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Construction and Start-Up of A High-Rate Wet Weather Treatment Facility in Nashua, New Hampshire

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This paper describes the construction and start-up of a wet weather treatment facility in Nashua, New Hampshire. This facility, which features ballasted flocculation technology, is the centerpiece of Nashua's innovative, cost-saving, and environmentally responsible wet weather pollution control program. Start-up of the wet weather treatment facility occurred in January 2009, with final construction completion expected in the spring of 2009.

Incorporated in 1853, the city of Nashua is located along the western bank of the Merrimack River, where it meets the Nashua River, and is the second largest city in the State of New Hampshire. The existing wastewater collection system serves approximately 96 percent of the population, and approximately 78 percent of the total land area. Approximately 25 percent of the collection system service area is served by combined sewers, and most of the combined sewer service area is in the older, densely developed center of the city.

In December 2005, the United States Environmental Protection Agency Region 1 (EPA) issued the City of Nashua a Consent Decree that required the City to implement an innovative program to achieve a high level of CSO control for the nine permitted CSOs which discharge to the Nashua and Merrimack Rivers during wet weather. At the heart of the program required under the Consent Decree is the wet weather treatment facility, which is currently being constructed at the Nashua Wastewater Treatment Facility (NWTF) site. The current Consent Decree superseded a previous Administrative Order which required separation of the city's combined sewer system by December 31, 2019. Nashua's current CSO control plan represents animprovement over the previous sewer separation plan in terms of greater water quality benefits, cost savings in excess of $100 million, and a shorter implementation schedule.

With a sophisticated hydraulic model of the collection system, it was possible to demonstrate that Nashua's existing, large-diameter interceptors were capable of delivering substantially more wet weather flow than the current 50 mgd NWTF capacity. By maximizing wet weather flow to the NWTF, discharge at several upstream CSOs was predicted to be eliminated in storms with recurrence intervals of between 1 and 2 years. Upstream CSOs that will continue to discharge in an average rainfall year following maximization of flow to the NWTF will be contro lled using arange of in-system and/or end-of pipe CSO control technologies.

In dry weather, the NWTF provides full secondary treatment for approximately 13 mgd, and can handle up to 38 mgd through the secondary process during wet weather events. An additional 12 mgd currently receives primary treatment during wet weather, with primary effluent blended with secondary effluent prior to disinfection. Nashua's current NPDES permit allows for this wet weather discharge of up to 50 mgd of blended primary and secondary effluent with typical 30 mg/l monthly average, 45 mg/l weekly average, and 50 mg/l maximum day BOD5 and TSS limits.

The collection system model showed that up to 110 mgd of wet weather flow can be delivered to the NWTF site. The capacity of the wet weather treatment facility was set at 60 mgd to enable up to 110 mgd to be treated at the existing NWTF site. The first 50 mgd will continue to be treated by existing primary and secondary treatment processes. Once the capacity of the existing processes is exceeded, the excess flow will be treated by the wet weather treatment facility. Effluent from the wet weather treatment facility will be blended with existing primary and secondary treated flows, creating a “3-part” effluent blend that will be required to comply with existing NPDES Permit limits.

The major facilities constructed as part of the wet weather treatment facility include:

Diversion structure


Wet weather flow pump station


Wet weather treatment facility


Disinfection system improvements


This paper presents introductory and background information on Nashua's collection system and CSO control plan, followed by a description of the wet weather treatment facility, including equipment, process selection, and operation and control strategies. Experience gained during facility start-up, including initial performance data, is also presented.
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Keywords: Actiflo®; CSO; ballasted flocculation; implementing long-term control plans

Document Type: Research Article

Publication date: 2009-01-01

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