Repairing sewer plant’s ‘Achilles' heel’

Pipe under Rte 95 is deteriorating

John Howell
Posted 8/6/15

It would seem to be simple enough: if a pipe fails, dig it up and replace it.

But now imagine the single pipe that carries all of the wastewater flowing to the sewer treatment plant. That can’t …

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Repairing sewer plant’s ‘Achilles' heel’

Pipe under Rte 95 is deteriorating

Posted

It would seem to be simple enough: if a pipe fails, dig it up and replace it.

But now imagine the single pipe that carries all of the wastewater flowing to the sewer treatment plant. That can’t be shut off like a tap without backing up toilets across the city.

The Warwick Sewer Authority is faced with that dilemma, only it’s more complicated.

That one pipe delivering an average of 4.7 million gallons of wastewater a day to the plant from 21,700 customers is buried under Route 95. It has been there ever since the road was constructed, and corrosion is causing it to break down. In the words of authority executive director Janine Burke, “It’s the Achilles' heel.”

In most situations the authority can bypass a problem to maintain service until the pipe is repaired.

“We can’t put a bypass across Route 95 unless we want to piss off everyone,” says Matthew Solitro, collection systems manager.

Plan B is to put a pipe inside a pipe, and that’s what’s going to happen over the next several months.

Last week, the authority awarded R. Zappo of Stoughton, Massachusetts a $753,902 contract to run a more than one-inch thick fiberglass composite pipe 650 feet from the east side of Route 95 in the vicinity of Viesmann Manufacturing to the headwork at the treatment plant. Zappo has worked on the Warwick system before, said Burke, and was the low bidder for the project.

Solitro explained that pits would be dug at either end of the pipe. The top half of the pipe will then be cut off, allowing the new pipe to be fitted within the old pipe in sections. Sections of the pipe are grooved so that they fit within each other and will be rammed in place.

Solitro was not sure at which end the company would start but thought Zappo would probably work “upstream” to eliminate the possibility of the pipes “floating.”

This “slip lining method” would be done with “wet” pipes, meaning the flow of wastewater won’t be interrupted. A form of grout will be pressured between the old and the new pipes to form a solid connection between the two.

Burke observed it takes “special people” to work under these conditions because “it [the wastewater] is pretty nasty by the time the flow gets here.”

Even at 4.7 million gallons a day, neither the existing pipe nor its replacement are near capacity. The system was designed to accommodate more than 7 million gallons daily.

Last year, in an effort to assess conditions and make preemptive repairs, a video camera traveled the length of the pipe. The video revealed that hydrogen sulfide gases had corroded the interior of the cement pipe. Often when that happens, Burke said, the pipe goes out of round and takes on an oblong shape. Fortunately, the pipe hasn’t deteriorated to that point, as it would have made the “slip-lining” method of repair difficult.

One would imagine reducing the diameter of the pipe from 48 to 42 inches would also reduce capacity, thereby restricting future growth of the sewer system.

That’s not the case, says Solitro.

As the new pipe has a smooth lining compared to the rough lining of the cement pipe, the capacity of the line will be increased by 5 percent, according to engineering studies. Solitro said the “friction coefficient” of the replacement pipe would actually improve flow.

Surprisingly, plant capacity may not be an issue going forward even with expansion of the system.

Burke pointed out that while the number of customers are increasing as property owners tie into the system, the volume of wastewater treated continues to decline. Nine years ago the average daily flow was 5.3 million gallons with 16,000 customers. Today that has dropped to 4.7 million gallons with an additional 5,700 customers.

Burke attributes the decline in volume to a number of factors but largely to water conservation and efforts to track down and repair system leaks. A major source of infiltration was the Cedar Swamp pumping station, where as much as 800,000 gallons of ground water and water from Buckeye Brook was being introduced into the system.

Lining the pipe should be completed in three months and, while costly, will not impact user fees.

Burke said funding would come from the industrial pre-treatment fund that currently stands at about $1 million. Money flows into the fund from fees assessed commercial users who stray from meeting requirements. She said use of the funds for pipe repairs was deemed legitimate.

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RISchadenfreude

Since no one apparently thought of it at the time the highway was built, you would think they'd install a parallel (bypass) line now in case that section ever needed to be shut down. A horizontal drill rig would bore a hole from one side of Route 95 to the other, the new pipe would be pulled through, and valves installed on both ends of the new line that tie in to the existing line. The new composite pipe would be one piece (with an excellent friction coefficient) and not in sections that need to be "rammed" together.

Friday, August 7, 2015