The Unified Government of Wyandotte County and Kansas City needed a cost-effective and compact wastewater treatment process to replace an aging package plant.

Space was at a premium because the plant site had to be elevated significantly with fill to lift it above the 500-year floodplain. The new facility was needed to accommodate growth but also to comply with a consent decree.

Today the treatment plant (2 mgd design, 1 mgd average) serving what is known as the Wolcott area uses the AquaNereda aerobic granular sludge process (Aqua-Aerobic Systems). The highly automated facility yields effluent that easily meets permit limits for BOD and TSS and is low in total nitrogen and total phosphorus, which do not yet have discharge permit limits.

Above the flood

The unified government faced a consent decree to correct sanitary and combined sewer overflows. At the same time, projected growth made it necessary to change the flow direction of a pump station that had fed another treatment plant and sent it to the new Wolcott facility.

A 200,000 gpd activated sludge package plant was decommissioned, and the new plant was built on a low-lying 36-acre city-owned property. To remove the plant from the flood plain, about 10 acres had to be built up to an elevation about 15 feet higher.

“One of the drivers for going with the AquaNereda process was the compact footprint that enabled us to build it on a smaller area while leaving room for expansion,” says Rick Bird, a wastewater and collection systems manager who oversees four treatment plants. Future plans call for adding a second AquaNereda plant on the same site. The receiving stream is Connor Creek, a Missouri River tributary.

Being innovative

In planning the new facility, agency staff investigated conventional activated sludge treatment and a couple of sequencing batch reactors. Before settling on the AquaNereda process, team members visited installations in Ireland and England, and in Foley, Alabama.

“Ultimately based on our research we thought, ‘Let’s be trailblazers,’” Bird recalls. “The process was just basically hands down what we wanted.” 

The plant was a design-build project with HDR as the consulting engineer and Garney Construction as general contractor. It went online in January 2022. “The compact footprint reduced the capital cost in elevating the site,” notes Paula Dorn, a process engineer with Aqua-Aerobic Systems.

“They were also interested in the low long-term maintenance and the flexibility to add another treatment train in the future. The AquaNereda process had the lowest life cycle cost among the alternatives considered, including capital, energy, maintenance and chemicals additions.”

The process provides biological removal of nitrogen and phosphorus, which at present are simply monitored but are likely to receive effluent limits in a new discharge permit to be issued in 2024, Bird observes.

Active granules

Aerobic granular sludge is the key to the AquaNereda process. As Dorn describes the process, in a fill phase, influent enters the reactor at a controlled rate through a distribution grid on the floor; treated water is displaced over the top of the reactor through a series of effluent weirs.

While influent is fed to the system, the aeration is off, and therefore the influent immediately contacts the settled sludge blanket and the biology it contains. The anaerobic environment optimizes the conversion of readily biodegradable substrate into storage polymers by slow-growing bacteria, resulting in release of phosphorus.

In the subsequent react phase, during which the aerators operate, phosphorus is taken up by polyphosphate-accumulating organisms. A byproduct of the phosphorus release is a naturally produced extracellular polymeric substance that becomes the substrate for formation of granules. “We don’t add any chemical or carrier to get the granules to develop,” Dorn says. Microorganisms in turn attach to the granules, providing a measure of fixed-film treatment.

During the settle phase, the aeration is turned off. A valve on the side of the reactors opens, and the lighter and more flocculant fraction of sludge is wasted from the system. The heavier and denser particles remain to aid in the formation of granules, which ultimately can grow to 4-5 millimeters in size.

“As the granules grow, they develop an oxygen gradient, with aerobic conditions on the outside and anoxic conditions toward the center,” says Dorn. “The granules are large enough so that oxygen can’t penetrate deeper into the interior.”

Putting it to work

The Wolcott plant was started up by seeding it with 180,000 gallons of conventional activated sludge from another treatment facility. “Within two weeks we were already at a sludge volume index of less than 100 mg/L,” says Bird. “We were settling in five minutes. The simultaneous nitrification-denitrification process created conditions for the sludge to want to settle.

“We were starting to grow granules within a couple of months. The way I look at it, we’ve got millions and millions of tiny little treatment plants doing the work. Every granule is its own treatment plant. It does phosphorus removal. It does ammonia uptake. It does everything for you.”

The aeration process is controlled automatically using ammonia analyzers. Operators monitor the system by taking and analyzing process samples and assessing the mixed liquor. They can easily adjust parameters such as settling and wasting rates as process conditions change. The three reactor tanks operate alternately in the fill, react and settle phases.

Solid performance

Bird notes that the process consistently yields effluent with BOD and TSS well below 10 mg/L, total nitrogen less than 3 mg/L, and total phosphorus less than 1 mg/L. That is upstream of the plant’s AquaDisk tertiary filters.

Assessing the system’s performance, Bird observes, “On a scale from 1 to 10, as long as we effectively maintain the ammonia analyzers, I would call it a 10. It’s such an intuitive process. It’s very exciting.”