Garden Ready

An auto-thermophilic aerobic digestion process helps a popular resort community deliver Class A biosolids in keeping with the public’s high expectations.
Garden Ready
The Stowe team, shown in front of SBR tanks, includes Mark Stirewalt, plant operator; Jesse Wilkesman, assistant chief plant operator; Greg Lewis, superintendent; Gwyn Wilkins-Mandigo, lab technician; and Bryan Longe, chief plant operator.

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Stowe is a small Vermont town with A BIG name: It’s world renowned as a ski resort that has grown to be a year-round destination. That has turned out to be a good news/bad news story for the Stowe Wastewater Treatment Facility.

Since its wastewater comes entirely from homes and the hospitality industry, the plant avoids dealing with potentially toxic industrial waste. But Greg Lewis, facility superintendent, believes the community’s fame has put a spotlight on the town’s wastewater treatment and biosolids recycling programs.

A lesser-known municipality might be able to avoid that spotlight, Lewis says. And the attention, fed at times by misinformation about the biosolids process and the nature of the material itself, has produced its share of challenges.

Over the last decade and a half, Stowe has made a series of treatment plant improvements, allowing it to go from producing Class B to Class A biosolids. The new product has become so popular as a topsoil amendment for landscapers and gardeners that the community has not needed to apply the material to cropland since 2008. “We generate a beautiful Class A biosolids,” Lewis says. “The people who use it love it.”

Population fluctuation

Like many resort town treatment plants, the Stowe facility sees wastewater volume that varies with population. The year-round population is 4,000 to 5,000, but it can go up to 30,000 or 40,000 during the ski season and at other times of the year. Events like a lacrosse tournament that filled the community last July have turned the town into a four-season resort, and flows can fluctuate dramatically.

The plant was built in 1979 and expanded in 1985. “It was originally a 167,000 gpd plant and got permitted to go to 250,000 gpd,” Lewis says. At the time, its territory was primarily the village at the base of Stowe Mountain. The ski resort had its own very small treatment plant for the lodge along with septic systems, and properties along the mountainside on State Route 108 also had septic systems.

Over time, and despite some opposition, the plant’s territory was extended to the mountainside properties and to the resort. Until the early 2000s, the treatment plant used aerobic digesters to make Class B biosolids. Beginning in 2000, the plant underwent a major renovation that expanded its design capacity to 1 mgd and completely revamped the treatment process.

Complete rethinking

A driving factor was to produce pathogen-free Class A biosolids. Stowe already had a fairly clean material, but given that its raw wastewater was essentially free of toxics and heavy metals, “We felt if anybody should generate a Class A biosolids, it should be us,” says Lewis. “The state wanted us to go to Class A, too.”

The project allowed the Stowe team to rethink almost every aspect of wastewater treatment.

The plant installed four Jet Tech sequencing batch reactor (SBR) tanks made by US Filter (now part of Evoqua Water Technologies [formerly Siemens Water Technologies]). “That gives us flexibility to bring the tanks on and off to try and match our flows,” Lewis explains. “We’re limited in space here. SBRs are a combination aeration and settling tank in one.”

Another benefit is that the SBR process “has a good organic uptake of phosphorus,” Lewis says. That allows the plant to reduce chemical treatments for phosphorus removal and meet its permit discharge limit of 0.2 mg/L in the environmentally sensitive Lake Champlain drainage basin.

The facility also uses ClariCone clarifiers (CB&I), which Lewis likens to “a clarifier for phosphorus removal with no moving parts.” He credits the choice with saving $20,000 a year or more on energy. The old clarifiers were converted to blending tanks for sludge.

The plant also replaced its tertiary sand filters with AquaDisk cloth filters (Aqua-Aerobic Systems). “If you have an upset and you get solids into your filters, within half an hour you can take a cloth filter offline, clean it up and get it back online,” Lewis says. “If you’ve got a sand filter, you can have that filter offline for days just for that reason alone.”

At the time renovations were being planned, Lewis says, cloth filters were used mainly in drinking water treatment. Impressed with their convenience, he persuaded one manufacturer to consider restructuring its pricing and market them to the wastewater segment.

For disinfection, Stowe originally used chlorine gas but switched to less hazardous liquid chlorine. The 2000 renovation made a complete break with chemical disinfection, installing an Aquaray two-channel UV system (Ozonia North America). Says Lewis, “We just felt getting away from chemicals was the right thing to do.”

The plant recently replaced the unit’s twin control panels with a single panel that has an alarm system and better controls, and integrates with the facility’s computer system. “We’re using far less energy and we can better control our UV bulbs,” Lewis notes. “It was an investment that will probably pay for itself within two years.”

Solids side

Activated sludge wasted from the SBR process contains about 0.8 percent solids — thinner in summer and somewhat heavier in winter. That material is boosted to 4.5 to 5.5 percent solids on a dual-drum gravity belt thickener (BDP Industries).

For biosolids processing, the plant installed two auto-thermophilic aerobic digesters (ATAD), also from Evoqua. The ATAD process uses the metabolic heat released by microorganisms during digestion to kill pathogens in the material.

Lewis acknowledges there was a learning curve in adjusting to the new equipment. It took a few years to work the kinks out of the system, during which the plant reverted to its former aerobic digestion process to generate Class B biosolids. Since 2008, the ATAD process has been reliably delivering Class A material.

With two ATAD units available, the plant is prepared for fluctuating wastewater flows. Biosolids are typically fed into ATAD Reactor 1, but not often actually treated there. “We use that as a quasi storage,” Lewis explains. “We do most of our reacting in ATAD Reactor 2. We can do that because our flows really aren’t all that high.” They range from 0.4 to 0.45 mgd in summer to 0.6 to 0.65 in winter.

“We feed Reactor 1 daily,” Lewis says. “About once a week we take material out of Reactor 2 to post-ATAD storage.” The material is fed into the ATAD system at about 5 percent solids. The ATADs reduce the volume by about 50 percent. Then the material is aerated and cooled down. Ammonia released in the process is vented through chemical and carbon scrubbers.

The digested material is then sent to a Sharpless dewatering centrifuge (Alfa Laval). The centrifuge runs about twice a week in lower-volume periods, a little more often when volume goes up. The machine typically produces material at 18-25 percent solids, about 22 percent on average. The centrate from the centrifuge is cycled back to the SBRs. Dewatered biosolids are stored in a concrete bunker until they can be tested and distributed to users.

The biggest challenge to the biosolids process has been dealing with grease from the community’s many restaurants. Food businesses are required to use and maintain grease traps, and most comply. “Where it’s a problem is when we have a septage hauler who pumps out a grease trap and tries to mix it with a septic tank load and sneak it by us,” Lewis says. That’s rare, but it does happen. Problems with some septage haulers led the facility to raise its rates for them and, as a result, fewer haulers now bring septage.

The ATAD system requires maintaining digester temperatures between 50 and 60 degrees C. “The problem with grease is it’s just too high a food source for an ATAD,” says Lewis. “It kicks into overload and you can’t control the temperature.” When the material heats up excessively, it causes odor problems. Lewis hopes at some point to retrofit the system with cooling coils, but for now, “We’re not set up to do that.”

Facing controversy

In 2013, Stowe’s land application permit was up for a routine renewal by the Vermont Agency of Natural Resources. The renewal was granted, but not without a flurry of opposition from environmental activists. “It got pretty personal in the hearings,” Lewis says. “I was called a polluter, a liar and a cheat.” One outside critic reportedly complained to local officials that the town should have digesters — not knowing it already had them.

Lewis chalks it up to “the nature of the community” — Stowe’s national high profile, he believes, draws outside opposition from organizations. In any case, the renewal was mainly a backstop. Although Stowe has a designated, town-owned farm where it can apply Class B biosolids, the community hasn’t produced any since 2008. “Hopefully I’ll never use it again,” Lewis says.

Stowe will go on producing its Class A material as topsoil and landscaping material. Lewis boasts that unlike cow manure, the Stowe product does not contain weed seeds: The digestion process kills them. This means gardeners don’t have to worry about seeding their flower and vegetable beds with weeds.

“It grows beautiful grass,” Lewis says. “There’s no weeds or anything growing up through it.”  

More Information

Alfa Laval - 866/253-2528 - www.alfalaval.us

Aqua-Aerobic Systems, Inc. - 800/940-5008 - www.aqua-aerobic.com

BDP Industries, Inc. - 518/527-5417 - www.bdpindustries.com

CB&I - 832/513-1000 - www.cbi.com

Ozonia North America, LLC - 201/676-2525 - www.ozonia.com

Siemens Water Technologies Corp. - 866/926-8420 - www.water.siemens.com



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