Influence Of Rainfall On Wastewater Treatment Plants And Optimization Strategies

The influent flow of wastewater treatment plant (WWTP) increases substantiallyduring rainfall events. Because of the flushing effect of rainfall, the rainwater containsan enormous quantity of pollutants. As a result, the deterioration of the WWTPperformance may occur during rainfall events. Based on the WWTP model built byBiowin software, the influence of rainfall on the example WWTP was studied and theoptimization strategies were proposed. The main contents and conclusions involved inthis research are:(1) Under different rainfall return period conditions, the influence of rainfall onthe influent flow and water quality of WWTP was studied. The results showed that,the influent flow greatly exceeds the designed treating capacity of the exampleWWTP and the WWTP cannot treat all the influent water. The overflows should bedelivered into rivers and the water environment is greatly polluted.(2) Three intensive processes were proposed, i.e. enhanced primary treatment,enhanced primary treatment combined with secondary treatment (the combinedprocess) and enhanced primary treatment combined with secondary treatment withdecreased hydraulic retention time (the combined process with decreased HRT). Theresults showed that, under the P=0.5a rainfall condition, all the influent water can betreated by the proposed processes and the maximum treating capacity is increased to114×104m3/d, which is2.53times larger than the designed treating capacity of theexample WWTP.(3) Under the rainfall condition, the effluent pollutant loads of the exampleWWTP are greatly reduced by the proposed processes. The effluent pollutants areefficiently reduced by the chemically enhanced primary treatment (CEPT) andbiologically enhanced primary treatment (BEPT). For the combined process, whenCEPT is applied, the effluent COD, TN, TP and SS are reduced by the ratios of59.87%,26.98%,62.04%and77.44%, respectively, compared with the existingprocess. When BEPT is applied, the effluent COD, TN, TP and SS are reduced by theratios of53.39%,47.26%,36.62%and77.53%, respectively, compared with theexisting process. The pollutant removal efficiency is further improved by thecombined process with decreased HRT. When CEPT is app lied, the effluent COD, TN, TP and SS are reduced by the ratios of70.79%,42.23%,72.13%and86.48%,respectively, compared with the existing process. When BEPT is applied, the effluentCOD, TN, TP and SS are reduced by the ratios of64.98%,59.06%,68.96%and84.14%, respectively, compared with the existing process.(4) Under the P=0.5a rainfall condition, the investment cost of BEPT is14.55%larger than that of CEPT, but BEPT is better than CEPT in terms of removalefficiency and operation cost. Therefore, BEPT is more preferable than CEPT for theexample WWTP.