Anoxic Transformations in Sewers

Anoxic conditions in wastewater of sewer networks require availability of nitrate or other oxidized nitrogen compounds and an extremely low DO concentration. Such conditions are generally not of major importance because of the low nitrate concentrations that typically occur in wastewater. However, anoxic conditions in sewers are artificially established when nitrate is used to control sulfide problems (Section 6.2.7). A low nitrate uptake rate (NUR) is preferred, in this case, to keep the nitrate consumption low for economic reasons. 

There is — in addition to the use of nitrate for control of anaerobic conditions in sewers — a potential for anoxic treatment in terms of removal of organic matter. The anoxic treatment is an alternative to aerobic treatment. An advantage is that the addition of nitrate is simple compared with the injection of oxygen. However, a NUR value that is of the same order of magnitude as the actual OUR value—compared in units of electrons transferred—is crucial to obtain a relatively high removal rate of organic matter. For this and other reasons it is important to compare aerobic and anoxic transformations (cf. Example 5.5). 

Example 5.5 Comparison between the stoichiometry of aerobic and anoxic transformations 

To compare the stoichiometry of aerobic and anoxic transformations, the electron acceptors 02 and NO3 must be considered by using electrons as a common and suitable unit for redox processes. The basis for this comparison is shown in Examples 2.3 and 2.4  

From these two reduction processes, it follows that the transformation of 1/5 mole of NO3-N equals the transformation of 1/4 mole of 02, i.e., the oxy-gen-to-nitrate ratio for comparison of the two substances as electron acceptors is as follows  

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