Digester Gas Hold-up and Rapid Rise Foam Formation

For decades, plant operators from certain wastewater treatment plants have been plagued with anaerobic digester foaming. Foaming within an anaerobic digester can present significant problems for a utility, including: off-site odors, foam/sludge spillage from overflows, loss of digester cover buoyancy on floating cover digesters, fouling of gas lines, and over pressurization of the digester through clogged pressure relief valves (PRVs). Cases of digester foaming problems have been identified in many digesters. Analysis of several severe foaming events has led the authors to hypothesize that foaming in a commonly understood sense may not be the cause of such events, but rather a phenomenon that can be generally termed as a “rapid volume expansion.” The cause of these rapid expansion events is thought to be related to a change in gas holdup, the volume of biogas retained in the digester liquid. Rapid volume expansion was first defined in the Proceedings of the Water Environment Federation Annual Residuals and Biosolids Management Conference, Sacramento, CA., 2011 (Chapman, 2011). This paper revisits the rapid volume expansion definition, summarizing the probable causes, and describes a well documented case study at the Brightwater Wastewater Treatment Plant (WWTP).