The Impacts of Wastewater Treatment Plant

The City of Boston and Boston Harbor are located in the northwest portion of Massachusetts Bay. Serving a population of more than two million people in the greater Boston metropolitan area, the waters of Boston Harbor and near shore Massachusetts Bay provide an important recreational, as well as commercial, resource. The waters of Boston Harbor have also served as a recipient of the region’s sewage. Historically, the input of this sewage had an adverse impact on the water quality of Boston Harbor. As a result of a court-ordered mandate, the Massachusetts Water Resources Authority (MWRA) has ceased the discharge of digested sludge to Boston Harbor and is presently constructing a 1,080 MGD secondary (1,200 MGD primary) treatment facility on Deer Island, with a 15.3 km outfall to Massachusetts Bay. While the cessation of sludge discharge has already improved the aesthetics and the "swimmability" of the waters around the outer harbor islands, it is expected that the planned treatment facility upgrades and outfall relocation will result in significant improvements in Boston Harbor's water quality.

However, there has been some controversy concerning the impacts that the proposed outfall relocation would have in the Massachusetts and Cape Cod Bays ecosystem. In particular, concerns have been voiced that the discharge of effluent into Massachusetts Bay would cause nutrient enrichment in Cape Cod Bay and might result in Bayside eutrophication. In order to address these and other environmental issues concerning this facility construction and outfall relocation project the MWRA has provided funding for a number of monitoring and research projects. One of these projects has resulted in the development of a coupled time-variable three-dimensional hydrodynamic/water quality model of the Massachusetts Bays system. The coupled model uses a novel algal growth model (Laws and Chalup, 1990) and a novel approach in aggregating hydrodynamic model computations for use in the water quality model.

The purpose of the coupled model is to provide a greater understanding of the relationships between circulation, stratification, nutrients, light, primary productivity and dissolved oxygen within the Massachusetts Bays system and to provide water quality managers with a tool for assessing the impacts of various pollution control/relocation strategies on water quality in Boston Harbor, Massachusetts and Cape Cod Bays.Other than exchange with the Gulf of Maine, the principal inputs of oxygen demanding material and nutrients entering Boston Harbor and the Massachusetts Bays system are:

  • source discharges from municipal wastewater treatment plants,inputs from combined sewer overflows (CSOs),
  • loadings associated with various tributaries draining to Boston Harbor,
  • nonpoint source inputs from study area rainfall runoff,
  • groundwater loadings, and
  • atmospheric inputs impinging directly on the water surface.

Data necessary to make estimates of the various pollutant loadings to the system were obtained from reports prepared by MWRA (Alber and Chan, 1994) and Menzie-Curia & Associates (1991). Data used for a model calibration were drawn from historical sources as well as from an ongoing outfall monitoring program funded by MWRA. A three-year (1992-1994) data record, available from the MWRA-sponsored outfall monitoring program, provided a good test of model performance, in that significant differences in water quality were observed between the years in Massachusetts and Cape Cod Bays. In particular, it was observed that bottom water dissolved oxygen was significantly lower in the fall of 1994, as compared to the fall of 1992 and 1993. Occasional values of less than 6.0 mg L-1(State of Massachusetts standard = 6.0 mg L-1) were observed in 1994. Model versus data comparisons for dissolved oxygen, as well as other key water quality variables, indicate that the water quality model reproduces the primary interactions between nutrients, primary productivity and dissolved oxygen in Massachusetts and Cape Cod Bays.

Upon completion of the model calibration, a projection analysis was conducted. In this analysis, the hydrodynamic and water quality models were rerun assuming that the MWRA outfall would be relocated from Boston Harbor into Massachusetts Bay at a distance approximately 15 km east of the current Dear Island outfall and at a water depth of 32 meters. Projection analysis indicates that with the upgrade to secondary treatment and outfall relocation, water quality in Boston Harbor should improve significantly with little or no adverse impacts on water quality in Massachusetts or Cape Cod Bays. The reason that the outfall will have little impact of Massachusetts Bay and Cape Cod water quality is due, in part, to the fact that the outfall plume will be trapped below the pycnocline of the water column during the summer months. Therefore, nutrient inputs to the surface waters of northern Massachusetts Bay will not be increased as a consequence of outfall relocation. This is an important finding, in that it suggests that nutrient reduction or removal at the MWRA wastewater treatment facility is not required in order to maintain present water quality in Massachusetts and Cape Cod Bays. Additional evaluation of model computations indicate that nutrient inputs from MWRA are on the order of 5-15% of the total nutrient loading to the Massachusetts Bays system; the major source of nutrients to the Massachusetts Bays system is via import from the Gulf of Maine.