For more than 30 years, ultrafiltration(UF) membranes haveproven to be the most effective technology to remove particles from a water source. The historic high cost of this technology, however, limited its use to only low volumes of highvalue products. Public water utilities have long known the water quality advantages of membrane-treated water, but have considered the technology too expensive for the high volume required in a municipal water plant. That is, until today.
Like many technologies, as more applications are put in service, the cost of manufacturing drops, making the technology more affordable for the next user. This is precisely what has happened in the municipal drinking water arena, where very large water plants can now be designed and constructed around the best available technology—membranes.
Making history
One of the world’s largest membrane UF installations, Twin Oaks Valley Water Treatment Plant (WTP), San Diego County, Calif., is the first large-scale membrane facility to be bid with capital construction cost, annual cost and life cycle cost significantly below that of competing conventional systems.
Three competing designs—two using conventional granular filters and one using UF membranes—were submitted to the owner of the San Diego County Water Authority (SDCWA). Following a detailed evaluation, the consulting and engineering firm, CH2M Hill, was awarded the $159 million design-build-operate bid, specifying Zenon’s ZeeWeed UF membranes. The winning bid was nearly 4% lower on capital cost than the closest competitive offer.
“Pricing has historically been one of the biggest challenges for membranes to overcome, but Twin Oaks Valley WTP demonstrates that this issue is now behind us, and membranes are without question the best choice for all future water treatment plants,” said Graham Best, director of water treatment at Zenon Environmental.
Inherent savings
While lower membrane costs are a big money saver for the SDCWA, the ZeeWeed system brings inherent benefits to water treatment that also help lower the total installation cost of the plant.
Size matters for the Twin Oaks Valley WTP because much of the land surrounding the site is considered environmentally sensitive and could not be disturbed in the construction of the plant.
The ZeeWeed system eliminates the need for bulky pretreatment systems such as coagulation and flocculation specified in the conventional technology offerings. This produces a significantly smaller plant, which maximizes land efficiency and spares the surrounding sensitive land.
The membrane filtration area occupies much less space than that of a conventional granular media system due to the high surface area and high packing density that hollow fiber membranes offer. Each ZeeWeed fiber is essentially a filter filled with microscopic pores that physically block suspended solids from entering the treated water supply.
Thousands of membrane fibers are closely packed into a membrane module, and although a module is only about the size of an average couch seat cushion, it provides 500 ft2 of filtration surface area. Eighty-one such modules will be loaded into each cassette at Twin Oaks Valley WTP, giving a cassette the ability to filter over 1 mgd of raw water.
Critical need for water
“The completion of the water treatment plant in 2008 will provide additional treated water capacity needed to address the ever-increasing demand for drinking water in San Diego,” said SDCWA Board Chairman James Bond.
The area’s three million residents currently receive drinking water from the Metropolitan Water District of Southern California (MWD), but growth in the area has exceeded the MWD’s ability to deliver sufficient quantities of water during the summer season. The Twin Oaks Valley WTP will help to mitigate any water shortages and also provide a self-sufficient, affordable, high quality supply that will meet the area’s demands for many years to come.
“One of the main goals of the Twin Oaks Valley WTP project is to ensure that the SDCWA can continue delivering high quality water to ratepayers without increasing prices beyond what MWD currently charges,” Best said.
How the plant works
ZeeWeed membranes provide direct filtration of State Project water and Colorado River water drawn from the Second San Diego Aqueduct.
After passing through a fine screen at the headworks of the plant, the water will be distributed among 14, 7-mgd process tanks, each containing six membrane cassettes.
With a nominal pore size of 0.02 microns, the membranes act as a physical barrier to virtually all suspended particles, and can consistently provide 4-log removal (99.99%) of Giardia, Cryptosporidium, bacteria and viruses. These removal rates ensure that the water from the treatment plant will meet the new Long Term 2 Enhanced Surface Water Treatment Rule and protect customers from the harmful effects of these microorganisms.
Following membrane filtration, the water will flow through an ozonation system, then granular activated carbon and finally to a chlorine contact tank. Two onsite 7.5-million gal clearwells will provide treated water storage. The innovative design by CH2M Hill is also significantly less costly to operate than similar plants. SDCWA has awarded the 15-year operating contract to CH2M Hill subsidiary OMI at an annual fee 8% less than the second place bidder.
The plant will be highly automated, including turbidimeters, particle counters, fiber optic communications and a supervisory control and data acquisition system to continuously monitor water quality. Each process train is separately monitored and can be shut down for maintenance or troubleshooting if turbidity rises beyond a safe level.
Optimum permeability will be maintained through a series of proven cleaning methods that ensure high performance and low energy consumption for water production.
Periodic backpulsing with permeate will dislodge particles that have adhered to the membrane pores, while simultaneous coarse bubble aeration will further scour debris from the fiber surface. Chemical backpulsing is another cleaning strategy done about once per day to further restore permeability and involves backpulsing membranes with a chemical such as sodium hypochlorite.
Finally, a few times per year, recovery cleaning is performed and involves in-tank chemical soaking to remove organic and inorganic contaminants from the fibers.
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