Improvements to the third layer of treatment at the Regional Wastewater Treatment Plant expected to be put out for bid next year will help the plant's effluent comply with state and federal standards during heavy rain events, the city said.
The city budgeted $500,000 from its wastewater fund to install disc filters next year at the Davidson Drive facility that treats wastewater before it reaches the city's permitted outfall into Lake Catherine.
In addition to the regular wastewater flow carried by a sprawling collection system that reaches south and west of Lake Hamilton, east of Hot Springs and north to the 4000 block of Park Avenue, stormwater from heavy rain events also gets conveyed to the treatment plant, Utilities Director Monty Ledbetter said, straining the plant's capacity to handle the additional flow.
Stormwater gets in despite improvements the city has made to the collection system in response to the consent administrative order it entered into with the Arkansas Department of Environmental Quality in 2011.
According to the request for release the city sent ADEQ in June, it's spent more than $6 million shoring up manholes and almost $12 million replacing and rehabbing two large mains that serve as the spine of the collection system. The capital outlays are part of the almost $70 million in ratepayer-financed improvements the city has committed to bring the regional wastewater system into compliance with the Clean Water Act.
Ledbetter said stormwater primarily enters where people have connected storm drains to the collection system, leading to high volumes of inflow during large rain events. Groundwater infiltration is less profound, but both contribute to sewer system overflows. The 359 from 2004 to mid-2008 put the city under the ADEQ mandate and forced the city to pay a $105,000 civil penalty in 2011.
The request for release listed 515 overflows from 2015 through April of this year, more than 100 of which may be duplicates, ADEQ told the city. Most of the overflows coincided with large rain events, but Ledbetter said improvements have shortened the duration of most wet weather overflows.
"The overflows we have now go away really quickly," he said. "It just lasts a few hours. It's gone after it quits raining."
The disc filters will help keep the plant's effluent in compliance with standards when heavy rains bring additional flow that requires bypassing upstream treatment steps. Their cloth-filtered medium will replace sand filters currently used as the third treatment step.
"It's a new kind of technology that's really going to help us," Ledbetter said. "They can treat a lot more water in a lot smaller space for a lot cheaper."
The filters follow primary and secondary clarifiers and aeration basins where impurities are given time to settle out of the flow and be digested by bacteria. Biosolids, or sludge, separated from the flow gets heated in underground digesters that divest it of organic material and squeezed dry by a conveyor-belt press before going to the city's compost facility.
Ultraviolet lamps that scramble bacteria's genetic material and keep it from multiplying are the final step in a process that treats about 12 million gallons on an average day.
Inflow and infiltration from a prolonged rain event this winter put the plant's 54 million-gallon equalization basin at capacity, requiring a bypass that led to ADEQ issuing an effluent violation warning.
The basin was holding flow while the plant's new headworks was under construction. Flow retained in the basin during construction led to another effluent violation later in the year that Ledbetter said could not be avoided, as retention time and warm weather provided a favorable environment for impurities that proved difficult to treat.
"I can't wait until (the disc filters) are online," Ledbetter said. "It's going to add another tool to our toolbox. We hate having violations, but when you're operating an old wastewater plant, sometimes you're going to have them. This new system will definitely help."
Ledbetter said the new headworks came online this summer. Its screens and grit chamber remove larger constituents that can damage treatment equipment downstream.
Local on 12/17/2018