Evaluation of Septic Tank and Subsurface Wetland for

system were measured to be 3.3 mg/l entering the septic tanks, 1.5 mg/l entering the
wetland and 1.2 mg/l exiting the wetland. The average mass load of phosphorus on the
wetlands was 0.28 kg/ha-d (0.25 lb/ac-d ) at Pisgah and 1.5 kg/ha-d (1.3 lb/ac-d) at
Retrieve.
Average phosphorus removal was determined to be 97% for the Pisgah sanitation system
and 64% for the Retrieve system. The septic tank removal efficiencies of 25% at Pisgah
and 55% at Retrieve were within the typical phosphate removal efficiency range of 20 –
65% (Seabloom et al., 1982; Rahman et al., 1999). Removal efficiency for wetland
influent phosphorus was determined to be 96% at Pisgah and 20% at Retrieve. These
results are outside the removal efficiency range of 26.7 – 65.0% reported in literature for
SSF wetlands (Vymazal, 2002). The harvest of the wild cane from the Pisgah wetland
contributed to the high phosphorus uptake. The high solids accumulation in the Retrieve
wetland effluent caused by the sampling method could have contributed to the low
phosphorus uptake that was measured.
Table 5-7 Total Phosphorus (P) for Pisgah and Retrieve Sanitation Systems
________________________________________________________________________
Sample Arithmetic Mean 90% Confidence Sample
( X ) mg/l Interval mg/l Size (n)
Pisgah First Septic Tank 12.8 X ± 7.6
5
Pisgah Wetland Influent 9.6 X ± 6.6
5
Pisgah Wetland Effluent 0.4 X ± 0.2
4
Retrieve First Septic Tank 3.3 X ± 1.6
6
Retrieve Wetland Influent 1.5 X ± 0.5
6
Retrieve Wetland Effluent 1.2 X ± 0.5
7
Long term phosphorus removal should not be expected for a wetland unless regular plant
harvesting is performed which can yield a maximum of 0.25 kg/ha-d (0.22 lb/ac-d)
phosphorus removal (EPA, 2000). The removal of phosphorus was determined to be 0.27
kg/ha-d (0.24 lb/ac-d) at Pisgah and 0.0088 kg/ha-d (0.0079 lb/ac-d) at Retrieve.
48