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Water Treatment Plant

Water Treatment Plant 


 

71 Stonehouse Hill Road
Holden, MA 01520
Phone: (508) 799-1513

 

 

 

 

Faced with urbanization in watershed and aquifer areas, an aging and deteriorating infrastructure, and stricter water quality standards, the City of Worcester, in 1984, began planning to protect, preserve, and expand its supply of potable water. The city set its sights on construction of a new water filtration plant to filter ---for the first time ever---the drinking water for the 200,000 people it serves. The Safe Drinking Water Act Amendments of 1986 dictate that all surface water supplies must use filtration to treat drinking water if they cannot meet highly stringent water quality and reliability criteria.

Thus, Worcester began the process of building a new 50-million-gallon-per-day plant to comply with federal regulations and, more importantly, meet the water quality and public health needs of the area.

From its inception, the plant was designed to be environmentally and aesthetically compatible with its natural setting. The location of the plant, a six-acre site adjacent to the eastern shore of Holden reservoir No. 2, was chosen carefully from among 10 potential sites. Both the architecture and the building façade of the plant were planned to blend harmoniously with the natural surroundings.

Efficiency was the second critical factor in constructing the plant. Before design or construction began, several water treatment processes were identified and analyzed to determine the most effective and economical approach. Population trends, water demands, and plant capacity studies were analyzed and pilot testing---a method for testing various treatment methods on a small, and therefore economical scale---were performed. The pilot testing revealed that pretreatment would best be achieved by pre-ozonation, coagulation, and flocculation using alum and cationic polymer. Combined with direct filtration through a deep-bed anthracite filter, these processes were the most effective and economical water treatment process for Worcester. Ozone, coagulation, and filtration reduce organic matter, which, if allowed through the system, lead to formation of disinfection by-products---potential heal hazards if not removed. The processes also remove particulate matter in the water that can harbor harmful pathogens and microorganisms.

Worcester’s Water Treatment Basics

  • Plant Flow - 50 Million Gallons Per Day (MGD)
  • Reservoir System - The treatment plant utilizes a series of ten surface water reservoirs located in Leicester, Paxton, Rutland, Holden, and Princeton. The ten reservoirs combined, hold over 7 billion gallons of water.
  • Primary Disinfection - Ozone, generated by four ozone generators (one standby) from air with a system capacity of 834 pounds per day. The applied ozone averages 1 mg/L, with a design maximum of 2 mg/L.
  • Rapid Mixing/Coagulation - Two-stages, utilizing vertical shaft radial turbine mixers. Coagulant chemicals are aluminum sulfate (alum) and cationic polymer.
  • Flocculation - Three stages, having a total of 15 min. detention time and utilizing vertical- shaft, axial-flow flocculators. nonionic polymer is provided as a filtration aid.
  • Filtration - Eight filters having a design filtration rate of 8 gpm/sf. Filter media consists of 60 inches of anthracite coal over 12 inches of sand. The filters are designed to accept activated carbon media if needed in the future.
  • Corrosion Control - pH adjustment using lime, followed by an application of a blended orthophosphate corrosion inhibitor.
  • Final Disinfection - Chlorine
  • Treated Water Storage - Two, 2.75-million-gallon storage tanks.

Worcester's primary source of drinking water is derived from two chains of upland surface water reservoirs. Water from Worcester's Lynde Brook Reservoir meets with water in the tailwaters of the Holden reservoir system, where the treatment process begins. The preozonation process treats the water with ozone, which is a powerful disinfectant. From there, the water goes through coagulation via a rapid mix process, and then flocculation. Next, the water is filtered, treated to control corrosion, and finally given post-disinfection treatment by chlorine. This keeps the water safe from bacterial growth while it is transported to residences and businesses. The entire treatment process removes bacteria viruses, and pathogens such as Giardia, which are regulated by the federal Safe Drinking Water Act. As well, the system removes Cryptosporidium, another potential health hazard.

The instrumentation and control system for the plant consists of field-mounted sensors and control devices electrically wired to a digital system back at the plant, hooked to the central operator's console in the control room. The control room, in fact, is the focal point of the plant, monitoring the entire system, as well as 10 remote system storage tanks and five remote pump stations. The computer system collects and stores accurate and reliable operating information, assists plant operators by noting and announcing off-normal operating conditions and equipment failures, performs calculations, accumulates and stores equipment running times and changes in plant operations, stores information for preventive maintenance and inventory control, and provides displays and summary reports.

Source waters contain organic materials such as fine silts and clay among others, these can be very objectionable to consumers. We add coagulants (aluminum sulfate) and Coagulant aids (polymers) to the rapid mix basin where they are mixed at a rapid rate, this Insures an instantaneous and even mix throughout the chamber. The reason for rapid mixing is so that the chemicals are rapidly, and evenly distributed, and all parts of the water will be equally well treated.

Flocculators are mixers that run at a slower rate of speed than rapid mixers, so that the floc will continue to form without being sheared. There are three chambers, each chamber is mixed at a slower speed, allowing the floc to become more dense, thus increasing the size and weight of the particles. The optimum size of the floc particles that we want are between 1-2 millimeters. The water is then channeled to the filters.

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