Advanced Water Treatment Technologies May Bring Repurified Water to San Diego
"In four years, half of San Diego may be filling its glasses with tap water that once ran through its toilets,"reported the San Diego Union-Tribune recently. A plan to mix "repurified water" with the City's raw drinking water reservoir could become a reality, thanks to the wonders of water treatment technology.
Such a possibility elicits various reactions including disgust, marvel and fear. Although the public generally trusts and admires modern technology, politicians and even scientists fear the grave repercussions a technological failure of this kind would bring. What is under debate is exactly how risky the project would be and what the alternatives and their cost would be.
Treated wastewater is nothing new, but using it for drinking water is. According to the Union-Tribune report, only one other city in the U.S. currently adds purified sewage water to its drinking water reservoir: Alexandria, Virginia. The idea is so novel that a new term was coined in San Diego to describe it: repurified water. One may ask why a city the size of San Diego would need to implement what many consider such a drastic measure?
San Diego County's robust economy and prosperity is a captive prisoner to water availability, and there isn't enough to keep up with the growth. The County's population of 2.7 million is expected to reach 3.8 million by 2015, which translates into an estimated demand of about 900,000 acre-feet by 2010. This represents a shortfall of 150,000 acre-feet over the currently imported 690,000 and locally supplied 60,000 acre-feet. Since over 200,000 acre-feet of San Diego's treated wastewater is discharged into the ocean each year, repurification became one of the logical steps to help make up the shortfall.
The initial plan is to repurify 22,000 acre-feet of wastewater and blend it with raw drinking water in a surface reservoir. The plan calls for "multiple barrier" treatment and constant monitoring of the repurified water's quality. Diversion options are included to prevent blending should pre-established water quality criteria not be met.
Specifically, the sewage water will undergo conventional and advanced treatment steps. Conventional tertiary treatment would include screen filtration, grit chamber treatment, primary clarification, aeration, secondary clarification, filtration and chlorination. Advanced treatment would include microfiltration pretreatment (including pH adjustment and disinfection), reverse osmosis (which will produce water with a concentration of 100 mg/liter of dissolved minerals, a concentration one-fifth that of the current City of San Diego water supply), disinfection (with ozonation due to its safety, reliability and superior capability) and nitrate removal (using ion exchange treatment).
Once these advanced treatment steps have been completed and purity is verified, repurified water would be pumped to a reservoir and blended with other local supplies. Upon withdrawal, the water undergoes final treatment including conventional coagulation, mixing, clarification, filtration and disinfection before introduction to the city's pipelines.
Thus, "multiple barriers" would exist between humans and water-borne pathogens including enteric viruses, bacteria and protozoan cysts such as Giardia and Cryptosporidium. Heavy metals and trace organics also would have multiple barriers as well as an aggressive source control program prohibiting discharge into the wastewater collection system to begin with.
As thorough as this may seem, the plan is not without its opponents. At question is safety to human health and cost. Even though proponents share cautious optimism, the fact remains that the project could still be abandoned at any time by the San Diego County Water Authority and San Diego City Council partnership even though $8 million already has been spent on the proposed $168 million project. In spite of the seemingly high costs and risks involved, repurification may still be the best of limited options.
Surprisingly, seawater desalination is not currently considered a cost-effective option for the County. The reasons cited include the high cost compared with repurification or water transfers, and the fact that large capital facilities along the coast are difficult to acquire, permit and mitigate. It continues to be considered a long range option, however, and will be viewed more favorably as costs are reduced.
Transferring agriculturally conserved water from San Diego's neighboring Imperial Valley has been proposed and aggressive negotiations are underway, but massive political hurdles must be overcome since this water is subject to the seven-state/three nation Colorado River compact known as "The Law of the River."
Other sources such as water saved through conservation and groundwater recovery projects are already in the plan and account for approximately 10 percent of the projected 2010 water resources.
Whatever the option, it is clear that reverse osmosis will play a central role in San Diego's ability to provide its people with a reliable source of water. Whether used to repurify wastewater or desalinize seawater, RO is the technological centerpiece. But it's a costly one. At current costs of $1,000 to $2,000 per acre-foot, it's no wonder scientists and officials seek alternatives. Perhaps not for long, though.
New products, techniques and technologies have emerged that significantly address the RO cost issue. Since energy consumption is one of the biggest contributors to cost, industry has diligently worked to produce low pressure RO membranes, some of which provide good performance at pressures less than 100 psig.
RO fouling also is being addressed. Since RO membranes are easily fouled (clogged) by particulate matter, extensive, expensive pre-filtration is required for units to work properly. To overcome this, the membrane industry is developing new membrane surface and element construction modifications that will decrease the fouling tendency. Larry Lien, manager of Osmonics Desal's Membrane Development Specialists, reports that his teams' modification show great promise in drastically reducing the costs for RO pre-filtration.
As the best minds in our industry continue to make RO advancements, costs will be reduced whether the application is for wastewater repurification or seawater desalination. As RO costs continue to decline, the technology will be considered more and more viable. This is good news to thirsty cities like San Diego and to our business.