with the continuing l'VI IiiIIIiill of landfill regulations and control of landfill gas, it has become very din iI IIII III mine landfills. Opening the cap of a landfill allows gas to escape to till' .1111111 sphere and permits rainwater to produce contaminated runoff. Finally, III IIItI materials have marginal economic value because they are quite dirty and d 11111I" to clean.

Landfill mining makes economic sense only if it can create new volunu 1111 continuing the life of an existing landfill and can be done at minimal enviro nuu 11111 cost. Only shallow and wet landfills that have few vertical lifts are cal1(\III,III for landfill mining, since they are most likely to have fully biodegraded. 1..11111111I mining operations consist of excavating the buried refuse and screening it t<l '1\ III rate out the useful new cover materials. Typically, the nonbiodegradable Wllilll have little value and are re-landfilled. Successful operations significantly ('1(111111 the time to closure for landfills and thus provide an economical alternative III III siting of new landfills."

8-7 FINAL THOUGHTS

Landfills are engineering projects that require an unusual mixture of lCt 111111II skills and public relations acumen, with the latter often outweighing the i()IJIIII Little is mentioned here relative to the nontechnical problems associated wit l: lit planning, design, and operation of landfills, but this is not to imply thai Ihl aspects are insignificant.

A popul.u' d 'f lillloll )/ 01 d W,IMt(' , 111111"I( Is Ill' 111111'1'('V 'I ()Ill' wlIllls II II I' I up 1111n ) one WIIIII I ut lown." Silldics Ol1 the I sy II )I()gy :111(/ S) 'i()logy II IiII d to (11)silillg of' I. n lfllls (I"' 'Lh r s in, Lingc n Ism what Crigll'l in . W' 11Il(IW'I tircly l. liulc about j ubli r acti n t and th lrnps l fsu h pro] ts, ,IIIl thc d si n ngin r is fl n I la I in a sorn what UD ornf rtabl positi n of II, ng th Solomon-like judge of public reaction and publi good. Nowh r lees III' human natur aspect of the engineering profession become as important as in III' Icsl n ofthe ultimate disposal of a community's residues, and too oft n, the 1IIIIn r becomes "public enemy number one." Why is this?

I\ngineering is an old and honored profession. Some ofthe earliest engin rs II th n wly founded United States were our best minds and leaders. Ceorgc

Wash ington taught himself surveying and supervised the construction of roa Is, 1,\11,Is, and locks. Thomas Jefferson was an inveterate tinkerer, and some or his III IS lS are clever even by modern mechanical engineering standards.

Engineers are indeed a special breed. A study performed some years ago I'V:1luatedthe characteristics of students who entered college intending to study Il1gin ering. The study found that those students who continued in the engin cr- lng program after two years differed markedly from those who left engine ring. ( )111y certain persons choose to become engineers and undertake the rigors 01' Ih demanding college curriculum that allows entrance to the field. Engineeri ng (udents and practicing engineers tend to have a very positive image of themselves

,\11I a lively esprit de corps. For them, engineering is fun. Engineers also see themselves as performing a public service. The American

.' iety of Civil Engineers motto describing the civil engineering profession as the /I[ ople-serving profession" neatly sums up engineers' perception of themselves. I\ngineers build civilizations. Engineers serve the public's needs.

But while engineers see themselves as successful problem solvers acting in th . public interest, the public's perception of engineers is often somewhat different, W II-publicized engineering failures resulting in damage to human and environ- 111ntal health have encouraged many people to perceive engineers as the creators, n t solvers, of problems.

Part of the public attitude toward engineers might be explained by the view that engineers tend to conduct social experiments." Engineers do not have all or the answers at hand when a problem arises and often cannot perform full-seal .xperiments to obtain such answers. For example, when designing a suspension bridge, engineers cannot construct a full-scale model of the bridge to test the t1 sign. Instead, they use the best knowledge and designs available and extrapolate, using sound judgment. Sometimes the extrapolation is faulty, and the experimenl ~ ils. Engineers learn from such failures and modify the design process in subse- [uent projects. What is important is that the experiments occur not in a private laboratory but on the public stage.

Engineering experiments affect humans in many ways. A road, for example, Is both a technical and a social undertaking. The road might affect homes an I businesses, create traffic and noise, or even divert funds from other projects. A new lawn mower is designed and manufactured, and it might cut grass effectively but t the expense of neighborhood quiet and perhaps even auditory damage to the

user. A reservoir is constructed but at the cost to recreational whitewater rafti ng,

lcsp ilutlon orn how 11l1,llld vll'gln '( os SI 'Ill, ()I 111,d ',~lllI('11)11 oj 1111 11111 ham st ad, 1\ n w bi I gi 81 wcap 11, I 'sigll xl to I ill P '01 lc, is \II' 1'/1'1

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the knowledge that its use would d vastat III gl bal 'yst 'Ill, Ilow III neer to deal with these conflicting public benefits?

The fundamental difference between how engin rs S" (111'111 • I I how the public sees them depends on how well engineers III ' '( plIlllil II III Sometimes engineers are labeled the "tools of the establish rn nt." I11(' 1ft/I /II of the environment," or the "diligent destroyers." With t11 publi, il ,~II/I II against "them," and "them" is too often the engineers. Why ar (l1e ('1111

11 11, I Ivillains?

The source of the problem is that engineers and the lay publi hold elillt ethical attitudes. Because engineers tend to be utilitarians, they 100 k .11 1111 II and aggregate net benefits, thus diminishing the importance or harnl I. I iii " vidual. Because engineers are positivists, they tend to ignore or dislnj,,~ 11111 'I! ations for which reasons of a certain type cannot be given-that is, qll,llIlIl or at least empirical data-thus ignoring intangibles. Engineers valu« 11111111 course, but they have a particular view of what is good for peal I :11111 1111 I good is to be determined in a given case. Finally, engineers think ()/ 1111 III I as doing applied physical science, not applied social science. 111e physit ,Ii • It I approach to engineering (ignoring the "people-serving profession" 1111111111ill engineers to think of their work as not being germane to the needs of Sill III conflict of ethical outlooks is a root cause of much of the problem wit II I "1111 interaction with the public.

References

1. O'Brien, J. 1977. Unpublished data. Durham, N.C.: Duke Environmental Center, Duke University.

2. Hershfield, S., P. A Vesilind, and E. 1. Pas. 1992. "Assessing the True Cost of Landfills." Waste Management and Research lO:471-484.

3. Nelson, A. c. J. Genereux, and M. M. Genereux. 1997. "Price Effects of Landfills on Different House Value Strata." Journal of Urban Planning and Development, ASCE, 123, no. 3:59-67.

4. Vesilind, P. A, and E. 1. Pas. 1998. "Discussion of A C. Nelson, J. Genereux, and M. M. Genereux, A Price Effect of Landfills on Different House Value Strata." Journal of Urban

Planning and Development, AS( I I no. 3:59-68.

5. Sleats, R., C. Harries, 1. Viney, .uu l I I Rees. 1989. "Activities anel J)'~llllilll of Key Microbial Groups in 1.111111111 In Sanitary Landfilling: Proce.l.l. Technology and Environmentill filII',,., London: Academic Press.

6. Pohland, F. G., and R. Englebr('1 Ii '" Impact of Sanitary Landfills; /11/ ( /1', I of Environmental Factors and ('Ol/fl,'/ Alternatives. New York: Repou Jill 'I for the American Paper Instiuu,

7. Fenn, D. G., K. J. Hanley, and ,/" V DeGeare. 1975. Use of the Willl'l 1",1 Method for Predicting Leachate (."11 I from Solid Waste Disposal su., I I' OSWMp, SW-168. Washingloll, II

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