Evaluation of Septic Tank and Subsurface Wetland for

BOD was determined to be 53% at Pisgah and 63% at Retrieve. These are within the
typical BOD removal efficiency range of 46 – 68% reported in literature (Seabloom et al.,
1982; Rahman et al., 1999). The Pisgah wetland removed 52% of its influent BOD. This
was below the 68.5 – 92.7% removal efficiency reported in literature for SSF wetlands
possibly because of the low influent concentration (Vymazal, 2002).
Table 5-1 Five-Day Biochemical Oxygen Demand for Pisgah and Retrieve
Sanitation Systems
________________________________________________________________________
Sample Arithmetic Mean 90% Confidence Sample
( X ) mg/l Interval mg/l Size (n)
Pisgah First Septic Tank 58 X ± 20
5
Pisgah Wetland Influent 27 X ± 9
4
Pisgah Wetland Effluent 13 X ± 5
3
Retrieve First Septic Tank 40 X ± 24
6
Retrieve Wetland Influent 15 X ± 10
5
Retrieve Wetland Effluent 20 X ± 7
7
The first two sets of water quality samples indicated that the water in the top of the first
chamber of the septic tanks had BOD ranging from 8 to 69 mg/l which was lower than
expected. Typical BOD for raw sewage ranges from 110 – 350 mg/l in the United States
(Tchobanoglous et al., 2003). Influent BOD to the Montego Bay treatment ponds and the
oxidation ditches in Ocho Rios ranges from 47 – 555 mg/l (NWC, 2005). Because of the
low influent BOD values, the collection point for subsequent samples was moved from
the center of the first tank to the inlet tee of the first tank. Although there were some
fecal particles in subsequent samples, the BOD of septic tank influent ranged from 24 to
83 mg/l.
The temperature for five-day incubation of BOD samples is to remain 20ºC (68ºF) ± 1ºC.
However, the temperature in the NWC incubator from August 17 to November 15 ranged
from 14.0 - 17.5ºC. After repairs the temperature ranged between 18.5 - 20.5ºC from
38