Systematic analysis of the operational response of activated sludge process to variable wastewater flows. A case study

Abstract.

Flow perturbation due to inflow of water (mainly rain water) and loss of wastewater from the pipes of a sewage network is a widely recognized phenomenon in the operation of sewage transfer systems. The most apparent result is the variability of wastewater flow in the input of the subsequent wastewater treatment facilities, which affects several operational parameters including the sedimentation processes and the biological activity. As part of an overall effort for the upgrade of the wastewater treatment plant of Ioannina (Greece) a 3-year study was undertaken to investigate the effect of wastewater flow fluctuation on the performance of the treatment process. In either case (water infiltration or wastewater evaporation) a decrease in the process performance was observed due to decrease in the hydraulic or solids retention time or due to the elevated feed concentration caused by the reduced flow, respectively. Handling these problems through control of the return activated sludge flow was not found to be totally adequate for all operating conditions, thus indicating the need for a different perspective in the design procedures in order to meet with the legislative mandated effluent limits.