Sewer work continues with manhole project

C.E. Goodall

Now that the Visu-Sewer company has finished lining 22,000 linear feet of sewer pipe running beneath the streets of Maryville, it’s nearly time for phase two of this year’s effort to significantly reduce “infiltration and inflow” — the amount of storm water leaking into sewer mains and swelling the volume of effluent ultimately piped to the city’s wastewater treatment facility.

The second half of the 2016 “I&I” reduction effort involves lining 139 manholes throughout the city, many of them porous brick structures installed decades ago.

When work begins next month, Visu-Sewer crews will return to Maryville from the company’s headquarters in Pewaukee, Wisconsin, and start coating the inside of the manholes with a spray-on material that hardens much like cement.

For the majority of folks who don’t think much about sewers, a manhole is simply a vertical structure buried in the ground with a round metal cover on the surface and a sewer main running through the inside. The main is open on top, allowing access so that city workers can clean out backed-up lines and perform other maintenance tasks.

Unlike the brick installations of yesteryear, modern manholes are made of tightly sealed sections of pre-cast concrete.

Public Works Director C.E. Goodall said Wednesday that the Maryville sanitary sewer system includes 1,072 manholes spaced between 400 and 600 feet apart.

The upcoming lining project is designed to prevent storm water from leaking into manholes and ultimately flowing into sewer mains. Goodall said the city was able to identify the manholes most in need of work either through either smoke testing or direct observation by city crews after it rains.

While the city’s manhole network won’t be completely watertight after Visu-Sewer completes its work in an estimated two or two and a half months, Goodall said the majority of major trouble spots should be taken care of.

Until fairly recently, I&I problems were only a moderate priority for the city because Maryville had a lagoon-based sewage treatment system. Essentially, effluent was pumped into a series of lagoons where human waste settled to the bottom as sludge. The relatively clean water on top of the sludge was then allowed to flow into the One Hundred and Two River.

The system wasn’t especially efficient by modern treatment standards, but it was cheap, since a lagoon doesn’t use any electricity.

By way of contrast, Maryville’s new $13.7 million “mechanical” treatment plant, completed last year under a state Department of Natural Resources mandate, requires a lot of power to operate, meaning that every gallon of effluent that can be kept out of the system saves the taxpayers money.

So municipal officials decided a couple of years ago that treating storm water after it works its way into sewer lines through leaky pipes and manholes during heavy rains is a luxury the city can no longer afford.

But getting years of cumulative I&I problems under control isn’t exactly cheap either.

Over the past two years, Maryville has spent tens of thousands of dollars for smoke testing in order to identify scores of locations where the system needed repair. Even more money was set aside to fix the leaks.

This summer’s manhole project carries a price tag of nearly $161,000. Adding in the pipe-lining work completed this spring raises the total I&I cost for 2016 to nearly three quarters of a million dollars out of a combined $2.5 million wastewater budget comprising maintenance, construction, and treatment.

For a city the size of Maryville, that represents a major investment, but Goodall said the effort seems to be paying off.

While this week’s rainfall caused excessive flow into the crucial southwest lift station across Route V from Maryville Middle School, Goodall said the backup appeared smaller than usual, and that other problem areas were experiencing lower volumes as well.

“From what we’ve seen initially it looks like it’s helped,” he said.