Evaluation of Septic Tank and Subsurface Wetland for

o Alkalinity
Nitrification and denitrification of 1 g NH3-N requires a net 3.58 g
of alkalinity as CaCO3 (See below).
Table 5-6 lists the favorable range of the parameters influencing biological nitrification
and the test results for the Pisgah and Retrieve sanitation system wetlands. The carbon
source availability in the wetlands as indicated by the BOD concentrations entering and
exiting the wetlands ranged from 9 – 38 mg/l at Pisgah and 9 – 32 mg/l at Retrieve.
These were close to the recommended range of 15 – 20 mg/l (Reed et al., 2001). The
wetland water temperatures of 22 – 26ºC at Pisgah and 23 – 26ºC at Retrieve were below
the optimum temperature range but within the necessary range of 5 - 40ºC (Hammer et
al., 1994). The pH ranges of 6.7 -7.9 at Pisgah and 6.7 – 7.8 at Retrieve indicate the pH
sometimes fell below 7.0 where reasonable nitrification can occur, but was normally near
the optimum range of 7.5 – 8.0 (Tchobanoglous et al., 2003). Insufficient DO data was
gathered to make a conclusive comment. Alkalinity of the rainwater at Pisgah was
measured to be 8.1 mg/l as CaCO3. Alkalinity of the municipal water supply at Retrieve
was measured to be 210 mg/l as CaCO3. Alkalinity can be shown to be a limiting factor
for biological nitrification at Pisgah.
Table 5-6 Parameters Influencing Biological Nitrification and the Measured Values
for Pisgah and Retrieve Wetlands. The alkalinity ratio applies to systems in which
biological denitrification also occurs.
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Optimum or Recommended Measured Range
Parameter Range Pisgah Retrieve
BOD 15 – 20 mg/l 9 – 38 mg/l 6 – 32 mg/l
Temperature 30 - 35ºC 22 - 26ºC 23 - 26ºC
pH 7.5 – 8.0 6.7 – 7.9 6.7 – 7.8
DO > 2.0 mg/l n/a n/a
Alkalinity > 3.58g CaCO3 : 1g NH4-N 0.14 : 1 36 : 1
46