The Effect of detention time, activated sludge, and oxygen concentration on treatment efficiency of extended aeration activated sludge system

74 SUMMARY Extended aeration system is a wastewater treatment method that the mixture of wastewater and activated sludge is aerated to obtain oxidation of organic matter in activated sludge. Aeration time is usually 18-36 hours and settled activated sludge is returned to aeration tank. Since activated sludge is oxidized at high rate as the result of endogenous respiration, the amount of waste sludge is less than that occurred in other activated sludge systems and easy to treat. In this study, variation of treatment efficiency by hydraulic detention time, activated sludge and dissolved oxygen concentrations were investigated using an extended aeration activated sludge model plant. Synthetic wastewater that includes organic matter approximately equal to medium strong municipal wastewater was used in the experiments. The reasons of using synthetic wastewater are that it can be prepared in desired and constant characteristics and does not include toxic matter. The system was operated as a continuos flow complete mix reactor. Three different MLSS concentration was applied: 2000, 4000 and 5000 mg/L For each MLSS concentration, 2 and 5 mg/L dissolved oxygen concentrations were applied and effluent COD values were measured for 1 8, 24, 30 and 36 hours detention times, and then COD treatment efficiencies were calculated. The results were discussed with literature together. According to the, the results obtained from the experiments, increasing hydraulic detention time from 18 h to 36 h did not effect treatment efficiency considerably. However, it is known that, increasing detention time decreases sludge production. This may be used in designing activated sludge treatment systems for different purposes.75 Beside these, increasing MLSS concentration from 2000 mg/L to 5000 mg/L decreased the treatment efficiency due to the lower F/M ratio. Increasing dissolved DO level from 2 mg/L to 5 mg/L did not increase the treatment efficiency but however it is affective on the sludge stabilization. As a result, in designing an activated sludge plant, special attention should be paid for these three parameters, especially when considering initial investment, operating costs and sludge disposal.