Norwich — Norwich Public Utilities is seeking help from the public to solve a strange criminal mystery.

During the past two weekends, someone apparently dumped “a very large amount of raw sewage” illegally into a street storm drain or manhole, likely in the Laurel Hill section of Norwich, said Larry Sullivan, NPU wastewater integrity manager.

The illegally dumped sewage is highly concentrated and could pose a serious public health risk by clogging sewage system equipment and causing raw sewage to back up into the streets, NPU officials said. The problem also has taken up staff time to unclog and repair damaged machinery and dilute the material so it can pass through the system.

Normal wastewater passing through the system has been diluted by discharges from dishwashers, washing machines and stormwater, Sullivan said.

NPU has detected the highly concentrated dumped sewage on the past two Mondays, indicating that someone might be dumping the sewage into the system over the weekend. Based on the position of the sewage, it likely came from the Laurel Hill area, Sullivan said. After it was detected the first time on Aug. 24, he said crews thought it could be “an anomaly.” But not after it was detected again this week.

NPU has a station at the sewage treatment plant on Falls Avenue for private septic pumping companies to deposit sewage. NPU charges $85 per 1,000 gallons, and a typical full truck holds about 3,000 gallons. The person could be charging customers for the pump-out, including the NPU fee, and then not paying to discharge the material legally, Sullivan said.

On weekends, the station is open only from 7 to 11 a.m. Saturdays, he said. He speculated the dumper might be looking to empty the truck immediately, rather than wait for the plant to reopen Monday.

NPU spokesman Chris Riley said the practice is not only illegal and poses a public health threat but could be costly to NPU sewer ratepayers, who would foot the bill for repairs to damaged equipment and staff time handling the problem.

NPU posted information on its Facebook page, facebook.com/NPUtilities, seeking the public’s help to stop the illegal dumping. With a graphic photo of raw sewage flowing from a pipe, the posting stated: “Do you want this dumped on your street?”

Anyone who sees a private wastewater tanker truck — NPU trucks are bright yellow or white with the utility’s logo on the doors — in the street or on the side of a road discharging into a catch basin or manhole, or who sees someone opening a manhole cover, is asked to report the incident to NPU at (860) 887-2555.

NPU is offering an unspecified reward, possibly including a bill credit, to anyone providing information that leads to the identification or arrest of the responsible parties, Riley said.

c.bessette@theday.com