Can Septic Systems Recycle
Wastewater and Protect
Water Wells from Viruses?
By: Michael Payne and Ian Ralston
For: WCOWMA Conference, March 2017
This talk is about basic septic
systems
How a septic system recycles water
4 stages of
treatment
1. Septic tank
2. Biomat
3. Unsaturated soil
4. Saturated soil or
bedrock
Treatment Stage 1:
The Septic Tank and Filter
Retains solids:
 Sludge
 Scum (FOG)
 Clear zone
 Screen
1. The Septic Tank and Filter
Anaerobic
digestion of
organic carbon
Plain English:
 Bacteria each the
sludge
 They love it!
1. The Septic Tank and Filter
Ammonification
a.k.a.
Mineralization
 Bacteria convert
organic nitrogen
into ammonia
1. Treatment in the Septic Tank
Pollutant
% Removed
IN
OUT
Suspended solids
95%
600
30
Fecal coliform
~ 50%
>1,000,000
~1,000,000
Nitrogen
2%
65
64
Phosphorous
25%
13
10
mg/L
MPN/100mL
Total N in mg/L
Total P in mg/L
Stage 2: The Biomat
The Biomat
Oxygen
Soil cover
Oxygen
●
Gravel
Biomat
Oxygen
Unsaturated soil
Limiting layer
Oxygen
What is this Biomat?
The biomat contains:
 Slime bacteria
 Organic chemical deposits
 Fine particles (clay, silt, lint)
 Mineral precipitates, especially iron sulphide
What is a good Biomat?
 Semi-permeable
 Biologically active
 Both anaerobic
and aerobic zones
 Stable
What is a bad biomat?
 Unstable
 Becomes almost
impermeable
 Becomes fully
saturated
 Low oxygen
 Aerobic bacteria die
 The trench floods
2. Treatment processes in the
biomat
 Retains solids
 Chemical precipitation and adsorption
 Biodegradation of sewage
 Anaerobic and aerobic bacteria
2. Treatment in the Biomat
Pollutant
% Removed
IN
OUT
Suspended solids
50%
30
15
Fecal coliform
99%
~1,000,000
10,000
Nitrogen
~ 10%
64
58
Phosphorous
~ 10%
10
9
mg/L
MPN/100mL
Total N in mg/L
Total P in mg/L
2-log
Stage 3: Unsaturated Soil
Soil = aerobic packed-bed bioreactor
Oxygen
Soil cover
Oxygen
●
Biomat
Oxygen
Soil + Water + Air
Soil bacteria “coating”
Limiting layer
Oxygen
Microscopic View
3. Treatment processes in
unsaturated soil
 Soil retains fine particles
 Chemical precipitation and adsorption
 Aerobic biodegradation
 Nitrification: Ammonia to nitrate
3. Treatment in unsaturated soil
Pollutant
% Removed
IN
OUT
Suspended solids
90%
15
1.5
Fecal coliform
> 99.99%
10,000
<1
Nitrogen
50%
58
29
Phosphorous
90%
9
1.0
mg/L
MPN/100mL
Total N in mg/L
Total P in mg/L
> 4-log
Stage 4: Saturated Soil
4. Treatment processes in
saturated soil or bedrock
 Chemical precipitation and adsorption
(especially phosphorous)
 Denitrification: Nitrate to nitrogen gas
 Dilution
 Limited biodegradation or oxidation
4. Treatment in Saturated Soil
%
Removed
IN
OUT
Suspended solids
75%
1.5
0.4
Fecal coliform
> 90%
<1
< 0.1
Nitrogen
70-90%
29
3-9
Phosphorous
90%
1.0
0.1
Pollutant
mg/L
MPN/100mL
Total N in mg/L
Total P in mg/L
> 1-log
This is “Reclaimed Water”
Concentration
Drinking Water
Guideline
Suspended solids
0.4
< 1.0
Fecal coliform
< 0.1
<1
Nitrogen
3–9
< 10
Pollutant
mg/L
MPN/100mL
Total N in mg/L
This is “Reclaimed Water”
Pollutant
Phosphorous
Total P in mg/L
Concentration
Guideline for
Discharge to
Surface Water
0.1
< 1.0
Conclusions
A basic septic system is:
 Cost effective
 Simple
 Reliable and durable
 True recycling
Implications and Cautions
 Effective rules are important
 Relies on work of qualified practitioners
 We need somebody to enforce the rules
 Nitrogen and phosphorous are cumulative
so sensitive to housing density