Ennix Digester Optimization Program for Energy Management and Biosolids Reduction

EXECUTIVE SUMMARY A seven-month study was carried out to compare a novel approach for digestion of wastewater sludge, the Ennix process, with a conventionally operated (Control) aerobic digester on a side-by-side basis. The Ennix process does not require aeration of the digester and employs mixing on an intermittent basis. Selected bacteria and biochemicals are periodically added to waste activated sludge entering the digester to maintain a favorable environment for sludge biodegradation.Hydraulic and volatile solids loading patterns for the Ennix and Control digesters were almost identical throughout the study. The mean hydraulic residence times were 21 and 20 days, respectively, and the mean volatile solids loadings were 36 and 39 lb/1000 ft3/d, respectively. The mean solids residence time of the Ennix digester was 24.4 days, compared to 25.4 days for the Control.Oxidation-reduction potentials in the Ennix digester were significantly lower and more consistent than in the Control. However, the mean dissolved oxygen and hydrogen sulfide concentrations in the two digesters were statistically equivalent. The Ennix digester had significantly higher ammonia and alkalinity concentrations and significantly lower sulfate concentration and pH. The pH in the Ennix digester was lower than expected, given the high alkalinity. This possibly reflects accumulation of carbon dioxide from biochemical sludge stabilization.The Ennix digester achieved higher volatile solids destruction than the Control on eight of the ten sampling dates. Its mean volatile solids destruction was 20.4%, compared to 17.2% for the Control. Fecal coliform MPNs in the Ennix digester were consistently below the EPA 503 Class B criterion of 2 million per gram total solids. Its mean log removal of fecal coliforms was 1.11, compared to 1.08 for the Control. Contents of As, Cd, Cu, Pb, Hg, Mo, Ni, Se, and Zn in Ennix digester sludge were similar to those of the waste activated and Control digester sludges and were well below EPA 503 Class B limits.Annual savings of $22,900 in electrical energy consumption at 6.5cents/kWH could be achieved using the Ennix process for both digesters at the 3 MGD facility where testing was conducted. Additional savings of $3,600 yearly would accrue through avoidance of electrical demand charges. Generally, a wastewater treatment plant employing 2% solids digestion would reduce electrical energy consumption by 3,100 kWH to 6,200 kWH annually per 1000 ft3 of digester volume with the Ennix digester optimization process.