MODELLING AND CONTROL OF ACTIVATED SLUDGE WWTPS UNDER INFLOW VARIATIONS DUE TO A COMBINED DRAINAGE SYSTEM

Flow variation influences the management of Wastewater Treatment Plants (WWTPs). Normally pollutants concentration variation is the most relevant consequence of the flow variations and thus influences the process behaviour in different plants sections. In WWTPs, receiving both sewage and rain water, the respect of law limits requires an accurate management of catching work with primary treatment phases forced to receive, in rainy conditions, a large amount of water volumes. Mathematic descriptions about active sludge (A.S.) WWTP performances in function of incoming and bypass fluxes show that, in a flow scheme based on the mixing of all plant bypass flows with the final effluent, it must be received by primary settling a peak flow included between 4 to 5 times the mean dry weather hourly flowrate together with a 2 times permitted to the secondary. Main part of the study consists in a detailed examination of experimental data in order to understand the effects of incoming concentration variations on a classic A.S. process. Three years monitoring results of Bologna WWTP flows permit to verify removal efficiency for BOD and TKN of primary and secondary phases of the urban wastewater treatment plant and reveal, both in dry and wet conditions, a different but substantially constant removal efficiency for BOD values above 100-110 mg/l. In wet conditions, when rain permits flowrate higher than 3*Q24, the efficiency of primary settling decreases from 26,9 % to 23,5 % and efficiency of secondary phases from 82,7 % to 77,3 %. Main wet functioning scenarios have been represented with WWTP modelling carried out by a ASM-IAWQ based model (STOAT 5.0) validated and calibrated in dry conditions. Output results have been compared with monitoring results. Calculated and measured efficiency values have permitted to verify the proposed bypass scheme focusing on the required plant performances in terms of BOD and TKN removal.