DESIGN OF A HIGH-RATE WET WEATHER TREATMENT FACILITY IN NASHUA, NEW HAMPSHIRE

This paper presents the design of a wet weather flow treatment facility (WWFTF) in Nashua, New Hampshire. This facility, which will employ a ballasted flocculation technology, is the centerpiece of Nashua's innovative, cost-saving, and environmentally responsible wet weather pollution control program. Nashua's CSO control plan, currently being implemented under an Administrative Order issued in 2003, represents an improvement over the previous sewer separation plan because the plan will result in greater water quality benefits by reducing annual bacterial loading to the rivers from CSOs without increasing stormwater discharges. Furthermore, the current plan will be implemented in less than half the time it would take to separate all of the sewers in Nashua and as a result will achieve environmental benefits much more quickly. The current plan will maximize the use of existing infrastructure by maximizing system storage through the implementation of system optimization measures, an approach which is consistent with State and National CSO control policies. The estimated cost of the current CSO control plan is 50 million compared to nearly 250 million for complete sewer separation.At the heart of the current plan is a high-rate WWFTF. With a sophisticated hydraulic model of the collection system, it was possible to demonstrate that Nashua's existing, large-diameter interceptors were capable of conveying wet weather flow in excess of the current Nashua Wastewater Treatment Facility (NWTF) capacity to the treatment plant site. By maximizing wet weather flow to the NWTF, discharge at several upstream CSOs will 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 controlled using site-specific off-line storage or flow-through treatment technologies.The high-rate WWFTF will be located at the existing NWTF site. 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. Flow in excess of secondary treatment capacity currently receives primary treatment during wet weather, with primary effluent blended with secondary effluent prior to disinfection.With a 60 mgd capacity, the high-rate WWFTF will enable up to 110 mgd of wet weather flow to be conveyed to the existing NWTF site. The first 50 mgd will continue to be treated by the primary and secondary treatment processes as described above. Once the capacity of the existing NWTF is exceeded, new pumps, screens, ballasted flocculation treatment units, and disinfection equipment will treat the excess wet weather flow. Following treatment, effluent from the high-rate WWFTF will be blended with existing primary and secondary treated flows, creating a “3- part” effluent blend that will be required to comply with the current 30 mg/l monthly average, 45 mg/l weekly average, and 50 mgyl maximum day BOD5 and TSS limits.