Driven by Invention

Manchester’s Lake Massabesic treatment plant thrives on innovation and has earned prestigious awards for outstanding drinking water.
Driven by Invention
Laboratory manager Cheryl Wood checks samples.

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The Lake Massabesic Water Treatment Plant in Manchester, N.H., recently achieved Phase 4 Excellence in Water Treatment recognition from the Partnership for Safe Water and was among the top five plants in the American Water Works Association's national "Best of the Best" taste competition last year.

But the award the facility really should get is for its innovative, homegrown sludge removal technique. "After 30 years, our old chain and flight mechanisms were essentially junk," says David Miller, water supply engineer with Manchester Water Works. "After we pulled them out, our operators had to wash down the sedimentation basins with a fire hose."

To reduce labor required for that approach, the crew carried the concept further, creating a spinning water blaster they call a Whirligig or Whirlie, and positioning the devices down the center of each basin to flush out solids. "It worked so well that we incorporated the design into the major plant upgrade we completed several years later," says Miller.

That's just one example of the way the plant team takes ownership of operations and resolves issues with minimal reliance on outside sources. Besides Miller, the plant leadership team includes director of waterworks Thomas Bowen, assistant director Robert Beaurivage, water supply administrator David Paris, watershed forester John O'Neil, and lab manager Cheryl Wood. With 11 experienced operators, they take care of a plant that outperforms regulatory requirements and even exceeds the expectations of the Partnership for Safe Water.

Plant renovation

The Whirlie invention was part of a major renovation of the plant in 2003-2006 designed to increase capacity from 40 mgd to 50 mgd, upgrade equipment, and minimize creation of disinfection byproducts in the 500-mile distribution system.

"We were getting higher than desirable levels of trihalomethanes," Miller recalls. "By implementing intermediate ozone as a primary disinfectant, and then adding sodium hypochlorite and ammonia to the water after filtration to form monochloramine, we now produce very high-quality water with very low disinfection byproduct in our system.

"We've been chlorinating since the 1920s. Now, however, a lot of our organics are removed before the final chlorination step, and our disinfection byproduct numbers are way down — almost eliminated." In fact, total trihalomethane levels are around 1.5 ppb, compared to a regulatory threshold of 80 ppb. Haloacetic acids have always been well under the 60 ppb compliance limit but now average 3.6 ppb.

The change in disinfection was just one of a number of changes involved in the $30 million upgrade, designed by CDM Smith. Major improvements included:

New bulk chemical storage facilities (Poly Processing Co)New chemical feed systems (Philadelphia Mixing Solutions)Monomedia filtration (AWI) to replace old automatic backwash filtersNew pumps (Peerless Pump Company, Flowserve Corp., Milton-Roy Company, Watson-Marlow Pumps Group)Particle counters (Hach Company)SCADA controls (GE Intelligent Platforms)

All power systems were replaced and a new laboratory and offices were built.

High-quality source

The plant gets its water from Lake Massabesic, a 2,500-acre spring- and stream-fed lake just east of Manchester. Lake water quality is protected by strict usage and watershed regulations, and the utility continues a program to acquire land and restrict lakeside development. "The water is very soft and has a turbidity of less than one NTU," says Miller.

A quartet of variable-speed pumps draw the water from the lake through a 60-inch high-density polyethylene pipeline. Stainless steel screens are mounted on the withdrawal end, which extends 430 feet from shore at a depth of about 15 feet.

Then, through a 48-inch reinforced concrete pipeline, water proceeds to the treatment plant, where soda ash is added to supplement alkalinity and boost pH before coagulation. In rapid-mix chambers, aluminum sulfate is added to initiate the coagulation-flocculation process.

From the rapid mixers, the flow passes to a set of four flocculation basins, each having two stages separated by a baffle wall, with a pair of mixers in each stage. The arrangement allows the plant to control mixing speed, which tapers to a slower rate as the flow moves to the end of the flocculation stage.

Four sedimentation basins are next, each 50 feet wide by 160 feet long. In the quiescent zone, flocculated solids settle to the bottom. On Monday of each week, one of the four basins is taken out of service, drained, and washed down with the Whirlies. "Once the chain and flight systems started coming out, our operators needed some way to remove the sludge," says Miller. "When the fire hoses proved to be too labor-intensive, they started thinking about how to improve the situation."

Cleaning innovation

The result was a series of wands positioned down the center of each of the four bays. From a six-inch header, a horizontal piece drops down and spins on a coupling, and water shoots out through nozzles on the end.

"All the wands start spinning around, and the waterjets break up the sludge into smaller pieces that get washed down the incline from the basin outlet end toward the sump at the inlet end," Miller says. The Whirlies run for about 10-15 minutes and are then shut off, leaving a clean floor behind.

The solids are dosed with a nonionic polymer to aid in dewatering and are pumped to one of four earth-lined sludge lagoons (a fifth lagoon is reserved for filter backwash water). Decanted water is routed back to the raw water pumping station.

After sedimentation, clarified water is pumped to the new ozone and filtration systems. Four ozone contact chambers operate in parallel. The chambers are brought online according to the volume of the flow. "Ozone is a powerful oxidant," says Miller. "It disinfects while removing color, taste and odor as well."

Ozone is generated on site by passing a high-voltage electric current across a dielectric discharge gap in a pure oxygen stream. "We have two 9,000-gallon liquid oxygen tanks that provide the oxygen gas to the ozone generators," says Miller. "Three 500-pound-per-day ozone generators (WEDECO – a xylem brand) produce the required ozone, which is injected into each of four ozone contact chambers through fine-bubble diffusers. The contact chambers provide the necessary time for completion of the ozone reaction."

Excess ozone gas that accumulates above the contact chambers is removed under vacuum through a thermal-catalytic ozone destruct process that vents to the atmosphere. The eight new filters, 720 square feet each, contain 6 feet of filter media. One filter train consists of four anthracite media filters; the other four contain granular activated carbon (Calgon Carbon Corporation).

The plant is running a comparison, trying to decide which media is the most cost-effective. "Anthracite has performed well but does not have the surface area of the carbon," Miller reports. "Because of the amount of oxygen in the water after ozonation, our GAC filters become very bioactive, and that improves their ability to remove organics and manganese. On the other hand, if we switch to carbon exclusively, we would need lots of it — about a million pounds." And that would be costly.

High and low

Next, the Lake Massabesic Plant adds sodium hypochlorite and ammonia, at a ratio of 4.0 to 1.0, forming monochloramine to satisfy the chlorine residual requirement while minimizing formation of disinfection byproducts and eliminating taste and odor. "Careful control of the chlorine-to-ammonia ratio is crucial. We don't want our water smelling like a swimming pool," says Miller.

The plant also adds fluoride for dental protection and phosphoric acid for corrosion control. From a 700,000-gallon clearwell, the treated water is pumped into the distribution system serving Manchester and parts of six surrounding communities. The city lies in the Merrimack River Valley with a dense downtown population at lower elevations near the river. These customers receive water pumped from the plant to a 20-million-gallon low-service reservoir at 367 feet elevation.

"Elevation increases farther from the river, and customers in the high-service pressure zone are served with water pumped from the plant to the high-service tank (8.8 million gallons) and high service reservoir (4 million gallons), both at 500 feet elevation," says Miller. Remote pumping stations fill tanks to serve customers at even higher elevations.

The water is of high quality, as reflected in the recent awards. "The Partnership for Safe Water standard is to produce water with turbidity of 0.1 NTU or less, 95 percent of the time," says Miller. "Our water is consistently at 0.06 or 0.07 NTU, and often as low as 0.02 or 0.03."

Trihalomethanes are no longer an issue, either. "Two things mitigate this," says Miller. "By switching to ozone as a primary disinfection step, we're delaying the addition of chlorine until after filtration, where fewer organics are present. Combine this with a conversion to chloramine secondary disinfection and we benefit from an even more dramatic reduction in disinfection byproduct formation."

The changes are helping the plant meet new U.S. EPA Stage 2 requirements for disinfection byproducts. "Previously, we sampled at four locations and averaged the readings," says Miller. "Beginning in April 2012, we began sampling at eight locations and now must report the average at each location, not an average of the entire system."

Operations at the center

The plant's quality performance would not be possible without the operation and maintenance staffs and a culture of helping one another and getting the job done. "This is a union shop, but our operators multi-task," says Miller. "We encourage them to gravitate toward their interests — plumbing, welding, carpentry, machining, whatever."

The team takes pride in getting things done in-house. "We don't rely on outside services," Miller says. "We do our own cleaning, electronics, SCADA system controls, electrical, plumbing, you name it. Our staff is encouraged to develop trade skills that facilitate a broad set of in-house capabilities. It helps build and maintain good morale."

That attitude was helpful during the three-year plant renovation project. "It seemed daunting at first, with all the new technology we were about to be responsible for operating and maintaining," Miller says. "But implementation was spread over three years, so we were able to get familiar with the new systems, one at a time."

The biggest adjustment was the new ozone system. A team of operators visited the manufacturer's site to witness a weeklong acceptance test and take factory training. The manufacturer also provided classroom training as part of the contract when the equipment arrived in Manchester. "You could see the looks of anxiety dissipate from operators' faces as the transition happened in a very controlled and systematic way," Miller reports.

Low turnover also helps: Only three of the plant's 11 operators have less than 10 years' experience. Finally, the team is frugal. "We watch expenditures closely," Miller says. "We serve about 160,000 folks and have more than 30,000 service connections. Our revenues and experienced personnel enable us to be self-sustaining. We're a very lean, well-run organization."


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