New wastewater filters benefit Lake Catherine, city says
The city is “breathing a little easier” since the Davidson Drive regional wastewater treatment plant’s tertiary filters came online earlier this month, Hot Springs Utilities Director Monty Ledbetter said.
The two Aqua-Aerobic Systems cloth-media filters added to the treatment process on Jan. 5 can treat four times the flow volume of the plant’s two sand filters and serve as a backstop when periods of prolonged rain prevent flow from going through the full treatment spectrum.
Stormwater infiltrating the warren of wastewater lines that carry flow to the plant washes out microorganisms the plant relies on to breakdown wastewater. Many of the 68 effluent violations the city reported to the state Department of Environmental Quality from January 2018 through May of last year happened while microorganisms were regenerating in aeration basins.
To preserve the microorganisms, plant operators occasionally route rain-diluted flow around the basins and clarifiers in the middle stages of the treatment process. The cloth-media filters can remove total suspended solids, phosphorous and other impurities from the rain-diluted flow before it goes through the ultra-violet light disinfection chamber, the final stage before effluent reaches the city’s permitted outfall at upper Lake Catherine.
“This is kind of the last line of defense,” Ledbetter said of the tertiary filters. “It takes 1.5 inches of rain to start bothering us. Two or three rain events in a row are the ones that hurt us. Not so much with the filters now. We’re kind of breathing a little easier.”
The new filters are part of plant upgrades the city has made to bring its wastewater system into compliance with the Clean Water Act. A $9 million upgrade completed several years ago paid for new headworks, a grit separation chamber and the UV disinfection chamber.
The $38 million of wastewater fund debt the city refinanced in 2020 freed up $17 million for more improvements. Ledbetter said $10.5 million in additional improvements are planned, including a new clarifier and new diffusers and air-handling systems in all three aeration basins.
The tertiary filters have yet to prove their mettle against large inflows of stormwater, but Harold Mauldin, the city’s wastewater facilities operation manager, said the city is already seeing a return on its more than $2 million investment. Levels of dissolved oxygen in the plant’s effluent have increased since the filters came online, a boon for aquatic life in the receiving waters on upper Lake Catherine.
Flow passing through the filters cascades down stair steps en route to the UV disinfection chamber. Ledbetter said dissolved oxygen gets added to the flow as it tumbles down the steps.
“By a simple design like this, it helps carry the dissolved oxygen to the lake,” he said.
The UV disinfection chamber that replaced the chlorine disinfection system has also benefited the receiving waters. The latter left a chemical residue in the effluent, but the UV system uses light to deactivate pathogens in the effluent.
“I promise you the water we’re putting into the lake is a lot better quality than what’s in the lake,” Ledbetter said. “We’re actually helping the lake by doing what we’re doing.”
Mauldin said the filters are capturing total suspended solids, or organic matter, that don’t settle out in the plant’s secondary clarifier, reducing TSSs by 70% compared to the week prior to the filters coming online.
“The secondaries can only do so much,” he said. “Most of our treatment process is slowing everything down, so the heavy particles can settle. What goes over the secondary will go to the filter,” he said.
He said the filters are also adept at removing phosphorus from soap and detergent that gets into wastewater, requiring fewer chemicals to treat the phosphorous.
The filters’ vacuum system cleans the rotating cloth discs while they’re in operation. Material the vacuum removes gets sent upstream to the plant’s headworks, where it goes through the full treatment process. The plant’s sand filters have to be taken offline while they’re being cleaned.
“Now when we backwash our sand filters, we have to take them out of operation,” Ledbetter said. “And we’re not treating any water during that time. (The vacuum system) doesn’t interrupt your cycle at all. You just keep on treating.”
Effluent samples are tested daily in the plant’s on-site, Environmental Protection Agency-certified lab. Test results are submitted to the state through discharge monitoring reports. Ledbetter said few treatment plants have on-site labs.
“Our lab is a certified lab, which is not easy to do,” he said. “It saves us a lot of money. We don’t have to hire outside labs to do our testing for us. Our permit requires testing seven days a week. That would be hundreds of thousands a year we would spend if we didn’t have our certified lab.”