Utilities Turn to Membrane Treatment for Drinking Water

In many communities north of San Antonio, TX, the demand for drinking water is on the rise. To meet this need and provide potable water in line with the turbidity and microorganism requirements...

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In many communities north of San Antonio, TX, the demand for drinking water is on the rise. To meet this need and provide potable water in line with the turbidity and microorganism requirements of the Safe Drinking Water Act, Canyon Regional Water Authority (CRWA) is expanding its Lake Dunlap water treatment plant with the latest ultrafiltration technology from Koch Membrane Systems (KMS).

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The MegaMagnum® reverse osmosis element is the world's first 18-inch-diameter brackish water spiral membrane. This increase in element size and membrane surface area reduces the footprint and cost of the installed RO system.
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In the Spring of 2002, Canyon Regional Water Authority started up a 4 mgd ultrafiltration system from Koch. The system consisted of two skids each containing 48 eight-inch-diameter UF cartridges and was designed to be expanded to 10 mgd by adding extra skids.

Turbidity can be a problem at the Lake Dunlap plant, which is fed by a river. Shortly after the system became operational, heavy rains resulted in serious flooding which caused very high turbidity spikes in the feed water to the ultrafiltration system. The ability of the system to continue producing quality water during this challenging period confirmed the wisdom of selecting a pressure cartridge system to treat this surface water supply.

CRWA had known from the outset that it would expand the Lake Dunlap plant and constructed a building capable of housing six UF skids. Late in 2002, CRWA decided to move forward with it's planned expansion. The expansion was purchased in 2003, and will increase the system's capacity to more than 15 mgd. The system will consist of six skids, each able to hold 44 of the new PMPW-10 ultrafiltration cartridges from KMS. The addition of cartridges to the open positions on the new skids will increase capacity by another 10%.

One of the advantages of the PMPW-10 cartridge is that the same floor space and building used to accommodate six stages in the initial system design will soon produce much more water. This is because the cartridges contain 60% more membrane surface area than the eight-inch-diameter cartridges used originally. Each new skid will be able to produce 30% more water than the original skid. Yet each skid will occupy the same amount of floor space.

The PMPW-10 used at CRWA is 72 inches in length, and contains 870 square feet of membrane. The same cartridge is also available 48 inches in length, containing 550 square feet of membrane. Both cartridges contain thousands of hollow fibers made from polysulfone, a membrane material that has been in use in membrane water treatment plants around the world for over a decade. Polysulfone can tolerate a broad pH range, and can withstand chlorine at high pH giving it distinct advantages when it is time to chemically clean the membranes.

The hollow fibers are effective in reducing turbidity and routinely achieve less than 0.1 NTU on a wide variety of water supplies. With a molecular weight cut-off of 100,000, the membranes can provide 4-log removal of cryptosporidium, 4-log removal of giardia and 4-log removal of viruses, thereby helping operators to comply with the Safe Drinking Water Act.

Recently the town of Franklin, MA, selected the PMPW-10 cartridge to treat water from the town wells. A pretreatment step involves adding ozone to oxidize iron and manganese found in the water. The KMS hollow fibers in the 48-inch cartridges are effective in removing the precipitated iron and manganese oxides. The town will use two skids, each containing 16 cartridges, to produce 1.2 mgd of potable water.

This approach to iron and manganese removal has been employed successfully in other Massachusetts communities using smaller KMS cartridges. The Town of Littleton began using a KMS ultrafiltration system to remove iron and manganese from well water under the influence of surface water back in 1997 and is still using the original cartridges today. Like Franklin, this system also uses ozone pretreatment to convert the iron and manganese to insoluble iron oxide and manganese oxide.

Like the PMPW-10, the membranes are also made from polysulfone, but they are housed in cartridges that are 5 inches in diameter and 43 inches long. Four skids with 50 cartridges on each skid produce 1.4 mgd of potable water from a well that once was shut down.

To minimize waste at the plant, a membrane wastewater recovery system was also installed at the Littleton plant. The concentrated waste from the water treatment system is directed to a wastewater storage tank. A wastewater treatment skid with 20 five-inch ultrafiltration cartridges treats the collected water further concentrating the iron and manganese oxides. The highly concentrated slurry of oxides in the tank is periodically pumped to drying beds. Approximately 99.9% of the feed water is recovered in this way.

Thanks to the larger diameter of the PMPW-10 cartridge, smaller buildings are needed for ultrafiltration systems today than just a few years ago. Water treatment plants using the cartridge are 30 to 50% smaller than those that house older 5-inch or 8-inch systems, and will process the same amount of drinking water at a significant savings in new construction costs.

Just as larger diameter UF cartridges have produced higher flows at lower cost in low-pressure ultrafiltration systems, the same is true of reverse osmosis (RO) membrane systems using larger diameter elements from KMS. While standard RO and nanofiltration elements are 8-inches in diameter and 40-inches long, the new MegaMagnum® elements from KMS are 18-inches in diameter and 60-inches long allowing for approximately seven times more active membrane area per element.

Compared with standard 8-inch diameter RO elements, the greater membrane area of the MegaMagnum® RO element requires fewer elements and pressure vessels for a given flow rate. This saves time in the loading of elements at system commissioning as well as the obvious benefits of less fittings and fixtures and smaller plant footprint.

Orange County, CA, has recently completed testing of a MegaMagnum® reverse osmosis element used for wastewater reclamation. Scottsdale, AZ, has had a prototype under evaluation for over a year. The Metropolitan Water District of Southern California plans to start up a demonstration pilot plant this year that will use several of the RO elements housed in three pressure vessels.

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