hundreds of years. The backbone of the present collection and transport system was a combined sewer system completed in 1904 which provided for the discharge of millions of gallons per day of untreated sewage into the harbor. Over the next four decades it became obvious that these untreated discharges presented public health risks for swimmers and shellfish consumers and were causing severe aesthetic problems. To correct these problems, two primary treatment plants that removed about half the total suspended solids (TSS) and about a quarter of the biochemical oxygen demand (BOD) were built—one on Nut Island built in 1952 and having an average daily flow of 110 million gallons per day (MGD), the other on Deer Island built in 1968 and having an average daily flow of 280 MGD. However, the digested sludge produced by these primary treatment plants, approximately 50 tons per day, was also discharged into the harbor until December of 1991. During wet weather, the wastewater system's hydraulic capacity was exceeded causing combined sewage and stormwater to overflow into the harbor through 88 overflow pipes. The discharge from combined sewer overflows (CSOs) would occur approximately 60 times a year, dumping billions of gallons per year of combined wastewater into the harbor and causing frequent closings of nearby shellfish beds and bathing beaches. Figure 1.5 provides a map of the Boston region and includes the location of the Massachusetts Water Resources Authority's wastewater and CSO discharge points.
The Metropolitan District Commission (MDC), the state agency responsible for managing water and wastewater treatment in the Boston metropolitan area until 1984, suffered from insufficient funding to maintain and upgrade the treatment plants. The result was equipment breakdowns, which in turn resulted in the further release of raw and partially treated sewage. The 1972 Clean Water Act mandated an upgrade to secondary treatment (85 percent removal of both TSS and BOD) for all wastewater discharges by 1977. In 1979, the MDC applied for a waiver from secondary treatment as provided by the 1977 Clean Water Act. The MDC proposed a seven-mile outfall to discharge primary effluent into Massachusetts Bay, cessation of sludge discharge into the harbor, and CSO abatement. The MDC studies on water quality determined that the impact of the discharge of primary effluent was acceptable and concluded that secondary treatment would not be cost effective. In June 1983, the EPA denied the waiver, primarily because of concerns about maintaining the dissolved oxygen standard in the bay and protecting the balanced indigenous population of marine life. The EPA also concluded that secondary treatment would result in fewer water-quality exceedences of priority pollutants, one-tenth the loadings of toxics to the sediments around the outfall, and a smaller area of sediment enrichment around the outfall. The MDC modified its waiver request by extending the outfall 9.2 miles into Massachusetts Bay to provide better dilution. The