Thirsty California Turns to Sea and Sewer

In April, California Governor Jerry Brown ordered the state’s first mandatory water restrictions in response to a fourth year of severe drought. The regulations are designed to cut urban water use by 25%in part by prohibiting the use of potable water to irrigate street medians and lawns on certain types of properties. To meet these requirements, water utilities are encouraging conservation efforts, including offering rebate programs for the installation of lowflow toilets and washing machines, and lawn replacement with drought-tolerant plants. For water-starved communities, “the most cost effective technology is conservation,” says Tzahi Y. Cath, director of the Advanced Water Technology Center at the Colorado School of Mines. But with aquifers and reservoirs running low, and dry conditions predicted to become more common across the state with climate change, many California municipalities are also planning for the future by considering two less conventional sources of drinking water: the sea and the sewer. Neither desalination nor wastewater recycling is new to California. One of the state’s first large desalination plants was built in Santa Barbara in 1991 in response to the region’s last major drought, but it only made it into a testing phase before being decommissioned when rains returned. Now the city plans to update it and put it back in service, at a cost of $40 million. Meanwhile San Diego County is building a new desalination plant in Carlsbad for $1 billion. When it opens in late 2015, it will provide 50 million gallons of water a day to the county, making it the largest in the United States. As for wastewater recycling, the largest plant of its kind in the world is the Orange County Water District’s Groundwater Replenishment System, which treats sewage wastewater to replenish inland and coastal aquifers. In service since 2008, it can produce 70 million gallons of treated water per day; the water district is now spending $143 million to expand it to yield a total of 100 million gallons a day, enough to supply 850,000 residents. Communities across the state are weighing the costs and benefits of the two approaches as they debate whether to follow in the footsteps of Orange County or San Diego. For coastal communities, desalination promises a reliable water source, but it is still extremely expensive and energy intensive, which is problematic in a state committed to reducing its expenditures and greenhouse gas emissions. Environmentalists are also concerned that the process can harm coastal ecosystems. In contrast, recycling wastewater into drinking water is cheaper and may have a smaller environmental impact, but it raises an “ick” factor for some people and poses health concerns about exposure to pathogens and micropollutants. In current state-of-the-art facilities, the linchpin of both approaches is a water purification method called reverse osmosis. It involves using pressure to force water through a semipermeable membrane, producing freshwater on one side and leaving concentrated brine behind on the other. But because the energy requirements of reverse osmosis are proportional to the salinity of the water to be treated, desalination is about twice as energy intensiveand expensiveas wastewater recycling.