Lesser of Two Evils – Digester Cleaning or Run to Failure

The complexity of anaerobic digester cleaning has forced numerous utilities to operate their anaerobic digester systems in a run-to-failure mode. The main disadvantage of the run-to-failure approach is that anaerobic digesters are cleaned only after several years of steadily declining process performance. The generally accepted symptoms of a digester requiring cleaning are:Foaming.Unstable operating conditions.Declining volatile solids reduction coupled with a reduction in digester gas production.A reasonable cleaning frequency is once every three to eight years, depending on grit accumulation within the digester, with the digesters losing 20 to 25 percent of volume during this time period. It is recommended that the digesters be cleaned after their fourth year of operation; however, some facilities have been able to extend their operation a few more years before cleaning was required. Monitoring grit accumulation within the digesters once per year after an initial three years of operation will help determine when cleaning is required at the facility.Utilities must also grapple with other cleaning issues that require effective risk mitigation from both known physical concerns (toxic gases, confined space entry and materials handling) and site specific unknowns, including the following:Internal and/or external valve/piping replacement.Digester roof or cover coatings failures.Concrete corrosion.Repair time and/or budget constraints to execute the necessary mechanical and/or structural repairs.Failure analyses of the digester mechanical, electrical, and instrumentation systems, as well as implementation of upgrades and/or replacement, should be considered prior to the execution of a digester cleaning project. Cleaning projects should also include the chemical and/or mechanical cleaning of the various piping systems.Three case studies will be presented including experiences at the Morris Forman Water Quality Treatment Center in Louisville, KY, Mauldin Road Wastewater Treatment Plant in Greenville, SC, and Blue River Wastewater Treatment Plant in Kansas City, MO. These case studies include lessons learned by the owner(s), including cleaning/disposal challenges, challenges to continue operating the facility during the cleaning process, and digester startup once cleaning is completed. The total cost for digester cleaning is presented for each case study, including costs for the contractor to clean the digester, hauling/disposal costs, and repair expenses.