Efficiency Evolution

Energy audits and enthusiastic operators help the Worcester Water Filtration Plant reduce electricity bills through process efficiencies and solar power.
Efficiency Evolution
Ground-mounted (upper) and roof-mounted solar panels have a combined 126 kW capacity. They produce 150,000 kWh per year, or 5 to 10 percent of the plant’s power needs.

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"The best water quality at the most reasonable price" has been the mantra for Bob Hoyt and his staff since the 1997 opening of the drinking water treatment plant in Worcester, Mass.

Hoyt, plant manager, has led his team to be vigilant at identifying ways to improve efficiency. Through findings from two energy audits and the operators' efforts, the city saves more than $100,000 annually in electrical costs. The gains have come mostly from process optimization and solar energy sources. Electric rates have increased 42 percent in 15 years while the plant's yearly electricity cost has increased 11 percent.

"We're good operators trying to do the right thing for the city," Hoyt says. "Operators need to be aware that improvements are possible, and often these improvements can be accomplished with existing staff at a reasonable cost."

Treatment steps

Worcester, in the heart of Massachusetts, is home to 182,000. Once dominated by industry, the city is now mainly residential, with several colleges and professional companies. The treatment plant, in nearby Holden, averages 22 mgd but has 50 mgd capacity to enable growth. The extra capacity allows the city to sell water to three neighboring communities.

Water is drawn from 10 surface reservoirs in five towns across the region. The reservoirs, which hold more than 7 billion gallons, are at high elevation, so water flows to the plant by gravity.

The first treatment step is disinfection using four ozone generators. The original thought was to use pre-ozonation only in summer, but because of its water-quality benefits, the city began using ozone year-round. Next, aluminum sulfate coagulant and cationic polymer are added, in low dosages because the source water quality is high.

After flocculation, eight deep-bed filters layered with anthracite coal, sand and gravel with clay tile underdrains provide filtration at a rate of 8 gpm/sf. Lime is added to adjust pH, and a blended orthophosphate is added to inhibit lead and copper corrosion in the homeowners' copper pipes. Chlorine disinfectant is added before the water flows by gravity to two 2.75-million-gallon storage tanks.

Energy audit

In 2000, a free energy audit conducted by local utility Holden Municipal Light revealed several opportunities to reduce energy usage. First came some simple changes: hundreds of overhead incandescent light bulbs were switched to fluorescents, control panel bulbs were swapped out for LEDs, and motion detectors were installed to keep lights off in unoccupied rooms. The biggest impact came from changing the filter backwashing process.

The audit revealed that the plant incurred a peak electricity demand charge on machines running for more than 15 minutes, including the blowers used for filter backwashing. The operations team began a filtration surveillance program to reduce blower run times.

"The operators looked at the entire filtering process for efficiency opportunities," Hoyt says. "Were we backwashing too much? Were flow rates too high? Were we flushing too long? When the engineering firm set up the original backwash program, they set it conservatively so they knew it would work. But if it's pumping longer than it needs to, electricity is wasted."

For four years, the operators reviewed backwash parameters, monitored filter performance, and evaluated effluent quality. The team made gradual parameter changes, reviewed energy usage after each process change, confirmed reductions in energy usage, and tested water quality. At full optimization, the backwashing process dropped from 21 minutes to 9 minutes with no negative effect on water quality.

Another factor affecting backwash flow rates was water density. During winter, cold water is denser, requiring a lower backwash flow rate. Based on research provided by the AWWA and after testing by Hoyt's team, 50 percent of the flocculation motors were turned off in cold conditions. The rapid mixers for coagulation were reduced from high to low speed, and the run time of the air scour blower for backwashing was reduced from 15 minutes to 5 minutes. Backwashing optimization saves $40,000 annually on energy and an additional $12,000 from reducing peak demand charges.

Renewable energy audit

In 2007, the Massachusetts Department of Environmental Protection conducted an energy audit at Worcester (and several other plants) in search of ways to use renewable energy. The audit provided recommendations for solar and hydroelectric installations, as well as an HVAC improvement to recirculate heated air during winter.

When a $1.5 million grant from the American Recovery and Reinvestment Act of 2009 became available, Worcester implemented most of the recommendations; the hydroelectric project proved to be unfeasible.

In 2011, Nexamp installed 565 Kyocera roof-mounted and ground-mounted solar photovoltaic panels with combined 126 kW capacity, locating them carefully for optimal effectiveness and safety. "We considered a location in front of the plant but worried about vandalism because it was closer to the road," Hoyt says. "But we didn't want to go too far away, because some energy would be wasted as resistance in the line."

The solar array annually produces more than 150,000 kWh — 5 to 10 percent of the plant's power needs. It eliminates the equivalent of 113 tons of carbon dioxide emissions and reduces energy costs by $18,000 per year.

Staff buy-in

Worcester executed a multi-phased energy-efficiency program using its own operations team, consisting of 10 operators and seven maintenance and laboratory staff members, without hiring an outside contractor. To Hoyt, the key to success was getting operators involved at the beginning.

"I had to get their buy-in to say why this was a good thing," he says. "For good operators, if you give them more responsibility, and if they feel they are involved, they do a better job. It's more fun than just sitting back and letting things go, thinking, 'Well that's how the engineers set it up, so that's how we do it.' "


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