The Effects of Temperature and Solids Retention Time on Activated Sludge Treatment Performance

A bench-scale treatment system with external clarifier was used to determine the effects of solids retention time (SRT) and temperature on treatment performance while simulating the full scale activated sludge process. Four bench-scale reactors, operating at SRTs of 20, 10, 5 and 2 days, were fed settled municipal wastewater collected after primary clarification from the Kuwahee wastewater treatment plant in Knoxville, TN. Three operational temperatures were investigated during this study, 22oC, 12oC and 7oC. The order and duration of operation at each temperature was first 22oC for 61 days, next 7oC for 47 days and lastly 12oC for 105 days. Experimental results indicate that carbon treatment performance improved, on average with increasing SRT; however, carbon treatment performance was not effected by operational temperatures studied to the same extent. On average, carbon oxidation improved with increasing SRT at each operational temperature; however, carbon oxidation was more efficient at 12oC than 22oC. CODT treatment performance at 7oC was poor irrespective of SRT. The difference in performance between 22oC and 12oC was attributed to acclimation of biomass from startup at 22oC to operation at 12oC beginning at 108 days. Sludge volume index (SVI) and effluent suspended solids (ESS) values also indicated that SRT had a greater impact on biosolids settling than did temperature. During this study, nitrification was observed to be viable at 22oC and 12oC despite reported sensitivity of lower SRTs (2 and 5 days) to process changes and low operational iv temperature. According to these observations, higher SRTs would be recommended for full-scale treatment especially during wintertime or sub-optimum temperatures. Nitrification at 7oC was observed to be almost nonexistent for all SRTs studied during this research. Based on the data presented in this study, an SRT of 5 days and a process temperature of 12oC are necessary to achieve effective carbon treatment, biosolids settling, and nitrification. A comparison of the full and bench scale (5-day SRT) systems, based on the operational and environmental parameters considered in this study revealed inadequacies of the bench scale system in simulating the full scale facility. Carbon treatment performance as well as MLSS levels, differed in the two systems. The full scale facility consistently discharged significantly lower carbon levels than did the bench scale system. Additionally MLSS levels were significantly higher in the full scale facility. Nitrification performance was better in the bench scale system at temperatures above 12oC but were significantly poorer at 7oC. Overall, the large variability in the performance data obtained for the two systems indicated that the bench scale system did not effectively simulate the full scale facility.