Keene to use non-toxic smoke to search for leaks in sewer system

June 26--At some point next month, smoke will be flowing through Keene's sewer system.

But don't be alarmed, city public works department officials say. It's just a test to find any potential leaks in the pipes. The test will focus on the east side of the city.

A while back, the city did the first phase of an inflow and infiltration evaluation of Keene's wastewater collection system, according to City Engineer Donald R. Lussier. The work was prompted by the city experiencing unexpected spikes in sewer flows associated with large rain events, which can be problematic for the wastewater treatment plant, he said.

"What that tells us is stormwater is getting into the sewer system somehow," he said. "The question is from where?"

Keene has essentially two wastewater systems, and they're designed to be separate. The sewer system is connected to homes, businesses, schools and other buildings, and sends sewage from those structures to the wastewater treatment plant. The stormwater system, most easily identified by drains along the sides of roads, collects rainwater and other precipitation as runoff. That system empties into the Ashuelot River.

The city's sewer system is designed to handle only sewer waste, and not stormwater, Lussier said. This can result in the city spending money to treat water that is already clean, he said.

In addition, in extreme circumstances, if there's too much sewer water running through the wastewater treatment plant, it could overwhelm the plant, he said. As a result, wastewater would be flowing through the facility so fast that it wouldn't get treated, he said.

The first phase of the project narrowed the problem to the east side of the city, Lussier said. The second and final phase will take place in July, and try to pinpoint the exact locations where water is getting in.

The project had been scheduled for this month, but was postponed because of an unexpected increase in rainfall, he said.

To identify where the stormwater is getting into the sewer system, a consultant hired by the city will use a machine to blow a non-toxic white smoke through the pipes, Lussier said. If the pipes don't have leaks, the smoke will stay underground until it finds a way out through manhole covers and vent pipes on building roofs, he said.

If pipes are broken, the smoke will start coming up through the side of roadways near those breaks, according to Lussier.

There's a possibility smoke could get into buildings through dry traps in basement floor drains or seldom-used plumbing fixtures, such as mop- or laundry sinks, Lussier said.

To prevent this, people should pour water down those drains before the testing, he said. The water will prevent smoke from getting in, and stop any sewer gas that may have been coming in because the drain and traps were dry, he said.

In extreme cases, sewer gas can be toxic, he said.

However, smoke could still enter a building that has defective plumbing, according to Lussier.

In a news release, city officials are asking residents to ventilate any smoke that gets into their houses, and report its presence to the crew doing the test.

"If possible, they will assist you in locating the source of the smoke entering the building," the release says.

The testing is expected to cost $45,000, and will be done by Flow Assessment Services LLC of Goffstown, Lussier said.

Once the city receives the results of the testing, it will use them to guide sewer maintenance work over the next several years, he said.

Meghan Foley can be reached at 352-1234, extension 1436, or mfoley@keenesentinel.com. Follow her on Twitter @MFoleyKS.

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