The Village of Hinckley needed to upgrade its rotating biological contactor plant to activated sludge to meet a consent decree.

Looking to minimize impact on ratepayers, the village’s engineers and its operations and maintenance contractor found a solution through a small-footprint aeration technology that would save the expense of a large capital upgrade, while being easy to operate and maintain.

They chose the Aero-Mod SEQUOX activated sludge process, which provides biological nutrient removal in a configuration that can be easily expanded to accommodate growth. The system’s successful performance has been partly responsible for the facility’s recognition as 2020 Class C 2020 Plant of the Year by the Illinois Association of Water Pollution Control Operators.

Need for growth

Hinckley, a community of about 2,100, was the site of the very first road game for the Harlem Globetrotters basketball team in the 1930s. The wastewater treatment plant was built in 1974. At the time the village determined that outsourcing operations and maintenance for the plant and sewage lift stations made economic sense. The plant is operated by Total Environmental Service Technologies.

As the years went by, the plant became hydraulically and organically overloaded, and TEST recommended modifying it to an activated sludge process to meet concerns about capacity and compliance. The consent decree sped up the timeline for change, and in 2014 an expansion was completed.

Instead of building a new plant, TEST and the village’s engineers looked to source components that could be integrated with existing structures, and to reuse or repurpose still-serviceable buildings. The engineering team investigated solutions, considering BOD loading and future regulatory requirements, including phosphate and nitrogen removal.

In visiting other treatment plants and interviewing operators, they encountered the Aero-Mod process in a neighboring community. “We learned from operators that the Aero-Mod system is very easy to operate, takes a relatively small footprint, and is easily expandable,” says Dale Youngers, contract operator with TEST and operations manager for water and wastewater systems in Hinckley.

“If the town were to grow, it is designed to accept additional treatment modules. The number of moving parts underwater is very limited, including only submersible mixers in a selector zone.”

Treatment in action

With the integration of the Aero-Mod process and a Hach dissolved oxygen control system, the plant capacity increased from 0.2 mgd to 0.5 mgd (average flow is 0.25 mgd).

Inflow comes through an automatic bar screen (Hydro-Dyne) and enters a wet well where pumps (Smith & Loveless) transfer it to the biological nutrient removal system. In that process, the water first goes through a bio-P fermentation zone in a tank that provides anoxic conditions to promote the growth bacteria that take up phosphate. From there it flows to a bio-P selector with submersible mixers (Wilo USA).

The flow then splits into two treatment trains where it undergoes two sequential stages of aeration. “If you were to look at this standing on top of the tankage, you have one first stage and one second stage aerating at the same time in a diagonal configuration. Then the process splits every two hours so what was aerating is no longer.

“The purpose of this is the destruction of nitrogen. Under aeration you’re converting the ammonia to nitrate. When it goes into the unaerated tank, the nitrate is converted to nitrogen gas and simply off gases into the atmosphere. It’s so simple it’s brilliant.”

The flow then enters a small tank to be commingled for third-stage aeration. From there it splits again into separate trains and into the final clarifiers. The clarified effluent flows by gravity through UV disinfection (Glasco) before discharge to Little Rock Creek. 

Regional challenges

Hinckley, about 60 miles west of Chicago, has harsh winters; operators are challenged when plant water temperatures drop to 50 degrees F, slowing biological activity. Youngers compensates by raising mixed liquor suspended solids concentrations and adjusting the wasting schedule.

During summer the wasting rate is relatively high; it decreases starting in fall. In extreme cold winter, Youngers may even skip a day of wasting at times. Wasting is performed out of first-stage aeration, into the Aero-Mod digesters, and the amount is controlled depending on the time of year.

Since the existing plant had two usable digesters, part of the upgrade included retrofitting these with aeration diffusers Aero-Mod supplies.

When the solids start to fill up the Aero-Mod digesters, operators run the Aero-Mod belt filter press to dewater the solids from the old, retrofitted digesters. These dewatered solids are then land-applied two times each year. Solids from the Aero-Mod digesters are then transferred to the older retrofitted digesters to fill them back up. This cycle takes about one week; the belt press runs one day each week.  

“That’s a little unique for us, as most plants only have one set of digesters being added to every day,” says Youngers. “So, you may not get digestion as complete as if you could just let the digester sit and let the microbes do the work uninterrupted with no more food introduced for a week at a time. We have found this transfer to be very advantageous.”

The Aero-Mod system offers flexibility with automated controls that allow operators to adjust for variable conditions. For example, inflow and infiltration during heavy rains can double plant flows within hours, creating a potential for washout of the mixed liquor.

To protect the mixed liquor in that scenario, all of the return activated sludge is pumped out of the final clarifiers into the aeration tanks to preserve the solids. Operators can then manually manipulate which of the aeration tanks are online and dial blower output down.

All of these adjustments are initiated and managed through the system’s computerized control system and software with just a few mouse clicks. “This is just one example of the flexibility of the system,” Youngers says. “Operators have the ability to adjust the system to manage multiple situations.”

Community impact

The plant expansion did not come without cost, and it followed a water treatment plant upgrade to deal with radium in the groundwater source. To limit the impact on ratepayers, the city increased the rates in four stages.

The upside to the expansion is that increased flows and sewer backups are no longer an issue. In addition, the end of the consent decree enabled the village to end a moratorium on new sewer connections and remove what had been a lid on development. As a result, new subdivisions are being built and businesses are looking at Hinckley as a place to set up shop.

Youngers notes that communications with village trustees and sound plant operation decisions were factors in winning the Plant of the Year award. “We’re also concerned about how things look,” he says. “When you work 40-plus hours a week at a plant, you spend many more waking hours there than at home. So, we keep a really nice-looking plant.”

Before the COVID pandemic, Hinckley often conducted plant tours for visiting operators, who often asked if the team actually ran the biosolids filter press because the room was so clean. “We are the guardians of the village’s investment, and it’s our job to keep it clean, well- operated and well-maintained,” says Youngers. “We’ve been recognized with awards for these efforts, and it’s nice to get that recognition.”

Youngers also credits some the successes to TEST’s relationship with the village: “They respect our advice. Sometimes there are things they can’t agree to because of cost, but overall, they have open arms to take our suggestions and help improve operations. We owe a debt of gratitude to the part they play in our peer recognition.”