Management of Rapid Rise Foam Formation at a Codigestion Facility

In 2011, Johnson County Wastewater completed the construction of its upgraded solids processing facilities at the 14.5-mgd Douglas L. Smith Middle Basin Wastewater Treatment Plant located in Overland Park, KS. The solids processing upgrades were needed to provide reliable capacity for the expanded liquids treatment processes and to meet future loading conditions that included treatment of trucked-in sludges from other Johnson County treatment plants and trucked-in fats, oils, and grease (FOG) waste and high-strength industrial wastes from food processing. A major focus of the project was incorporating sustainable features to maximize energy capture. The upgrades included cleaning of existing two primary and one secondary anaerobic digesters, construction of a third primary anaerobic digester, installation of a pumped-mix system into all digesters, and construction of a FOG waste and high-strength waste receiving and processing facility to enable codigestion of the waste material with the treatment plant sludges. The project also included the installation of upgraded digester gas piping, two dual-fuel fired boilers, upgraded digester gas flares, digester gas treatment system, a flexible-membrane gas storage bubble, and two 1060 kW cogeneration units.Following completion of construction of the new primary digester (Digester No. 4) in July 2010, this unit was put into service prior to sequentially removing from service, cleaning, and upgrading the other three digesters. Improvements to the digesters were completed in November 2010 and all were in service when FOG and high-strength wastes were received at the plant and codigestion was initiated in December 2010.One significant foaming event occurred in August 2010 shortly after start-up of new primary digester. The cause of this event was likely overfeeding as digester feeding was controlled manually by the operations staff. Major foaming events started in January 2011 after the initiation of codigestion. Digester discharges exceeded the capacity of the effluent control boxes for 6 events, four for Digester No. 4 and two for Digester No. 1, resulting in sludge spilling onto the roof of the digester building and subsequently draining down the storm water system and onto the adjacent ground. Many smaller foaming events also developed that did not exceed the capacity of the effluent control boxes. Following foaming events, significant drops in the digester levels and were believed to have been caused by the apparent abnormal discharges, both contained and uncontained. Most of the level drops were associated with rapid rise foaming events. An extensive review of the operating data identified where inadvertent overfeeding caused several of the major events; however, no apparent cause was determined for other events.Multiple controls methods were implemented to minimize the development of rapid rise foaming events including operational and physical controls. SCADA-controlled automated valves on the individual influent lines to the three primary digesters cycled the feed to each digester after a pre-selected volume to avoid slug feeding of the digesters. The digester levels were manually controlled by the operations staff to keep the level below the gravity effluent control boxes for several months to prevent discharges from exceeding their capacity. Feeding of the FOG and high-strength wastes was reduced to low volumes and were extended throughout all three shifts, if possible. The pumped mixing systems were cycled on and off to reduce the mixing intensity and avoid overmixing. The addition of a secondary gravity effluent lines (termed emergency overflow lines) and level indicators and alarms were installed in each of the three effluent control boxes. Following installation of the control measures, the levels of the digesters were returned to normal. Rapid rise foaming events still develop, but they are much less severe and do not exceed the capacity of the effluent control boxes.