Evaluating Digestion Compliance Calculations

The Orange County Sanitation District (OCSD) operates and maintains two wastewater treatment facilities. Both plants treat an average of 200,000 cuft/day of primary and secondary sludge through the digestion process. An average of 650 tons of biosolids is hauled daily to reuse options. Both plants operate a combined total of 25 digesters and 5 holding digesters. As part of the Districts Biosolids Environmental Management System (EMS) an objective was initiated to evaluate the existing compliance calculations and monitoring data for quality assurance.As part of that review, OCSD compared the use of the Van Kleeck and mass balance method to calculate the volatile solids destruction (VSR) then tracked the results for more than two years. The two methods produced results that differed 5–10% for some digesters. In addition, changes in primary treatment ferric chloride addition were monitored to demonstrate the impact on the VSR. Digester working volumes were changed to reflect improved grit accumulation estimates and to expand digester cleaning maintenance cycles increasing digester capacities.