Case Study: Causes and Solutions for Anaerobic Digester Foam Blankets

The City of Phoenix (COP) owns and operates the 23rd Avenue WWTP (63 MGD) whose liquid stream is configured with preliminary treatment, primary treatment, activated sludge with nitrogen removal, effluent filtration, disinfection, and dechlorination. Solids handling includes centrifuge secondary sludge thickening, 15-18 day HRT mesophilic anaerobic digestion, centrifuge dewatering of digested sludge, and land application of the dewatered biosolids. Currently, primary and thickened waste sludges are pumped to four 1.5 MG anaerobic digesters. Prior to December 1999, only primary sludge was feed to the digesters, and waste sludge was diverted to the 91st Avenue WWTP regional facility.Since the Solids Handling Facility was commissioned during December, 1999, digester foaming at the COP 23rd Avenue WWTP has caused operational problems ranging from the need for increased operator intervention, to structural damage (digester dome) and reduced solids handling capacity (required for tank protection). Digester foams have been caused by liquid stream filamentous bulking, anaerobic filamentous bulking, liquid stream pass-through events, and digester design and operational issues. Digester foam solutions have included tank and plant upgrades, additional instrumentation, and changes to operational procedures.This case study will look at:Four different types of foam experienced, and factors that contributed to their formation.Digester modifications and operational strategies that have prevented, minimized and controlled digester foam blankets.