Case Study: Causes and Solutions for Anaerobic Digester Foam Blankets

Abstract

The City of Phoenix (COP) owns and operates the 23 rd 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 91 st Avenue WWTP regional facility.Since the Solids Handling Facility was commissioned during December, 1999, digester foaming at the COP 23 rd 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.

Coping with Digester Foaming in an Age of Increasing Incidence

“…Total Solids (TS) concentrations of two to three times that in the digesting sludge have been reported during digester foaming episodes in Phoenix (Jacobs, 2008), Sacramento (Pagilla, 1997) and San Francisco (Jolis, 2003). This results in a longer SRT for solids in the foam layer.…”

“…The filaments become incorporated in the waste sludge (WAS) and are "seeded" into the digester as part of the feed solids. A number of wastewater utilities have reported digester foaming problems in recent years which they attributed to filamentous organisms (Ghosh, 1995;Jacobs, 2008;Jolis, 2003;Pagilla, 1997). In all likelihood, many more filament-induced foaming incidents have been experienced at other locations, but filaments have gone unnoticed as the primary cause.…”

“…Jacobs et al (2008) reported on several digester foaming incidents at the 23 rd Avenue Plant in Phoenix, the most serious of which occurred during startup of the Torpey process (sidestream thickening of digester contents to allow higher loading and extend the digester SRT). A previously unknown "Anaerobic filament B" was identified during microscopic analysis of digester foaming that subsequently occurred during a second Torpey startup.…”

“…Install radar level-detection instrumentation to operate in conjunction with an effective liquid level detection system, such as pressure sensors. This is a relatively low-cost modification which, according to Jacobs (2008), his been particularly useful in enabling staff at the 23 rd Avenue plant in Phoenix to identify foaming episodes quickly and working to eliminate them.…”