Digesting California’s Innovations

Conference tour visits wastewater treatment plant, dairy digester
By Anna Austin | February 21, 2012

The Pacific West Biomass Conference & Trade Show kicked off with a day-long tour that featured one of the country’s most innovative wastewater treatment plants, as well as an on-farm dairy digester power system.

The city of Santa Rosa’s 21-million-gallon-per-day Laguna Wastewater Treatment Plant houses four 1-million-gallon anaerobic digesters that turn sludge separated from the water into Class B biosolids, which are sent across the street to a facility that transforms them into Class A biosolids.

Biogas emitted from the digesters is blended to a 50/50 mix with utility gas and is then combusted by three 0.8 MW engines to create a total of one-third of the plant’s electrical needs. A combined-heat- and-power (CHP) project to replace the existing facility is expected to be finished in December, according to Cayden Hare, city intern and aquatic biomass program lead researcher.

When that’s complete, four 1 MW engine generators will burn the biogas, and serve as standby power for the plant’s needs during utility outages.

Uniquely, some of the plant’s wastewaster is diverted to six channelized wetlands, where aquatic biomass cleans the water, absorbs pollutants and is allowed to grow. The biomass is eventually transferred into two 1,500-gallon vertical anaerobic digesters, Hare explained, along with waste glycerol and winery leaves. The resulting biogas is used to power small cars used on site, according to Hare, and the remaining half of the materials are composted.

At the Giacomini Dairy at Point Reyes, Calif., tour attendees viewed an ambient temperature, covered lagoon anaerobic digester fueled with manure from about 350 cows.

With financial aid from the USDA and other government grant programs, the digester has been operating 24/7 for the past two years, according to digester technician Douglas Williams. Biogas emitted from the digester is captured through a gas collection system and then piped to a gas handling system.

Electricity produced from the biogas via an 80 kilowatt generator powers the farm’s operations, and excess electricity is purchased by electric utility PG&E, according to Williams. “There’s also a CHP aspect, as heat coming off the engine is used in the dairy and creamery,” he said.

—Anna Austin