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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
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3
2
4
1) Septic Tank- settling tank where wastewater is stratified, liquid
effluent exits tank, solids remain
2) 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.,
Plan-View of On-Site Wastewater
Treatment and Disposal System
Distribution Box
Soil
Soil
Septic Tank
Drainfield Trenches
On-site Wastewater Treatment and
Disposal Systems
25-30 % of US population use On-site systems
% Septic
10-25
26-40
>40
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
> 700,000 systems
Wastewater, Water-Borne Diseases and
Public Health
Raw Sewage Characteristics
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
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
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 and Public Health
Wastewater and Public Health
Wastewater Pre-treatment
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
1)
Sewage contains pathogens, if the sewage remains
below the surface direct exposure will not occur. If
sewage surfaces, there are immediate public health
concerns.
2)
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
Septic Tank
Aerated soil
Groundwater
Wastewater Treatment
Group II-IV Soils:
Loams, clay loams
and clays
12”
Group I Soils: Sands
18”
Principles of On-site Wastewater
Treatment and Disposal
III. On-site systems should apply effluent to the soil only
in suitable and prepared treatment and disposal field.
1) 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.
2) Septic system components (tank, distribution
devices, pipes, etc.,) should not leak- may cause
contamination.
3) 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’
Contact Areas
Trenches = 500ft2
1
11
3
1
3
1
1
6
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
Previous Septic System Studies
• 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
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 19911995 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)