Dover residents could see their sewer rates increase by more than 50 percent if the Environmental Protection Agency imposes stiffer nitrogen limits to benefit Great Bay.

The discharge permit for Dover’s wastewater treatment plant has expired, and the city now must negotiate a new permit with the EPA. If the new permit includes the EPA’s proposed nitrogen discharge limit of three milligrams per liter, the plant will require about $25 million worth of upgrades, according to a notice posted on the city’s website. That would result in a rate increase of more than 50 percent.

In response, Dover has joined Portsmouth, Exeter, Rochester and Newmarket to form the Great Bay Municipal Coalition, which is contesting the science behind the EPA’s proposed limits. The coalition has proposed an alternative approach to the permit, which would include a nitrogen limit of eight milligrams per liter and would also address other nitrogen sources. The plan would require $10 million worth of upgrades to Dover’s wastewater treatment plant, resulting in a 19-percent rate increase.

The EPA’s proposed stricter limits come in response to the declining health of the Great Bay Estuary, where increased nitrogen loads have led to reductions in water quality, eelgrass habitat and oyster populations. Researchers estimate 25 percent of nitrogen in the estuary comes from wastewater treatment plants. The remaining 75 percent comes from “non-point sources” like septic systems, fertilizers and stormwater runoff.

For more information, Dover residents are encouraged to call the city’s Community Services Division at 603-516-6450.