The First Seven Years of Operation at an Ultrafiltration Plant
p> Littleton, Mass. is a town of approximately 7,000 residents with 2,500 water connections. The town is supplied by four wells, of which the Spectacle Pond well has the highest yield. Manganese is present in the water at 0.75 mg/l, and iron concentrations are around 0.30 mg/l. Prior to the installation of the ultrafiltration (UF) system, complaints were received from consumers about ugly brown stains on laundered clothing and bathroom fixtures.
The largest commercial user of town water, a fruit juice bottler, was concerned that deteriorating water quality would ultimately affect the quality of the beverages that they sold. The well produces 1,100 GPM but the poor aesthetic quality of the water had it relegated by the town’s water department to emergency backup only.
The well is located in a residential area adjacent to Spectacle Pond and under the influence of the pond. The location is a Zone One area, which means that there is no waste discharge allowed at the site. A solution to the well water problem would need to account for the fact that the town had no sewer system.
In the 1990s, the town hired the engineering firm of Haley & Ward to conduct a series of pilot studies at the well, and then later worked with another engineering firm Tata & Howard.
A set of primary and secondary goals for improving the quality of the water was established, and the results of the studies were measured against these goals. The primary goals were to meet the current requirements of the Safe Drinking Water Act. This included reducing iron, manganese, and corrosivity, eliminating coliform bacteria and minimizing trihalomethane formation. Given that the well is under the influence of surface water, the secondary goals were established to meet the anticipated requirements of the Surface Water Treatment Rule, and the Enhanced Surface Water Treatment Rule for the elimination of Giardia, Cryptosporidium and HPC bacteria, and for the reduction of radon and disinfection byproducts.
The studies compared the use of manganese greensand adsorption and oxide coated sand filtration with UF. The use of ultrafiltration technology, specifically hollow fiber UF with ozone oxidation as pretreatment, proved to be more effective in meeting both the primary and the secondary goals for water quality. Additionally, the UF technology produced less waste, and was better suited to Zone One restrictions.
In a second ultrafiltration study, two separate systems were installed: a primary system that filtered the ozonated raw water, and a secondary system that treated the backflush and bleed from the primary system. The purpose of the secondary system was to improve the overall feedwater recovery of the plant, thereby minimizing the amount of wastewater that would require disposal.
A consistent system
In 1996 a 1.4 MGD UF system was purchased from Koch Membrane Systems, Inc., and by 1997 it was installed and running. Since then, the system has delivered drinking water to the town that has consistently met the town’s primary and secondary goals for water quality, while producing very little wastewater.
The system that was installed in Littleton uses 1.25 mg/l of ozone to oxidize the dissolved manganese and iron. The oxides of manganese and iron are insoluble, and precipitate out of the water as brown particles that turn the water to the color of weak tea (7 to 8 NTU). These particles are large enough to be readily filtered out of the water as it passes through the hollow fibers. The ozone is generated on-site using clean, dried air, and flows counter current through the first of three pretreatment tanks, where oxidation takes place. In the second tank remaining ozone is stripped away by a counter-current flow of air, which also carries away any radon that may be present. The air enters an ozone destruct unit prior to release to the atmosphere. The third tank is used for detention time, allowing residual ozone, which is unstable, to break down. Because ozone will rapidly degrade the polysulfone that makes up the hollow fiber membranes, it is essential that all ozone be removed from the treated water in the last two tanks.
The brownish effluent from the ozonation process is then pumped at 1100 gpm to 200-micron prefilters that remove large particles that might clog the hollow fibers. Water is then fed to four parallel skids of hollow fiber cartridges. Each skid contains 50 cartridges, and each cartridge houses hundreds of fibers. Approximately 95% of the feedwater passes through the membranes, from inside to outside, and becomes permeate. Filtrate flows to the clearwell where it is stored prior to distribution to the town. The remaining 5% contains the retained manganese and iron oxides and is bled away from the filters to the collection tank where it is later treated by the secondary UF system.
Once every 60 to 120 minutes the filters are backflushed for one minute with permeate from the clearwell, and this waste is also sent to the wastewater collection tank.
Minimization of waste disposal
An important design objective was to minimize waste disposal requirements. The secondary ultrafiltration system consisting of a skid containing 20 cartridges housing hundreds of polysulfone fibers accomplishes this by batch treating the waste stored in the collection tank. These fibers are slightly larger in diameter than the fibers on the primary UF system to accommodate the much greater concentration of particles in the wastewater. Bleed and backflush water from the secondary system is returned to the collection tank. Permeate is directed to the head of the primary UF system. Nearly all the waste from all five skids is concentrated in this tank, and the tank is periodically drained to drying beds where the wastewater is filtered through sand and gravel and is sent to Spectacle Pond, leaving behind a fine deposit of manganese and iron oxides. The deposit is non-hazardous and is used as fill in road-bed construction. As a result, the system recovers 99.9% of the feedwater as product, and there is very little waste with nothing to haul away.
For over seven years, operations at the plant have been steady. Approximately once every week one of the skids on the primary system is taken off-line for about six hours, and is cleaned with chemicals. A 1,000-gal tank is used for this purpose, and water heated to between 80-120°F is introduced. Citric acid is rinsed through the membranes every five weeks to remove the build-up of oxides, and every three months chlorine and caustic are added to the regimen to remove any build-up of organics and microorganisms from the fibers.
“The membranes at this facility have been operating since 1997 with a high level of integrity,” said Chris Allen, water production engineer for the Littleton Water Department.
In fact, the plant has operated for nearly seven years and has never had a need to replace any of its 220 membrane cartridges.
“With regard to the membrane plant, there have been no problems, no issues of any kind since start-up,” added Allen.