Water, water everywhere and all we want to drink, shower in, swim in, grow food with, etc.
by Jim Bell
Many experts are projecting doom and gloom, scenarios of decreasing water supplies and increasing cost, yet the San Diego/Tijuana Region can easily become renewable water self-sufficient and even become a net water exporter.
Even if we assume the worst case scenario of zero precipitation and the complete cutoff of all imported water, the San Diego/Tijuana Region could completely replace all the freshwater it currently uses by installing PV panels over 4.3% of its roofs and parking lots. In 2015, 4.3% 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. A yearly average of 5-hr. of sunlight per day,
2. 1,000 sq. feet of roof and parking lot per capita,
3. An average potable water consumption level of 180 gallons per capita per day,
4. A 2015 regional population of 6 million people,
5. That 70 gallons of freshwater can be extracted from seawater per kWh of electricity consumed through reverse osmosis (RO)
6. PV (photovoltaic) panels 15% efficient at converting sunlight into electricity, (Commercially available panels are already pushing efficiencies of 20% 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 freshwater through while excluding salt, other minerals and contaminants in general.
The issue of sucking marine life into reverse osmosis system 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 near ocean wells, until the water to be returned to the ocean is no more than 20% 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 size of the “worst case scenario” RO system discussed above could be cut in half, if recycled sewage water was filtered and disinfected, then used for irrigation. Using graywater 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.
Combining water recycling and efficient water use with better rainwater runoff collection and storage systems, our region would only need to install 15% efficient PV panels on 2% 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.