It's Sound, Not Chemistry, That Conquers Algae for This Water Supplier

A southern California utility uses electronics to detect impending algae blooms in a recycled water reservoir — and then destroys them with ultrasound.

It's Sound, Not Chemistry, That Conquers Algae for This Water Supplier

The Vallecitos Water District serves about 100,000 customers in northern San Diego County.

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It’s an old project winning awards with new technology.

Vallecitos Water District in Southern California has supplied reuse water to neighboring communities from the Meadowlark Reclamation Facility since 1982. It was named Recycled Water Agency of the Year in 2009 by the WateReuse Association California Section.

The project’s latest honors, a 2021 national Excellence in Action award from the WateReuse Association and a 2021 award for Innovation and Resiliency from the California Association of Sewerage Agencies, are the result of a partnership with LG Sonic technology at the 54 million-gallon Mahr Reservoir, where the district stores recycled water.

A buoy manufactured by LG Sonic measures pH, temperature, turbidity, dissolved oxygen and other parameters to monitor algae levels and help control algae blooms. If a bloom appears imminent, the buoy can send an ultrasound signal that keeps the algae from rising to the surface, essentially stopping the bloom in its tracks.

Multiple users

The district still uses traditional methods, such as bleach and aeration, to help keep algae under control but “ultrasound is another tool in the toolbox,” says Chris Robbins, public information and conservation supervisor. “It’s not the only thing that gets done. We still use chemical treatments and so forth, but we don’t have to use as much.”

The district serves about 100,000 customers in northern San Diego County. Most wastewater is treated at the Meadowlark facility, which opened in 1958 with a capacity of 250,000 gpd. In 1982, it was converted to a water recycling plant with a capacity of 2 mgd. In 2005, it was upgraded to 5 mgd. The process includes sedimentation, roughing filters and a fixed-film biological process, along with activated sludge, chlorination and tertiary filtration.

The recycled water is sold to the City of Carlsbad for irrigation at Legoland, city hall and some golf courses, and to the Olivenhain Municipal Water District. The two municipalities are contracted for 2 to 4.5 mgd, depending on the season. Surplus water is stored in the Mahr Reservoir.

Collaborative approach

In a typical year, the reservoir level rises in winter, when irrigation demand is lower, and falls in summer. The abundant sunlight and the high nutrient level of the reclaimed water make the reservoir an ideal breeding ground for algae. Since the sonic buoy was installed in December 2018, algae levels are about half what they were previously.

Thanks in part to the region’s geography, all of the recycled water goes to customers outside the district. The reservoir can supply Carlsbad and Olivenhain by gravity. “The tricky part with any water, and with recycled water especially, is that it’s easier to let it flow downhill,” Robbins says. “We’re a little bit up, and we’re sending it down toward the coast.

“It’s just much cheaper not to have to pump it. If we were to send it back to our own system, we’d wind up pumping it.” The water gets pumped to the reservoir from the treatment plant through a 24-inch pipe. Vallecitos Water District is part of the North San Diego Water Reuse Coalition, a group of 11 water and wastewater agencies.

“The local entities all kind of work together to figure out what we should do with the recycled water to make sure that someone isn’t producing it and has no place to use it or put it,” Robbins says. “The coalition works together to define where there is a good distribution area.”

Big on solar

The Vallecitos district has emphasized sustainability for a long time. In 2006 it installed solar panels over the fleet employee parking area; that array produces about 90% of the headquarters building’s power, and sometimes a surplus delivered to the utility grid.

In 2017 the district has worked to add more solar energy. Solar arrays over two underground drinking water storage tanks (40 million and 33 million gallons) came online in 2021. Those projects and another at a lift station were built under 25-year power-purchase agreements with energy developers. They are projected to save $8.3 million over the terms of the agreements.

The district has won awards for its Sustainable Demonstration Garden and the headquarters building. The garden features native plants and two fountains supplied only with rainwater and operated by solar power. That garden, intended to be a demonstration for visitors to see what they could do with their own landscapes at home, was redesigned in 2010.

“Sustainability is something our board of directors has been pushing for about 15 years or so,” Robbins says. “It goes in stages.” 

Smart technology

The buoy that helps control algae at the reservoir was installed a couple years ago, and now the district is winning awards for using the technology. Besides pH, dissolved oxygen and turbidity, the buoy can monitor chlorophyll to detect emerging green algae and phycocyanin to detect emerging blue-green algae. It sends out different-frequency ultrasound to control the different varieties.

“It tends to predict what’s coming,” Robbins says. “It kind of samples, tells you what conditions you have, indicates the type of algae that may show up in the next few days. We can treat it chemically if we want to, but we can also use the sound function.”   


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