Manchester Water Works
Treatment Process Description
The Manchester Water Works operates a 50 million gallon per day (MGD) conventional water treatment facility utilizing rapid mixing, coagulation, flocculation, sedimentation, intermediate ozone disinfection, deep-bed anthracite and granular activated carbon filtration, and residual disinfection processes. The treatment facility was completed in 1974 and significantly upgraded in 2005.
Lake Massabesic, located on the east side of Manchester, serves as the sole source of supply for a population of over 159,000 consumers in the greater Manchester area and portions of six surrounding communities.
Raw Water Pumping
“Raw” water from Lake Massabesic is conveyed through a 60-inch high density polyethylene pipeline intake that reaches 430 feet from the shoreline into a new, low lift pump station constructed in 1997. The original station, built in 1906 and renovated for raw water service in 1974 is maintained for redundancy. Three variable speed pumps deliver the raw water through a 48-inch pipeline to the rapid mix chambers. This pipeline is equipped with a soda ash feed point where alkalinity is boosted prior to coagulation.
In the rapid mix chamber, the primary treatment chemical, aluminum sulfate is added to begin the process of coagulation. Two rapid mix chambers are configured in series with the capability of adding the coagulants into either or both chambers. High speed mixers ensure complete dispersion of these chemicals enabling them to react with the natural dissolved and particulate matter in the water and cause them to cluster together.
The next process, known as flocculation, involves gentle stirring of the minute particles formed in the rapid mix/coagulation process causing them to collide and stick together and form larger, denser, snow-flake-like particles called “floc”. Flow from the rapid mix chambers is distributed evenly into each of the four flocculation basins. The flocculation basins are configured in two stages separated by a baffle wall with the second stage mixers set at a slightly slower speed than the first.
The sedimentation process is achieved by allowing the water to flow slowly through a long, deep, quiescent basin that allows sufficient time for the floc particles to settle to the bottom forming sludge, a treatment process by-product. The sludge is periodically removed by isolating one of the four basins, decanting, and pumping the sludge layer to a lagoon where it is dried and moved to a landfill.
Settled water flows into the intermediate pump station where it is lifted into the ozone contact chambers. Ozone is a powerful oxidant and disinfectant that removes tastes and odors, along with killing or inactivating harmful organisms in the water. Ozone is generated on-site by passing a high-voltage electric current across a dielectric discharge gap through a pure oxygen stream. A combination of three, 500-pound-per-day ozone generators produces the required ozone gaseous stream that 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. Residual (excess) ozone is removed from the water by applying sodium bisulfite prior to exiting the contact chambers and continuing on to the filters.
Anthracite and Granular Activated Carbon Filtration
Following intermediate ozone, the water passes either through one of four deep-bed anthracite filters, or one of four deep-bed granular activated carbon (GAC) filters. Each filter contains six feet of biologically active media that completes the physical removal process. While both filter media have proven to be effective, they are being studied side-by-side to determine which will be the most beneficial and economical in the long-term. Anthracite is an excellent filter media but does not have the adsorption capability of GAC, however GAC requires periodic reactivation or replacement to restore is adsorption characteristics. Comparing each side-by-side will enable Manchester Water Works to make an informed decision in the future as to which will most benefit our customers.
Hydraulic Control Structure
Combined filter effluent flows by gravity into the hydraulic control structure that provides constant back-pressure on the filters. The hydraulic control structure also provides an application and mixing point for post-filtration chemicals that include ammonia (chloramine residual disinfection), zinc orthophosphate (corrosion control), fluoride (dental protection), and sodium hydroxide (pH adjustment).
Finished Water Pumping
From the hydraulic control structure, water flows into a 700,000 gallon clearwell and finished water pumping station. A series of seven vertical turbine pumps lifts finished water into the distribution system.