MWRA Advances Thermal Drying Art

The Massachusetts Water Resources Authority (MWRA) is nearing completion of a 3.5 billion-dollar upgrade of its Boston Harbor wastewater treatment facilities. Once completed, the upgrade will significantly expand average daily capacity of the facility to 370 MGD, providing secondary wastewater treatment for over two and one half million people in 43 communities.The MWRA had been mandated by federal court order to cease dumping sludge into the ocean by December 31, 1991. Seven days before the deadline, the MWRA permanently closed the sludge outfall pipe and began converting municipal biosolids into a commercial fertilizer product at a stand-alone interim sludge processing and disposal facility (ISP&D) remotely located in Quincy, MA. The ISP&D was intended to function as an interim facility bridging the gap between the court-ordered ocean dumping ban and the yet-to-be-determined final residuals treatment facilities.After two years of successful operation, the Authority initiated a study to expand the capacity of the ISP&D, to improve the performance and longevity of the process equipment, and to determine additions and enhancements necessary to convert this interim facility into a long-term solution. Three construction contracts were awarded: the first, in 1994, to construct and evaluate full-scale prototype modifications, and two in 1996 and 1997 to expand and implement these process upgrades.This paper focuses on the major thermal-process optimization upgrades implemented by the MWRA at the expanded biosolids pelletizing facility. Topics discussed include exhaust-gas recirculation; more efficient air pollution controls; high-solids centrifuge dewatering; product cooling, inerting and coating; and increased processing capacity. Historical performance of the original system was evaluated and modifications or replacements made as appropriate. System throughput capacity was increased by an average of 50% per train and, with the addition of two new trains, overall capacity increased 100%. New air pollution controls, when combined with exhaust gas recirculation, reduced particulate emissions by over 80% and all other priority pollutants by over 50%.