The sewer district joins more than 40 American municipalities renovating their sewer systems to comply with the CWA.

The Metropolitan St. Louis Sewer District, the nation’s fourth-largest system, will overhaul its sewer infrastructure at an estimated cost of $US 4.7 billion over 23 years, according to a settlement announced on Thursday by the U.S. Justice Department and the U.S. Environmental Protection Agency.

It is the largest municipal Clean Water Act settlement in U.S. history, according to the EPA.

The sewer district, which serves the city and parts of two counties, will build three storage tunnels and increase capacity at two of its seven treatment plants. These improvements are expected to eliminate 49 million cubic meters (13 billion gallons) of untreated sewage, otherwise discharged illegally each year from the district’s 15,500-kilometer (9,600-mile) network of pipes.

“The people of St. Louis, including those who live in minority and low-income communities, will receive tangible, lasting benefits from this significant settlement,” said Ignacia Moreno, Assistant Attorney General for the Justice Department’s Environment and Natural Resources Division, in a statement.

The agreement also includes a provision that the sewer district spend at least US$ 100 million on green infrastructure in low-income or minority areas that have traditionally borne the brunt of polluted water and air.

Reducing the amount of raw sewage and contaminated storm water dumped into the nation’s rivers and streams is a priority area for the EPA. Since 1998, more than 40 municipalities — including Atlanta, Baltimore, Indianapolis, Los Angeles, Louisville, and San Diego — have reached similar settlements with the federal government. These settlements mandated, among other things, sewer improvements estimated to cost a large city anywhere from hundreds of millions to billions of dollars.

And some of the largest settlements have come in just the last few years. In December, for example, the Northeast Ohio Regional Sewer District, which serves the Cleveland area, agreed to a plan for US$ 3 billion in system improvements to keep untreated sewage out of Lake Erie. And 15 months ago, Kansas City, Missouri, agreed to pay US$ 2.5 billion in renovations to its sewer system.

In many cities, all wastewater — street runoff, toilet flushes, and commercial operations — flows into the same set of pipes. These ‘combined sewers’, as they are called, often exceed the treatment capacity at wastewater plants during storms or other high-flow periods, resulting in the surplus of sewage being spilled into rivers, still untreated. Separating these systems and increasing treatment capacity has been quite expensive.

The Metropolitan St. Louis Sewer District has invested US$ 2.3 billion over the last 20 years to reduce both combined and sanitary sewer overflows, but the city does not dispute that it needs to invest even more. And the burden of payment will likely fall on its citizens in rate increases.

The agreement is fair, the sewer district said in a statement, but “the fact remains that this is billions of dollars that will come from the pocketbook of St. Louis ratepayers — with little-to-no state or federal assistance — and will be unavailable for other critical needs in our community.”

Because the sewer district is under a federal gag order, the statement is the only comment it can make at this time.

Note: This article has been corrected to clarify that the Metropolitan St. Louis Sewer District serves not just the city of St. Louis, but parts of the surrounding counties.