THE FLORIDA STATE UNIVERSITY ARTS AND SCIENCES ...

• Microbial acclimation
• Hydraulic retention time
1.2.1. Nitrate concentration (Electron acceptor concentration) and Carbon
concentration (Electron donor concentration).
The stoichiometric equation for denitrification by organic carbon is (Thayalakumaran et
al., 2004; Puckett, 2004):
5C + 4NO
+ 2H
O → 2N
+ 4HCO
+ CO
−
3
2
2
−
3
2
5
---- Equation 1.2
Based on this equation it can be stated that a minimum ratio of 5:4 is needed for
denitrification to proceed with organic carbon being used as an electron donor. There are
various other reports of minimum ratios; for example, Canter (1997) states that a C:N
ratio of 3:1 is required for denitrification to occur in low levels or absence of oxygen.
Hiscock et al. (1991) reported a minimum required ratio of 5:1. The above stoichiometry
indicates that 1mg carbon (C)/l of Dissolved Organic Carbon (DOC) is capable of
converting 0.93 mg-N/l of nitrate to nitrogen gas. The DOC however first needs to be
oxidized by dissolved oxygen. This requires 1mg-C/l DOC to convert 2.7 mg-O2/l. In air
saturated groundwater (10.3 mg-O2/l at 12ºC), up to about 3.8 mg-C/l must therefore be
oxidized before denitrification can commence (Rivett et al., 2008). Hence it suggests that
in order for denitrification to commence a total minimum ratio of organic carbon to
nitrate in any given system would be 4.8:1, perhaps suggesting that the ratio of 5:1
suggested by Hiscock et al. (1991) is more realistic. There is thus a direct correlation
between the amount of available carbon and the amount or rates of denitrification.
Several studies have shown this direct relationship between the amount of available
carbon and the amount of nitrate removed due to denitrification. (Stanford et al., 1975,
Anderson, 1998; Smith & Duff, 1998; Brettar & Hofle, 2002; Hill et al., 2004; Fellows et
al., 2011).
The ratio between organic carbon and nitrate also controls the pathway between
denitrification and Dissimilatory Nitrate Reduction to Ammonium (DNRA) (Kornaros et
al., 1996). The lower the ratio between the available carbon to nitrate the greater is the