Recycling sewage to drinking water could save city of San Diego money: Study
The city of San Diego could save hundreds of millions of dollars in upgrade and expansion costs for its Point Loma sewage treatment plant , and get more drinking water in the bargain, according to a final draft study given to the city in late May.
With the savings, recycling sewage to drinking water standards costs roughly the same as importing more water, according to the final draft of the San Diego Recycled Water Study. The precise numbers depend on what assumptions are made about which costs can be avoided.
If the study's recommendations are followed, new sewage treatment plants would be built in Point Loma, University City and the South Bay. The cost of upgrading the existing Point Loma sewage plant would drop from an estimated $1.2 billion to $710 million.
Release of the final draft study comes as the county's main water supplier is in the late stages of negotiating for another new water supply, desalinated sea water from a plant to be built off the coast of Carlsbad.
Poseidon Resources Corp. is negotiating a water purchase agreement with the San Diego County Water Authority, which supplies most of the water used in the county. The authority says it expects to get a completed contract ready this summer for a 60-day public review before a final vote is held.
Under the draft study, sewage treated to drinking water standards would yield 100 million gallons a day, about 20 percent of the region's water use. Assuming the study's numbers are accurate, the net cost to the city per unit of water amounts to about half that estimated for water from the proposed desalination plant.
The study presented three scenarios for the cost of repurified sewage in dollars per acre foot. One acre-foot is about 326,000 gallons:
- $1,200, counting avoided costs from reduced usage of the Point Loma sewage treatment plant,
- $1,100, including a "salt credit" for removing salt from the treated sewage
- $700, including those savings along with avoided costs of upgrading of the Point Loma plant, which discharges treated sewage into the ocean.
"These costs compare well to the existing untreated water cost of $904 per acre foot, and are more economical than most other new water supply concepts being proposed," the report stated.
Peace with local environmentalists would be another potential benefit. Environmentalists have long complained that the treated sewage, expelled into the ocean 4.4 miles off the coast at a depth of 320 feet, harms marine life. The city disputes the claim.
Livia Borak , an attorney from Coast Law Group, urged the city to forgo the Poseidon desalination proposal and choose sewage recycling instead. Borak spoke at a May meeting of the Water Authority, She pointed out that the city has great leverage with the agency because of its weighted vote.
The city of San Diego contains a little more than half the 3.1 million population of San Diego County, with proportionate clout on regional agencies such as the Water Authority. So what San Diego decides to do will greatly influence the water supply picture for the whole region.
Moreover, the city faces water- and sewage-related financial considerations that don't apply to North County.
San Diego's potential costs for upgrading the Point Loma plant exceed $1 billion . So far, the city has obtained waivers, the latest of which expires in 2015. There is no guarantee the city, already reeling from high water bills, will receive another waiver.
Counting those avoided costs, the net cost to the city of San Diego for recycled sewage amounts to about half that of buying water from the proposed desalination plant.
In water industry jargon, recycling sewage to drinking water standards is called indirect potable reuse , or IPR. The purified sewage is sent to reservoirs or underground storage, then drawn off with new supplies for treatment and distribution. This contrasts with direct potable reuse, in which reclaimed water is directly put back into treated drinking water pipelines.
The study's numbers for IPR seem reasonable, said water policy expert David Zetland , who has long researched Southern California's water infrastructure. Zetland said he did not review the study, but was making a general observation about the cost of indirect potable reuse compared to seawater desalination.
"IPR can easily be cheaper than desalination, as it takes partially-treated wastewater and further cleans it for consumption," said Zetland, a senior water economist at Wageningen University in the Netherlands. "That's why the "marginal" cost will be quite low. A new IPR plant can also be cheaper than desal, since there are fewer salts to remove."
The desalinated ocean water from the Carlsbad plant would cost about $1,865 per acre-foot , according to an estimate made last year by Poseidon Resources, based in Stamford, Conn.
While the city of San Diego would not directly buy the desalinated water, the plant's 50 million gallons a day capacity would increase the region's total supply by about 10 percent. Desalinated water from Carlsbad would also bring a new source of local water to the arid county, which has seen its traditional sources of imported water threatened by drought, environmental restrictions, over-optimistic assumptions, and legal challenges.
Recycling sewage to drinking water would also provide a reliable local supply to the county. But the city's leaders and the San Diego Union-Tribune, now U-T San Diego , have rejected it because of discomfort of what's been dubbed "toilet to tap."
Regardless, the approach is being used in Orange County, and scientists have said evidence indicates indirect potable reuse can provide water as safe as that from untreated imported water. A new report from the respected National Research Council encouraged exploration of its use.
"The report presents a brief summary of the nation’s recent history in water use and shows that, although reuse is not a panacea, the amount of wastewater discharged to the environment is of such quantity that it could play a significant role in the overall water resource picture and complement other strategies, such as water conservation," the NRC study stated.
With water costs skyrocketing and imported supplies increasingly more difficult to come by, the city of San Diego commissioned the study to get a fresh look at the pros and cons of repurified sewage.