COMBINED SEWER SEPARATION – NOT NECESSARILY THE ANSWER TO CSO CONTROL

The city of Nashua, New Hampshire currently has nine permitted combined sewer overflow (CSO) outfalls which discharge excess combined sewage to the Nashua and Merrimack Rivers during wet weather. On April 20, 1999, the United States Environmental Protection Agency Region 1 (EPA) issued an Administrative Order that required the City to separate its combined sewers by December 31, 2019, in order to mitigate its CSO problem and related water quality impacts. Nashua began implementation of its sewer separation program, but following completion of several early construction contracts, the city decided to reassess its CSO control approach. At a cost of 2.3 million per mile (based on recently completed sewer separation work), the total cost to separate the 110 miles of combined sewers in Nashua has approached 250 million in today's dollars. In addition to being costly, the early sewer separation construction contracts demonstrated that sewer separation was disruptive to Nashua's neighborhoods and businesses, and increased the volume of pollutant-laden urban stormwater runoff that is discharged to receiving waters. Realizing the significant cost, increased stormwater pollution, and disruption of the urban environment that would result from continuing with the sewer separation program, in the year 2000 the City undertook a reassessment of its CSO control approach. The objective of the reassessment was to define a water quality improvement program that would achieve maximum benefit at a reasonable cost in the shortest reasonable time frame. With a sophisticated hydraulic model and watershed-based planning approach, an alternative CSO control plan was developed that maximizes the use of existing infrastructure to retain combined sewage in the collection system for conveyance to the Nashua Wastewater Treatment Facility (NWTF) for treatment during storm events. The alternative CSO control plan will achieve greater environmental benefits in a shorter timeframe at significantly lower cost than the previously approved sewer separation program.