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Easy water:
Transforming wastewater's murky image into gold
story by Gordy Slack
photos by Peg Skorpinski When a newcomer to California asked writer Wallace Stegner what he should know about the state, Stegner answered, “Water. It’s about water.”
Californians are notoriously desperate for water and willing to do just about anything to get it. Some of our most remarkable engineering accomplishments are monuments to both our water needs and our determination to slake them. California’s bitterest environmental battles are fought over water: Owens Valley, Mono Lake, Hetch Hetchy Reservoir, and the Peripheral Canal. And some of the state’s most outlandish ideas have been water related, too. Remember the proposals to tow icebergs from the Polar Regions, or notions to create a floating offshore water pipeline to transport water to California from the state of Washington?
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The Monterey Regional Plant irrigates 12,000 acres of farmland in Monterey County, suppying up to 21 million gallons of processed wasetwater to nearby farms each day.
PHOTO COURTESY OF THE MONTEREY REGIONAL WATER POLLUTION CONTROL AGENCY |
Despite efforts to conserve and redistribute water, our thirst is outpacing our ability to secure more sources. By the year 2020, says the State Department of Water Resources, California will face a shortage of between two and nine million acre-feet of water per year. A million acre-feet is water enough to keep some five million Californians hydrated each year.
“The struggle to secure safe water affects just about everything that’s important to us: environment, industry, public health, economy, agriculture, and simply having enough safe drinking water,” says Kara Nelson, Berkeley professor of civil and environmental engineering and an expert on wastewater reuse.
While some engineers ponder how to conserve water or get more of it delivered where it’s needed, Nelson and her colleagues are taking a different tack. They hope to increase the safety and efficiency of reusing water that would otherwise be disposed of. “As water becomes more scarce, we are more reluctant to throw it away once it’s been used,” says Nelson. “I view wastewater as a valuable resource. By irrigating with wastewater, we can return the organic matter and nutrients it contains back to the soil, mimicking the same cycles that occur in nature. But we must be extremely careful to protect public health by removing pathogens from the water first.”
Nelson is currently focusing her efforts on Monterey County’s Salinas Valley, where she hopes to help boost the amount of water the regional recycling plant can process and deliver to farmers each year.
For decades now, as both local agricultural and municipal water demands grow in the Salinas Valley, and groundwater is extracted faster than it can be replenished, salt water from the Pacific is seeping further and further into the aquifer. Water must now be drawn from very deep down, and once pumped up, it may be too salty even to use on crops.
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Berkeley CEE professor and wastewater reuse expert Kara Nelson. Above, she is dwarfed by the filtration plant's vast system of underground pumps and pipes.
PEG SKORPINSKI PHOTOS
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Seven years ago, the Monterey Regional Water Pollution Control Agency (MRWPCA), the regional water recycling body, began taking urban wastewater from nearby municipalities (Monterey, Pacific Grove, Seaside, Sand City, Del Rey Oaks, Marina, Salinas, Castroville, and Moss Landing) and running it through an extra filtration process, making it pure enough to use again for crop irrigation. By sating much of farmers’ irrigation needs with recycled water, they have been able to reduce the amount pumped from underlying aquifers by 20,000 acre-feet per year. If farmers use less groundwater, that leaves more for residential customers without overtaxing the aquifers. The effort has already paid off, says Nelson—2004 was the first year in decades that saw a rise in the aquifer’s water level.
Although only seven years old, the MRWPCA‘s tertiary treatment plant already has a bottleneck. The need for irrigation water is greatest during the peak summer tourist season when full hotels and restaurants at the seaward end of this popular area are creating more municipal wastewater than the plant can pass through its filters. More accurately, the plant receives more than is legally permitted by Title 22 (the state law regulating water quality standards). That, as it turns out, is a key distinction, one that may point the way toward significant relief for the Salinas Valley’s—and California’s—water woes, says Nelson.
When Title 22 was adopted by the state legislature in the 1970s, it set the rate at which water could be loaded into tertiary filtration plants at five gallons of water per square foot of filtration media per minute (5 gal/ft 2 – min). Along with much of Title 22, that loading, or flow, rate has become the worldwide “gold standard” for tertiary filtration plants, says Nelson. And because it was known to be a product of the widely respected 1977 Pomona Virus Study, the figure was never critically assessed.
“They just picked 5 gal/ft 2 – min as the highest rate they were going to study and never looked any higher or explored the possibility that higher rates might also be safe,” says Nelson.
But four years ago a group of water reuse specialists looked critically at the flow rate limit and realized that it was more of a cultural artifact than a scientifically established limit. When the 1977 study was conducted, 5 gal/ft 2 – min was the highest loading rate tested.
“When it comes to finding new water sources, we thought all the low-hanging fruit had already been picked,” says Bahman Sheikh, a co-PI with Nelson and three other scientists on the Monterey project, known as Filter Loading Evaluation for Water Reuse, or FLEWR. In this case, however, some choice low-hanging fruit was hiding right behind some mistaken assumptions, says Sheikh, who was among the scientists who first thought of re-testing the loading rate for tertiary filtration plants.
“This was an area where a huge amount of good water might be safely recycled without a lot of new infrastructure or expense,” adds Nelson. The state is in desperate need of recycled water. If we can safely raise the loading rate to 7.5, we could increase the capacity of such plants by 50 percent, which is just incredible. It means more water. It’s as simple as that.”
To test the hypothesis, Sheikh and the team at MRWPCA recruited Nelson to design a study establishing the actual top end of safe filter loading rates for tertiary plants like Monterey’s. The challenge appealed to Nelson in part because of its potential international ramifications. She did her doctoral work on water recycling near Mexico City and has always been interested in finding affordable ways for developing nations to recycle wastewater for agriculture.
If the rate could be safely increased at the Monterey plant, it might be safe to raise it in nearly 100 other California tertiary treatment plants as well. “Beyond that,” says Nelson, “there are thousands of tertiary treatment plants around the world that take the Title 22 loading rates for granted. If many of them could safely increase their filtration rates too, the worldwide impact could be very significant.”
To find the safe upper limit for plants like Monterey’s, Nelson and graduate student Gordon Williams set about designing what they call “the Mercedes Benz” of pilot-scale plants. It is an impressive $300,000 apparatus, designed to mimic as closely as possible the workings of the tertiary wastewater treatment plant nearby.
The pilot plant is 18 feet tall, a model built at a scale of roughly 1/1000th of the actual plant. Its five test filters—made of clear PVC pipe so everything is easily observed inside—are however, the same height as those in the full-scale plant. “In the vertical direction, everything has to be exactly the same,” says Nelson. The water that enters the filters is the same water that passes through the main plant. And in both plants, the water is filtered through four feet of anthracite coal and one foot of sand before being disinfected with chlorine to kill any remaining pathogens.
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At the pilot plant he helped design and build, doctoral student Gordon Williams collects water samples to test for pathogens.
PEG SKORPINSKI PHOTO |
Each of the five experimental filters is fitted with an array of computerized systems to control flow and monitor water quality. About 150,000 data points are recorded in the computers per day, says Williams, who worked with specialists to develop the data analysis programs. The roughly five million data points accumulating over 40 filter runs will be analyzed to compare the effectiveness of the filters at different loading rates.
From the research, Nelson and her colleagues hope to learn more about the fundamental mechanisms by which wastewater particles and pathogens are removed by filtration. On the practical side, they are working closely with the State Department of Health Services to develop four criteria to assess performance of the pilot plant’s filters to see if higher loading rates are safe.
First, the team monitors removal of total coliform bacteria, which indicates removal of any pathogenic bacteria from the wastewater. Second, at regular intervals they add MS2 coliphage, a specific virus that can only infect bacteria, and monitor its removal. Third, they monitor particle removal in the 2-15 micrometer range—the size of the ubiquitous water-borne pathogens Cryptosporidium and Giardia—using an instrument that continuously shoots a laser across a vial of filtered water to count the suspended particles. The final indicator, turbidity or cloudiness, is a measure of the concentration of particles and a crucial indicator of the overall filter performance.
“Our preliminary studies suggest that a rate of 7.5 may be safe,” says Nelson. “The highest flow rate we’ve explored was twelve and one-half, and it was definitely too high. At that rate the filters clogged too quickly and treatment was poor. But the difference in performance between the 5 and 7.5 rates has been rather insignificant,” she says.
“The treatment process we are studying is not so terribly innovative,” says Nelson. “What’s innovative is that we’re trying to come up with an upper limit for safe filtration loading rates, and in the process, we are beginning to understand the effect of loading rate on fundamental treatment mechanisms. This is the first study looking in depth at these higher loading rates.”
The final results for FLEWR’s first phase will be completed this summer. If they are as promising as the preliminary results suggest, Nelson and her colleagues will petition the state to allow the actual Monterey tertiary plant, as well as four other treatment plants in the state, to operate their full-scale filters at higher flow rates. “We’ll analyze that data over about a year,” she says, “and if we get good results, we’ll ask the state to adopt a new state standard, changing the law so that all facilities can operate at this higher loading rate.”
Right now, wastewater recycling is already producing 500,000 acre-feet of usable water per year in California. If we could increase that by 50 percent, says Nelson, we’d have an extra 250,000 acre-feet of water annually. That would surely be one of the largest recent contributions toward mitigating the intensifying water crunch in the Salinas Valley, California, and beyond. It sure beats towing icebergs.
Gordy Slack is an Oakland-based science writer specializing in evolution and the environment. He is a frequent contributor to Forefront. His work appears in California Wild, Wired, Mother Jones, Bay Nature, and Sierra. |
