Effect of low dissolved oxygen concentration on effluent turbidity

An activated sludge plant in Atlanta, Ga. (R. M. Clayton Plant) with a design treatment capacity of 5.2 m3/s (120 mgd) was operated for 9 months with average dissolved oxygen (DO) concentrations of less than 1 mg/L in the aeration basins as a result of aeration equipment failure. During this period, plant effluent was turbid regardless of changes in sludge age, aeration basin contacting pattern (conventional versus use of aeration and reaeration basins), amount of solids recycled from sludge handling facilities, and seasonal variations. Industrial wastes did not seem to be the cause of high effluent turbidity. The only operating parameter that was consistent throughout this period was low DO concentration. Once the aeration equipment was repaired and the DO concentration increased to more than 2 or 3 mg/L, plant effluent cleared markedly within a few days. Concurrently, effluent suspended solids concentration which had typically been 40 to 60 mg/L, was reduced to 15 to 30 mg/L. Further verification that effluent turbidity was related to DO concentration occurred when a deterioration followed by im provement in plant effluent corresponded with failure and repair of the aeration system. Others involved in the operation of activated sludge plants12 have observed the same phenomenon. However, little reference to this phenomenon could be found in the literature. The purpose of this research was to demonstrate this relationship and to identify responsible mechanism(s).