Wastewater treatment plant starts nutrient trading to improve discharge

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To help reduce nitrogen and phosphorus discharge to California’s impaired Laguna de Santa Rosa, the Sotoyome Resource Conservation District is working with City of Santa Rosa officials and wastewater treatment plant staff on a nutrient credit trading program.

The nutrient trading plan allows the wastewater facility to earn credits from projects to reduce nonpoint source pollution in the area. Such projects include improving stormwater runoff and reducing sedimentation.

Improving impaired waters
The city’s 21 mgd (design) Laguna Wastewater Treatment Plant in 2002 discharged 2.8 billion gallons to the Laguna de Santa Rosa, which flows to the Russian River. Because the laguna is an impaired water, the facility needed to reduce its discharge drastically.

Sotoyome district program director Valerie Minton says the city has made improvements to the plant to improve effluent quality, and has reduced treated wastewater discharge by reclaiming wastewater for irrigation on 6,000 acres of farmland and on parks and school landscapes. “But in very wet years, they have to discharge treated wastewater directly to the laguna, so they’ve been working with the North Coast Regional Water Quality Control Board for decades to cut back on pollution,” Minton says.

To further reduce discharge, the city developed the Geysers Recharge Project, a geothermal power plant that pumps recycled wastewater to the Geysers steamfields in the Mayacamas Mountains. When injected water comes in contact with rocks that have been heated by underlying magma, steam is created and travels through a pipeline to spin turbines, generating enough electricity for 100,000 homes. 

Improvements to the wastewater facility include changing the nitrification process. “In 1996, our activated sludge system was upgraded with the addition of anoxic selector zones in the first zone of the aeration tanks,” says plant superintendent Joe Schwall. “The upgrade was made with sludge conditioning in mind to improve settleability by reducing Nocardia. Nitrate reduction was an added benefit, but the system was designed for sludge conditioning, not nitrogen removal.”

City deputy director of subregional operations David Guhin adds, “We currently nitrify through a standard activated sludge process, then denitrify in anoxic selectors with pumped mixed liquor recycle for nitrate feed.

“This process reduces nitrate by about 30 percent, to about 10 mg/L. Phosphorus is reduced by about 25 to 50 percent through our activated sludge system. We also have a study under way to look at other process changes that could provide additional nitrogen and phosphorus removal.” 

Discharge limits
Despite all that, the city received notice a few years ago from the Regional Water Board that it had to achieve zero net discharge of nitrogen and phosphorus because the city does not have a current Total Maximum Daily Load (TMDL) in effect for the laguna. Getting discharge levels down to zero was just not possible.

“The board saw huge expenditures and efforts the city had made to improve its treatment facilities and find every way to recycle water,” says Minton. The board still required zero net discharge but allowed the city to achieve it through offset projects.

The municipal water quality credit trading market plan allows the wastewater facility to assist other areas of the city. “The city can pay for projects that will prevent nonpoint source pollution, and calculate how much nitrogen and phosphorus those projects keep out of the laguna,” says Minton. “For example, if it keeps 20,000 pounds of nitrogen and phosphorus out of the water, that means the city is within their legal right to discharge that much if they need to.”

The credit trading program is in its early stages. “The program provides us with a method to attain compliance with the zero or net loading requirement until a TMDL is developed and offers a market place for obtaining or trading credits as needed post-TMDL,” says Guhin.

The city works directly with the Sotoyome district to organize the projects for trading credits. “We help the City of Santa Rosa with their immediate offset credit needs, and we help them develop the projects and get them done,” says Minton. “We are the organization that works directly with the landowners.”

Santa Rosa is the only area city now involved in credit trading, but surrounding municipalities might eventually need credits, as well. “Most surrounding cities send their wastewater to the Santa Rosa plant,” says Minton. “When the TMDL is completed, cities will have to make some pretty significant reductions in pollution from their stormwater systems, and it may be effective for them to use a combination of upgrades and the offset credits.”

With a TMDL in effect, cities will be able to trade credits with each other, and Santa Rosa then could sell unneeded credits to another city. “We foresee a market with multiple buyers,” says Minton.

In the works
The Sotoyome group is working with the Beretta Dairy in the Laguna de Santa Rosa basin to improve stormwater runoff from manure handling. “The dairyman was more comfortable working with the us rather than with the city,” says Minton.

Another project in the works is reducing sedimentation into local creeks at the 3,120-acre Pepperwood Preserve northeast of Santa Rosa, at the headwaters of three creeks that are key tributaries to the Russian River watershed.

“I think the nonpoint source projects we do will have benefits to the watershed beyond nitrogen and phosphorus,” says Minton. “For example, reducing erosion keeps sediment out of the water, but that’s also a huge fishery issue. Fishery benefits are really important to the beneficial uses of the entire watershed.”

Credit sharing between water departments and even across city limits might be an avenue for other treatment plants to consider when TMDLs come knocking. “It’s a really creative way to deal with water-quality issues, and I think it’s going to end up with a net benefit for the watershed,” says Minton. “Because the water board has to be conservative about how many credits they allow, the projects we do could end up having more benefit than what they’re offsetting.” 



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