Known as Battery E, the four-year project will be built on the southwest side of the site. It includes an aeration tank and final settling tanks, plus additional structures, equipment and process improvements. O’Brien, one of the MWRD’s seven water reclamation plants, serves Evanston and neighbouring municipalities and releases its treated effluent into the North Shore Channel.

The main aim is to reduce the amount of phosphorus in the treated water that the plant releases. The nutrient reaches wastewater from fertilisers and urban runoff, among other sources, and drinking water utilities even add it to protect pipes against corrosion. In excess, phosphorus can trigger algae growth and deplete the oxygen in the water. It is considered a major driver of the dead zone that forms in the Gulf of Mexico, one of the largest in the world and roughly the size of New Jersey. Nutrients from across the vast Mississippi River basin feed into it, including those carried by Chicago area waters, which travel through the North Branch of the Chicago River and the Des Plaines River before joining the Mississippi on their way to the Gulf.

The project is one of the largest single investments made by the district since its creation 137 years ago. MWRD President Kari K. Steele explained that, beyond enabling compliance with new phosphorus discharge limits that come into force by 2030, the additional battery will allow the facility to be modernised and to keep serving the region for the next hundred years.

Contributing to a basin-wide goal

To shrink the hypoxic zone, the U.S. Environmental Protection Agency (EPA) and a task force are pursuing a 45% cut in the nitrogen and phosphorus reaching the Mississippi River basin by 2035. The target is not a federal law in itself, but the EPA and state agencies can enforce it using tools provided by the Clean Water Act. According to MWRD estimates, the nutrient removal technologies being deployed at its three largest facilities, including O’Brien, will deliver 20% of Illinois’s reduction goal.

Wastewater plays a major role. Citing several studies, the Sierra Club Illinois Chapter reports that sewage effluent is responsible for an estimated 48% of the phosphorus and 16% of the nitrogen in the state’s rivers. Runoff from soils treated with fertilisers and manure accounts for an equal share of the phosphorus.