Two-Phase Anaerobic Digestion of Municipal Sewage Sludge Optimization of the Pathogen Destruction

The Two-Phase Thermo-Meso Anaerobic Digestion of municipal sewage sludge has gathered a lot attention in recent years for its capability to produce Class A biosolids. During 1997 and 1998 Suez and Ondeo Degremont, Inc. ran a pilot study of the Two-Phase Thermo-Meso Feed-Sequencing Anaerobic Digestion at the Belmont AWT facility in Indianapolis, Indiana. Consistent production of Class A biosolids as well as 3-log and 2-log reduction of enteric virus and viable helminth ova respectively were documented. Most of the pathogen destruction occurred in the first phase of the process. Data from the literature was carefully reviewed in an attempt to find the cause of such extensive pathogen reduction. The use of a well-mixed “draw and fill” digester, a thermophilic temperature, the presence of free ammonia and volatile fatty acids (VFA) were all found to be germicidal. A combination of all these factors in the first phase of the process is believed to be at the source of the pathogen destruction observed. The pilot test results can be reasonably extrapolated to full-scale systems as long as proper operation of the thermophilic phase is ensured and sufficient mixing is provided with the industrial size digesters. The Two-Phase Thermo-Meso Feed-Sequencing Anaerobic Digestion process (2PAD™) has currently a site-specific PFRP equivalency recommendation. However, this recommendation is in the process to be changed over to a “conditional” national PFRP equivalency recommendation.