Making It Fit

A 2009 upgrade brings new and different secondary treatment and solids processing systems to a treatment plant in Missouri
Making It Fit
The Washington Wastewater Treatment Plant team includes, from left, Kevin Massmann, operator 3; Doug Grafrath, operator 3; John Zimmermann, chief operator;Kevin Quaethem, water/wastewater superintendent; David Pickett, wastewater collections operator

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Hikers and bikers on the trail past the newly upgraded Washington (Mo.) Wastewater Treatment Facility probably have no idea about the innovative technology at work beyond the fence. But Kevin Quaethem, John Zimmermann and their staff already have learned a great deal about their new Vertical Loop Reactor (VLR) treatment and Cannibal sludge reduction system, and they’re gaining important process knowledge with each passing day.

The new 4 mgd (design) plant replaced an old trickling filter facility that was over design capacity, had deteriorating concrete structures, and was held together “by duct tape and wire.”

“It’s been a pretty steep learning curve,” Quaethem says. “Before, there was not much to monitor, but now we’re monitoring regularly for sludge blankets, mixed liquor, suspended solids, return activated sludge suspended solids, settleability, pH, oxygen reduction potential (ORP), dissolved oxygen (DO), and more. With the addition of so many new processes, maintenance requirements have increased, as well.”

Quaethem adds that his team has handled the upgrade with no increase in staffing. The key is communication. “We need to make sure we’re doing things properly,” Zimmermann says. “We talk every day. Communicate, communicate, communicate.”

Old days

Washington, population 15,000, nestles alongside the Missouri River some 60 miles southwest of St. Louis. Before the recent upgrade, wastewater was treated in a plastic-media trickling filter plant with a design capacity of about 2 mgd.

By the early 2000s, flow had increased to 2.3 mgd, and it was obvious that capacity had to be expanded. But as in many municipal treatment operations, space was an issue. The riverfront around the plant had been declared a flood zone, and no new structures were allowed, forcing the city’s design consultant, Jacobs Engineering of St. Louis, to come up with a new process that wouldn’t appreciably increase the facility’s footprint.

The answer was the VLR biological technology coupled with the Cannibal sludge reduction process, both supplied by the Envirex products group of Siemens Water Technologies.

“The VLR is basically an oxidation ditch turned on its side,” says Zimmermann. “We have four VLRs in series here.” The series of looped reactors allows for DO stratification, and while treatment capacity is the same as an oxidation ditch, the VLR consumes much less space.

The Cannibal process combines a non-biodegradable material removal step with a series of reactors where solids interchange and cycle between aerobic and non-aerobic environments. As a result, solids are destroyed, and the amount of biosolids is significantly reduced.

As for the rest of the plant, a new preliminary treatment headworks building accepts influent at an elevation that allows gravity flow to carry the water completely through all the processes to the Missouri River.

Moving on through

The preliminary treatment building houses a pair of influent channels, one for normal flow and the second for high flows due to rainfall. A third channel is available as a bypass. “With these processes inside a building, we avoid equipment failures due to freezing,” says Zimmermann.

Parkson 5 mm bar screens remove debris, automatically activating when they blind. “We get a lot of debris here, especially in the first flush of a rainstorm,” says Zimmermann. A Eutek SlurryCup/Grit Snail system supplied by Hydro International further cleans the wastewater before it moves on to the series of VLR units.

“From our discussions with the manufacturer, we understand that VLRs can be operated in a number of different configurations,” says Zimmermann. “In our case, we run them in series, one through four. We have flexibility. With air adjustments, we could probably run three, or two, and still meet our treatment requirements. We can handle over 18 mgd with an automated storm flow mode set up on the VLRs to prevent our sludge from becoming unbalanced in our system.”

Washington operates the first VLR under anoxic conditions, maintaining an ORP of negative 250, then adds oxygen in the subsequent units so that the dissolved oxygen level is about 1.5 ppm in the fourth or final VLR.

Each rectangular VLR is about 20 feet deep, extending about 10 feet above grade. Upper and lower compartments are separated by a horizontal baffle running the length of the tank. The process has been adapted from oxidation ditch technology and uses surface-mounted discs to provide mixing and deliver oxygen. “The system generates a lot of velocity,” Zimmermann observes.

The process achieves significant ammonia nitrogen reduction, even though the plant is required only to monitor that parameter. Zimmermann estimates 25-27 mg/l of ammonia nitrogen enters the plant, while the effluent contains less than 0.1 mg/l. He reports some phosphorus reduction, as well.

After biological treatment, the flow splits and passes to a pair of 90-foot-diameter Tow-Bro clarifiers (Siemens). A TrojanUV 3000+ system disinfects effluent from April through October, the period of high public contact with the river. The units consist of two channels with six UV modules in each and have an automatic lamp cleaning system. During winter, the staff pulls and thoroughly cleans and maintains the modules. Coliform counts are less than 10 CFU per 100 ml.

Sludge reduction

The Cannibal process is designed to reduce biosolids ultimately needing handling. At Washington, the process was retrofitted into two existing clarifiers and two former sludge digestion basins. First, a wedge wire screen removes fibrous materials, such as hair, and other non-biodegradable solids from the stream. A compactor presses this material to 60 to 70 percent solids, and it is stored in a dump container and taken to the city-owned landfill.

Then, in two material trains, the solids pass through the series of tanks where they are interchanged and cycled to promote biodegradation and solids reduction. In the first tank, ORP is maintained at very low levels. In this environment, aerobic bacteria are conditioned for destruction and biodegradation, making their byproducts available for facultative bacteria that can survive in this environment.

Zimmermann says the plant has seen a reduction in solids production from 290 to 300 dry tons per year to 280 dry tons per year with the new process, but plant influent TSS is somewhat higher than before. In addition, where plant effluent used to average 25 to 30 mg/l TSS, it now averages less than 4. “We’re keeping a lot more solids out of the river,” he says. “The Cannibal process has been very effective for us.”

In cold weather, solids are wasted from the Cannibal system every two to three days. “In the summer, when the bugs are more active, we usually go several weeks between wasting,” Zimmermann says.

Washington was the first plant in Missouri to use the Cannibal system, and one of the first in the country. Zimmermann’s team talks with the process experts at the manufacturer by phone once a week to report results and tweak the system. “We adjust aeration rates, and transition times,” he says, to get the best results. The staff has found that cycling the biosolids contents between the tanks at night lessens the chance of odor complaints.

A 1993 vintage Ashbrook belt press dewaters solids to about 18 percent, and the cake is hauled by city trucks to the local landfill, where it is used for cover. “Topsoil is very expensive these days,” says Quaethem, “so we’re saving the city money.”

All plant processes are monitored and controlled by a SCADA system supplied by ECC. “In the original design, we just had a monitoring function,” says Quaethem, who has experience with automation. “But we thought since this was a state-of-the-art process, we should have a SCADA system with full operational capability so we added it to the SCADA system in-house.”

Expandable

As Washington underwent the 2009 upgrade, plant management and the design engineering firm also kept an eye on future needs. As a result, the new facility is completely expandable to 6 mgd should the need arise. “We’ve made it easy to expand,” says Quaethem. “We can add a third clarifier if necessary; the splitter box is already set up for three basins. We also have room for two additional VLRs and one more RAS pump.”

The UV system can easily be expanded by adding additional modules. Says Quaethem, “We’re all plumbed up and ready to go.”

WATCH THEM IN ACTION

To learn more about Washington (Mo.) Wastewater Treatment Facility, view the video at www.tpo mag.com.



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