After being thickened by the gravity thickening process and the dissolved air flotation thickening process, the solids removed from the primary and secondary treatment are subject to the process of anaerobic digestion. This process has three major purposes: sludge stabilization, pathogen destruction, and sludge volume reduction. Anaerobic digestion is accomplished by retaining the thickened solids in the absence of oxygen within the large, airtight digestion tanks for an average detention time of 20 to 25 days. Within these tanks, microorganisms digest the sludge and convert the solids into cells and gaseous byproducts, resulting in a stabilized sludge and a significant volume reduction. To increase the rate at which the biological digestion process takes place, the digester contents are heated to 100-127 degrees Fahrenheit. The gases that are produced are collected and used as a fuel source in the energy recovery process. The stabilized solids are conveyed to the dewatering process.
