Evaluation of Septic Tank and Subsurface Wetland for

coliform by 88%. The Retrieve septic tank reduced total coliform by 95% and fecal
coliform by 97%. All of these septic tanks performed better than the 25 – 66% coliform
removal rates reported in literature for septic tanks (Seabloom et al., 1982; Rahman et al.,
1999). The very high coliform reduction in the Retrieve septic tank may have been
promoted by dilution from the continuous flow of chlorinated water through the urinal.
The Pisgah wetland had a measured 3 log (99.9%) reduction of total and fecal coliform.
The Retrieve wetland had a measured 2 log (99%) reduction of total coliform and 3 log
reduction of fecal coliform. A 2 to 3 log reduction of fecal coliform is expected for a
wetland with 5 – 10 day HRT (Reed et al., 2001). HRT averaged 2.2 days in the Retrieve
wetland and 41 days in the Pisgah wetland.
Table 5-8 Total Coliform of Pisgah and Retrieve Sanitation Systems
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Sample Geometric Mean 90% Confidence Interval Sample
( X ) MPN/100ml Mean MPN/100ml Size (n)
Pisgah First Septic Tank 3,500,000 X ± 2,780,000 5
Pisgah Wetland Influent 422,000 X ± 1,450,000 5
Pisgah Wetland Effluent 234 X ± 20,200
4
Retrieve First Septic Tank 2,040,000 X ± 11,300,000 6
Retrieve Wetland Influent 94,400 X ± 58,700
6
Retrieve Wetland Effluent 386 X ± 1,140
7
Table 5-9 Fecal Coliform of Pisgah and Retrieve Sanitation Systems
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Sample Geometric Mean 90% Confidence Interval Sample
( X ) MPN/100ml Mean MPN/100ml Size (n)
Pisgah First Septic Tank 3,200,000 X ± 3,280,000 4
Pisgah Wetland Influent 379,000 X ± 1,930,000 4
Pisgah Wetland Effluent 140 X ± 1,600
3
Retrieve First Septic Tank 3,360,000 X ± 14,000,000 5
Retrieve Wetland Influent 85,000 X ± 75,100
5
Retrieve Wetland Effluent 73 X ± 831
5
50