Evaluation of Septic Tank and Subsurface Wetland for

Table 5-5 Nitrate (NO3-N) for Pisgah and Retrieve Sanitation Systems
________________________________________________________________________
Sample Arithmetic Mean 90% Confidence Sample
( X ) mg/l Interval mg/l Size (n)
Pisgah First Septic Tank 0.2 X ± 0.1
5
Pisgah Wetland Influent 0.5 X ± 0.3
5
Pisgah Wetland Effluent 0.7 X ± 0.4
4
Retrieve First Septic Tank 0.4 X ± 0.3
6
Retrieve Wetland Influent 0.4 X ± 0.3
6
Retrieve Wetland Effluent 0.5 X ± 0.3
6
Unionized ammonia (NH3) is toxic to many fish (Reed et al., 1995) and can evaporate out
of wastewater whereas ammonium (NH4 + ) cannot. The low pH found in a wetland
causes ammonium to be the predominant species so that very little unionized ammonia is
available to be removed by volatilization. The percentage of ammonia in the unionized
form may be determined by the chemical equilibrium equation
+
[NH 3][H
]
+
[NH 4 ]
= K where
K is the equilibrium constant. At pH 9.25 the NH3 and NH4 + species are in equilibrium
so that = 10 -9.25 . At the typical wetland pH of 7.5, [H + ] = 10 -7.5 K
so that the percentage
of ammonia (NH3) equals
[NH ] × 100
[NH ]+[NH ]
3
+
3 4
100
= +
1+[H ]/K =
1+
100
−7.5
10 ( −9.25
)
10
= 1.8%
Ammonia and ammonium removal by plant uptake is more rapid with growing plants
than mature plants (Koottatep et al., 1997; Reed, 2001). Ammonia removal will reach
steady-state once plant density reaches maximum and the cycle of plant growth and death
equalizes. Media adsorption will contribute to ammonia removal until all the absorption
sites are saturated (Reed et al., 2001).
Biological nitrification and denitrification has been reported to be the primary removal
mechanism for nitrogen in SSF wetlands (Crites et al., 1998; Reed et al., 2001).
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