Creating a Resource

By Diane Gow McDilda

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Biosolids produced at the Moccasin Bend Wastewater Treatment Plant weren’t always considered a resource. Before improvements to the plant in 2005, all biosolids were shipped to the local landfill. Renovations moved the solids closer to a Class A product.

“I call it a Class A-minus/B-plus,” says Jerry Stewart, director of the waste resources division of the plant, in Chattanooga, Tenn. “After the filter press is optimized, we won’t necessarily go through EPA certification for Class A. If we do, we’ll still handle it as a Class B.”

Previously, the plant handled solids from two 65-foot-diameter anaerobic digesters using two 90-foot-diameter solids thickeners. The digested material was blended with waste-activated sludge, and the mixture was dewatered using plate-and-frame filter presses or centrifuges. The final product was then taken to a landfill owned by the Chattanooga Public Works, which also owns the treatment plant.

Changes in the treatment train now deliver a better product that is beneficially reused by land application to cropland on farms in Tennessee and Alabama. “The renovation will never pay for itself, but we no longer have to put the material in the landfill,” says Stewart. “Ninety-eight tons is one year’s space in the landfill and that’s worth $2.7 million.”

Using the old

Renovations to the system made use of existing equipment to the extent possible. The two thickeners, installed as part of the original plant, were refurbished with new piping, and a protective coating was applied to the interior walls. The existing digesters were also refitted with new equipment that modified the digestion process.

“Infilco Degremont is the designer and provider of the entire set of equipment that makes up the thermophilic-mesophilic digestion process, called TPAD for temperature-phased anaerobic digestion,” says Stewart. “The process and its controls consist of Cleaver-Brooks boilers, a Cannon Mixer system (Infilco Degremont Inc.), fixed and floating digester covers, heat exchangers, Amot valves, and other components.”

As part of the renovation, digester gas compressors and associated piping are now housed in a building on top of the digester building. This eliminated the need to meet explosion-proof electrical requirements during design and construction. Six digesters now operate as thermophilic (135 degrees F) or mesophilic (95 degrees F) digesters in two trains and destroy 50 to 70 percent of the volatile solids.

“After spending five days in thermophilic digester number 1, the material is transferred into mesophilic digesters 3 and 5,” says Stewart. “The material that spends five days in thermophilic digester number 2 is transferred into mesophilic digesters 4 and 6. After about 10 days in the mesophilic digesters, the material goes to the storage tank, where it is mixed with the thickened waste activated sludge and remaining primary sludge. Then it is sent to be dewatered by the centrifuges.”

The storage tank holds 0.5 million gallons and was installed as part of the upgrade. From the storage tank, the mixture is pumped to one of the centrifuges.

“All material from the storage tank goes into any of the centrifuges,” says Stewart. “The solids supervisor decides which ones are available for the day and how many are needed, based on the daily flow rates and volumes. We keep a sufficient number of centrifuges available, between maintenance requirements, to handle the necessary volume.”

Two high-G Flottweg centrifuges, each rated at 60 dry tons per day, deliver biosolids cake at about 23 percent solids. Dewatered biosolids from the high-G centrifuges are stabilized using limekiln dust, bringing the solids content up to 30 to 32 percent. The plant also uses two low-G Kruger centrifuges that produce 50 dry tons per day at 18 percent solids. When treated with limekiln dust, the solids content increases to 24 to 28 percent.

Operators are flexible and have their own way of expressing what equipment they’re running. “Operators have their own language,” says Stewart. “They say they’re either running pudding or running cake.”

Farm and zoo

Ultimately, the plant produces more than 30,000 dry tons of biosolids a year. The material is spread over 4,000 acres in Tennessee and 2,800 acres in Alabama. Private contractor Recyc LLC, of Boaz, Ala., handles hauling and land application.

After dewatering, the biosolids are stockpiled on site on a concrete pad storage area. Solids are normally stored one to two days, and older material is taken off site first. The trucks are weighed before loading and again before leaving. Front-end loaders move the solids to the trucks. Every truck is tarped and hosed down before making the drive to the farmland where the nutrients will support crops.

Accounting for the nitrogen, phosphorus, potassium, and ag lime, the plant staff estimated that in 2008, the nutrient value of the biosolids was $3.7 million. The material is surface applied, not tilled in. “Farmers grow hay and pasture grass,” Stewart says. “Once they cut the hay off, then they let the cows graze. We have some farmers that raise corn for silage.”

The local community was skeptical about the safety of biosolids because they were transported not just off the treatment plant property, but out of the county, and even out of the state. To foster public confidence, the staff cordoned off a 15-acre plot on treatment plant property that is now farmed for hay and uses biosolids.

“We have two cuttings a year and get about 5,200 bales,” says Stewart. “It’s used as part of public works construction projects, or we’ll run it out to the landfill, and they’ll use it for bank stabilization.”­

Another beneficiary of the crops grown on the treatment plant’s property is the Chattanooga Zoo. After a fire in late 2006, Moccasin Bend offered the zoo hay, free of charge, to help feed the animals. The donations have continued, to the apparent pleasure of the camels who eat the biosolids-enriched hay.

Toward Class A

Meanwhile, more improvements are in store for Moccasin Bend biosolids. Six J-Vap hot-water vacuum filter presses (Siemens) were installed as part of the upgrade and are being re-tested to make sure they meet performance requirements for dewatering and drying waste activated sludge.

“They are not yet in full-time use, but we expect them to be placed into full-time service soon,” notes Stewart. “They will supplement production of biosolids that is now being fulfilled by the centrifuges. They will produce a Class A material that has been through a pasteurization process. That makes many more biosolids reuse options available to us.”

If that plays out successfully, another option for final disposition of the material will be to grind it and distribute it for use as a soil amendment. The city Parks and Recreation Department has expressed interest, as have municipal contractors. While Stewart appreciates the time needed to tweak the system, he admits, “I’m looking forward to getting through the performance tests.”

He is also working with a polymer contractor who suggests different polymers to increase the solids content. But he has seen too often that success at one plant doesn’t mean the same results at another. “We look at everybody’s polymers and have seen plants that use rotary drum mixers and run sludge with polymers,” he says. “But what works in Chattanooga may not work in Knoxville or Atlanta.”

The plant renovation also included a heat recovery and transfer system. Two boilers (8.3 MMBtu each) run on digester gas, or natural gas if needed, to heat the biosolids to temperatures adequate to destroy pathogens. Six solid-to-water heat exchangers capture heat entrapped in the solids going to the mesophilic digesters and use it to pre-heat solids entering the thermophilic digesters.

Expertise and enthusiasm

It’s the drive to innovate that keeps Stewart looking to the future of biosolids. After seeing an announcement in the Mississippi State Alumnus magazine, he contacted his alma mater asking to be involved in a study the university was conducting on creating biofuel from biosolids. The same drive leads him to keep up with an anti-biosolids blogger, just to know what’s being said.

Stewart understands the opposition to land applying biosolids, and that’s why he and his team work to exceed, not just meet, regulatory requirements. When it comes to negative press coverage, Stewart believes it’s sometimes warranted. “Either they’re not applying it right, or they’re applying more than they’re supposed to or where they shouldn’t be applying it,” he says.

As part of its desire to bring integrity to the Moccasin Bend biosolids program, the plant joined the National Biosolids Partnership (NBP) in August 2005 and began work on an Environmental Management System. In February 2009, the plant became the 23rd agency in the U.S. and the first in Tennessee to receive NBP certification. To celebrate, representatives from NBP, the Water Environment Federation, and National Association of Clean Water Agencies were invited to the plant. An enthusiastic group hoisted a flag in honor of the event.

But Stewart understands that a good reputation is only as good as the next act and he intends to continue running a tight ship. “One ‘aw shucks’ wipes out 100 ‘atta-boys,’” he says. With a chuckle, he admits he doesn’t always use the word “shucks.”


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