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| Permeate pump draws water inside of membrane fibre. | Membrane floor. Treatment capacity can be increased by adding immersed membrane cassettes. |
By Bruce Baldwin, Seekonk Water District, Massachusetts,
and Michael Stadnyckyj, ZENON Environmental Inc., Ontario
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| Permeate pump draws water inside of membrane fibre. | Membrane floor. Treatment capacity can be increased by adding immersed membrane cassettes. |
A 16,000 m3/d iron and manganese immersed membrane filtration plant in the Town of Seekonk, Massachusetts, is the largest of this type in the United States and the only one of its kind in the State. It incorporates a unique reject water reuse process to increase plant recovery.
The Seekonk Water District relies on groundwater for both domestic water and fire protection. For nearly two decades, the District had been experiencing high levels of iron (up to 2 mg/L) and manganese (> 6 mg/L) in the product water, exceeding the US National Secondary Drinking Water Regulations.
Historically, the District used an in-ground treatment system consisting of oxygenated water injection around the wells to oxidize and settle both minerals. The process was generally effective, but when one of the wells was taken off line, it became a challenge to meet peak summer water demands. In 1998, the District conducted an assessment to determine the best available technology for upgrading the existing treatment process. The evaluation included pilot testing of the following processes:
Raw water for these pilot tests was obtained from an existing two and a half inch monitoring well with a manganese concentration range between 2.5 and 3.1 mg/L.
Membrane filtration was very effective in meeting the water quality goals of 0.03 mg/L manganese and 0.01 mg/L iron. The permeate exceeded the goals consistently throughout the testing period, even at maximum flow rates, and the pilot demonstrated extremely high production efficiency (99%).
Pressure filtration using intermittent regeneration with either sand/anthracite or greensand media proved ineffective in meeting the product water quality manganese goal of 0.03 mg/L, and these were eliminated as treatment options for the District.
Pressure filtration using continuous regeneration with greensand media was marginally effective in meeting the water manganese goal of 0.03 mg/l at a loading rate of (US) 3 gpm per square foot of filter surface area. The pilot test demonstrated a production efficiency of 96.5%.
During the piloting process the region experienced severe flooding, resulting in E.coli contamination in one of the District's wells. Due to this flooding, the Massachusetts Department of Environmental Protection changed the status of the District's water supply from a "groundwater" source to "groundwater under the influence of surface water" source, which would require full treatment under the Surface Water Treatment Rule.
In September 1998, the design engineers, P.C. Amory Engineers, of Duxbury, MA, presented the Board of Water Commissioners with an in-depth report detailing the entire pilot study, along with relative costs. After considerable discussion, it was determined that the immersed ultrafiltration membrane system provided by Zenon Environmental Inc. was the most advantageous for the District. The membranes not only met the iron and manganese removal requirements, but would also provide a positive physical barrier to prevent bacteria and pathogens from entering the distribution system.
Raw water is pumped from the District's wells into a common intake line. Potassium permanganate and sodium hypochlorite are injected prior to a compact "static" mixer, to oxidize both iron and manganese. Sodium fluoride is also added at the same location. After the static mixer, the water flows into a common 21 m3 influent channel. From the channel, the water flows by gravity into three separate process tanks having a combined capacity of 280 m3. Each process tank can be isolated from the influent channel by manual watertight slide gates. Immersed directly in each process tank are ten membrane cassettes, with eight modules each.
Filtration is achieved by drawing water to the inside of the membrane fibre under a low-pressure suction of -1 to -9 psi created by a permeate pump. With an absolute pore size of 0.1 microns, the ZeeWeed® membranes retain the precipitated iron and manganese solids, and at the same time provide a physical barrier to bacteria, pathogens and certain viruses (Table 1).
While the process tanks are in operation, dedicated aeration blowers supply air to the bottom of each membrane cassette. The air minimizes the settling of solids and scours the surface of the fibres.
To maintain concentrate levels of oxidized iron and manganese within the process tanks, each 'process train' is equipped with a reject pump. The reject pumps withdraw a pre-set volume of concentrated iron and manganese and supply it to a 80 m3 tank. The reject water is allowed to settle, decant over a weir into a pump chamber, and is returned back to the plant to be re-processed through the ultrafiltration membranes. This unique reuse process achieves a recovery rate >99%.
Each process tank is equipped with its own particle counter to ensure the integrity of the membranes. If a particle counter determines that there is a break in a membrane, the process train is immediately taken off line. Installed on the common clearwell feed line is a colour monitor that will shut down the plant and sound alarms in the event of a problem. A common feed line discharges into a 300 m3 clearwell, located beneath the floor of the facility.
There are four vertical turbine water discharge pumps installed directly over the clearwell having a combined capacity of 16,000 m3/d. In the discharge piping are two chemical injection taps that are used to add both sodium hypochlorite and sodium fluoride if needed.
This facility was built on a footprint of only 7,488 ft.2. There is also a 714 ft.2 blower room that is attached to the main building. During the erection of the building, skylights were included to minimize the interior lighting needs, and to reduce energy costs.
Designed with room for expansion, the capacity of the plant can be increased simply by adding immersed membrane cassettes.
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