Nitrification inhibition assessment of industrial effluents and influent to Amanzimtoti wastewater treatment plant.
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The aim of process industries is to produce products and intermediates from raw materials and other intermediates. Inevitably, there are waste products to be disposed of and if these are of no use, they must be returned to the air, water or land environments. Such returns should be carried out in such a way as to minimise any adverse effects on the environment, otherwise the waste is bound to cause pollution to the environIDent. Wastewater is one such product that has to be returned to the environment. A weakness in the current practice of wastewater treatment is that the potential toxicity of the effluent is only addressed through the prevention of specific types of waste being discharged to the sewer. The discharge of effluents containing toxic or inhibitory substances is currently not directly addressed or controlled by many industries and local authorities. While cost recovery is important, due consideration must be given to the possible effect on the receiving environment. The magnitude of the problem of toxic components in the inflow to wastewater treatment plants in South Africa is largely unknown. However, it is thought by some treatment authorities to be relatively serious. In addition, there has been no attempt to quantify the effect of individual toxicants on the performance of the treatment processes and thus put a monetary value to individual discharges. Nitrification is one of the important biological processes that takes place in wastewater treatment plants, which may be affected by toxicants from wastewater. The toxicants may inhibit the nitrification process and create problems in the treatment plant. The aim of this study was to determine if the Amanzimtoti Wastewater Treatment Plant is experiencing inhibition of nitrification, and if so, determine whether large industries discharging into the plant contribute to this problem. The study site used in this research was the Amanzimtoti Wastewater Treatment Plant, located at Isipingo, in Durban, together with some selected industries that discharge their effluents into this treatment plant. In this study, the Amanzimtoti Wastewater Treatment Plant together with lO industries that discharge effluent into it, were surveyed for inhibition of nitrification. A screening method for estimation of inhibition of nitrification at municipal wastewater treatment plants described by Jonsson (2001) was used in the investigations. This involved testing inhibition of nitrification at various dilutions of wastewater effluent from 20% to 80% dilution. An investigation was conducted of inhibitory substances within influent wastewaters to the Amanzimtoti Wastewater Treatment Plant, and inhibitory substances were detected in all four sampling weeks. The level of inhibition was in general up to 29%, with the greatest inhibition being observed at 20% and the least at 80% dilution. In order to investigate the source of inhibition, inhibition of nitrification was measured in the sewage influent during times when industries are open and when they are closed. Inhibition was significantly lower during December when industries close, supporting the hypothesis that industrial effluent contributes to inhibition of nitrification. Comparison of wastewater from different industries showed that of 10 surveyed industries, 9 generated wastewaters that were found to be inhibitory, with Industry D showing the highest inhibition of approximately 30% over the 4 dilutions. The least inhibitory effluent was from Industry C with an average of 10%. Industry A was found to stimulate nitrification. There was no correlation found between the daily volume contribution of the industries to the treatment plant, and the inhibition of nitrification. There was also no correlation found between the inhibition of nitrification and the chemical oxygen demand and settleable solids concentration of wastewater from each of the industries.. At 80% dilution, the nitrification inhibition results obtained for all nine industries were similar and it was difficult to distinguish between them, whereas at 20% dilution, the differences among the industrial effluents on nitrification could be clearly evaluated. Industries B, D, E, G and J were found to have higher inhibition than the other four surveyed industries. Results obtained at the 20% dilution could therefore be used as a decision making tool by wastewater pollution officers to identify industries requiring close monitoring. From the study, it was clear that the inhibition of nitrification that resulted from mixtures of industrial wastewaters cannot be readily predicted from nitrification inhibition by the individual wastewaters. New compounds may be formed during mixing in the sewer network that are more or less inhibitory than if the wastewaters are not mixed.