An Integrated Fixed-Film Activated Sludge Secondary Treatment Process Helps an Illinois Plant Boost Capacity

The award-winning Mahomet Wastewater Treatment Plant packs a lot of treatment capacity and quality into a severely confined footprint.

An Integrated Fixed-Film Activated Sludge Secondary Treatment Process Helps an Illinois Plant Boost Capacity

Jason Heid, water and wastewater superintendent

The Mahomet (Illinois) Wastewater Treatment Plant sits in a tight 5-acre triangle: on one side a railroad track, on another side the Sangamon River, and on the third side a subdivision.

That posed a major challenge about 10 years ago as growth in the community, a bedroom suburb of Champaign-Urbana, required a capacity expansion. Complicating things further, the Illinois EPA issued total maximum daily limits for nitrate and phosphorus discharges to the river.

Jason Heid, water and wastewater superintendent, along with his operations team and engineering firm, were up to the challenge. They converted two existing activated sludge package treatment plants into a new extended aeration facility with biological nutrient removal, without having to expand the plant’s footprint.

The key to the project was an integrated fixed-film activated sludge (IFAS) secondary treatment process that enabled an increase in design capacity from 0.5 to 0.9 mgd within the existing tankage. The facility now produces effluent containing monthly averages of 6.15 mg/L nitrate and 0.63 mg/L total phosphorus, both well below permit limits.

For its consistently excellent performance, the facility received a 2017 Plant of the Year award (1 to 7.5 mgd) from the Illinois Association of Water Pollution Control Operators and a 2017 Wastewater Plant of the Year award from the Illinois Rural Water Association.

Handling growth

The Mahomet plant was built as a package facility in 1961. An extensive 1986 upgrade converted that package plant to an aerobic digester still in service today and replaced it with two package activated sludge extended aeration plants (Sanitaire - a Xylem Brand). The upgrade also included a filter building with three rapid sand filters (Evoqua), along with a clearwell and mudwell.

“In the mid-2000s, knowing we needed to stay ahead of our growth, we looked at another expansion,” says Heid, who holds Class 2 Wastewater and Class B Water licenses. “As we sorted out what we wanted to do, we got a letter from the IEPA saying we now had to remove nitrates and phosphorus to meet the new TMDLs.” Construction began in fall 2011 and was substantially complete in 2014.

“The engineers came up with a technology that enables us to use the two existing activated sludge plants and reconfigure them to achieve our desired design capacity without expanding the footprint of the tanks,” Heid says. “Each of the existing plants had five cells for activated sludge extended aeration with a 30-foot-diameter clarifier in the middle.

“Those clarifiers were converted to aerobic digesters. The outside rings of the tanks were modified to create an anaerobic cell and two anoxic cells, along with three aerobic cells. Two of those cells, amounting to 40 percent of the tanks, have the IFAS media in them (Veolia Water Technologies). The IFAS media allows us to do ammonia and BOD removal without a bigger footprint.

“The IFAS media is just brilliant. A conventional activated sludge facility can treat about 15 pounds of BOD loading per 1,000 cubic feet. Now we can treat 200 pounds. It’s just a huge difference. The design engineers, Larry and Matt Johnson and Narendra Patel, did a really good job of looking to the future.”

The facility was built for easy and affordable expansion to a 1.25 mgd design capacity. Two new 46-foot-diameter secondary clarifiers are sized for that next phase. They’re installed on the site of a former excess-flow lagoon; a concrete 0.5-million-gallon concrete clarifier in a corner of the site near the river now receives excess wet-weather inflows. The expansion also included rehabilitation of the sand filters with PLC-based controls and new backwash valves.

Essential headworks

The only major new addition to the plant was a headworks building containing a rotary-drum microscreen (HUBER Technology) with quarter-inch holes. “We had the Chevy model on the old plant, and now we’ve got the Mercedes,” Heid says. “It’s one of our most important pieces of equipment because we can’t let trash get through and still operate the plant.

“The IFAS tanks have screens that keep the media inside. You can’t let rags in there like you could with a regular activated sludge plant. If rags get into the tanks, they’re not coming out. They’re going to get stuck on a wall screen or on the screens between the cells.” The headworks also includes a rapid-rate chlorine disinfection system for high wet-weather flows, along with alum feed storage tanks for phosphorus removal and a climate-controlled electrical room for all the plant’s submersible pumps and mixers (Flygt - a Xylem Brand).

After screening, the flow enters a splitter structure before entering the two secondary treatment trains. “In those tanks we have a lot of internal recycling to accomplish nutrient removal. In each tank, the anaerobic cell is for phosphorus reduction; the anoxic cells are for nitrate. Then we have two aerobic cells with IFAS media and control logic and a third cell with fine-bubble aeration and no media.

“There are several flow schemes we could use to accomplish BNR, but we use the modified University of Cape Town process, since we have found it to be the most robust.”

From secondary treatment, the flow moves on to the clarifiers, which have fiberglass weir covers to free operators from weir cleaning. The effluent then goes to the sand filters for tertiary treatment. A nonpotable water system supplies water to clean the headworks screen and other plant equipment, and for grounds irrigation.

Land-applied biosolids

Waste activated sludge is pumped to the aerobic digesters, equipped with coarse-bubble diffusers. An airlift pump decants the digesters; thickened biosolids gravity flow to a pump station that delivers the material to a 1-million-gallon holding lagoon, also decanted periodically. Each fall, a contractor land-applies the biosolids on farms by knife-injection.

“At some time in the future, there will probably be some phosphorus regulation for the farmers and we’ll have to change to a different process,” Heid says. “For now, our storage capacity allows us to land-apply just once a year after the crops are out in fall.”

The plant functions with a small team who also takes care of the village’s water plant, the water distribution system and wastewater collections system. Team members are Matt Gregory, water and wastewater operator (Class 2 Wastewater and Class B Water licenses), and Shawn Rideout, Reed Coleman and Kyle Welborn, maintenance workers.

The small staff makes efficiency paramount. Heid believes in performing maintenance and repairs in-house to the fullest extent possible: “When you pay someone to do maintenance, it doesn’t get done as often because it costs a bloody fortune. If you want to pull a submersible pump to change a seal, it’s going to cost $3,000 because they’re going to come with a truck crane to lift the pump out of the ground.

“I’m not going to buy an $80,000 truck to pull 15 pumps. We use a composite Davit crane. There are two pumps to a wet well. We put a crane mount at every wet well that’s bolted into the concrete. The crane breaks down into three pieces so we can take it anywhere we want. We put the pumps on a stainless steel chain and crank them up.

“I sent Shawn Rideout to the firm where we bought the pumps. At their maintenance shop outside St. Louis, they have every pump and mixer we use. They showed him how to break everything down and do every repair. Our philosophy is that we don’t Band-Aid anything. If something needs fixing, we fix it right and be done with it. We don’t have enough staff to be fixing things more than once.”

Stress on prevention

“Another philosophy I have is to carry spares wherever I think there might be a need. For example, our HUBER screen has a variable-speed drive that controls the speed of the drum. If that drive goes out, we’re out of service. I asked HUBER to send me a spare and program it. It’s on the shelf and ready to go. Any spare part I can think of that might be useful, I try and get it so if something goes wrong, we can deal with it and move on.”

Preventive maintenance is also a top priority. The plant has three Howden Turblex 100 hp high-speed turbo blowers connected to Hach dissolved oxygen meters in the aerobic tanks. “We can run the whole plant with one blower operating at 75 percent,” Heid says. “We have the two other blowers for backup and for the next phase of expansion.

“We have somebody come out once a year and go through those blowers from top to bottom, in addition to the regular maintenance we do. They get a laptop out and download everything off those blowers, making sure there are no issues, making sure everything is balanced correctly, and going through the machine histories.”

The preventive approach applies to the collections system as well. Since 2005, in alternate years, Heid has ordered cured-in-place lining in the sewer system. “That way we don’t have to go out and cut tree roots out of the sewers,” he says. “If you have a 10-inch line and one joint is full of roots, you don’t have a 10-inch line anymore. You have maybe a 4-inch line. So in wet weather, you could have backups and surcharges.”

The collections system maintenance program now also includes regular line cleaning and televising: “We had 18,000 feet done last year, and it only cost $18,000. It showed us potential candidates for lining.”

Relying on teamwork

In leading his team, Heid strives to make work life as pleasant as possible: “I treat everybody the same, with respect. I try and help them enjoy the work. Anything I tell them to do is not something I haven’t done.”

The career has been rewarding for Heid: “I’ve been here for 24 years, and the job has just grown with the town. The plant has doubled in size. That means the treatment doubles and the technology doubles. There was no reason for me to go anywhere else. And the village takes good care of me. The board is extremely supportive.”

And the Mahomet plant continues to churn out high-quality effluent from its little corner of town.

From wings to wastewater

Jason Heid launched his wastewater treatment career in the U.S. Air Force. That role wasn’t his first choice.

“The Air Force had a very high retention rate, and for the other jobs I wanted, there was up to a year wait,” Heid recalls. “The job I took was called environmental support specialist. It was something to get me in the door.”

Before he knew it, after basic training and schooling, he was treating water and wastewater at Air Force facilities, ending with Chanute Air Force Base (since closed) in Champaign County, Illinois. Upon discharge from the service, he returned to his native Pennsylvania.

Soon after the move, he got a phone call from the Urbana & Champaign Sanitary District: “I packed up, got in my car and found an apartment in Champaign. I can say I was probably unemployed for one week in my whole life.”

He worked six years as an operator at what was then a 17 mgd wastewater treatment plant. Then he and a friend left for Mahomet to work as part-time employees, with plans to also operate plants at other small communities on a contract basis. 

After eight months, the water and sewer and street and alley superintendents in Mahomet died tragically in a trench accident. Heid observes, “The next thing I knew, I was here full time.”


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