System overflow the problem

According to the federal Environmental Protection Agency, combined sewer systems are sewers designed to collect a mix of rainwater, household sewage and industrial wastewater in the same pipe. Most of the time, the systems transport all of the wastewater to a sewage treatment plant, where it is treated and then discharged into a body of water – in Nashua’s case, the Merrimack River.

During heavy rain or periods of snowmelt, however, the volume in a combined sewer system can exceed the capacity of the treatment plant.

For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly into water bodies, according to the EPA.

However, the overflow in Nashua has exceeded the amount allowed by the EPA, causing the agency to require the city to lessen the amount of pollution entering the Merrimack and Nashua rivers. The EPA has been working with the city on a timeline for the improvements.

So far the city has spent $15,000 on a study by a Quebec company called BPR to help determine the most cost-effective path. Two representatives from the company attended the Board of Public Works meeting to discuss the work.

The current plan includes the construction of a $25 million screening and disinfection facility to treat wastewater upstream from the city’s wastewater treatment and wet weather facilities. The plan also includes the construction of two retention tanks.

BPR is exploring another option – to construct a second sewer line and to implement a series of gated controls to help the sewer system better handle the wastewater volumes during times of heavy flow.

The city’s wastewater treatment plant and wet weather facility have enough capacity to handle the sewer flow during periods of heavy rain, BPR engineers said. However, sewer lines designed a century ago can’t handle times of heavy flow, the engineers said.