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Series of upgrades & process optimization drastically improve water quality, operations
In 2004, when the sustainability movement was still in its infancy, Pennichuck Water Works (PWW) engaged Fay, Spofford & Thorndike (FST) to conduct a comprehensive evaluation of the company’s water treatment plant (WTP). Serving the city of Nashua and 10 surrounding communities in southern New Hampshire, PWW is the state’s largest private utility, handling a population of more than 110,000. The project included assessing the circa-1978 plant’s capability of producing a consistent, high-quality finished water to meet projected maximum demands of 35 million gal per day (mgd).
Following the initial evaluation, FST designed a series of improvements to meet more stringent drinking water regulations, while also improving the plant’s energy efficiency, minimizing chemical use and increasing its reliability. As a result, in addition to being the state’s largest private utility, PWW also has become a pioneer in promoting sustainability.
Totaling $35 million, the WTP improvements were constructed under four separate contracts incorporating numerous energy efficiency improvements. These measures helped PWW achieve improved treatment performance while minimizing the impact on the environment at the time of construction and during future plant operation.
Contracts No. 1 and 2 included the addition of 2,000 ft of power, chemical feed and instrumentation conduit, a new aeration building and passive (non-powered) in-reservoir structures for raw water quality management, resulting in significant improvement to the reservoir’s water quality. These in-reservoir structures consisted of a series of flow-routing baffles and a depth-selective aerator. As a result of these enhancements in raw water quality, PWW has reduced the amount of treatment chemicals necessary to meet drinking water quality standards.
Contract No. 3 included the installation of a new 10,000-sq-ft, 35-mgd finished water pumping station with chemical storage and feed facilities, an emergency generator, and a new 6.5-million-gal finished water storage tank. The non-electric finished water pump options included a 6-mgd hydropowered pump and a 10-mgd natural-gas-powered, engine-driven pump. The hydropowered pump derives its energy from a renewable resource. Water from the supply pond is diverted through a penstock where the 15-ft difference in elevation drives a waterwheel as well as the high-lift pump, which was among 10 pumps installed
at the treatment plant—some with variable-
frequency drives, others with modulating valves, and all with energy-efficient motors and automated sections to minimize power use.
The finished water storage tank enabled gravity operation of the plant’s granular activated carbon (GAC) filters, eliminating the need for the previous 100-hp backwash pumps and reducing the facility’s peak electric load. The storage tank also allowed pumping of the treated water to the distribution system during off-peak hours, which further reduced the plant’s peak load.
Contract No. 4 consisted of constructing new operations and administration facilities, improvements to the existing clarifiers, complete rehabilitation of the gravity GAC filters, two new chemical storage and feed systems, boiler and generator replacement, and new field instruments and SCADA system. There also were building modifications, including new sanitary facilities and an operations and training area. The clarifier upgrade featured the selection of an upflow floc-blanket clarification process in lieu of higher energy dissolved air flotation technology—cutting process horsepower requirements by a factor of 10.
The administration facility improvements were energy efficient and operator friendly, providing better indoor air quality, maximizing daylight in occupied spaces and optimizing natural ventilation. Occupant and motion sensors were installed to control lighting, and light fixtures were renovated to be energy efficient. The building was reused, and previously unused space was converted to a new office area.
Existing oversized unit heaters were replaced with smaller, more efficient units, and two new high-efficiency boilers were installed to replace a single oversized low-efficiency boiler to produce only the amount of hot water that is required. Laboratory renovations included refinishing the existing metal cabinets and casework, and locker room renovations featured low-flow fixtures. Building renovations were designed to maximize natural daylight to reduce dependency on electric light fixtures. To address heat island effects, a light-colored ballasted roof was installed. The design generally maximized use of recycled materials and those that could be obtained regionally—two parameters that again reduced the carbon footprint of the construction.
A Holistic Approach
FST’s approach of reusing existing buildings, process optimization, and upgrading mechanical and electrical systems with lower-energy-use options has resulted in a more sustainable and energy-efficient facility. In recognition of PWW’s efforts to reduce its energy consumption, the area’s electric utility, Public Service of New Hampshire, provided a rebate for energy-efficient light fixtures, and for high-efficiency pump, blower and HVAC unit motors.
The holistic approach of implementing improvements at the source of supply in combination with major upgrades at PWW’s water treatment plant has resulted in:
• Elimination of algae blooms and associated taste and odor complaints;
• Elimination of clarifier and filtration upsets, allowing consistent production of a high-quality drinking water;
• Reduction in chemical and energy use; and
• Significant improvements in reliability, redundancy, operator safety and ease of operation.