Many experts are projecting gloomy scenarios of decreasing water supplies and increasing costs, yet the San Diego/Tijuana Region can easily become renewable water self-sufficient and even develop into a net water exporter.
Even if we assume the worst case scenario of zero precipitation and the complete cutoff of all imported water, the region could completely replace all the freshwater currently used by installing solar panels over 4.3 percent of roofs and parking lots. In 2015, 4.3 percent of our region’s roofs and parking lots will be about 9 sq. miles, or 4.5 sq. miles on each side of the border.
The above statement is based on the following assumptions:
1. Yearly average of five hours of sunlight per day
2. 1,000 sq. feet of roof and parking lot per capita
3. Average potable water consumption level of 180 gallons per capita each day
4. Regional population of 6 million people in 2015
5. Assumption that 70 gallons of freshwater can be extracted from seawater per kWh of electricity consumed through reverse osmosis (RO)
6. PV (photovoltaic) panels 15 percent efficient at converting sunlight into electricity (Commercially available panels are already pushing efficiencies of 20 percent or better).
The electricity produced by this system would be used to power large scale reverse osmosis (RO) pumps to convert seawater into freshwater. The pumps push seawater through filters that let in freshwater while excluding salt, other minerals and contaminants in general.
The issue of sucking marine life into reverse osmosis systems can be solved if seawater to be processed into freshwater is extracted from wells close to the ocean above high tide instead of direct ocean extraction. Since seawater coming into such wells would be sand filtered, marine organisms will be eliminated from the process.
Similarly, since “waste water” from the RO process will be twice as salty as seawater, it will have to be diluted by mixing it with seawater— also extracted from the nearby ocean wells— until the water to be returned to the ocean is no more than 20 percent saltier than seawater. Once diluted, its release into the ocean would be defused as an additional precaution against negative ecological consequences. Other sand filtering technologies have also been proposed.
Mining RO waste water for salt and other minerals opens up other local business and employment opportunities for the region and could potentially eliminate the need to return RO wastewater to the ocean at all.
The impact of the “worst case scenario” RO system discussed above could be cut in half if recycled sewage water was filtered, disinfected and used for irrigation. Using gray water at home would also be a plus for efficient water use. This is because half of the potable water currently used in our region is used for irrigating landscaping and crops.
Water-use efficiency improvements could reduce the role of renewable energy-powered RO as well.
By combining water recycling and efficient water use with better rainwater runoff collection and storage systems, our region would only need to install 15 percent efficient PV panels on 2 percent of its roofs and parking lots to provide equal or superior water use services in the future, compared with what we have today. Plus, if we want more freshwater, we can cover more roofs and parking lots with PV panels to power expanded RO capacity and create all the freshwater we want.
Additionally, all this can be funded through a water purchase agreement model that will pay for itself by redirecting the dollars we now export to pay for imported water into hiring local businesses and workers to make our region renewable water self-sufficient, with renewable energy powered RO being our back-up for water if all else fails.
by Jim Bell
Jim Bell is an internationally-recognized expert on life support sustaining development. His projects include the design and construction of the San Diego Center for Appropriate Technology and Ecoparque, a prototype wastewater recycling plant in Tijuana, Baja California, Mexico. Currently, he serves as Director of the Ecological Life Systems Institute and the San Diego Center for Appropriate Technology, and as representative of the Sierra Club on the City of San Diego’s Regional Advisory Energy Committee. Jim has more than 40 years experience in the design and construction industry and ran for Mayor of San Diego in 1996 and 2000, as well as 2nd District City Council in 2002. The views expressed here are his own.