PLANNING FOR THE FUTURE: EXISTING ANAEROBIC DIGESTION FACILITIES UPGRADED TO ACHIEVE CLASS A BIOSOLIDS

Implementing pasteurization ahead of an existing digestion facility may be an effective and simple means to obtain greater pathogen destruction to Class A standards. Heat and energy balances show that implementing heat recovery will allow plants of sufficient size to utilize the biogas to maintain process temperatures with excess biogas for building heating or cogeneration. Heat balances will be given for a 50,000 gallons per day sludge flow facility to illustrate the overall process.The process to destroy pathogens and stabilize solids has two distinct stages. The first stage solubilizes the biosolids and destroys pathogens in a pasteurization tank at 65.5°C (150°F). This achieves Class A pathogen reduction by fulfilling the time and temperature requirement as stated in Alternative 1 of the United States Environmental Protection Agency (USEPA) 503 Regulations. This tank would be located upstream of the anaerobic digestion. The anaerobic digester would provide stabilization and energy recovery through biogas generation. The following table, Table 1, summarizes bench-scale results achieved by pasteurization.Pasteurization tank size and its impact on the sludge heating rate (boiler firing rate needed to process the daily sludge flow in batch mode) will be presented. Furthermore, the biogas quantity generated in anaerobic digestion maintains process temperatures if heat recovery is implemented. Heat recovery is defined as the utilization of the pasteurized sludge to prewarm the raw sludge. Detailed information on the sludge heat recovery system and equipment, benefits and possible drawbacks will be discussed.