Overflow Control

A Massachusetts plant acts voluntarily to improve handling of CSOs while making other major changes to enhance treatment performance
Overflow Control
Robert Trombley checks data on the supervisory control and data acquisition (SCADA) system in the main office (GE iFIX).

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While stormwater can overwhelm treatmentsystems, a project to reduce CSOs didn’t interfere with operations at the Montague (Mass.) Water Pollution Control Facility. In fact, the plant’s management and staff took advantage of the construction to make other improvements.

While adding primary effluent disinfection and smoothing out storm surges from a hilly part of town, superintendent Robert Trombley and his staff added a much-needed biosolids dewatering system, replaced the roof on the operations building, installed a state-of-the-art SCADA system, replaced the bar screens in the headworks building, and made changes to the biological system that are saving the town hundreds of thousands of dollars a year.

Through it all, the Montague team kept the existing treatment facilities running smoothly. “Our operators are integral to our success,” says Trombley, a Navy and Massachusetts Air Guard veteran with 28 years of service to the clean-water profession. “They’re a proactive group with lots of good ideas.”

His team includes lead operator John Little, operators Tim Little, Eric Meals and Michael Little, and assistant lab technician/administrative assistant Tina Tyler.

 

Meeting the permit

Montague is located in hilly central Massachusetts. The original treatment facility dates to the early 1960s and provided primary treatment. An activated sludge secondary process was added in 1982, and the 2010 upgrade changed the process to extended aeration.

In addition to domestic wastewater, the plant receives industrial flows from a paper mill, a soybean processor, a fish farm, and others. The plant also receives Franklin County septage, which is pumped to the headworks via a diaphragm pump.

The treatment system starts with a Headworks Mahr Bar Screen and Screwpactor washer/compactor, and FMC aerated grit removal and grit washing (WSG & Solutions). After primary clarification, the wastewater is raised by Internalift screw pumps (Siemens) about 25 feet to the secondary process. Montague uses a tandem of rectangular aeration basins operating in parallel in the extended aeration mode and equipped with Sanitaire (Xylem) coarse-bubble diffusers. Torin blowers supply the air.

Treated water settles in two circular clarifiers powered by Falk (Rexnord) drives. Then the effluent is disinfected in a chlorination system (Fischer Porter, now Severn Trent) and discharged to the Connecticut River just downstream from its confluence with the Deerfield River. Chlorination is required April through October.

Trombley and his team have a portable BioTriad odor-control unit available for masking odors at various points around the plant if necessary. A GE iFIX SCADA system controls the new CSO treatment system and soon will be expanded to full plant coverage.

Biosolids wasted from the system pass to a storage tank and gravity thickener and then are mixed with polymer in a system powered by variable-frequency drive motors and mixers. On a new Fournier rotary press, the material dewaters to a cake averaging 30 to 45 percent solids. It is stored in a 40-cubic-yard container, and New England Organics hauls it to its Hawk Ridge composting center in Maine.

The plant meets a 30/30 permit for BOD and TSS with plenty of room to spare, and tracks nitrogen, although it does not have a nitrogen limit.

 

Handling CSOs

A challenge tougher than meeting the permit comes from the local topography and the combined sewers in an older section of town. These conditions contributed to frequent stormwater overflows into the Connecticut River. The plant staff stepped up voluntarily to correct the situation.

The $6.7 million CSO project, funded through sources including the state revolving loan fund, the state’s Tribal Assistance Grant Program, and the U.S. Department of Agriculture, involved major changes at the treatment plant and throughout the sewer system. During construction and startup of the new facilities, the plant continued to meet its permit and effectively serve the town’s 7,200 citizens.

To better manage sewer flow, the project modified three regulators by raising the weir height to control the volume passing through to the plant. Downstream of the portion of the system where combined sewers still exist, an 800-foot-long, 4-foot-diameter buffer line was installed to slow down storm flow before it passes to the treatment plant.

“During wet weather, as much as 45 percent of our flow might be stormwater,” Trombley says. The plant is rated for 1.83 mgd and sees average daily flows of 1 mgd. Peak flow capacity is 4.86 mgd. With the changes to the CSO regulators, more flow now comes to the plant, and less is discharged to the Connecticut River.

To reduce the risk of secondary treatment system washout, the plant is equipped with a chlorinated bypass system after primary treatment. The shortcut is activated when storm events push the hydraulic flow above 4 mgd. To improve removal of rags and debris, a new automatic bar screen replaced the previous manually operated unit.

Finally, the CSO reduction project called for a SCADA control system. The system has automated the overflow treatment process, and plans are to extend it to monitoring and control of the entire treatment plant in the future.

 

Work-arounds and improvements

How did Montague manage the project while continuing to treat wastewater? “Very carefully,” says Trombley. He credits his operators with managing the changes. “They stayed in contact with the contractors in the field on a daily basis,” he says. “They made sure the project didn’t interfere with our normal operations.”

The new heavy-duty automated bar screen was critical because, as the influent line was cleaned out, the plant had to deal with a freer flow and more debris and grit that used to simply accumulate in the line. To accommodate the improvement, Trombley’s team continued to operate the old manual bar screen during construction.

Solids handling was affected as well. The Montague plant lies on a long, narrow strip of land between a highway and the river, and the construction crowded the normal flow scheme. That, plus the need to sandblast and reseal the sludge storage tanks, forced the plant to route liquid biosolids around the sludge handling facilities, directly to 9,000-gallon tanker trucks that hauled the material away.

A spare storage tank came in handy as a way to store influent and equalize flow through the existing treatment system as the construction proceeded.

 

Smart financing

A thoughtful approach to financing enabled the plant to accomplish other improvements during the CSO reduction project. “We couldn’t have received funding for these improvements, but CSO reduction is sort of the hot item of the day,” says Trombley. “As we applied for funding for that project, we included requests for a number of other things we needed.”

The upgrades included a new roof on the operations building, the new bar screen, and — probably the most important addition — the rotary press for biosolids dewatering. “That alone added nearly $1 million to the project,” Trombley observes.

The project also brought significant changes to the secondary treatment system, saving the town at least $200,000 a year in operating costs. Lead operator John Little explains that after experimenting with the activated sludge system, the plant settled on alternately running one of the two basins in the aeration mode while shutting off the air in the other.

“We changed the whole process in order to meet budget constraints,” he says. In the new arrangement, operators run the air into just one basin for two to three hours and put the return activated sludge (RAS) into the other basin, where the air is turned off. “We run the RAS down to the mixing box and feed it through four entry points,” says Little. “That provides more carbon source for denitrification.

“When the oxidation reduction potential (ORP) gets into the hundreds, we switch and shut off the air in the other basin. It’s like a sequencing batch reactor (SBR), except we don’t have an SBR.” Automatic timers turn the airflow on and off.

Cutting nutrients

The operational changes have reduced effluent nitrogen, ammonia and phosphorus. BOD and TSS are so low they are sometimes hard to measure. In turn, chlorine usage has dropped. “We used to receive three or four 2,000-pound cylinders each year,” says Little. “Now we’re down to less than two.”

Power consumption has also been reduced. “For years we ran the blowers at 100 horsepower,” Little explains. “Now we can run at only 50 hp.” Even more savings result from holding more solids in the system. “Our sludge blankets have increased to two to three times the normal thickness,” says Trombley. “And we’re holding as much as 35,000 pounds of solids in the system, where before our sludge inventory was between 6,000 and 8,000 pounds.” That means fewer solids out of the plant and a reduction in dewatering and cake hauling costs.

“In the old days, our liquid sludge was trucked out and incinerated,” says Trombley. “We’d see eight to 12 truckloads a week, and maybe spend $375,000 a year for solids management. It’s much less now.”

 

Ambitious goals

In another innovation, Montague returns waste activated sludge to the head of the plant, where it is allowed to co-settle with primary solids. The result is increased cake solids content.

All in all, Montague has met the ambitious goals it set at the outset of the project. “We’re achieving our CSO reduction targets, even though we’ve never had so much flow through the plant,” says Trombley. “Our main goals are to prevent overflow of untreated water to the river, and to protect our secondary system from solids blowout. We’re at the mercy of Mother Nature.” He’s proud that the remodeled plant can handle the variations in storm intensity and duration.

Little expresses a degree of amazement when he thinks about what the team has accomplished in the last few years. “Experts said we couldn’t do this,” he says. “It’s not by the book. They said we were crazy, but it’s working. It’s just amazing.”



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