Variation in dissolved oxygen concentration and its effect on the activated sludge properties studied at a full scale wastewater treatment plant

Aeration of activated sludge is very costly and therefore it is desirable to operate the treatment plant at as low DO concentration as possible without risking poor effluent quality. This paper presents a study of the impact of DO concentration on sludge properties at a full scale wastewater treatment plant which utilizes pre-denitrification followed by aeration in activated sludge and with post-nitrification in nitrifying trickling filters. The DO concentration has in the past generally been kept high at the plant (> 5 mg/l) as it has been believed to give better effluent quality. During later years attempts have been made to reduce the DO concentration to 2-3 mg/l. In this study the DO set-point was changed every 3 weeks between 2 and 4 mg/l for a few months. Also short term effects of change in DO concentration was assessed by changing the DO concentration in the three parallel activated sludge tanks to 1, 3 and 5 mg/l for a few hours. The results show that the impact of DO concentration is hard to assess as the process is subjected to constant changes in the environmental conditions. The settling properties were not influenced significantly whereas slight negative effects of reduced DO concentration on effluent quality could be seen. The microscopic investigation showed a tendency towards poorer sludge properties at a DO concentration of 2 mg/l compared to at 4 mg/l.