Treated Waste Water May Be Used Soon for Irrigation Along Freeway
Five years after San Diego’s innovative aquaculture project set out to make drinking water from sewage, the water that it produces may finally find a practical use.
The experimental plant in Mission Valley uses water hyacinths, crayfish, catfish, koi (large goldfish) and other water creatures to transform sewage into drinkable water. Although the plant has attracted curious visitors from around the world, its practical benefits so far have been nil.
Because of the county Department of Health Services’ hesitation about its biological alchemy, the 300,000 or so gallons of water that the plant produces each day have been dumped back into the sewers. But if a proposed collaboration with Caltrans receives final approval from the Health Department, the reclaimed water will be irrigating the greenery along Interstate 15 by the end of the month.
By 1988, a new, larger plant based on the techniques developed at Mission Valley may be supplying all the irrigation water for Balboa Park if the proposal receives City Council approval. The proposed plant, whose hyacinth-filled pools would cover seven acres, would have the capacity to produce 1 million gallons of water each day--water that officials say is clean enough to drink.
Aside from providing all the water the park needs to keep its wide variety of plant life verdant, the plant would be a scenic addition to the park, according to Allan Langworthy, a senior water plant supervisor.
“We’d probably have a jogging path around the pools, and picnic tables,” he said. “It’s not at all what people think of when they think of water treatment plants.”
The project represents a fortuitous confluence of the streams of interest of the city Park and Recreation Department and the Aquaculture Project, according to project engineer Frank Maitski. “We have an $8-million grant to produce a 1-million-gallon plant, and they have a 1-million-gallon-per-day need of water. It really fit together quite nicely,” he said. Without the grant, construction of the plant would be economically infeasible at this stage.
For the Park and Recreation Department--which spends $500,000 a year to water Balboa Park--the system’s economic and ecological features make it attractive.
“It’s going to save us a whole bunch of money,” said Deputy Director Jack Krasovitch. Asked about the conservation aspect, he said, “Oh, that too. . . . Definitely recycling and financial reasons both.”
The project is the first to use this particular arsenal of biological weaponry to clean up the water that human beings befoul. After going through a series of progressively finer screens--the final one being 250 microns in diameter (a micron is a millionth of a meter)--the raw sewage flows into tanks filled with the plants and animals that feast on human waste. The most voracious appetites belong to water hyacinths, whose broad leaves cover the surface of the still tanks as the plants quietly soak up the swill at a furious rate.
The greatest difficulty that the project has run into is what to do with the hyacinths, which must be harvested at the rate of two tons per acre per day. They are now used for landfill, but eventually the hyacinths may be decomposed in turn in order to yield methane gas. Other potential uses include animal fodder, compost and soil enrichment, Maitski said.
Another potential problem is mosquitoes. Because the treatment plant resembles swamp conditions, it provides a fertile breeding ground for mosquitoes--one byproduct that would be particularly unwelcome in a public park.
“Mosquitoes are our greatest concern. When they solve that, the project will have a green light,” said Councilwoman Gloria McColl, who chairs the City Council’s Public Services and Safety Committee.
“One solution is chemical control,” Maitski said, “but because of the fear that mosquitoes will mutate to be immune to poison, the Health Department is against this.” Instead, the plant will stock the tanks with gambusia--small fish whose favorite food is mosquito larvae.
Technically, the water that emerges after spending 12 hours to three days in the tanks is clean enough to drink--meeting strict standards for content of BOD (Biochemical Oxygen Demand), coliform bacteria and suspended solids, as well as other hazardous materials in sewage, according to Maitski. But the Health Department has its doubts.
“Right now no recycled sewage can be used as drinking water under Health Department regulations,” he said. The reclaimed water is limited to irrigation, industrial uses and ground water injection, according to Maitski.
But the Water Utilities Department eventually hopes that San Diego will obtain 40% of its water needs through recycling--including that which flows out of the tap in private homes.
Beyond health regulations, public opinion may also have to be changed.
“If some day we hope to reuse a significant portion of our water, there is going to have to be a whole educational effort in order to convince people that drinking water derived from sewage can be safe,” Maitski said.
San Diego’s sewage is particularly ripe for reclamation both because of the warm climate, which is well-suited to the sun-loving hyacinths, and the city’s status as a major water importer, Maitski says. “Because we are so dependent on other places for our water, we are trying to find ways to cut down,” he said.
Ultimately, the growth of water reclamation will depend on economic factors, however. “Right now, it’s still cheaper to pipe the water from Northern California than it is to recycle it,” Maitski said. “But as the supply becomes more limited and water needs grow, we will reach a point at which that becomes reversed. Then you’re going to start hearing a lot of talk about reclamation.”
In addition to saving water, reclamation can reduce the amount of sewage that is pumped into the ocean, Maitski said. Right now, San Diego’s sewage, after being filtered, is pumped through pipes 2 1/2 miles offshore.
So far, the Balboa Park plan has received the tentative approval of a City Council committee and will go before the full council sometime later in the summer, after final technical questions are ironed out. If the project proves feasible, other small plants may follow.
“When we heard about the Balboa Park proposal, we started to think of a whole lot of other places in San Diego that needed irrigation,” McColl said. “A city that imports more than 90% really needs to look into the re-use of all kinds of water.”