How A Sewer Plant Cleans Wastewater

Oct 7, 2019

When we hear about purifying water in the news, it’s often about what comes out of the tap. But how do we clean it once it’s been used?

Whether it’s from a toilet, sink or bathtub, dirty water eventually makes its way into the local sewer lines.  And those lines, provided they’re in working order, make their way to a water treatment plant.  In the City of DeKalb, that responsibility falls on the Kishwaukee Water Reclamation District.

Vehicles, ranging from small buggies to bulldozers and septic trucks, shuttle between buildings. Each of those, in turn, is responsible for a different part of water treatment.  Our first stop is garbage screening.

Quarter-inch filters catch garbage in the water. That debris is swept up by a mechanical rake (device at top) and transferred to a dumpster.
Credit Chase Cavanaugh

The water passes through metal grates, with gaps a quarter-inch thick.  District Assistant Engineering Manager Mike Holland says from there, mechanical rakes pick up the “debris.”  

“It screens out all the rags and goofy stuff that people can flush down the toilet. Think dental floss, toothpicks, think of inorganic material,” he said. 

Anything that slips through the filters is sucked into a vortex and, along with the garbage caught before, pumped out, dried, and hauled away in dumpsters.

A pipe (right) dumps the garbage into nearby dumpsters after it's been picked up from the rakes (center).
Credit Chase Cavanaugh

District Manager Mark Eddington says garbage screening has the most “presence.”

“You’ve now moved through the worst smelling portion of the wastewater treatment process, so if you can handle that, you can handle anything,” said Eddington.

From there, the water goes outside to a primary clarifier.  It’s an open-air settling tank that constantly stirs the water. Eddington says this process isolates some of the organic waste.

We skim off those fats, oils and greases off the top," he said.  "We pump the heavier solid material off the bottom, feed it all to our digesters, which act as the stomach of the plant.”

Digesters are vats filled with bacteria. They eat the waste and release a biogas, mostly methane.  As for what’s left:

“We then de-water those solids with centrifuges and the byproduct is an inert dirt-smelling (and looking) substance that we contract with local farmers to take out and spread it on their farm fields,” Eddington said.

Trickle filters water through a layer of rocks about six feet deep.
Credit Chase Cavanaugh

The water that’s left over splits to feed two groups of microbes.  The first is a trickle filter. It’s an outdoor tank filled with rocks about six feet deep.  Water is then sprayed on these rocks like a sprinkler.

“All the rock naturally is that red rock, but where it’s green is where we’ve been spraying the wastewater onto it, and we’re getting that biological growth on the rock,” Holland said.

The growths consume waste as the water sinks through the cracks.  Birds scramble on the rocks, even with the threat of a rotating sprayer.

Water is sprayed onto the rocks through a rotating arm. The green color is a colony of microbes that can eat organic waste.
Credit Chase Cavanaugh

"They’ll be picking worms out, and they will hop over it each time when it comes around, so they’ll eat for ten seconds, hop over it. Eat for ten seconds, hop over the next one,” Eddington added.

Water from the clarifiers also goes to several basins that are filled with waste-eating bacteria.  Holland says oxygen gets pumped into the mix to feed these microbes.

“The clean water goes over the top. That denser material with the organisms in it -- that we want to be doing the biological treatment for us -- we recycle right back into here, and they just keep feeding,” he said.

The plant's rotating biological contactor removes pollutants such as ammonia. Eddington and Holland say it works like a giant fish filter.
Credit Chase Cavanaugh

The treated water then goes through two final stations.  First, a large plastic device called a rotating biologic contactor filters out pollutants like ammonia. Eddington and Holland say its works like a giant fish filter. 

After this, germs like E. coli are removed.  Eddington says the plant previously used chlorine to kill these germs. But they switched to ultraviolet lamps to avoid chemicals harming the local ecosystem.

"The UV does not kill it, but renders it sterile. So it sterilizes the system with bacteria," Eddinton explained. "It's okay if bacteria's in it as long as it isn't able to reproduce itself." 

Before being dumped in the Kishwaukee River, the treated water is exposed to racks of ultraviolet lamps. This sterilizes any remaining germs in the water, such as E. coli, so they can't reproduce.
Credit Chase Cavanaugh

The final product flows into the Kishwaukee River, and possibly even further.

"North to the Rock, over the Mississippi, and down to the Gulf of Mexico," Eddington said.

The Kishwaukee Water Reclamation District has been treating the water in DeKalb since 1928. But the sewage plant is near the end of a nearly $47 million construction project that will make their treatment even more efficient.