The effect of temperature on activated sludge plant operation

Three laboratory and pilot scale, diffused air activated sludge units were operated as continuous plug-flow tapered aeration systems at constant temperatures of 10 C, 20 C and 30 C. Dissolved oxygen was maintained at about 2 mg/l and the sludge was recycled at the same rate as the sewage feed rate for all the three units. Synthetic sewage was fed to each unit at four different detention periods namely 5, 3, 1.5 and 1 hour in turn including the return sludge. At each detention period the effect of four different concentrations of mixed liquor suspended solids of 2,000, 3,000, 4,000 and 5,000 mg/l were investigated. Operational parameters for each run measured included BOD 5 , COD, mixed liquor suspended solids, suspended solids, dissolved oxygen, turbidity, pH, free and saline ammonia, nitrite, nitrate, organic nitrogen, sludge volume index, sludge production, specific resistance to filtration of sludge, anionic surface active agent, air flow rates and total phosphates. In addition microscopic examination of the activated sludges vas,carried out occasionally. The results obtained indicated that BOD5 and COD removal efficiencies, air requirements, nitrification increase with an increase in temperature from 10 C to 30 C. Nitrogen loss is caused by aerobic denitr1fication. Complete removal of nitrogen in sewage is possible by aerobic nitr1fication-denitrif1cation process in the aeration tank of a single-stage activated sludge unit at 30 C and at high loading. Specific resistance to filtration of sludge, sludge production rate and phosphate removal decrease with an increase in temperature from 10 C to 30 C. The sludges· possess better resistance to bulking at higher temperatures and at low loadings. The average values of detergent removal efficiencies are well over 88% for all the three units at 1O C, 20 C and 30 C . Metazoa such as rotifers, earth worms and water mites were numerous at higher temperatures. There is only a marginal saving in cost for plant operating in tropical country as compared to that in developed country in spite of the better plant performance in tropical country