RELATIONSHIP BETWEEN ENDOGENOUS DECAY AND DENITRIFICATION IN THE CONTROL OF pH IN OPERATING AEROBIC DIGESTERS

The City of Lebanon, Indiana operates a 3.3-mgd average daily flow wastewater treatment plant. During a recent expansion project the facility was converted to aerobic digestion for sludge stabilization. Over the course of the first year of operations the facility has experienced numerous operational problems consisting of difficulty in pH control, limited digestion ability, insufficient storage volume, and the production of significant odors. Review of digester and storage operating procedures, historical responses, and operational data, was undertaken in an effort to understand the digestion process more fully. During this review several issues arose. pH control in the digesters during the limited digestion achieved could be controlled by extending the decant cycle times to allow denitrification to occur in the digesters to maintain the alkalinity of the sludge. The nature of the sludge wasted to the digesters from the facilities oxidation ditches limits the extent of volatile solids destruction ability in the digesters and the SOUR of the waste activated sludge was determined to meet vector attraction reduction requirements. Storage requirements imposed by limited land application opportunities required mechanically thickening of the sludge in the digesters to levels of up to 5% total suspended solids. Elevated solids levels in the digesters can impair the aeration system and resulted in the upset of the digesters and the production of odors.