Mason Farm WWTP
The Mason Farm WWTP in Chapel Hill, N.C., operated by the Orange Water and Sewer Authority (OWASA), was perhaps the first treatment plant in the United States to be brought online with a configuration to produce Class A biosolids using thermophilic-anaerobic digestion. OWASA originally investigated configurations that would have qualified as an equivalent PFRP before settling on a time-and-temperature configuration.
In the mid-1990s, the EPA's PEC denied OWASA's initial application for a 51° C operation. The basis for that application was mathematical modeling of fecal coliform kinetics in the reactors, with rate constants consistent with the Part 503 time-and-temperature equation. The application was denied based principally on three factors: a lack of empirical data supporting the process description, perceived likelihood of pathogen short-circuiting in the described operational scheme, and concern over the ability of a 51° C process to adequately inactivate Ascaris.
OWASA subsequently built the facilities based on a plan to operate the process near 56° C. This allowed the second reactor, with a greater than 20-hour batch time, to reliably provide the necessary batch time to meet the Part 503 time-and-temperature requirements and qualify under Alternative 1. Most plants that have considered time and temperature options have gravitated to operational criteria near 55° C, with approximately 24 hours of batch detention time. This is due largely to the operational stability of thermophilic systems at temperatures in the 55° to 56° C range, while allowing the batch times to be maintained at practical times and volumes.
In the Mason Farm Class A digestion system, the thermophilic stages are followed by a mesophilic stage creating a temperature-phased system and increasing volatile solids reduction and gas production. The additional gas produced at OWASA is used to fuel an engine-driven aeration blower.
The digestion system at OWASA was started up in late 2000 and reached stable, Class A operation in the spring of 2001. The plant currently is undergoing an upgrade from 12 to 141/2 mgd capacity. As part of that project, the digestion system is being enhanced to eliminate odors and improve operational reliability.
— Willis is a vice president and southeast wastewater practice leader with Brown and Caldwell Consulting Engineers in Atlanta. Schafer is a vice president and Brown and Caldwell Consultants' National Biosolids practice leader in Sacramento, Calif.
Portions of this article were part of a 2005 WEFTEC presentation titled “The State of the practice of Class A anaerobic digestion: Update for 2005.”