Evaluation of Septic Tank and Subsurface Wetland for

solids than a sample point that pulled from the top or mid-stream of flow, and it would
allow non-buoyant solids to continue to enter the bottle even when full of liquid. Another
contributor to high solids may have been the atmospheric exposure of the effluent prior to
sample capture. On some occasions there were water bugs or mosquito larvae in the
sample bottle.
Table 5-2 Total Suspended Solids for Pisgah and Retrieve Sanitation Systems
________________________________________________________________________
Sample Arithmetic Mean 90% Confidence Sample
( X ) mg/l Interval mg/l Size (n)
Pisgah First Septic Tank 92 X ± 44
5
Pisgah Wetland Influent 57 X ± 57
5
Pisgah Wetland Effluent 13 X ± 9
4
Retrieve First Septic Tank 41 X ± 21
6
Retrieve Wetland Influent 6 X ± 4
5
Retrieve Wetland Effluent 98 X ± 67
7
An overall TSS reduction of 85% was measured in the Pisgah system but an overall
increase in TSS was measured at Retrieve. The Pisgah septic tanks removed an average
of 38% of their influent and the Retrieve septic tank removed 85%. These are near the
removal efficiencies of 30 – 81% expected for septic tanks (Seabloom et al., 1981;
Rahman et al., 1999). The Pisgah wetland removed 77% of influent TSS. This was
within the 63.0 – 89.8% removal efficiency range for SSF wetlands reported in literature
(Vymazal, 2000).
Entry zone solids loading rates for both wetlands averaged well below the recommended
design limit of 0.008 lb/ft2-d (39 g/m2-d) (Crites et al., 1998). The average loading was
0.0013 lb/ft2-d (6.4 g/m2-d) at Pisgah and 0.0038 lb/ft2-d (19 g/m2-d) at Retrieve. Entry
zone loading reached an estimated maximum of 0.0081 lb/ft2-d (40 g/m2-d) during the
week of highest flow rate at Retrieve.
40