For a facility serving a population of 3,300, the City of West Liberty (Iowa) Wastewater Treatment Plant handles large flows and produces a high biosolids volume. That's because a major turkey processor accounts for nearly 60 percent of the plant's flow and by itself contributes some 3,400 pounds of BOD per day.

That organic content from West Liberty Foods, processor of about 23,000 turkeys per weekday, boosts biosolids production to 310 dry tons per year. Producing that material and moving it to local farms has been a challenge at times, but the plant team has met it successfully, on the way winning the 2011 Biosolids Award for Exemplary Management Practices among small plants from the Iowa Water Environment Association.

David Clark, project manager and area manager for CH2M HILL OMI, which operates the plant and the biosolids program, leads a team that distributes material in both liquid and solid forms to about 450 acres of corn, soybean and hay ground per year.

It's a simple yet effective program that since 1991 has applied sound management principles to deliver a quality Class B product that helps farmers while operating free of public complaints.

Long partnership

West Liberty is a rural community about 20 miles southeast of Iowa City. For 22 years, the city has run its treatment plant under a public-private partnership with CH2M HILL OMI, the operations arm of the CH2M HILL consulting engineering firm. "We're in the second year of a 10-year contract, which means we'll be around for at least another eight years," says Clark.

The plant is designed for 2.16 mgd and sees average flows of 1.38 mgd – equivalent to flow from a residential population of about 24,000. Wet-weather flows were a substantial challenge until the 2009 addition of an 800,000-gallon stormwater tank.

"Before that tank was installed, we had problems with the sludge blankets in the clarifiers getting out of control," says Clark. "High-flow rain events would exceed the clarifiers' hydraulic and solids loading capacity. Now, with the addition of the stormwater tank, we are able to ensure better process control."

Today, flows within the plant's design parameters go directly into the headworks; any excess flow is diverted automatically to the stormwater tank. When the high flow subsides, the tank automatically drains to the headworks.

The influent first passes through a fine screen (Lakeside Equipment Corp.) that removes particles 0.25 inch or larger. Flow then passes into two aerated grit tanks where grit is removed by a WEMCO Hydrogritter system. Two screw pumps (Lakeside) then convey the liquid to a Carrousel oxidation ditch (Ovivo). After settling in three final clarifiers, the effluent is discharged to Waspinonoc Creek (the plant permit requires no disinfection).

The solids side

Sludge wasted from the oxidation ditch is pumped to six aerobic digesters (580,000 gallons total capacity) that achieve 52 percent volatile solids reduction in a 30-day detention time. The digested material at about 1 percent solids is dosed with polymer and pumped to a 2.5-meter gravity belt thickener (PHOENIX Process Equipment Co.) in a process that boosts the solids content to as high as 4 percent.

The dewatered biosolids then go into two concrete storage tanks: a 275,000-gallon unit with coarse-bubble aeration and a 500,000-gallon unit with a jet mixing system. When those tanks fill to capacity between land application seasons, material is stored in a 1-million-gallon geotextile tube on a concrete pad on the plant property.

The two storage methods mean the city actually operates two land application programs – one for liquid material and one for solid material from the geotextile tubes, which dewater to 8 to 10 percent solids. Plant personnel operate the liquid application program, and a local contractor handles the solid program.

Into the fields

Overseeing the entire program is biosolids coordinator Ken Riley, whose team also includes Brian Goldesberry and Chris Gerstbrein, both equipment operators II, and Craig Juergens, lead operator/lab analyst.

"We like to stay within a 10-mile radius of the plant to keep our fuel costs and labor costs down," says Riley. "We have never charged for the material. We have more farmers than we need, and the only marketing is word of mouth, farmer to farmer."

The team hauls liquid biosolids to farms using two semi-tractors pulling 7,200-gallon-capacity trailers (typically carrying 6,000 gallons per load). At the farms, a TerraGator applicator (Ag-Chem Equipment) with a 2,000-gallon tank applies the material to the soil surface using pressurized discharge over a deflector plate.

"We typically get three TerraGator loads out of each trailer load," says Riley. "By the time one trailer is empty, the next truck will be pulling in, and the other one goes back for another load."

In 2011, the team land-applied 218 dry tons of liquid biosolids covering 414 acres at an average application rate of 0.53 dry tons per acre. The typical solids content was 2.3 percent. At that rate, farmers received plant-available nutrients in amounts of:

25 pounds per acre nitrogen26 pounds per acre phosphorus14 pounds per acre potassium

"At those rates we're not meeting the needs of the crop – the farmers have to supplement with other fertilizers," says Riley. "The reason we don't apply at agronomic rates is that with the liquid product, with its low solids content, we would have to cover the field multiple times, and that raises the issue of soil compaction with the farmers.

"We apply the material in spring and fall (and to a limited extent in summer on hay land). About 70 percent is applied in fall. The issue we have in springtime is that when we want to get out into the fields, so do the farmers," he says. "The window of opportunity is very limited. Typically, we don't get all the material out in spring that we would like to, and that's where the geotextile bags come in."

More concentration

The city contracts with Dvorak Farms for application of the solid material from the tubes. "When we get all our permanent storage filled up, we use the bags to get us through until fall," says Riley. "Typically, the bags will sit anywhere from four to eight weeks before they get cut open and land-applied. If we fill up a bag early in the year, say in May or June, it's going to sit until probably September.

"We work with our contractor to coordinate that. He provides all the labor and equipment, and he also owns one of the main sites where we land-apply. He keeps fields available for us. When the material is ready to be land applied, we use a utility knife to cut a hole in the bag, and the contractor starts removing material. As we need more material, we work our way down along the bag.

"The contractor uses a front-end loader and loads the material into a side-dump semi-trailer that has a cover on it to prevent spillage. He takes it out to the application site and dumps it there, and then uses a manure spreader to apply it to the fields."

Clark notes that biosolids from the geotextile tubes are applied agronomically. "In 2011, we land-applied 120 dry tons agronomically over 45 acres," he says. "The typical application rate was 2.67 dry tons/acre, and at that rate the crops received 110 pounds per acre nitrogen, 115 pounds per acre phosphorus, and 60 pounds per acre potassium." The typical solids concentration of the material from the geotextile tubes was 8.3 percent.

Riley observes, "We're unique in the use of the geotextile tubes. We are among very few facilities in the state that use them on a regular basis. When we land-apply the thicker material, there's a cost savings in hauling 10 percent solids versus around 3 to 4 percent. Because we contract for that service with a local farmer, the money we spend stays in the local economy."

Keeping close tabs

The West Liberty team keeps meticulous records on its program and does regular sampling of both the farm field soils and the product itself.

"On the biosolids side, every quarter we get a sample analyzed for nutrients and heavy metals," says Clark. "Also once a quarter, we get a sample tested for fecal coliform to make sure it meets the requirements for Class B material. On the soils side, we take a core sample from each site and test it to make sure the soil has the proper pH."

If the farmer requests it, the team will order an agronomy report on the soil. "We send a sample away to a lab, and the results tell us if the soil is low on lime, or anything else that the ground needs to be productive," Riley notes. "It helps the farmers determine what their ground needs. It's a service we perform gratis as part of operating a quality program, and it makes the farmers very happy."

Each year, as required, the team prepares a complete annual report on the program that describes the program history, lists all farmers receiving material, provides the sampling schedules, tells how land requirements were determined, describes application methods and site management practices, and more. A copy of the report goes to the Iowa Department of Natural Resources and the regional office of the U.S. EPA.

Running smart

The team keeps a close eye on its internal functions, too. Riley and his team perform all basic maintenance (such as oil and filter changes) on the trucks and the TerraGator unit. "It's all set up in our computerized maintenance management system," says Riley. "It automatically prints out work orders for required maintenance on the vehicles."

Always looking to improve, the team plans to solicit bids to replace the TerraGator unit, which has performed well but is 20 years old. A longer-term possibility being explored is replacement of the gravity belt thickener with a belt filter press or rotary fan press, which would yield a higher-solids cake product.

Meanwhile, the treatment plant performs effectively despite the high loadings from the turkey processor. Clark notes that West Liberty Foods has a pretreatment system that includes four dissolved air flotation (DAF) units for removal of TSS and fats, oils and grease. Wastewater from the turkey operation contains about 500 mg/L BOD, and the total influent to the plant averages about 450 mg/L BOD.

Nonetheless, says Clark, "In our effluent that goes out into the creek, the BOD is about 1 mg/L, TSS is about 2 mg/L, and ammonia is essentially zero. All that loading coming in is being converted to sludge, and our issue is keeping up with production."

Farmers, the citizens of West Liberty, and the award judges at the Iowa WEA all would attest that Clark and his team are keeping up just fine.