MISSISSIPPI, USA (AP) — Greenville, among cities up and down the meandering Mississippi River, is working to keep it that way.

Greenville’s Wastewater Treatment Plant comprises a sprawling network of pipes, above and below ground. There is also a series of tanks that filter, separate and chemically treat sewage before discharging the largely purified water into the river.

“We’re fortunate to have the largest discharge stream in the country, and that’s the Mississippi River,” said Greenville Public Works Director Brad Jones, who oversees the treatment plant.

The plant treats, on average, up to 10 million gallons of wastewater a day, said manager Brenda Gales.

The process begins with a technique known as extraction.

Assistant facility operator, James Parks, said four intake pipes bring sewage from toilets, showers or sinks, to the plant from all across the city.

The wastewater is then funnelled into a raw-sewage distribution box, where it is separated into one of four large pods.

“The first step in cleaning the water is the bar screen,” he said.

The bar screen is a large, claw-like piece of equipment. It reaches into the water and filters out larger debris from the sitting water, which then makes its way to aeration tanks, where it is forced through a pocket of compressed air.

Aeration brings water and air into close contact to remove dissolved gases, including carbon dioxide, and to oxidise dissolved metals, such as iron, and remove volatile organic chemicals.

The microbial world, not unlike the human world, has its good guys and its bad guys. The treatment plant takes advantage of that.

Once wastewater is aerated, it is mixed with what is known as activated sludge.

“The word activated is used because the sludge contains many active bacteria that can feed on the waste and other harmful bacteria in the water,” Parks said.

It’s biological warfare, of a good sort.

“Adding oxygen to the water increases the bacteria so that it eventually destroys itself,” facility chemist Aldrick McMiller said. “It’s creating bacteria-killing bacteria.”

The wastewater, in its elaborate travels through the plant, flows into one of the facility’s several clarifier tanks. Each tank is 90 feet in diameter and roughly 15 feet deep. Each can hold more than two million gallons of water, Parks said.

There, the wastewater sits for several hours as the sludge sinks to the bottom, and the water is then disinfected with chlorine.

At that point, Jones said, the wastewater can safely be discharged into the river, even as the sludge is returned to the aeration system to react with newly arriving, untreated wastewater.

The separated, screened, aerated and otherwise treated effluent is then pumped underground five miles to where it is discharged near Warfield Point Park.

Before the water is discharged, it is tested on-site.

“We check the pH levels and the oxygen content of the water regularly,” McMiller said. “We also monitor for nitrogen and phosphorus.”

“We want to see what’s going on with the water we discharge” so that if, or when, the federal Environmental Protection Agency (EPA) sets new standard, the facility will be prepared to adapt, Gales said.

“For example, if sometime down the line, the EPA puts a smaller number on how many parts nitrogen your water can be, we’d have to revamp our entire process. We want to be prepared for that,” he said.

The process from start to finish takes 18 hours, and the water entering the river is 95 per cent pure, McMiller said.

“We help nature re-acclimate the water,” he said. “If this process was done naturally it would take hundreds of years. We’re just helping it along. Nature still has to take care of that remaining five per cent, though.”

That will take some 27 years, during which time Mother Nature plays her role, and, eventually, that water, once sewage, will be returned to the tap, McMiller said: “That’s something a lot of people are grossed out by, but what they don’t understand is almost all water is used water.”