Desalination isn’t the simple drought solution it might appear to be
By Paul M.J. Suchecki
In the middle of an historic drought, one can’t help but notice there’s more water due west of Los Angeles than anyone could dream to drain. For the foreseeable future, however, we may be able to boat and swim to our hearts’ content, but we’ll hardly be able to drink a drop of it.
Without the water imported from increasingly dry Northern California and the Colorado River, L.A.’s regional drinking water resources could only sustain a population of about 500,000 people, according to the Los Angeles Dept. of Water and Power.
“We have a huge reliance on imported water. The L.A. Aqueduct is probably going to have another record close to low year,” Marty Adams, senior assistant general manager of the LADWP’s Water System, said during a conversation in early summer.
Despite a few big summer storms, things haven’t gotten much better since.
The good news is that LADWP customers have been conserving water —we used 20.9% less water in December 2014 than that month the previous year, for example — but population growth is expected to erode any such gains. The Public Policy Institute of California projects that by 2050, California’s population could reach 50 million.
So if we aren’t able to conserve our way out of the drought, how can we produce more water?
In recent decades, Australia, Saudi Arabia, Israel and other places with low stores of fresh water have increasingly relied on desalination — the production of freshwater from saltwater.
Earlier this year, the Orange County Water District entered negotiations for all of the 50 million gallons per day of fresh water to be produced by a Huntington Beach desalination plant that could be online by 2018.
In Carlsbad, a desalination plant currently under construction is expected to provide San Diego County with a similar amount of water as early as next year.
By contrast, the only desalination plant in Los Angeles County, located on Catalina Island, produces a paltry 200,000 gallons of drinking water per day.
A decade ago, LADWP announced plans for a desalination plant that also had a potential output of up to 50 million gallons per day that would have been built practically under the Los Angeles Airport flight path, next to the Scattergood power plant in El Segundo.
What happened to it?
“We’re not pursuing,” Adams said. “It’s not the best choice for us in L.A. right now.”
Several factors went into that decision, he explained.
Desalination works by forcing seawater through semi-permeable high-tech polymer membranes that hold back salt, mineral, organic and biological compounds, including viruses and bacteria, while letting freshwater through.
It’s a process called reverse osmosis and is used in 15,000 desalination plants in 120 countries worldwide as well as every cruise ship that plies the seas. It takes about two gallons of salt water to produce a gallon of fresh. Seawater gets pushed through at 1,000 to 1,200 pounds per square inch.
“It’s a very energy intensive process,” Adams said, “which makes it expensive.”
Water from the Carlsbad plant will cost $2,257 per acre-foot, about double the cost of the San Diego Water Authority’s most expensive supply. Although the Carlsbad plant would only provide 7% of the county’s water needs, the extra cost is projected to raise home water bills by $5 to $7 a month, according to the authority.
There are also major environmental impacts from desalination. Intake tubes suck up not just seawater but plankton, larvae and fish eggs, degrading the marine environment. The waste water has to be disposed, usually right back into the sea. The high concentration of salts and dissolved solids in the brine can have an adverse impact on sea life.
Near Scattergood, desalination waste water was to be mixed with the outfall from the Hyperion Wastewater Treatment Plant next door. A detailed study of the project for the Scripps Institute of Oceanography concluded that at full capacity, one of the Hyperion outfalls would present a hazard to marine life 100% of the time — the emergency outfall pipe that runs a mile into Santa Monica Bay. The outfall at 1,200 feet offshore would “exceed the marine biology tolerance threshold” 18% of the time. Discharge from the pipe at five miles out would be sufficiently diluted to present no risks to sea creatures. Still, brine is heavier than seawater. Without wave action to break it up, it tends to spread along the sea bottom.
“We have so many projects that will deliver water at a lower cost to the customer that we’ll pursue those projects first. If we can’t make recycled water work, than de-sal is the way to go.” Adams said.
Recycled water is not going directly from toilet to tap. At the Groundwater Replenishment System in Orange County, water is purified in several stages. The waste water is first pumped into holding basins filled with 270 million micro-filters then forced under high pressure through reverse osmosis membranes. Next it’s treated with hydrogen peroxide, then blasted with ultraviolet light.
Still, the output is not added directly to the water supply; 35 million gallons per day are injected into the earth to create a freshwater buffer so seawater doesn’t intrude on freshwater ground supplies. Another 35 million gallons per day are pumped to percolation basins where the water seeps through sand and gravel to replenish Orange County aquifers. The plant has been operating since 2008.
The city of L.A.’s Terminal Island Water Reclamation Plant is using both microfiltration and reverse osmosis for what’s called tertiary treatment, bringing 4.5 million gallons per day up to drinking water standards, although it’s not used directly as potable water.
“It’s being used as a seawater barrier to create a fresh water wall to protect the ground water and also to recharge it,” Adams said. “Advanced treatment, what is being done in Orange County, is one step beyond this.”
Adams said that the focus for replenishing LADWP’s water supply with recycled water was in the San Fernando Valley for several reasons — one of the biggest being that the Valley is upstream of most of LA’s water system, where the pipes are bigger.
By contrast, pumping water from a seaside desalination plant or from Hyperion after advanced treatment would require laying more pipe and essentially pumping water uphill.
In the Valley, “We have a recycling project to recharge the groundwater that should be in place in six years,” Adams said. Effluent, he said, is “better quality source water than seawater, which is why it is so much cheaper to treat. Our cost is projected to be $750 to $1,000 an acre foot, competitive with water we’re purchasing now.”
Another source of L.A. drinking water is a resource that so far has been squandered. Right now the sewer system is designed to flush rainwater out to sea as quickly as possible to prevent flooding. It’s a system that worked great for the 20 wet years from 1978 to 1998.
The biggest problem is that so much of Los Angeles is paved with asphalt and concrete.
“We do have our large spreading grounds which capture water on some of the main storm channels. We’re enlarging those. We’re doing distributed storm water capture, green street installations and medians — places where storm water instead of running off would settle in a swale and percolate into the groundwater,” Adams said.
He stressed the need for engaging the public to capture rainwater, using as an example the parking lot of the L.A. Zoo where water runoff is no longer sloughed off but replenishes the aquifer beneath it.
“We’re fortunate here that we have a number of resources available to us, but some need cleaning up, some need developing further, and in the meantime we’re just glad that the public is responding to our calls for conservation. Right now we’re using the same amount of water we did in the early 1970s. It will be a few years, but we’ll be building a lot more reliability and local reliance into the equation,” Adams said.