Like many municipalities in urban and suburban areas, San Bruno, Calif.’s source water comes both from its own groundwater supply and through a...
Founded in 1947, the West Basin Municipal Water District in Carson, Calif., wholesales imported water to cities, mutual water companies, investor-owned utilities and private companies in South Bay and unincorporated areas of Los Angeles County, serving a population of more than 900,000.
In 1995, West Basin constructed the West Basin Water Recycling Facility to meet the growing demand for a sustainable, reliable water supply in Southern California and to reduce the demand on scarce potable water sources. It became one of the first water agencies in the U.S. to implement wastewater reclamation using membrane technology.
In 1997, the first Memcor low-pressure membrane plant at the recycling facility began producing 2.5 millions gal per day (mgd) of high-quality water for reverse osmosis (RO) pretreatment. The water was used initially for injection into a hydraulic barrier to control seawater intrusion into the groundwater. High-quality water that exceeds drinking water standards is required to ensure that the groundwater quality does not degrade.
From 1998 through 2002, the district installed three additional Memcor systems to supply recycled water to the nearby Exxon Mobil and British Petroleum refineries. Water from these systems was used as pretreatment to RO for boiler and cooling water feed.
Previously, the district used a conventional treatment process consisting of lime softening, recarbonation and multi-media filtration. The conventional process produced a large quantity of sludge and was unable to efficiently remove materials that can foul RO membranes. Moreover, the process was difficult and expensive to operate.
Memcor low-pressure membranes consistently produce a silt density index (SDI) of less than three, versus an SDI of five with conventional pretreatment technology. The reduced SDI results in less RO membrane fouling and longer durations between cleaning, which translates into lower operating costs and longer RO membrane life.
By 2004, West Basin was operating roughly 2,500 sq ft of low-pressure membranes to produce nearly 20 mgd of reuse water; however, as the demand for recycled water increased, the district required an additional 11.7 mgd of low-pressure membrane filtration. Thus began the facility’s Phase IV expansion.
A different approach
Although West Basin had been using the Memcor Classic CMF pressurized membrane system for more than eight years, it opted to go with a submerged membrane system after running a successful pilot test. The district liked the advantages of submerged membrane technology, which included a smaller footprint, lower operating costs, reduced waste production, greater flexibility and the ability to visually inspect the membrane modules.
In the Classic CMF pressure system, individual modules are contained in housings, which are then arranged onto skids, with each typically containing 90 modules to produce approximately 500,000 gal per day. In a submerged system, four modules are grouped together into a clover. Up to 10 clovers are then placed on a rack that is submerged in an open tank. By comparison, 90 pressure modules require approximately 70 sq ft, while 90 submerged modules require 27 sq ft, a significant footprint reduction.
In January 2004, West Basin purchased the Memcor CS submerged membrane system—its fifth Memcor system. Started up in September 2006, the system consists of six membrane cells, each containing 384 modules. Each cell produces roughly 2.4 mgd, allowing one cell to remain offline to meet capacity demands. The extra cell was installed to provide flexibility for operators and to achieve constant flow while another cell is undergoing the scheduled four-hour chemical cleaning that occurs approximately once per month.
In addition, each cell can be expanded to hold another 32 modules, for a total of 416 modules per cell.
West Basin is now the largest facility of its kind in the U.S. treating more than 30 mgd of secondary effluent, which equals about 8 billion gal of water annually for the South Bay. The facility produces six customized “designer” classes of water for a variety of industrial and municipal applications, including irrigation for parks and golf courses, seawater barrier injection, makeup water for oil refineries, cooling towers and for high-quality boiler feed.
These six classes are:
1. Tertiary water for a range of industrial and irrigation uses;
2. Amended tertiary water specifically for sports turf;
3. Nitrified water for industrial cooling towers;
4. Softened RO water, using secondary treated wastewater pretreated with Memcor membranes followed by RO and disinfection, for groundwater re-charge. This water is superior in quality to that required by state and federal drinking water standards;
5. Pure RO water, using secondary treated wastewater that has passed through Memcor membranes followed by RO, for low-pressure boiler feedwater; and
6. Ultrapure RO water (secondary treated water that has passed through Memcor membranes, RO and second- pass RO), for high-pressure boiler feedwater.
What began as a proactive measure to ease a potable water shortage has ultimately saved the region more than 65 billion gal of drinking water.
Today, West Basin is recognized as a leader in water conservation and water recycling and was recently named “Large-Size Recycled Water Agency of the Year” by the California section of the WateReuse Association.