For decades the Montague Water Pollution Control Facility received pulp and paper waste from two mills, and some parts of a 2010 upgrade were tailored around their flows.

Both mills ceased operations, and the largest (70,000 gpd) closing abruptly in August 2017. Almost overnight, sludge consistency changed to a mix of residential, industrial and septage waste streams.

“Before we lost the mills, our new four-channel rotary press dewatered material to 30-45% solids,” says Chelsey Little, plant superintendent and pretreatment coordinator for the facility in Northwest Massachusetts. “Now we were seeing cake at 15% solids and couldn’t keep up, even with our contract hauler making weekly trips to Rhode Island.”

Consequently, TSS in the treated wastewater accumulated until the plant was running at 60,000 total system pounds. The backlog of TSS short-circuited into the effluent, triggered monthly permit violations, and led to U.S. EPA administrative orders.

Seeking solutions, Little worked with Russell Resources, a vendor that arranged for two manufacturers to demonstrate their spiral screw dewatering presses for a week. Both produced cake at 18% solids. Little put the project out to bid, and PWTech (Process Wastewater Technologies) won it.

Within a month, the ES-302 two-channel Volute dewatering press (AMCON) reduced backup loadings to 9,000 total system pounds and restored the plant to compliance.

Setting the stage

The 1.83 mgd (design) activated sludge plant averages 780,000 gpd, of which 150,000 gpd is industrial process wastewater. Some 10% of the collection system consists of combined sewers.

Installing the Volute (meaning spiral) dewatering press fell to Little’s team, which includes Tim Little, foreman and operations manager, and operators Tim Puera, Adam Kleeberg and Samuel Stevens. The unit’s 149-by-52-inch footprint (64 inches high) enabled the crew to put it next to the previous press, which they retained for dewatering primary sludge.

Over two months, the operators poured the concrete pad and trenched in the drainage. Kleeberg, a licensed plumber, oversaw plumbing the return activated sludge feed and flow meter, while Stevens supervised running the conduit and pulling the wires. Outside control engineers installed and connected the control panel. Then the press arrived in two crates.

Although specifications came with the press and PWTech provided a checklist of what to do, the plant staff had to figure out how to put it together. In addition, they couldn’t test what they were doing. “Only a PWTech representative could activate the press after inspecting the installation, so we didn’t even know if the machine would turn on and run,” Little says. It did both, in early January 2022.

How it works

The press thickens and dewaters in a single, compact operation within the dewatering drum. Inside the drum, rows of tie rods hold spacers and fixed rings in position. Alternating between these rows are rows of moving rings that rotate as the screw turns.

The moving rings are slightly smaller than the outer diameter of the screw and slightly narrower than the spacers, enabling them to clean the fine gaps between all the rings. The moving rings also cut into the cake to release more moisture.

Operators can pull feed sludge directly from the 37,700-gallon gravity thickener, from the return or waste activated sludge lines, and from two 10,000-gallon mixing tanks. “We’re still fine-tuning the mix of primary and secondary sludge,” Little says. “The primary has coarse particles, but the waste activated is thin with fine particles. The occasional cake at 40% solids is too dry and binds up the press, so we’re still looking for that sweet spot.” 

Management options

Before installation of the Volute press, material at a sloppy 15% solids limited management options and made it difficult to secure hauling contracts.

“Most disposal facilities will only accept cake at 18% solids or higher, and we had no time to establish a track record on which vendors could bid,” Little says. “Consequently, cake is transported once every two weeks through three states to Canada at an annual fee of $368,000. But because the percent solids is now higher, the cost almost evens out with what we paid the previous hauler.”

Little also is exploring on-site composting. During the paper mill days, the plant superintendent tried making Class A biosolids, but the watery cake was difficult to compost and he abandoned the idea. Recently, Little ordered a feasibility study based on opening a regional composting facility.

“I sized the press for just that purpose,” she says. “My goal is to dewater our sludge and liquid sludge from neighboring facilities, then compost the cake.” 

Reaping the rewards

Initially, Montague operators ran the rotary press low and slow for 10 hours to optimize dewatering. “They constantly adjusted the pressure based on the feed solids and always worked overtime,” Little says. “The fully automated, self-cleaning Volute dewaters the same volume in two hours.”

Her team made the project affordable, easy, and successful — doubly important because the utility is an enterprise fund (customers fund the budget). Little estimates that keeping the installation work in-house saved more than $100,000.

There also were energy savings, as the previous press has a drive train powered by a 5 hp motor, and the Volute unit has twin motors using a combined 1.9 hp. “We’ve reduced overtime as well because the rotary press isn’t run too often,” Little says.

In April, the plant averaged 26% cake solids with a low of 18% and a high of 28%. “At 18% solids, our return on investment is 8.6 years,” Little says. “The payoff with 26% solids is 3.5 years, giving us an annual cost saving of $74,957.” Little anticipates even better results once operators find the sweet spot for the mixed sludges.