Contact: Cathy Rofshus, 507-206-2608
St. Paul, Minn. — Reducing phosphorus that causes algae in lakes and rivers is the goal of innovative approaches proposed in wastewater permits for the city of Princeton and Metropolitan Council Environmental Services. Phosphorus “trading” provisions in the Princeton permit and a watershed approach to Met Council permits will allow them to achieve water quality goals more efficiently and economically.
“Our ultimate goal is always clean water. This is just an easier, more cost-effective route for permitted operations as small as the city of Princeton, or as large at the Metropolitan Council,” said Katrina Kessler, water assessment section manager for the Minnesota Pollution Control Agency (MPCA), the agency responsible for issuing the permits.
North of Minneapolis, the city of Princeton, population 4,700, discharges its treated wastewater to the Rum River, a tributary to the Mississippi River upstream of Lake Pepin. To offset the phosphorus in the discharge to the rivers and lake, the city is restoring and maintaining streambanks on the Rum River in five areas. These efforts have reduced the amount of soil — and phosphorus attached to it — released into the river. The city’s draft permit gives them credit for the reduction that serves as an offset for the phosphorous load discharged from the wastewater facility.
This is the first trade of this kind involving a municipal wastewater facility in Minnesota, using unregulated pollution (nonpoint source) to make up for regulated pollution (point source). Nonpoint source refers to indirect sources of pollution such as stormwater runoff, whereas point source refers to a specific origin such as discharge from an industrial plant. MPCA officials are hopeful that this will serve as a successful example for other communities around the state looking to meet low phosphorus discharge limits without installing additional treatments at wastewater facilities.
The Princeton permit includes a trade ratio that requires the city to remove 2.6 times the amount of phosphorus through the restoration projects than the discharge amount permitted from its facility. The five streambank restoration projects are preventing sediment, containing about 10,700 pounds of phosphorus per year, from entering the Rum River. This innovation reflects statutory changes made by the 2014 Legislature to support pollutant offsets between permitted and non-permitted sources.
“The amount of phosphorus added to a watershed from a wastewater treatment plant can border on insignificant when compared to the amounts contributed from nonpoint sources. Therefore, if a city is going to spend hundreds of thousands of dollars for phosphorus reduction, it only makes sense to use those funds to address phosphorus from non-point sources,” said Mark Karnowski, Princeton’s city administrator. “The city of Princeton truly appreciates the MPCA’s willingness to try a new approach to improve the quality of Minnesota watersheds.”
Water quality goals for Lake Pepin — and the Mississippi River — require significant reductions of phosphorus from many upstream wastewater facilities. In addition to the trading provisions for Princeton, a pending permit establishes one phosphorous limit for five Met Council facilities, allowing the council to manage its phosphorous load among the five facilities. This flexible permitting approach is designed to achieve the overall water quality goals for the Mississippi and Lake Pepin, while allowing the Met Council to optimize its investments and accommodate future needs in a growing metropolitan area.
To understand the concept, think of the limit in the permit as a pie that represents the total amount of phosphorus that the five facilities can discharge to the Mississippi. This permit allows the Met Council to decide which facilities get the bigger pieces — more of the phosphorus pie — and which get smaller pieces — less of the phosphorus pie. The end result — the total phosphorus level discharged from the five plants combined — is the same but Met Council has the ability to decide how they will manage their phosphorous treatment to achieve the required reductions.
“The Metropolitan Council appreciates the work MPCA has put into developing and implementing this umbrella approach to phosphorus management,” said Leisa Thompson, general manager of Metropolitan Council Environmental Services. “This approach will maximize our flexibility to meet water quality standards, and in the process will save money that we can use to further improve wastewater treatment, and thus water quality, in the region.”
For more information about wastewater treatment, read the MPCA's On Point e-newsletter.