694 flushes per minute end up at Mitchell's new $15.3M north wastewater facility

Yahoo

08-02-2025

Feb. 7—MITCHELL — Safety, technology and cleanliness are staples of Mitchell's new $15.3 million north wastewater treatment plant.
The wastewater treatment plant opened in March 2024, and provides safe disposal of Mitchell's flushed items. The plant's automated software enables city workers to remotely monitor systems and the new screening hardware replaces the need for workers to manually remove human waste products.
"When you flush, it all comes here," said Jon Vermeulen, Mitchell's utility superintendent for water, sewer and stormwater.
The wastewater plant has a design peak of 5.8 million gallons. The average flow is 1.6 million per day, which is 66,666 gallons per hour or 1,111 gallons per minute.
The average shower in America is 8 minutes and uses 16 gallons of water. The average toilet flush uses 1.6 gallons. In other words, in one minute in Mitchell, there's 694 toilet flushes, or for every eight-minute block of time, there's 555 showers.
Previously, the solid waste removal process would allow sediment and small pieces of waste to leave the facility and travel along with the wastewater to a holding basin, where the sediment would gather.
"The new system is 40% more efficient," said Terry Johnson, Mitchell's engineering project supervisor.
The old way of cleaning required workers to wear masks with oxygen tanks and wade into a large holding basin to clear out the solids with a high-powered vacuum. The new system eliminates this duty.
"They're standing in a couple of feet of sewage. Most people would not do that," Vermeulen said.
The city funded the project with a $15.3 million loan from the South Dakota Clean Water State Revolving Loan Fund (SRF) program.
"We ended up saving about $700,000 on this project," said Vermeulen, who has been with the utilities department for 31 years.
The city's utilities department is an enterprise fund, which means it acts as a business and collects revenue. Collections and loans are used to pay for pipes and upgrades to facilities.
Each resident's water bill has a sewer charge, which is a base fee of $30.66 per 750 gallons used and $4.22 per volume as of Oct. 1, 2024.
The City Council recently voted to increase the rates for water and sewer services.
The sewer charge has $26.22 built into it to cover various capital improvement projects, such as the waste water treatment plant, the Dailey Drive lift station in 2022, and the Sanborn Boulevard project in 2020, which all required SRF loans to complete.
"Our base fee is how we pay for our projects. The more SRF projects you have, the higher the base fee. So that kind of tells you what you're spending money on for sewer," Vermeulen said.
The city has two wastewater facility locations. The north wastewater plant off East Havens Avenue, which originally had a November 2023 completion date, separates solid organic waste from wastewater. The
$32 million south wastewater treatment plant
, which is by Mitchell's Regional Landfill and was also funded through an SRF loan, has a fall 2026 completion date. The new facility will dehydrate any leftover sludge and send it to the landfill, which will finish the process of making the organic material in the wastewater safe for release.
The team of 12 utility department workers who keep all of Mitchell's various lift stations, sewer lines, and storm drains flowing will have an easier time maintaining the new wastewater plant than the previous one. Automated technology and safety alerts play a big part in daily operations.
The utility department uses Mitchell-based Dakota Pump's software platform that controls the hardware of the valves, monitors water flow, and alerts workers when a system is down or needs maintenance. From a dashboard, a digital display of each lift pump station and how much water is flowing can be seen, as well as how much water is flowing through the facility at any given time.
The plant sees high consumption times, with mid-day as the slowest for city-wide water usage. Morning has a peak and then slows by 11 a.m., and then picks up again between 7 p.m. to 11 p.m. Peak times see a 30% increase in usage, according to Vermeulen.
Green or red lights outside of doorways indicate the safety of the air inside the building. A green light means the air is safe, a red light means that personnel should not enter. Additionally, alarms sound inside the building if the air quality becomes life-threatening.
This usually occurs when a hauled waste contractor empties a septic tank on site, which releases toxic air.
"You get out of the building, and usually within 5 to 10 minutes, the air has turned itself over and it's safe to enter," Vermeulen said.
Cranes and hatchways in floors allow for heavy pumps, pipes and the like to be maintained and replaced. There is a redundancy of lift pumps, generators, and waste disposal systems so that one machine can be under maintenance while the other is able to keep the city running.
Fourteen lift stations pump sanitation waste through the city until it arrives at the lowest point in Mitchell, the new wastewater treatment plant. Here, it goes through an enormous roto-screener to remove anything larger than an eighth of an inch. The city discourages the flushing of toys, feminine care products and adult wipes, even ones labeled flushable, because they do not break down in the same way as organic items.
Solids are dumped onto a conveyor belt headed to an open-top trailer.
Smaller items, such as human waste and toilet paper, are gravity fed to channels underneath the plant that lead to a cone, which slowly separates the water out until only a grit remains and can then be sent back up to above ground level. The grit classifier, which is fed by conveyor belt, sifts silt, small particles and undigested corn and then deposits into the same trailer.
Every Monday morning, the trailer is emptied of about 1 to 2.5 tons of solid waste at the Mitchell regional landfill. During the summer, when tourists visit the Corn Palace or stay in hotels, there is an increase in the amount of solid waste that is deposited at the landfill, according to Vermeulen.
Wastewater, with some sediment, then travels to an aeration basin to remove ammonia. Afterward, it is pumped 2 miles to the south wastewater treatment facility, currently under construction, to undergo additional mixing and breakdown before being emptied into the city lagoons. The aeration process allows for greater oxygen coverage so that microbes, which require oxygen to live, can eat the organic waste.
There are five city lagoons, at various depths, covering a 240-acre area holding about 700 million gallons of water.
Three farmers have long-term contracts with the city of Mitchell to channel nutrient-rich lagoon water through nine pivots to irrigate their farms.
Lagoon water is transferred from one lagoon to the next, with a focus on moving top water with the least amount of sediment. In the spring and the fall, the lagoons are emptied into the James River by permit from the South Dakota Department of Agriculture and Natural Resources.
"The guys out there are working hard. They don't complain about it being cold. I feel really blessed to work with them and I'm glad I'm given the opportunity," Vermeulen said.