Hot Springs improves wastewater filtration
HOT SPRINGS — The city is “breathing a little easier” since the tertiary filters at the Davidson Drive regional wastewater treatment plant came online earlier this month, the manager of Hot Springs Utilities said. , Monty Ledbetter.
The two Aqua-Aerobic Systems fabric filters added to the treatment process on Jan. 5 can handle four times the flow volume of the plant’s two sand filters and act as a safety net when prolonged periods of rain prevent the flow to go through the full spectrum treatment.
Storm water seeping through the maze of sewage pipes that carry the flow to the plant washes away the microorganisms the plant relies on to break down the sewage. Many of the 68 effluent violations the city reported to the state Department of Environmental Quality from January 2018 to May last year occurred while microorganisms were regenerating in ponds. ventilation.
To preserve microorganisms, plant operators occasionally route the rain-diluted stream around the ponds and clarifiers in the intermediate stages of the treatment process. Cloth media filters can remove total suspended solids, phosphorus and other impurities from the rain-diluted stream before it passes through the ultraviolet light disinfection chamber, the last step before the effluent reaches the city’s authorized outlet at upper Catherine Lake.
“It’s sort of the last line of defense,” Ledbetter said of tertiary filters. “It takes 1.5 inches of rain to start bothering us. Two or three bouts of rain in a row are the ones that hurt us. Not so much with the filters now. We’re breathing a little easier.”
The new filters are part of the city’s upgrades to the plant to bring its sanitation system into compliance with the Clean Water Act. A $9 million upgrade completed several years ago paid for new headworks, a grain separation chamber, and the UV disinfection chamber.
The $38 million sewage fund debt the city refinanced in 2020 freed up $17 million for other improvements. Ledbetter said $10.5 million in additional upgrades are planned, including a new clarifier and new diffusers and air handlers in all three aeration basins.
Tertiary filters have yet to prove themselves against large stormwater inflows, but Harold Mauldin, the city’s wastewater treatment facility operations manager, said the city is already seeing a return on its investment of over $2 million. Dissolved oxygen levels in the mill’s effluent have increased since the filters were commissioned, a boon to aquatic life in the receiving waters of Upper Catherine Lake.
The flow passing through the filters descends the stairs towards the UV disinfection chamber. Ledbetter said dissolved oxygen added to the stream as it came down the steps.
“By a simple design like this, it helps transport dissolved oxygen to the lake,” he said.
The UV disinfection chamber that replaced the chlorine disinfection system 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 that the water we put in the lake is much better quality than what’s in the lake,” Ledbetter said. “We are actually helping the lake by doing what we do.”
Mauldin said the filters capture total suspended solids, or organic matter, that doesn’t settle in the plant’s secondary clarifier, reducing TSS by 70% from the week before the filters went live. .
“Secondaries can’t do much,” he said. “Most of our treatment process slows everything down, so heavy particles can settle. What goes through the secondary will go to the filter,” he said.
He said the filters are also adept at removing phosphorus from soap and detergent that enters wastewater, requiring fewer chemicals to deal with the phosphorus.
The filter suction system cleans the rotating cloth discs during operation. The material removed by the vacuum is sent upstream to the mill heads, where it goes through the complete treatment process. Factory sand filters should be taken offline while they are being cleaned.
“Now when we backwash our sand filters, we have to take them out of service,” Ledbetter said. “And we do not treat any water during this period. [The vacuum system] does not interrupt your cycle at all. You just keep processing.”
Effluent samples are tested daily in the plant’s laboratory certified by the Environmental Protection Agency. Test results are submitted to the state through release monitoring reports. Ledbetter said few processing 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 license requires testing seven days a week. That would be hundreds of thousands a year that we would be spending if we didn’t have our lab certified.”