There are 49 miles of water main pipelines under the surface of Sonoma streets, providing water to the residents and businesses of town. But those pipes need to be flushed out every now and then to continue supplying clean, clear water.
That’s what hydrants are for – that, and emergency use by the fire department, of course. Traditional practice is to open hydrant valves out periodically – once a year is ideal – to flush out the sediment, corrosion and “bio-film” that adhere to the pipes over time.
But each time one of those hydrants is flushed out, it wastes 6,000 gallons of good drinking water. With 476 hydrants in town, those gallons could add up to almost 3 million a year.
At this end of the four-year drought that sort of extravagance seems unnecessary, if not uncool. “You can just imagine the outcry during the drought if we were sending thousands of gallons out into the street,” said Dan Takasugi, Sonoma City engineer and pubic works director.
Enter the Neutral Output Discharge Elimination System, or NO-DES. The closed-loop flushing system, like that tested on the residential streets in the southeast section of Sonoma last week, taps into two adjacent fire hydrants with separate high-volume fire hoses, then pumps a reverse flow inside the water main between them.
The “upstream” flow scours the pipes clean, but instead of flushing the water into the street and down the storm drain, the water stays in the main. (“The rain in the main stays mainly on Spain Street,” as Eliza Doolittle might sing.) The closed-loop system filters out the particulates down to 1 micron and adjusts the chlorination, too.
“We can probably flush about three miles of water main per day,” said Takasugi, who sounded impressed with the demonstration provided by Valvetek Utility Services of New York. “It’s the first time we tried it. It’s kind of an innovative system, I don’t think it’s very commonplace.”
One reason for its rarity might be the price tag: a truck- or trailer-based pump and filtration system costs about $365,000, far more than most small-town utilities can afford. Which is why Takasugi invited representatives from Cotati, Petaluma and the Valley of the Moon water districts to witness the demonstration.
“We like the concept and will continue discussions with the City of Sonoma on the feasibility of either finding a grant to help offset the cost or a partnership approach,” said Daniel Muelrath of VOMWD. “It is not likely to pay itself off by recycling the water, since system flushing accounts for only a quarter of one percent of our total water demands.”
As well as the reduced waste of water if a million gallons or so isn’t just flushed away into storm drains, advocates point out that discharge permits, fees and fines are avoided as well.
Other advantages, as stated on the no-des.com website, is that the system allows “Flushing at night to avoid customer outrage, and reducing overtime costs.”
Takasugi realizes the expense is a hurdle. “At this point, we are only considering the possibility of purchasing this system as a shared resource along with other neighboring water utilities,” he said. “No budget requests have been made toward a purchase.”
The demonstration wasn’t entirely free, however. “We paid a little bit of their costs, because they spend a whole day up here flushing hydrants,” said Takasugi. But he said they ended up “flushing a pretty good portion of our system – maybe one twentieth of our system.”
The technology has already been put in place in several South Bay communities, including San Jose and Hillsborough. Santa Cruz County’s Soquel Creek Water District approved the purchase this week, supported by funds from the district’s water demand offset program.
They are considering renting the $380,000 machinery to neighboring water districts to offset the investment, according to a spokesperson.
If the main purpose is to flush out sediment, some cities are far worse off than Sonoma, said Takasugi. “We tend to have less sediment, because most of our water travels all the way from the Russian River, so it goes through several tanks where some of the sediment settles out. Even still, we have some sediment and what we call a bio-film that forms on the inside of pipes. It’s good to flush that out.”
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