Evaluation of Septic Tank and Subsurface Wetland for

Evidence from 14 horizontal flow SSF wetlands operating in the United States indicated
deep roots and long HRT improved ammonia removal through nitrification and
denitrification (EPA, 1993). Ammonium removal in pilot scale SSF wetlands was found
to be better in beds planted with softstem bulrush than beds without plants (George, et al.,
2000). This evidence supports the root zone concept of sufficient oxygen for biological
nitrification being supplied to the wastewater by the roots of the aquatic plants. Another
report stated that full-scale SSF wetlands built for ammonia removal performed below
expectations and some other technology besides the single pass horizontal SSF wetland is
required if ammonia removal is critical. It was concluded that plants cannot contribute a
significant amount of oxygen to a SSF wetland (EPA, 2000).
Nitrification is often the limiting step in converting ammonia to nitrogen gas because of
the low concentration of dissolved oxygen (Reed et al., 1995). Biological nitrification
occurs under optimal conditions for growth and sustenance of the aerobic autotrophic
nitrifying bacteria. These conditions include the following:
o Carbon Source
A BOD of at least 15 – 20 mg/l is recommended (Reed et al., 2001).
Too much BOD will result in competition with heterotrophic bacteria
(Tchobanoglous et al., 2003).
o Temperature
Optimum temperature is 30 – 35ºC with little nitrification occurring
below 5ºC or above 40ºC (Hammer et al., 1994). Microbes will work twice as
fast at 24ºC compared to 12ºC.
o pH
Optimal range is 7.5 to 8.0 but reasonable nitrification can occur at 7.0
(Tchobanoglous et al., 2003).
o Dissolved Oxygen (DO)
Rate of nitrification is reduced at DO concentrations below 2 mg/l (Hammer et
al., 1994) and the conversion of nitrite to nitrate is greatly inhibited at DO
concentrations below 0.5 mg/l (Tchobanoglous et al., 2003).
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