“You should have seen it,” says Frisbie. “They came up from southern Illinois in a truck, and were shot out of pipes into the river.”

This is the biggest fish release in the Chicago River’s history, and Frisbie says it will take four or five years—the time it takes a catfish to mature—for advocates to see the long-term effects. Back in the 1970s, there were only an estimated five to seven species in the water (channel catfish not among them), compared to more than 70 species today. That long-term gain in ecological health is what made the project possible. “It wasn’t worth doing until we knew it would be successful. We’ve finally gotten the river to the point where we could do this,” Frisbie notes.

Although channel catfish are native to the region, until recently they have been in limited in numbers due to poor water quality and lack of habitat. Two of the biggest contributors to the river’s degradation: phosphorus and nitrogen, which reach the river through human waste, among other things, and cause algae to grow like mad.

Scientists actually refer to phosphorus and nitrogen as nutrients. Dale Robertson, a research hydrologist for the U.S. Geological Survey, describes it this way: “At first, the nutrients help the algae to grow overabundantly, and the fish can eat more. But then the algae becomes so much, then it dies and decomposes, taking the oxygen out of the water. It’s like putting food into an aquarium—if you put too much in, you have problems.”

Right now, Chicago is dealing with an aquatic buffet where the food is rotting.

Phosphorus is a mineral found in humans, animals, and plants. Although it is a pollutant and can cause algae blooms (the reason many states have banned it in dish detergent), it is also an essential element which helps repair tissue and build strong bones and teeth, and a non-renewable resource that can be recovered and reused.

Unlike other states in the Midwest, Illinois hasn’t set limits on how much phosphorus can be allowed in rivers and streams. There are plenty of factors that contribute to Chicago’s high phosphorus and nitrogen levels. The city is only starting to disinfect its treated sewage before dumping it back into the river. Another factor is combined sewer overflows. Although these have declined substantially in recent years, raw sewage is still regularly released into Chicago’s river system.

In anticipation of state regulation, the Metropolitan Water Reclamation District (MWRD) will open the world’s largest phosphorus-recovery plant in February. The district will add chemicals to sewage to make a fertilizer product it can sell.

It’s also begun testing another method to help recover phosphorus. Surprisingly, the idea involves cultivating algae inside one of its plants. The algae eats the phosphorus while it’s in wastewater. After being cultivated, the algae can be scraped off conveyor-like belts and sold for a profit.