Challenges Abound In Effort To Capture Heat And Energy From Digestion

A Massachusetts plant team takes heroic measures to integrate a new biogas-fueled CHP system with an older treatment process.
Challenges Abound In Effort To Capture Heat And Energy From Digestion
The installation includes two fixed-cover Aquastore digester tanks (CST Industries) and dual membrane DuoSphere container (WesTech Engineering).

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Hundreds of pounds of solids slipped downstream through worn equipment at the Fairhaven (Massachusetts) Water Pollution Control Facility. The volume made it difficult for the plant to meet its discharge permit.

Looking for solutions, the town took part in a pilot study on anaerobic digestion and combined heat and power (CHP) systems. “It concluded that although we are a small plant, the process could be cost-effective if we used glass-fused-to-steel tanks instead of concrete structures,” says Linda Schick, sewer and wastewater superintendent.

The $8 million project was financed by a principal-forgiveness loan through the American Recovery and Reinvestment Act (federal stimulus) and the State Revolving Fund Green Infrastructure. “We had two years to deploy the system or face repaying the money with 2 percent interest,” says Schick.

Challenges accompanied the integration of new technology with 28-year-old equipment, requiring operators to be constantly on call at first. “We almost didn’t make it,” says Schick. “Besides the generation gap, small-scale digestion is most successful on farms, where there are no other processes to integrate or sidestreams to absorb.” The team’s two-year effort made Fairhaven the second wastewater facility in the state to generate its own heat and energy.

Setting the table

Built in 1969 and upgraded in 1989, the 5 mgd (design) activated sludge facility averages 3 mgd. Three 1987 plunger pumps on timers (Komline-Sanderson) feed primary sludge to two fixed-cover Aquastore digestion tanks (CST Industries). Two chopper pumps (Vaughan Co.) circulate solids through mixing nozzles in the tanks. Two other pumps move material through the heat exchangers to maintain a constant 98 degrees F in the digester.

A gas conditioner removes water from the gas before it enters the MAN 100 kW CHP generator (Kraft Power Corporation), which runs on natural gas, biogas or a mix. Excess biogas is stored in a dual-membrane DuoSphere containment (WesTech Engineering) or flared through a burner stack (Varec Biogas, a division of Westech Industrial).

Digested biosolids are treated with ferric chloride, then dewatered in two 1-meter gravity belt thickeners (Komline-Sanderson). The resulting cake is incinerated by a vendor.

Into the fray

Plant designers were present during the CHP system’s startup in September 2012, but Tom Bienkiewicz, director of the Massachusetts State Board of Certification of Operators of Wastewater Treatment Facilities, was pivotal in providing technical and moral support.

“He spent a lot of time and energy explaining to state Department of Environmental Protection officials why it was taking so long to get the process going,” she says. “Tom was our moral support. We even had his personal number, and he’d talk us through problems no matter when we called.”

Most repairs involved shutting down the digester, then starting up again from square one — a time-consuming process. It began the first day of startup when solids in the feed system never reached the digester. Increasing pressure burst an underground pipe, sending the material bubbling to the surface.

After investigating for weeks, Bob Gomes, CHP operator, Joe Frates, plant mechanic, and Ray Paczosa, plant electrician, pinpointed the cause. Rags had clogged a static mixer on the feed line in the CHP building, creating back-pressure on the line. Once they removed the mixer, solids flowed to the digester. They also retrofitted the plunger pumps, rerouted piping and installed new timers.

“Bob came to the plant every weekend to check on the digester,” says Schick. “Without his commitment, I doubt we would have been as successful. While others lost faith, Bob always rose to the challenge.”

Severe indigestion

More challenges were in store. A miscalculation placed the sludge level too close to the top of the digester tanks, causing violent agitation. The burping, foaming material infiltrated and then blocked the output lines. Seeking an escape route, the material entered the gas piping, forcing operators to open pressure release valves instead of flaring the gas.

“After we reduced the sludge level by 10 to 15 percent, Joe Frates installed drainlines on the gas piping, enabling operators to flush the system,” says Schick. “Assistant superintendent Rene Robillard was the lead on purging the entire gas system, beginning at the top of the digester and working back to the CHP generator.” The fire department stood by to assist in case of an emergency.

The struggle to meet the startup deadline heightened the operators’ stress. After learning new techniques, they had to relearn them as components and processes were re-engineered from within. Schick’s team spent two years modifying piping, adding chemical feed sites and reformulating how the facility wasted sludge.

While the adjustments benefited the digester, they often were not conducive to the main plant’s treatment train. Wastewater usually entered the primary settling tanks at about 50 degrees F, but mixing it with hot solids from the digester’s return sidestream adversely affected the process.

“We’re trying to balance how we optimize the digester to get the profit we were hoping for, but also minimize the effect it has on the other side of the plant,” says Vincent Furtado, Board of Public Works superintendent. The successful marriage of new with old was hard-won. “The town had budgeted $500,000 for startup adjustments, but they cost an additional $1.5 million,” says Schick.

Evaluating performance

Because the system is still in its infancy, projected savings are moving targets. The process generates only enough biogas to heat the digester tanks and power pumps in the digester building. To increase methane production and reduce purchased gas from a third to less than a quarter, the town is considering adding restaurant grease to supercharge the system.

Electric rates doubling in March also moved the number. “My normal electricity budget is $300,000,” says Schick. “I’m probably realizing a 20 percent savings from the CHP system and solar panels on three buildings after deducting maintenance and other costs.”

One solid benefit is the 50 percent reduction in biosolids sent for incineration. “We had 10 to 12 trucks hauling it away every week,” says Schick. “That’s down to four or five, reducing hauling costs by $130,000 last year.”  


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