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Waste Management & Research 30(8) 849 –858 © The Author(s) 2012 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X12448515 wmr.sagepub.com

Introduction

The slow biodegradation rate of municipal solid waste (MSW) in the landfills creates land-reuse bottlenecks. Land that could be used for development is instead rendered unusable due to unstable landfills from the past. Numerous studies have shown that leachate recirculation in bioreactor landfills can accelerate the breaking down of rubbish and can reduce long-term risk. There are several advantages to bioreactor landfills (Barlaz et al., 1990; Pohland and Kim, 1999; Reinhart and Townsend, 1997). Leachate is reinjected back into the landfill to promote in-situ anaerobic biological deg- radation. The advantages of leachate recirculation have been dem- onstrated by many researchers who performed numerous lysimeters and field tests (Chugh et al., 1998; Doedens and Cord-Landwehr, 1989; El-Fadel, 1999; Erses and Onay, 2003; Kinman et al., 1987; Onay and Pohland, 1998; Otieno, 1994; Pohland, 1980; Pohland and Kim, 1999, 2000; San and Onay, 2001; Tittlebaum, 1982; Townsend et al., 1996). In addition, this method reduces the nega- tive effects from co-disposal of both organic or inorganic hazard- ous waste with MSW (Pohland and Kim, 1999). A study on an in-situ bioreactor landfill in Nepean, Ontario, Canada showed the significance of leachate recirculation in increasing biodegradation efficiency (Warith, 2002). Some mechanisms, such as increasing moisture content, offering better contact with the low dissolvable material, thus improving its availability also enhance micro-organ- ism activity. The improved activation of micro-organisms helps

degrade organic matters more quickly, thus shortening the poten- tial pollution time (Barlaz et al., 1990). The increased moisture content (field capacity) by leachate recirculation enhanced biologi- cal stabilization in bioreactor landfills with a field capacity of about or higher 45% of MSW (Valencia et al., 2009). In-situ lea- chate recirculation leads to the high decomposition efficiency of more than 163 L m−2 of waste in North America (Benson et al., 2007).

The landfill problem of how to accelerate biodegradation and shorten the costly pollution monitoring period becomes impor- tant. One of the solutions is to supply air to maintain the aerobic environment in the landfill site. The aerobic landfill technology has been evaluated over the last few years to rapidly stabilize and detoxify the waste, reduce methane gas, volatile organic com- pounds and odour emissions as well as eliminate off-site leachate

Enhanced leachate recirculation and stabilization in a pilot landfill bioreactor in Taiwan

Fu-Shih Huang1,2, Jui-Min Hung1 and Chih-Jen Lu1

Abstract This study focused on the treatment of municipal solid waste (MSW) by modification and recirculation of leachate from a simulated landfill bioreactor. Hydrogen peroxide was added to recirculated leachate to maintain a constant oxygen concentration as the leachate passed again through the simulated landfill bioreactor. The results showed that leachate recirculation increased the dissolved oxygen concentration in the test landfill bioreactor. Over a period of 405 days, the biochemical oxygen demand (BOD5) in the collected leachate reduced by 99.7%, whereas the chemical oxygen demand (COD) reduced by 96%. The BOD5/COD ratio at the initial stage of 0.9 improved to 0.09 under aerobic conditions (leachate recirculation with added hydrogen peroxide) compared with the anaerobic test cell 0.11 (leachate recirculation alone without hydrogen peroxide). The pH increased from 5.5 to 7.6, and the degradation rate of organic carbon was 93%. Leachate recirculation brings about the biodegradation of MSW comparatively faster than the conventional landfill operation. The addition of a constant concentration of hydrogen peroxide was found to further increase the biodegradation. This increased biodegradation rate ultimately enables an MSW landfill to reach a stable state sooner and free up the land for further reuse.

Keywords Leachate recirculation, landfill, bioreactor, municipal solid waste, hydrogen peroxide

1 Department of Environmental Engineering, National Chung Hsing University, Taichung City, Taiwan

2 Environmental Protection Bureau, Taichung City Government, Taichung City, Taiwan

Corresponding author: Chih-Jen Lu, Department of Environmental Engineering, National Chung Hsing University, No.250, Kuo Kuang Rd., South Dist., Taichung city 402, Taiwan Email: [email protected], [email protected]

448515WMR30810.1177/0734242X12448515Huang et al.Waste Management & Research 2012

Original Article

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treatment needs (Cossu et al., 2003; Jacobs et al., 2003; Kim, 2005; Purcell, 2000a, b; Read et al., 2001). Aerobic reactions are favorable because of the high removal efficiencies for the buried matter, thus decreasing the time required for stabilization (Giannis et al., 2008). When the BOD5/COD ratio is below 0.1 with aerobic reactions, the decomposition of waste is almost accomplished (Pohland and Harper, 1986). Aerobic pretreatment of recirculated leachate might inhibit methanogenesis in the MSW landfill site (Stegmann et al., 1987). Aerobic treatment of MSW also has been proven to reduce the potential for metha- nogenesis (Comilios et al., 1999). However, another additional advantage is that aerobic decomposition can decompose lignin to cellulose and half-cellulose improving the follow-up biodegrada- tion (Szegi, 1988). In conventional landfill management, site operations and leachate monitoring continue for many decades after the closure of the landfill. The use of aerobic techniques can shorten that time by 10 years. This period might be further reduced, as combining with leachate recirculation. This reduced monitoring cost and earlier availability of the land for reuse are the prime reasons for adopting the aerobic biodegradation approach (Mehta et al., 2002; Price et al., 2003; Reinhart et al., 2002).

The present situation in Taiwan, a country that has limited available land, has forced the closure of landfills. The high cost of providing power for compressed air operations means that such

methods can only be used for short periods of time. Leachate recir- culation is an option, but being an anaerobic environment in land- fill site, it does not have some of the aforementioned advantages of aerobic techniques. To obtain the advantages of both leachate recirculation and aerobic decomposition, this study has adopted the process of hydrogen peroxide enhanced leachate recirculation to increase dissolved oxygen concentration in the bioreactor land- fill. The application of experimental result from the pilot study on a full-scale landfill bioreactor could be summarized as followings. The collected landfill leachate has been recirculation into the land- fill site using: (a) overflow ponds, (b) injection into the gas wells (methane well), (c) sprinkling on the surface of closured landfills, and (d) injection into multiple-pore pipes buried under the top soil cover about 1 m. The above mentioned methods have been oper- ated at different operating or closured landfill site in Taiwan.

Material and methods Equipment

This research employed three bioreactor landfills. The bioreac- tors were designed to be 2.58 m in diameter (ψ) and 3.66 m in height (H). The effective sectional area was 5.22 m2. The reactors were composed of high-density polyethylene (HDPE). Figure 1 shows a schematic diagram of the in-situ field-scale pilot setup

Grand stone

Reactor cell

Municipal Solid Waste

Municipal Solid Waste

Collection pipe

Soil layer ≈ 15 cm

Soil layer ≈ 50cm

Gas collection

Buffer tank

Pump

Air compresor

Air pipe

Leachate collection tank

Check valve

Check valve

Leachate recirculation

Gravel

Leachate recirculation

Figure 1. Schematic of experimental landfill reactor.

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used in this study. Each pilot bioreactor landfill included a main landfill cell, leachate collection pipes, a leachate equilibrating tank, an aeration mixer with an air pump (0.45 hp), gas collection pipes, a leachate recirculation motor (0.5 hp), and water sprin- kling system. After all of the piping was installed, the cell con- necting parts were sealed to provide effective leak protection.

The MSW used in this study was collected from a transfer sta- tion in Dali city, Taichung County, Taiwan. The MSW (by weight) consisted of 45.8% paper, 37.1% kitchen waste, 11.7% plastics, 2.2% branches and leaves, 1.97% glass, 1.3% fibres/cloth, 0.03% scrap iron, 0% non-iron scrap metal, and other constituents (pot- tery, sand) totalling 0.2%. As a result of the Taiwanese custom of disposing of rubbish in plastic bags, rubbish was inserted into the cells after breaking the bags.

The MSW was buried in the cells in two layers. About 1.5 m of rubbish was buried in two layers, separated by 15 cm of soil in the middle, then covered with an additional 50 cm layer of soil on the top. The cover soil was collected from a sanitary landfill site in Dali City, and it consisted of sandy loam with a small amount of gravel. The cover soil was handled and applied in accordance with the regulation of the Municipal Waste Recycling, Cleaning, and Treatment Regulation of Tiawan.

In the final setup, leachate was recirculated from the leachate circulation tank and sprinkled back onto the top layer of soil in the cells to maintain the moisture content in the cell. Every pilot bioreactor had a packed weight of 3500 ± 175 kg of MSW, and the average specific weight of the compacted waste in each waste cell was 224 kg m−3. Hydrogen peroxide, H2O2, at 35% was added to one of the recycled leachates.

Operation

The three pilot landfill bioreactor’s leachate recirculation were operated as the SBR sequencing batch reactor mould with differ- ent recirculation conditions. The operated pilot bioreactors with or without the additions of H2O2 into the leachate recirculation are as follows: (1) leachate with the addition of H2O2 to bioreac- tor A as the aerobic cell; (2) anaerobic circulation with leachate recirculation alone into bioreactor B as the anaerobic cell; and (3) aerobic/anaerobic circulations with the leachate that had intermittent additions of H2O2 as the bioreactor C. Water com- mensurate with the average rainfall of 5 years in Taiwan was added to the collection tank and mixed with the collected lea- chate to simulate the rainfall. After mixing for 3 min, the mixed leachate was then recirculated into the bioreactor. In bioreactor C, at 2-month intervals, hydrogen peroxide was added at a con- centration of 200 mg L−1. After 121 days, this was increased to a concentration of 500 mg L−1 until the end of the 405-day experiment.

The sampling frequency was once per week in the first 3 months in the post-closure cell, every 2 weeks from the 4th to 6th month, once per month from the 7th to 10th month, and every second month from the 11th to 16th month (until the end of the experiment). The leachate recirculation frequency in the first 6

months was the same as that used for the sampling. In subsequent months it was once per month until the end of the experiment. The experiment period was 405 days from 26 July 2008 to 4 September 2009.

Analysis method

The conductivity (EC), pH, redox potential (ORP), and dissolve oxygen concentrations were measured with a portable meter WTW Cond 315i, pH 330, and Oxi 330, respectively. BOD5 and COD concentrations were determined according to the standard methods of the American Public Health Association (APHA, 2005). The volatile fatty acid (VFA) analysis was car- ried out with a high-performance liquid chromatograph and vari- able wavelength ultraviolet (HPLC/UV) (Hewlett Packard 1100 series) equipped a stainless steel Chrompack column (TC-C18, Agilent) of dimensions 4.6 mm × 250 mm × 5 µm film. The con- centrations of CH4 and CO2 gases were determined by a gas chro- matography with thermal conductivity detector (GC/TCD) (Hewlett Packard 6890) equipped with a capillary column (DB- 624, J & W scientific) of dimensions 0.53 mm × 30 m × 3 µm film. All samples were tested in triplicate.

Results and discussion pH and ORP

The pH value of the A, B, and C cell leachate, with the three dif- ferent operation modes, ranged from 5.5 to 7.6. The pH value was more acid in the initial stage, and latter stage leaned toward slightly alkaline. The bioreactor biochemically reacted from hydrolysis in the initial stage to initiate acidic decomposition, and through the methanogenesis process, then gradually reached the mature steady stage. These trends are illustrated in Figure 2. The change of pH values from the initial stage to steady state tended to be similar to the pattern found in an actual field experi- ment in Canada (Nepean, Ontario) and in pilot-scale studies of MSW in Toronto (Warith, 2002). Figure 2 shows that the pH value in the aerobic (A) cell about 7 after 3 months, and in the anaerobic (B) cell it was below 7 after 4 months of operation. This shows that bioreactor A had accomplished the acidic stage earlier than bioreactor B.

Most measurements of ORP were negative, approximately −159 ± 45 mV. The lower ORP values show in Figure 3 indicated there was little available oxygen after the covering with soil. Even the addition of H2O2 (200–500 mg L

−1) was not enough to change the ORP value. Hence, the oxygen provided by this con- centration of H2O2 was unable to make the test cell completely aerobic. This means some of the biodegradation belongs to facul- tative aerobic digestion because the majority of the ORP was in anaerobic conditions. By transforming organic matters to organic acid, ORP decreased gradually. This phenomenon is generally associated with anaerobic environments (Pohland and Kim 1999).

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The fact that the leachate temperature was higher than the sur- rounding temperature shows that the decomposition reaction must be considered mesophilic biological decomposition. In addition, the leachate had higher conductivity values (EC) (Figure 4), showing that the leachate (including the cover soil composition) contained various kinds of salt. This effect may have been produced from kitchen waste, because of the use of high amounts of salt in food, as is the Taiwanese habit.

The system was acidogenic at the initial stage and then gradu- ally changed into a methanogenesis stage. A large amount of organic acid (VFA) was produced in the acidogenesis stage and was then gradually transformed to methane and carbon dioxide. Therefore, the pH value slowly became slightly alkaline. The lea- chate had an increased bicarbonate (HCO3

−) because some of the produced carbon dioxide was dissolved in the leachate. That meant the pH value did not rise again after trending toward alkaline.

BOD5 and COD

Figures 5 and 6 present the BOD5 and COD concentrations, showing the biodegradation of organic matter in the anaerobic process (cell B) and the aerobic processes (cells A and C) was different. The BOD5 and COD values showed that the biodegra- dation in leachate from cells A and C was more rapid than that from cell B. The BOD5/COD ratio (Figure 7) concurs with this. This result proves that the cell A with partial aerobic environment is an advantage to landfill biodegradation.

After bioreactor A had been buried for half a month, the BOD5 and COD of the leachate were found to be higher than those of bioreactor B. The main reason was due to the hydrolysis of organic matter and the biodegradation of the fatty acids produced during this stage. The partially aerobic system could have aided the hydrolysis and acidification reaction of the organic matter in MSW, causing the sugar and fatty acid/amino acid concentration

Figure 2. pH of leachate collection from pilot landfill tanks with different operating conditions.

Figure 3. ORP of leachate collection from pilot landfill tanks with different operating. conditions.

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Figure 4. EC of leachate collection from pilot landfill tanks with different operating conditions.

Figure 5. BOD of leachate collection from pilot landfill tanks with different operating conditions.

Figure 6. COD of leachate collection from pilot landfill tanks with different operating conditions.

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of cell B to be higher than that of cell A after 2 weeks of opera- tion. The rate of anaerobic decomposition was relatively slow. Finally BOD5 reached approximately 100 mg L

−1, suggesting that the biodegradation was already nearly accomplished, and the BOD5/COD ratio was less than 0.1 at that point. The leachate concentration of the organic matter showed the decomposition of the organic matter had nearly achieved steady state after 8 months of operation. In other words, this operation had significantly reduced the time required for the buried MSW to reach the steady state. It was less than one year.

The experimental results suggest that bioreactor A and C with their partial aerobic environments had an advantage with buried MSW biodegradation. The result of this pilot-scale experiment did show that the addition of H2O2 led to a

somewhat improved aerobic environment, further enhancing biodegradation.

VFAs and DOC

Bioreactors A, B, and C produced VFAs, including formic, ace- tic, propionic, butyric, and valeric acid. The VFAs shown in Figure 8 were represented as acetic acid. It must be noted that in the present experiment no formic acid was found after 4 months of operation, so formic acid is not listed here. Figure 9 also shows that the VFAs of the anaerobic cell digested more slowly than those of the aerobic cells. However, bioreactors A, B, and C had no organic matter accumulation after initiating biodegradation for 5.5 months. This phenomenon proves that all three cells had

Figure 7. BOD/COD of leachate collection from pilot landfill tanks with different operating conditions.

(m g

V L

− 1 )

Figure 8. Total VFA of leachate collection from pilot landfill tanks with different operating conditions (The VFA concentrated were represented as acetic acid).

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already reached a steady stage with no more biodegradation in either aerobic or anaerobic conditions. A similar situation caused DOC (dissolved organic carbon) (Figure 9) and BOD5 concentra- tions of the leachate to decrease significantly.

Gas

Gas samples were collected using a pumping process. Figure 10 shows that the CO2 concentration increased continuously, so bio- degradation appears to have still been proceeding. In comparison with the concentration of organic acid (VFAs) and the methane produced, bioreactors A, B, and C had all become gradually stable after being buried for 5.5 months. Methane production was already reduced and, as mentioned above, the leachate already had a sig- nificant decrease in BOD5 value. This phenomenon proves that all three cells had already tended toward a stable condition.

It is notable that in cell A (the aerobic cell) the low CH4 volume signalled that very rapid hydrolysis of organic matter was taking place. This is illustrated in Figure 11. The appear- ance of biodegradation caused a rapid conversion to the metha- nogenesis stage; therefore, the aerobic cell was quicker to reach the methane stage than the anaerobic cells in the middle period of the experiment. At the later stage there was no such phenomenon in either the aerobic or the anaerobic cells due to the deprivation of oxygen. The ratio of methane to carbon dioxide was greater than 1 during the first 6 months (Figure 12); this is similar to the in-situ field and the above-mentioned research values. As the ratio of BOD5/COD in leachate was less than 0.1, the biodegradation was already significantly decreased, and the concentration of carbon dioxide was greater than the concentration of methane after 7 months of operation.

Figure 9. DOC of leachate collection from pilot landfill tanks with different operating conditions.

Figure 10. CO2 of the gas collected collection from pilot landfill tanks with different operating conditions.

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After the cells had been buried for 5 to 6 months, methane continued to be produced. Even though the organic acid had decreased below a measurable level, the system had already reached a relatively stable stage. This proves that organic acid was almost instantaneously transformed to methane. Although there appeared to be a lack of organic acid, this was simply a result of easily biodegradable organic matter being rapidly con- verted. Therefore, almost no organic acid was observed in the final stage. In contrast, at that point methanogenesis had become the dominant stage. This means that the rate of organic acid pro- duction was lower than the rate of transformation. For this rea- son, methane could be quantified but no accumulation of organic acid could be found in the system.

Discussion and suggestions

According to the BOD5, COD, and DOC concentrations in the leachate, the produced gas (CH4 and CO2), and the reduction

of buried MSW volumes, the rate of organic matter biodegra- dation in bioreactor A was higher than that in bioreactor C, and bioreactor C was superior to bioreactor B. In the reduction of VFAs, bioreactors A and C biodegraded more rapidly than B. During the initial period of leachate recirculation (operated in the first 2 months), the advantage of leachate recirculation was not apparent with respect to the amount of organic matter degraded. Therefore, recirculation does not need to be oper- ated initially. From the point view of in-situ operation, it can even be said to be unsuitable to use recirculation in the initial stage, so as not to influence the field operation. This approach can be an advantage when applied to operate post-closure landfills as the addition of liquid hydrogen peroxide into lea- chate is relatively easily operated for landfill leachate recircu- lation. If the biodegradation rate in the landfill site can be enhanced, the landfill site can reach a stable state quickly. The site area can then be developed for other beneficial uses, such as a park.

Figure 11. CH4 of the gas collected collection from pilot landfill tanks with different operating conditions.

Figure 12. CH4/CO2 of the gas collected collection from pilot landfill tanks with different operating conditions.

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In addition, the area should be divided (layered) when burying MSW. In the pilot landfill, the buried mass of organic matter weighed approximately 1035 kg, and during the operational period, cell A used up to 4.79 kg of H2O2 altogether (4.79/0.35 = 13.69 kg; 35% purity). This indicates that each kg of H2O2 can offer 0.47 kg of oxygen by weight. A key aspect is to understand over what operational period hydrogen peroxide needs to be added to keep an aerobic condition. The addition of 35% H2O2, did not inhibit the bacteria growth when the concentration was increased from 200 to 500 ppm. The use of a H2O2 at a concentra- tion of 500 ppm in the cell was still within the tolerable range for the micro-organism growth in the bioreactors. The addition of H2O2 to the aerobic bioreactor accelerated the decomposition of MSW, but the most appropriate addition of hydrogen peroxide from the efficiency and cost point of view is still uncertain at the present time. Therefore, the following further research is going to carry on, such as: (1) the extent of biodegradation as BOD/COD ratio less than 0.1, (2) the effect on various H2O2 concentration on the microbial activity, (3) the volatile suspension solid analy- sis in post-closure cell, and (4) the comparison of the efficiency of decomposition and the cost of operation of the hydrogen per- oxide addition method to the force aeration method.

Conclusions

The results of the addition of H2O2 to a recirculated leachate sys- tem are presented in the following list.

1. In terms of its effect on the biodegradation of organic matter, the addition of H2O2 resulted in a faster biodegradation rate than leachate recirculation alone. The continuous addition of H2O2 was found to be superior to an intermittent operation mode.

2. The addition of the hydrogen peroxide at 200 to 500 ppm did not inhibit the microbial growth in the landfill bioreactor.

3. Biodegradation tended towards stability when the BOD5/ COD ratio was less than 0.2. The CO2 concentration was higher than the CH4 concentration, when the system reached the stable stage.

4. Leachate recirculation or the addition of hydrogen peroxide could shorten the MSW landfill operation time in comparison with the conventional landfill operation. The reduced opera- tion time and pollution monitoring cost are economic benefits for the operation of MSW landfills with leachate recircula- tion. Furthermore, freeing up the landfill land enables the land to be utilized much earlier, providing additional eco- nomic and environmental benefits.

5. Some major principles of leachate recirculation has been sug- gested to be of value in Taiwan: (a) avoidance of resulting run-off; (b) no recirculation during wet weather; (c) control of the recirculation rate to be less than the evaporation rate, (d) no recirculation in the operating area, and (e) recirculation of the pre-treatment leachate (with grit removal or with both grit removal and aeration).

Acknowledgements The authors gratefully acknowledge the financial support of the Environmental Protection Administration of the Executive Yuan, Taiwan, ROC, in funding and subsidizing this research and also the landfill in Dali District, Taichung City for offering a field study location.

Funding This research was supported by Environmental Protection Administration of the Executive Yuan, Taiwan, ROC.

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