Denitrification only partially offsets the alkalinity loss caused by nitrification, as the alkalinity gain per mg of nitrogen is only one-half of the loss caused by nitrification. This is because the alkalinity gain per mg of nitrogen is only one-half the loss caused by nitrification.
A value for alkalinity production suitable for engineering calculations would be 3,0 mg alkalinity as CaC03 produced per mg nitrogen reduced.
In the design of systems where alternating nitrification and denitrification are used, a sudden high load of ammonia in the waste water can cause a self-destruction of the system, because of the high H+ concentration developed during nitrification (Fig. 4.4) The denitrification will not occur because of the decreased pH, as the denitrifying organisms can not denitrify under a low pH condition.
Table 4.9 presents observations from the literature of the effect of pH on denitrification rates. It would appear that for most systems the denitrification rate is depressed below pH 6,0 and above pH 8,0. Different studies indicate different pH values as the optima for denitrification, but most studies show the highest rates of denitrification occur within the range of pH 7.0 to 7.5.
All results are presumably long-term pH dependence studies, but this is impossible to determine from the information available.
The influence of pH on denitrification is also dependent upon the duration of the effect. The short-term effect of a pH change is the most interesting, because a pH change generally does not vary over a long period.
In Section 4.3 it is shown that denitrification produces alkalinity, which will result in an increase in the pH value. The magnitude of this increase depends upon the buffering effect of the sewage, because nitrification, on the other hand, produces acidity.
In a combined nitrification-denitrification process, the pH of the two processes should thus balance each other out, the result being a constant pH. (Barth etal. 1968; Halling-Sorensen and Hjuler 1992).
Timmermann and Van Hauten (1983) determined the growth rate n as a function of pH in batch reactors at 25 °C. The biomass of the bacteria was measured as a MLVSS- concentration. Figure 4.5 shows that a maximum growth rate was found at pH 8,5.
According to Hartmann and Laubenberger (1968), a deviation of the pH from the optimum pH reduces the bacterial activity according to the mechanism of noncompetitive inhibition (see Section 3.13).
Table 4.9 pH variation in denitrification experiments, and pH studies.
Johnson and Schroepfer (1964)
Meiring and Stander (1964)
Barth and Ettinger (1968)
(Table 4.9 continued)
5.5-8,5 Kiff (1972) 6,0-10,0 Ide etal. (1972) 5,0-8,0 Clayfield (1974)
7.7 - 7.8 Halling-Sorensen and
Timmermann and Van Hauten (1983) also showed the methanol/nitrate-N ratio as a function of pH. At optimum pH (=8,3 proposed by Timmermann et al. 1983) the methanol/ nitrate-N ratio was found to be 2,52 g CH3OH / N03" - N, (Fig 4.6).
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