The Research On The Accidental Environmental Risk Of Secondary Sewage Treatment (A~2/O Process)

The urban secondary sewage treatment plants play an important part in improving water quality of the cities and providing safety and comfort living environment to human. However, because of some internal managerial problem of sewage treatment plants as well as other issue that is or isn't controlled by people, the sewage treatment plant influent or the internal operational components will happen some paroxysmal accidents. So it sometimes caused the effluent to be superstandard , as a result the effluent will have a certain risk to the accepted water.In this paper, the forecasted effluent models of BOD₅,TN and TP after the accidents have been deduced based on the design and calculation procedure of the A²/O process. Combining the specific quality of influent and the design parameters, the factors that affect the rate of pollutant removal have been studied. Moreover, the accidents of the sewage treatment plant have been analyzed.Than the environmental risk assessment methods of the sewage plants in different conditions have been proposed. And the measure of reducing the risk has been proposed based on this method, i.e. the special controlling measures has been proposed to modify the parameter in order to reduce the risk. From this way, the environmental risk of the secondary sewage plant can be minimized or eliminated by modifying the parameters of the sewage plants operation. Finally, a special town sewage plant's operation is exampled to study how to reduce the environmental risk of the effluent by modifying the parameters.Regarding the secondary sewage plant that have been built, the accidents have two kinds: one is when the inflow excess the design flow of the sewage treatment plant in rainy season for the combined gathering system, or the secondary processing units occur accidents and can't operate normally, the sewage discharge directly through the oveneach canal after the first treatment, at this time the disposal ratio of the pollutant is limited; The other is the variation of the inflow quality and water mount that induce the change of the effluent quality, although the inflow is under the design flow.The research on the example indicated that the environmental risk of the first effluent is inevitable, the only way is to strengthen the management so that as little as possible occurrence or non-occurrence. If the accident belongs to the second kind, when the consistency of BOD₅ exceeds the allowed figure, the environmental risk can be reduced by enlarging the aeration quantity or increasing the reflux ratio. When water leakage TN does not attain a designated standard, the effluent density of TN might be reduced by increasing the reflux ratio. When the water leakage TP does not attain a designated standard, the density of the leakage TP may be reduced by reducing the sludge backflow or increasing the quantity of sludge that would release through the adjustment of the sludge density, it's mean that reduce the sludge reflux ratio. For the example in this paper, four programs have been proposed, at the same time the simple analysis to these four programs have been done, it has been concluded that the fourth program is best when only consider the processing effect.