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Fig 3.14 The influence of thiocyanate on the nitrification rate. After Neufeld ef al. (1986).

Beg and Atiqullah (1983) conducted experiments with a fixed film reactor and showed that As3+, Cr6+ and F" were reversible non-competitive inhibitors, having inhibitor constants of 305, 65,3 and 1276 mg/l, respectively. Also, interaction between the three inhibitors showed that it did not affect the zero-order kinetic of nitrification with respect to the NH4+ - N substrate concentration.

When the concentration of a strong inhibitor was kept constant, and that of a weaker one was varied, two phenomena were observed (Beg and Atiqullah 1983). Firstly, for a shock dose at lower concentrations of the stronger inhibitors such as chromium and arsenic, the degree of inhibition was increased with the increase in the concentration of the weakest inhibitor, fluoride. This tendency was more pronounced at lower concentrations of the weaker inhibitor than at higher concentrations. Secondly, at higher concentrations of the stronger inhibitors, chromium (> 40 mg/l) and arsenic (> 300 mg/l), the degree of inhibition initially decreased to a minimum value, and then increased with the increase in the concentration of the weaker inhibitor. Beg and Atiqullah developed the following rate expression for the shock load of As3*, Cr6* and F", in pairs:

where; I is the concentration of the stronger inhibitor l2, that of the weaker one in the pair.

Table 3.20 show a list of inorganic compounds that lead to inhibition of the nitrification process. It is important to remember that the inhibition of inorganic compounds is dependent on the actual pH in the environment, because it is often the free inorganic compound, for instance copper ion, that inhibits the nitrification process. As an example, the free copper ion concentration increases with decreasing pH.

Table 3.20 Inorganic compounds that lead to inhibition of the nitrification process.

Compound Concentration mg/l Type of Plant References

CN"

SCN"

Toxic in all conc.

65,3

1267

Toxic in all conc.

Coke plant waste water

Fixed bed reactor

Plastic media trickling filter

Fixed bed pilot pilot plant.

Neufeld et al. (1986)

Beg and Atiqullah (1983)

USPHS (1965)

USPHS (1965)

USPHS (1965)

USPHS (1965)

USPHS (1965)

Hjuler (1992) (unpublished)

Figure 3.15 Effects of As3"1", Cr6+ and F" on the nitrification rate. After Beg and Hassan (1987).
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