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CSO Master Plan Update

The City of South Portland has provided wastewater collection services to its residents and businesses for over 80 years. Like many New England communities, sanitary wastewater and stormwater were originally conveyed within the same collection system and discharged untreated into the nearest water body.

In the 1970s, a central wastewater treatment facility and multiple pump stations were constructed. Three dozen combined sewer overflow (CSO) discharge locations were left in place to allow system relief in the event flows exceeded system capacity. In the 1980s, portions of the collection system were separated into sanitary sewer and stormwater systems, and the number of CSOs was reduced to 15.

In the 1990s, Maine DEP approved a CSO facilities plan outlining a control strategy to eliminate or reduce CSO volumes and discharge frequencies. In the following 13 years, upgrades to the treatment plant and pump stations, and continued sewer separation projects resulted in a further reduction of CSOs to six. However, the remaining six are among the most challenging to abate or eliminate.

Recently Wright-Pierce updated the facilities plan, which included the following tasks:

  • Evaluated progress made from 1994 to present
  • Completed several long-term flow monitoring and short-term "instantaneous" flow monitoring tasks
  • Developed a SWMM hydraulic model of the collection system
  • Developed a comprehensive plan to minimize or eliminate the remaining six CSOs
  • Recommended $12 to $14 million in improvements over a 10 year period with the goal of eliminating CSO discharges for the 2-year storm at five of the six CSOs
  • Recommended improvements including additional separation projects, two pump station upgrades, and an increase in capacity within sections of the City's major interceptor

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Project Highlights


As part of their Maine Pollution Discharge Elimination System (MEPDES) permit, the City was required to update their 1994 CSO facilities plan.


The CSO facilities plan update required gathering and evaluating data from 1994 to 2008 to develop control strategies for the next five years that were both technically feasible as well as economically viable.


Wright-Pierce performed long and short-term flow monitoring within the City's collection system to gather data used to develop and calibrate a SWMM model of the collection system.

The model was then used to evaluate how the system responds to various storm events and how various upgrade alternatives would impact the remaining CSOs.