Getting Ahead of Foam: Digester Foam Control Success Stories

Digester foaming is a common problem in anaerobic digesters. Digester foaming problems can range from being a periodic minor nuisance to a major problem leading to catastrophic failure of a digester structure. Foaming conditions that get out of control can block gas collections system and safety relief systems causing increased gas pressure high enough to lift a digester cover resulting in structural failure. How a facility addresses its foaming issues will enhance the reliability of the digestion process and prevent catastrophic events from occurring.This paper presents case histories of digestion facilities that have experienced moderate to severe foaming problems and how they have been able to solve the foaming issues. Case histories covered include:Port Angeles, WA: The Port Angeles treatment plant operates two anaerobic mesophilic digesters, one fixed cover digester (West Digester) and one floating cover digester (East Digester), in parallel. In 1994, the digester mixing systems were upgraded to gas mixing systems with unconfined gas diffusers located on the digester floor. Following normal digester cleaning and removal of significant amount of debris, the digesters experienced an upset, marked by high volatile acid concentrations, decreased gas production, and increased foaming in the digesters. The digesters were remediated with the addition of seed sludge and modification of digester feeding into a series mode of operation. The system returned to normal operating conditions as noted by lower volatile acid concentrations, increased gas production, and reduced foaming. When the two digesters were placed back in service, in parallel, the digesters showed signs of upset that precipitated into a significant foaming event. The gas mixing system was operated at reduced flows as the gas mixing exacerbated the digester foaming. Evaluation of the system biology indicated the foaming condition was caused by the insufficient mixing and excessive HRT provided by operating the digesters in parallel. Stable operation was restored when the digesters were placed in series mode of operation and the gas mixers were turned on continuously. The plant has continued operating in this mode and foaming has not occurred since.Eugene – Springfield, OR: The City of Eugene – Springfield Water Pollution Control Facility began operation in 1984 with three one million gallon anaerobic digesters. For several years after startup, the plant experienced continuous foaming problems. Several potential causes of foaming were considered, including the presence of Nocardia in the secondary sludge, heating and mixing systems, digester retention time, and digester feeding rates. Finally, the problem was traced to the way primary solids were being thickened before being fed to the digesters. Prolonged solids detention times in the primary clarifiers led to fermentation causing lower pH. By maintaining the pH level in the primary sludge above 5.4 and the digester alkalinity level below 4,800 mg/.L, the plant has been able to virtually eliminate foaming problems in their digesters.City of San Diego, CA: The City of San Diego's 175 dry tons per day (dtpd) regional solids handling facility anaerobic digestion process has three 105 foot diameter, 45 to 50 foot side water depth digesters. Due to low raw solids feed rate coming to the facility, the plant has been operating with one of the three digesters in service. After several years of operation, the plant began experiencing foaming conditions such that foam entered the gas collection system plugging the flame arrestors. This, in turn, caused the gas pressures to rise triggering the high pressure alarm. Investigation of the digestion system revealed that the digester was under loaded and the digesters were being over mixed. Modifying the digester mixing rate reduced the amount of foam to manageable levels.Lessons learned from these success stories can be helpful for other plants to get ahead of their foaming problems and promote a healthier, more stable anaerobic digestion process.