By Dawn Guendert
Southern California is a semi-arid desert with a growing population, and current water supplies are inadequate to meet future water demand. As a result, several agencies like Orange County Water District (OCWD) have pioneered the use of Advanced Wastewater Treatment (AWT) systems that follow a microporous membrane process such as Microfiltration (MF) with Reverse Osmosis (RO) for reclaiming municipal wastewater. This reclaimed wastewater can be used for agricultural, industrial and indirect potable reuse applications.
OCWD Water Factory 21 (WF21) has treated reclaimed wastewater to potable water quality since 1976, injecting it into coastal aquifers to prevent seawater intrusion into the groundwater basins and to recharge the reservoir capacity. The reinjected water is stored in the groundwater basin for approximately two years before it is used.
The treatment process at the Water Factory consists of flash mixing and flocculation at pH 11.4, using slaked lime, clarification, recarbonation for pH control and granular media filtration. The media filter effluent is treated simultaneously through granular activated carbon (GAC) to reduce organics and through RO to reduce salinity before it is blended with deep well water and then injected underground for use as a seawater intrusion barrier.
In 1997, OCWD and Orange County Sanitation District (OCSD) released a joint report proposing the Groundwater Replenishment System (GWRS) project. An indirect potable reuse project and a supplemental source of high-quality water, the three-phase system will take secondary treated wastewater from OCSD and further treat it to drinking water standards, with the first phase estimated to be complete in 2005.
Once finished, the replenishment system will send nearly 80 percent of the potable-quality water to OCWD. (The remaining 20 percent will be used to expand the existing seawater intrusion barrier.) This will almost double the district's capacity over the next 20 years to approximately 130 mgd of treated reclaimed water, as well as further augment the local groundwater basin.
The groundwater basin currently supplies about 75 percent of Orange County's potable water. By 2020, the replenishment system will be able to supply approximately one-fourth of the water needed to recharge the groundwater basin.
Although Water Factory 21's current water treatment process has served it well for more than two decades and has continually met or surpassed drinking water quality standards during that time, the GWRS project proposes replacing the facility's conventional RO pretreatment process with MF. OCWD's new treatment scheme will consist of using MF, RO and ultraviolet disinfectant (UV) to treat the secondary effluent from OCSD's treatment plant.
Pretreatment Systems
In comparing MF to pH-lime clarification as pretreatment to RO, Water Factory 21 began using pilot scale MF and RO systems in 1992. The MF system included a 0.2-micron polypropylene hollow fiber membrane and a patented compressed air backwash to control fouling, and the RO system contained cellulose acetate (CA) spiral-wound membrane elements.
The fouling precursors to RO membranes on secondary effluent generally include microorganisms, fine particles, colloids and macromolecules, which, if not successfully removed during pretreatment, can cause irreversible failure to the downstream RO system as well as irreparable membrane damage, reduced flux rates and increased operating costs from frequent chemical cleaning.
To the contrary, the filtrate water produced by this particular MF system contained exceptionally low levels of microorganisms, suspended solids and turbidity (in comparison to the lime-treated conventional process).
The Silt Density Index (SDI), which is commonly used to measure the effectiveness of RO pretreatment, employs a 0.45-micron membrane filter pad to assess the fouling potential associated with particles in the RO feed stream. Although there are limitations to the SDI test, it was used at WF21 to illustrate the differences between the MF pretreatment and conventional pretreatment. The SDIs of the MF effluent were consistently 50 percent lower than the best value for the media filters.
During the MF/CA RO pilot study, the CA RO membranes operated at a flux of 12 gallons per square foot per day (gfd) for up to 13 weeks before requiring chemical cleaning. In comparison, the CA-membrane RO system located downstream of the conventional process was operating at 10.4 gfd, with the membranes requiring chemical cleaning every four to six weeks.
The next phase of the pilot testing included replacing the CA membranes in the RO pilot system with polyamide (PA) thin film composite membranes. The improved microbial quality of the MF filtrate reduced the biofouling previously experienced with conventional pretreatment. Substituting PA thin film composite membranes for the CA membranes reduced operating pressure by up to 150 psi (330–350 psi for CA versus 200 psi for PA), thus decreasing the RO system's energy requirements by 30 percent to 50 percent.
Memcor® Membrane Modules in a CMF-S Cell.
As a result of the successful pilot testing, a 500 gpm MF/RO demonstration system was installed in 1994.
MF Pretreatment System
Located in a densely populated residential area where limited space hinders expanding the flocculation and coagulation basins, OCWD's WF21 needed a cost-effective MF system with a small footprint. The selected system, USFilter's Memcor® continuous microfiltration – submerged (CMF-S) system, fit the requirements. The system produces water with exceptionally low numbers of bacteria and suspended solids, as well as significantly reduced turbidity.
In comparing the CMF-S system to its conventional pretreatment counterpart, it is evident the system reduced turbidity of the pretreated water by 60 percent to 70 percent, and consistently produced SDI results well below the minimum cut-off of three recommended by the RO manufacturers.
In addition to its compact modular units that provide more than twice the capacity of a conventional treatment system housed in the same footprint, the CMF-S system also offers operational cost-savings normally associated with lime clarification. At 1.2 tons per mgd, adding lime, as well as collecting and recycling the resulting sludge, can often be prohibitive.
Using the MF system in lieu of conventional RO pretreatment will benefit WF21 in several other ways, including:
• Occupying 50 percent less space than does the conventional system for the same effluent capacity;
• Requiring no chemical pretreatment, other than pre-chlorination;
• Being easily automated, and requiring less maintenance;
• Improving the downstream RO system's performance;
• Allowing for change to PA membranes (versus CA membranes), and reducing power costs by 60 percent; and
• Providing overall cost savings of 41 percent in operation and maintenance, resulting from changing from conventional pretreatment with CA membranes to MF pretreatment with PA membranes.
Foreseeable Future
Without technological advancements made to MF and RO systems over the last few years, it would be very difficult to meet the objectives of such a large-scale project. The AWT facility, which will include MF, RO, UV and other ancillary facilities, will eventually replace OCWD's existing WF21. This treatment process is quickly becoming the industry standard for treating municipal wastewater in indirect potable reuse projects.
The Ground Water Replenishment System is an example of how integrated membrane technology can be used to address environmental problems associated with potable water supply and wastewater disposal. The project will use the latest membrane filtration technology to provide a cost-effective source of water that meets federal drinking water standards while satiating the water needs of Orange County's thriving population, which is expected to increase by another million within the next two decades.
When finished, the GWRS project and the combined MF/RO technologies used there will have an important impact on both OCWD and OCSD, as well as on the communities they serve. Conservatively, it is estimated that reclaiming secondary treated wastewater will only consume 50 percent of the energy required to import water from Northern California and 66 percent of the energy costs for water imported from the Colorado River. The project is also expected to postpone by at least a decade a $150 million investment in a new ocean outfall.
Submerged MF technology, similar to what is installed at OCWD, is being used in more than 20 membrane-based water reclamation systems globally.
About the Author Dawn Guendert is a product manager with USFilter. Dawn joined USFilter in 1999 as a technical sales manager. She has a BA degree from the University of California at San Diego.
