Increasing Water Conservation through Reuse

Despite recent rain and snowfall ushered in by El Niño, California still faces water scarcity that threatens the area's long-term water supply. The U.S. Drought Monitor indicates that, even after 58 inches of snow in the Sierra Nevada mountain range at the end of March, 73 percent of the state still endured severe drought conditions. In light of the ongoing threat of water scarcity on the West Coast, the State of California has been implementing new techniques and technologies to ensure adequate drinking water supplies for years to come. Water reuse, in particular, has been a main method of conservation. In 2014, the Santa Clara Valley Water District invested $72 million to create the Silicon Valley Advanced Water Purification Center, the largest facility of its kind in northern California. Employing what an article in The Atlantic refers to as "some of the most advanced technology in the world," the facility uses microfiltration, reverse osmosis (RO) and ultraviolet (UV) light to transform secondary-treated wastewater into 8 million gallons of purified water suitable for drinking every day. During the microfiltration process, solids and microorganisms are filtered out through membranes made of fibers with pores that have a diameter of 0.1 micron, or about 1/300th of a human hair. The next step is reverse osmosis, where high pressure forces the water through membrane holes that filter out even smaller substances such as viruses, pesticides and pharmaceutical products. Finally, the cleaned water is passed through chambers that further disinfect by emitting powerful ultraviolet light. According to a document from the water district, the benefits of this multimillion-dollar facility are crucial. Not only does the project create a locally controlled water supply, but it also increases water reliability, protects the region's groundwater supply and reduces wastewater discharge to the San Francisco Bay. The Silicon Valley facility, examined on the following pages, represents one of a growing number of water reuse plants around the world. While there are still many stigmas surrounding the concept of water reuse, the process of recycling this precious resource is becoming more important every day. The following outlines the step-by-step purification process in Silicon Valley, highlighting the intake and transfer pumps, microfiltration, reverse osmosis and ultraviolet disinfection.

The Process of Purifying Water in Silicon Valley

1. INTAKE PUMPS At the Silicon Valley Advanced Water Purification Center, the 8 million gallons start their journey at the intake pumps. This water has already gone through two levels of treatment at the wastewater plant before arriving at the purification center. Four 200-horsepower (hp) pumps, including one backup pump, move the incoming, secondary-treated water from the neighboring San Jose – Santa Clara Regional Wastewater Facility. 2. MICROFILTRATION PROCESS In this initial process, water is forced through filtration membrane modules made of thousands of hollow fibers, similar to straws. These fibers have very fine pores that are about 1/300th the width of a human hair. As the water is drawn through the pores into the center of the fibers, solids, bacteria, protozoa and some viruses are filtered out of the water. 3. INTER-PROCESS TANK & THE TRANSFER PUMPS The filtered water from the microfiltration system is conveyed to a 225,000-gallon stainless steel tank. From here, the water flows to the RO transfer pumps, where it is pumped through cartridge filters as a pre-treatment step before going to the RO feed pumps. 4. REVERSE OSMOSIS FEED PUMPS The center has three 500-hp RO feed pumps that boost the pressure very high and send the water to the next phase of RO purification. Before the water enters the RO system, two chemicals are added to eliminate scaling, or the buildup of minerals, and protect the RO membranes. 5. REVERSE OSMOSIS PROCESS In this process, high pressure forces the treated water through tightly wound sheets of thin membranes with pores so small that a water molecule is almost the only substance to pass through. RO removes contaminants, viruses, pesticides, salts and other materials from the water, producing highly purified water. This is the same process that is used by some bottled water companies and baby food manufacturers, as well as for kidney dialysis. 6. DECARBONATION TOWERS The water produced by the RO process contains a lot of carbon dioxide (CO₂), which would make the water corrosive to the pipelines. Decarbonation removes CO₂ and raises the pH of the water, reducing corrosion in downstream facilities, including the distribution pipelines. Water cascades from the top of the tanks, and the air blowers, adjacent to each tower, provide an upward airflow, which removes excess CO₂. 7. ULTRAVIOLET LIGHT DISINFECTION PROCESS Now the water is very clean. But as a further safety backup, the water is disinfected using UV lights. There are six UV trains, each consisting of a pair of UV vessels. Each vessel holds 40 high-intensity UV lamp bulbs, similar to extremely concentrated sunlight. This technique is often used to sterilize medicines, food and fruit juices. Hospitals and dental offices use it to sterilize instruments. 8. HIGHLY PURIFIED WATER STORAGE TANK After UV light disinfection, the highly purified water is sent to a 2.25-million-gallon stainless steel product water storage tank. From the storage tank, the purified water is sent to the transmission pump station where it is blended with tertiary-treated recycled water. From here, the improved recycled water is sent to the South Bay Water Recycling distribution system and on to more than 600 customers. Images and text provided by Santa Clara Valley Water District.