Halfway through a sugar beet processing campaign (September through May) a Midwest refinery noticed something was wrong with their wastewater processing system. The plant discharge was within limits for their NPDES permit parameters, but the chemical oxygen demand (COD) readings from their 8.9 million-gallon anaerobic digester were very different at the top, center, and bottom of the digester. The system engineer suspected the mixers were not operating properly.
Without functional mixing, the digester was unable to process the high COD wastewater from the refinery. To fix the problem, the facility’s processing and engineering team decided to repair the mixing blades in the digester at the end of the campaign. The digester was emptied and cleaned in the process, which removed all active biology in the tank. After maintenance was completed, the anaerobic digester needed to be reseeded to begin properly processing high COD wastewater.