100 MGD Zenon Membrane Plant Sets New Standards
Highest Quality Drinking Water at the Lowest Cost
Oakville, Ontario — For over 30 years, ultrafiltration (UF) membranes have proven to be the most effective process to remove particles from a water source. The historic high cost of this innovative technology; however, limited its use to only low volumes of high value products such as pharmaceuticals, beverages, and microelectronics fabrication. Public Water Utilities have long known the water quality advantages of pathogen-free, membrane-treated water but have considered the technology too expensive for the high volumes required in a municipal water plant. That is, …until today!
Like many advanced technologies, as more applications are put in service, the cost of manufacturing drops, making the technology even 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. Ten years ago a "large" membrane plant capable of treating about 1 million gallons of water per day. Five years ago a "large" plant was 5 MGD, and as recently as two years ago, the largest membrane plants were in the 50 MGD range such as those in Racine, Wisconsin and Thornton, Colorado. Today, 100 MGD is a reality enabling municipalities of all sizes to offer their customers the ultimate in cost-effective public health protection.
100 MGD Twin Oaks Valley WTP Makes History
Currently defined as the world's largest membrane ultrafiltration installation, the recently awarded Twin Oaks Valley Water Treatment Plant (WTP) in San Diego County, California is the first large-scale membrane facility to be bid with capital construction cost, annual cost and lifecycle cost significantly below that of competing conventional systems. A recently completed competition revealed that using advanced technology no longer costs a premium. Three competing designs with two using conventional granular filters and one using ultrafilter membranes were submitted to the Owner, 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 four percent lower on capital cost than the nearest competitive offer.
"As worldwide acceptance of membrane technology increases, we continue to make greater investments in our manufacturing processes and product development; yielding higher system performance at more cost-effective prices," says Graham Best, Director of Water Treatment at ZENON Environmental. "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."
Membranes Inherently Save Money
While lower membrane costs are a big money saver for the SDCWA, the ZeeWeed system brings inherent benefits to water treatment that also help to lower the total installed cost of the plant.
Size matters for the Twin Oaks Valley WTP since 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 that were specified in the conventional technology offerings. This will produce a significantly smaller plant which maximizes the efficiency of land at this space-constrained site and will spare the surrounding sensitive lands from damage. The absence of the pretreatment equipment further increases the cost-effectiveness of the plant by reducing construction costs as well as chemical costs and residual production since coagulants are not required.
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 (81) 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. This high efficiency means that a membrane system can be up to 10 times smaller than conventional systems of the same treatment capacity, depending on the system configuration and the raw water quality.
Critical Need for Water in San Diego Area
"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," says SDCWA Board Chairman James Bond.
"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 says. "ZENON and CH2M Hill worked closely during the proposal process to provide maximum cost savings, which will help SDCWA achieve this objective."
How the Plant Works
ZeeWeed membranes will 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 ZeeWeed 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 (LT2), 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 MG clearwells will provide treated water storage. The innovative design by CH2M Hill is also significantly less costly to operate than similar sized, conventional plants. The Owner has awarded the 15 year operating contract to CH2M Hill subsidiary OMI at an annual fee 8 % less than the 2nd place bidder.
The plant will be highly automated, including turbidimeters, particle counters, fiber optic communications and a supervisory control and data acquisition (SCADA) system to continuously monitor water quality. Each process train is separately monitored and can be shut down for maintenance or troubleshooting if turbidity levels rise 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.
Looking Forward
Twin Oaks Valley WTP is the first water treatment plant to be built by the SDCWA and is part of a $3.2 billion capital improvement program to reduce over-reliance on a single water supplier and improve the reliability of the supply by diversifying the region's water portfolio. It will join many other California membrane plants such as Sweetwater WTP (38.5 MGD) and Olivenhain WTP (34 MGD). Each of these plants plays a critical part in providing higher quality potable water through a cost-effective, multi-barrier approach that meets or exceeds EPA regulations such as LT2 and the new Stage 2 Disinfection Byproducts Rule.
Such membrane systems are also helping smaller communities build new treatment plants or retrofit aging or underperforming facilities with advanced UF technology. As ZENON and its partners work to continue reducing system costs while improving effectiveness, more communities than ever will be incorporating UF membranes to protect public health and cost-effective delivery of the highest quality potable water.
SOURCE: GE Water & Process Technologies - ZENON Membrane Solutions