Septic wastewater, characterised by the appearance of sulphide, is known to cause problems in sewage systems (corrosion and odour), at treatment plants (e.g. inhibition, sludge bulking) and for human beings (toxicity). Sulphide formation in sewers may be prevented by increasing the redox potential, either by oxygen/air injection (aerobic conditions) or dosage of nitrate (anoxic conditions). The effect on the nitrification capacity in a biofilm process of an anoxic wastewater as compared to a septic wastewater has been studied. The main change in wastewater quality as a result of nitrate dosage is reduced concentrations of organic matter and insignificant sulphide concentrations. The results show that a sulphide concentration of 0.5 mg/l had a considerable negative effect on the nitrification activity. The sulphide and the higher concentrations of organic matter in the septic wastewater caused together a 30-40% reduction of the nitrification capacity as compared to the anoxic wastewater, even with pre-aeration and pre-precipitation with Fe3+. The removal of organic matter in the sewer as a result of the anoxic conditions created by the addition of nitrate, resulted in a maximum nitrification capacity when particulate organic matter was removed by pre-precipitation.

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