The Effect of Detention Period on the Clarification Efficiency of Nitrified Activated Sludge

Proper performance of the final clarifier in the activated sludge process is vital to total system efficiency. Inadequate clarification can adversely affect substrate removal efficiency due to the biochemical oxygen demand exerted by solids lost in the clarifier effluent. Detention period is one factor which influences clarification performance and thus is of interest to the designer engineer. Also of interest is the relationship between detention period and rising sludge conditions in the clarifier, which can lead to a deterioration in clarification efficiency. Rising Sludge conditions are believed to be caused by the denitrification of nitrified activated sludge in the final clarifier. An experimental program was developed to provide the necessary data to support the research objective of determining the singular significance of detention period to the steady-state effluent suspended solids concentration from the final clarifier for nitrified activated sludge. Side-by-side experimental units with a common feed slurry source were utilized in order to minimize complications associated with time-dependent biological characteristics of activated sludge. The steady-state effluent suspended solids concentration was determined to be significantly affected by the detention period in accordance with a parabolic model which suggests a reduction in effluent suspended solids with increasing detention period of up to a 5-hour optimum, above which the concentration increases. The steady-state model was developed through inclusion of additional terms that could be statistically proven to improve model accuracy. The analysis of variance technique was utilized to confirm the model. Through visual evidence of rising sludge phenomena beyond the optimum detention period and the indication of increasing denitrification activity with increasing detention period, there is evidence to suggest that the impairment of clarification efficiency was due to denitrification within the experimental clarifier units. However, it appears that increasing the detention period to improve clarification efficiency for nitrified activated sludge can be practiced within reasonable limits without experiencing impairment of performance due to denitrification activity