DNR wastewater permit will cut storm-water flows to sewers.
More rooftops in the Milwaukee area will be enlisted to grow grasses, flowers and other plants now that the Milwaukee Metropolitan Sewerage District has been given the nation’s first wastewater discharge permit mandating “green infrastructure” to collect and absorb storm water.
The state Department of Natural Resources issued MMSD a new five-year pollution discharge permit this week that requires the district to establish 1 million gallons of so-called green storm-water storage capacity each year, said Ted Bosch, a DNR wastewater engineer in Milwaukee. Sewer pipes, bedrock tunnels and concrete reservoirs cannot be used to meet the mandate.
The district must use plants and soil, as well as some trendy rain barrels, to comply with the storage requirement. Apart from green roofs, the tools available include planting rain gardens at the ends of downspouts, installing porous pavement in parking lots that allows storm water to seep into the ground, creating landscaped swales on the sides of streets, and protecting wetlands and floodplains.
Use of green infrastructure will cut storm-water flows to sewers, reduce sewer overflows to waterways and lessen the risk of sewage backups into basements, DNR and district officials said. Buying an acre of open space in a wetland or in a river’s floodplain as part of the district’s Greenseams conservation program adds between 65,000 and 651,000 gallons of storm-water storage, depending on soil.
No more than 75% of the 1 million gallons each year can come from new purchases of wetlands or floodplains, under the permit. The district must work with municipalities and private property owners to achieve 250,000 gallons of other green storage each year. “That will be a stretch for us,” MMSD Executive Director Kevin Shafer said Tuesday. A green roof or rain garden can capture up to 3 gallons of rain per square foot. One rain barrel holds 55 gallons.
The new state permit does not ignore the district’s traditional facilities, such as the Jones Island and South Shore sewage treatment plants. Both plants will face “significantly tighter limits” on phosphorus discharges under the new permit, Shafer said. Phosphorus is a nutrient that spurs algae growth in Lake Michigan.
The South Shore plant in Oak Creek had been under a limit of 1 part per million of phosphorus in wastewater discharged to Lake Michigan. The limit drops this year to an average of 0.8 parts per million over six months and must be cut to no more than 0.6 ppm by November 2016. MMSD will be able to comply with this year’s tighter limits at South Shore with the use of additional chemicals to precipitate more phosphorus out of wastewater treated at the plant, Shafer said. To meet the 2016 limit, however, the district will evaluate adding treatment steps, he said.
Jones Island’s limit drops immediately from 1 part per million of phosphorus in wastewater to a monthly average of 0.66 ppm. Jones Island already achieves that limit regularly, so additional chemical use will ensure compliance with the new permit, Shafer said. Bosch warned the district that the Jones Island limit might be cut to just 0.22 parts per million in a future permit. But two ongoing research projects could result in even tighter limits on the district’s phosphorus discharges before the end of this five-year permit, according to Shafer. The U.S. Environmental Protection Agency is studying phosphorus concentrations throughout Lake Michigan and sources of the nutrient.
Locally, MMSD is working with municipalities in its service area, the DNR and engineering consultants to determine the total maximum daily load of phosphorus that can be added to waterways without reducing water quality. Once the load is determined, all sources of phosphorus in the watershed will be allocated a share of the total. Discharges from storm sewer or treatment plant discharges cannot exceed those shares.
For all the changes incorporated in the new wastewater permit, there is one item that remains the same: The state permit continues to allow up to six overflows of combined sanitary and storm sewers in central Milwaukee and eastern Shorewood to rivers and the lake. The limit was unchanged even though the district has reported an average of 2.4 combined sewer overflows a year since the deep tunnels’ first full year of operation in 1994. Since 1994, there have been six combined sewer overflows in just one year: 1999. There was one in 2011 and none in 2012.
Shafer defended the status quo as being consistent with EPA’s national policy on combined sewer overflows. Find this article here.