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EHST 3370
Wastewater Management
Spring 2016
Unit 1:Introduction to On-site Wastewater Treatment and Disposal Systems

Septic System Treatment and
Disposal Mechanisms

Septic Tank- settling tank where wastewater is stratified, liquid effluent exits tank, solids remain
Distribution device to convey effluent to trenches

3) Drainfield trench is aerobic environment where wastewater is stored until it infiltrates the soil

4) Soil beneath trench is aerobic, and most pollutant

transformation and removal occur

5) Setbacks- required horizontal distance from system to point of interest such as a surface water, well, property line, etc.,

Septic Tank

Drainfield Trenches

Distribution Box

Plan-View of On-Site Wastewater

Treatment and Disposal System

Soil

On-site Wastewater Treatment and

Disposal Systems

25-30 % of US population use On-site systems

50% of NC residents (4.5+ million people) use On-site (2 million systems in NC)

30,000 - 40,000 new systems/yr installed in NC

~7,000 repairs each year

60% Coastal NC residents use on-site

Pitt County, 460/yr

% Septic

10-25

26-40

>40

> 700,000 systems

Wastewater, Water-Borne Diseases and Public Health

Raw Sewage Characteristics

mg/L = milligrams per liter

s.u. = standard units

CFU/100 mL = Colony-Forming Units per 100 milliliters

Adapted From: US EPA Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008, US EPA Office of Water, 2002

Component	Range of Concentrations	Typical Concentration
Total Suspended Solids, TSS	155 – 330 mg/L	250 mg/L
BOD5	155 – 286 mg/L	250 mg/L
pH	6 -9	6.5
Total Coliform Bacteria	108 – 1010 CFU/100mL	109 CFU/100mL
Fecal Coliform Bacteria	106 – 108 CFU/100mL	107 CFU/100mL
Ammonium-Nitrogen, NH4-N	4 - 13 mg/L	40 mg/L
Nitrate-Nitrogen, NO3-N	Less than 1 mg/L	Less than 1 mg/L
Total Nitrogen	26 – 75 mg/L	60 mg/L
Total Phosphorus	6 - 12 mg/L	10 mg/L

These parameters are typical for domestic sewage, but not for sewage from restaurants, commercial facilities, or other sources that are not residential.

NC Design Flow = 60 g/d

Water Use Characteristics

Microbial Waterborne Disease

Prior to the late 19th century, outbreaks of epidemic waterborne disease claimed heavy tolls in human lives and suffering.
As late as the 1880s typhoid killed 75-100 people per 100,000 population in the US every year.
Cholera was also a serious issue in the US in the 19th century.
Contamination of waterways in the developing world continues to persist as the most pressing environmental health problem.

Wastewater and Public Health

Almost 5,500 people die every day due to fecal contaminated drinking water (developing nations).

Wastewater and Public Health

Wastewater Pre-treatment

Improving Tank Performance

Principles of On-site Wastewater Treatment and Disposal

I. On-site systems should ensure that the effluent is absorbed by the soil and does not come to the land surface or flow directly into streams, rivers, lakes, the ocean or the groundwater

Sewage contains pathogens, if the sewage remains below the surface direct exposure will not occur. If sewage surfaces, there are immediate public health concerns.
On-site systems treat and dispose of wastewater, via the septic tank and soil beneath the trenches. For treatment to happen, wastewater must stay in soil beneath the surface and above the water table.

Principles of On-site Wastewater Treatment and Disposal

II. On-site systems should maximize the aerobic treatment of the sewage.

1) Aerobic treatment occurs in aerated soil beneath the drainfield trenches (unsaturated zone).

2) Aerobic treatment is the fastest and most complete treatment the effluent can receive in the soil.

3) On-site systems should be located where the effluent must travel the farthest distance before reaching the water table or saturated layers.

Wastewater Treatment

Groundwater

Septic Tank

Aerated soil

Wastewater Treatment

18”

12”

Group II-IV Soils:

Loams, clay loams and clays

Group I Soils: Sands

Principles of On-site Wastewater Treatment and Disposal

On-site systems should apply effluent to the soil only in suitable and prepared treatment and disposal field.
The treatment and disposal field includes trenches or beds lined with porous media, where effluent is discharged into via pipes with holes. The soil beneath the trenches/bed absorbs the wastewater.
Septic system components (tank, distribution devices, pipes, etc.,) should not leak- may cause contamination.
Systems should only be installed in areas with suitable soil and site conditions, as determined by trained professionals (EHS).

Principles of On-site Wastewater Treatment and Disposal

4) Treatment and disposal field trenches should be designed to maximize the effluent contact with soil, thus improving treatment.

1) Long, narrow trenches provide more wastewater/soil contact area than shorter, wide trenches (Beds).

2 trenches - 50’ x 3’ x1’ 1 Bed- 50’ x 6’ x 1’

1 1 1 1 1

3 3 6

Contact Areas

Trenches = 500ft2

Bed = 400 ft2

Principles of On-site Wastewater Treatment and Disposal

5) Treatment and disposal field trenches should have level bottoms and should be level along their entire length to distribute effluent as evenly as possible.

1) Slanted or sloped trench bottoms will make effluent flow to the lowest area, possibly causing overload and ponding of wastewater to the surface.

Biomat
Formation

Reduces infiltration rate
Promotes unsaturated flow
Can improve treatment
May cause hydraulic failure

Indicator Bacteria Conc. Near Septic Systems
Cogger et al., 1988
Scandura and Sobsey, 1997
Humphrey et al., 2011
Conn et al., 2011
Harris et al., 2013
Relatively high concentrations adjacent to drainfields, and sandy, shallow soils more prone to bacterial loadings

Indicator Bacteria Conc. in Surface Waters Adjacent to Septic Systems
Booth et al., 2003
Ahmed et al, 2005
Cahoon et al., 2006
Harris et al., 2013

Previous Septic System Studies

North Carolina Environmental Issues

Shellfish Waters (SA) – Over 1,157 acres of SA waters have closed since 1990 (NC DWQ, 2007)
High bacteria concentrations
Recreational Waters- swimming advisories due to bacteria concentrations
46 advisories in 2006 due to excessive bacterial concentrations (NC DENR, 2007)
More stringent coastal stormwater rules enacted in 2008
Are septic systems efficient at reducing bacteria densities in wastewater before discharge to shallow groundwaters?

Previous Septic System Studies

Nitrogen Concentrations in Groundwaters Beneath and/or Adjacent to Septic Systems

Serving residential areas with shallow, sandy-sandy clay loam soils
Carlile et al., 1981
Corbett et al., 2002
Reay et al., 2006*
Del Rosario et al., 2014*
Serving residential areas with deep, sandy soils
Robertson et al., 1991*
Postma et al., 1992*
Buetow, 2002*
Humphrey et al., 2010*

All studies showed higher N near systems than background. Some (*) showed higher than water quality standards for NO3- (10 mg/L).

Previous Septic System Studies

Phosphorous Concentrations in Groundwaters Beneath and/or Adjacent to Septic Systems

Serving residential areas with shallow, sandy soils

Corbett et al., 2002*

Reay et al., 2006

Humphrey et al., 2014*

Serving residential areas with deep, sandy soils

Robertson et al., 1998*

Postma et al., 1992

Humphrey and O’Driscoll, 2011*

Elevated P concentrations relative to background conditions (3 or more times higher)

North Carolina Environmental Issues

Eutrophic Conditions and Fish Kills in 1990’s
Neuse River Nutrient Sensitive Waters Management Strategy
30% N loading reduction from 1991-1995 baseline implemented in 1998
Tar-Pamlico River Nutrient Sensitive Waters Management Strategy
30% N loading reduction from 1995 baseline implemented in 2000
No increase in P loading from 1995 baseline
Lake Jordan Watershed Strategy
Neuse, Tar –Pamlico and Lake Jordan Rules targeted most point and nonpoint sources of nutrients (agriculture, urban runoff, wastewater treatment plants), but not septic systems)

Neuse River Foundation (2009)