Mike Hogan walks toward a small pile of treated sewage sludge, a moist, black, lumpy, faintly malodorous substance inside a newly expanded processing plant at Encina wastewater treatment facility in Carlsbad.
“That’s what biosolids look like before going through the new heat drying process,” he yells over the drone of fans and heavy machinery.
Nicely dressed in a blue button-down and dark blue slacks, the 58-year-old Hogan is the general manager at Encina, the second-largest wastewater facility in San Diego County, serving 330,000 North County residents. Set to retire only days after we talk, Hogan is pleased to say that Encina has finally addressed the concerns that plague wastewater facilities throughout the county: how to dispose of biosolids. The new drying system turns the nutrient-rich biosolids into pellets that are cheaper to transport and can be put to more productive uses.
Most wastewater facilities produce class B biosolids, a highly regulated cakelike material containing low levels of pathogens. Class B biosolids can be used to fertilize crops and amend soil, although often they are trucked to landfills. The Encina plant produces class A biosolids, a pathogen-free dry pellet that has few restrictions on usage. The pellets can be sold as fertilizer for homes, golf courses, and parks and are also permitted for use as a biofuel.
“We produce a product now, instead of a residual material,” says Hogan. “Being a class A biosolid, the product has more outlets in the marketplace.”
The new system, which was finished in January and has been operating since February, dries the biosolids in a 30-foot-long drum. “The heart of the drying system is a triple pass rotary drum direct drier,” writes Encina project engineer Duane Larson in an email. “Heated gas is drawn through the rotating horizontal drum by an induced draft fan. Wet biosolids are fed into the drum at about 25 percent solids and move laterally through the drum emerging dry at the discharge end.”
Larson says temperatures must reach anywhere from 700 to 1100 degrees Fahrenheit to evaporate the water and kill the pathogens.
“The pellets are further processed by separation from the air stream, cooling, and crushing oversized material and screening it to a uniform size. The final product is a one-millimeter pellet with a solids content of 93 percent.”
During 2007, San Diego County’s seven sewage facilities produced nearly 302,000 wet tons of biosolids. Of that, 160,000 tons were hauled to farms outside Yuma, Arizona, to fertilize crops. Fallbrook sent its biosolids to Riverside County to be composted. The rest was sent to landfills in Otay Mesa and Yuma, where the material was used to cover garbage at the end of each day.
In recent years, communities near areas where biosolids have been trucked have enacted ordinances to restrict or ban them.
“Type B biosolids sometimes contain levels of pathogens that are not acceptable for food production, and then there are the industrial chemicals and medical waste,” explains Ron Lew of the California Integrated Waste Management Board. “People just don’t want those things on the food crops.”
“Also,” adds Fernando Berton, the board’s senior specialist, “many of these counties don’t want to be the recipients of these kinds of materials from other counties. They’re saying, ‘You produce it over there, you manage it over there.’ ”
The new ordinances have forced San Diego facilities to truck their biosolids ever farther. Over the past seven years, the Encina Wastewater Authority has spent $2.2 million each year to haul its biosolids 160 miles to a 12,000-acre farm in eastern Yuma County, where the material is used to fertilize alfalfa and Sudan grass.
In 2003, Encina officials responded to the rising costs and environmental concerns by exploring new ways to dispose of their biosolids. They settled on building the new drying system.
“We were already in the stages of expanding the plant,” says Hogan. “We couldn’t get rid of our biosolids where we thought we could, and our costs doubled. So we thought about our options, and this is where we ended up.”
Hogan says that the expansion project, including the drying system, cost $47 million. “The cost of doing what we did, and the cost of what we would have done was basically a wash.”
By pelletizing the biosolids, transportation costs will be lower. Hogan estimates that the weight will be reduced by about 80 percent, from 38,000 to 7000 tons per year.
Although Encina is still trucking biosolids to Arizona, Hogan thinks within the next two years the product can be packaged and sold to the public as fertilizer. Encina can even sell its class A pellets to counties that banned the class B biosolids.
And there’s the energy value.
“What we’re finding is the cement industry in California is very interested in diversifying their fuel portfolio with alternative fuels,” says Hogan. “They approached us, and we have been in discussion with several of them to buy our product.”
According to an October 2008 study by the Environmental Protection Agency on the cement industry’s use of alternative fuels, a performance test using biosolids reduced the coal feed rate “from 10 tons to 3 tons per hour, with good emissions results.”
While Hogan didn’t want to mention specific cement companies, in a February 2009 newsletter, Encina announced that negotiations had begun with Cemex, a cement producer that has its largest plant in Victorville, California.
“Cemex’s primary fuel for the plant kilns is coal,” reads the newsletter, “but they are in the process of expanding their alternative fuel supply to include biosolids pellets along with used tires and wood chips. Securing an agreement with Cemex meets one of the priority objectives of [the Encina Wastewater Authority’s] 5-Year Biosolids Business Plan, which is to secure a position in the evolving biofuel marketplace.”
Jennifer Borgen, spokesperson for Cemex, said the company does not have any contract with Encina to use biosolids for biofuel but said Cemex is “always looking at and researching several alternative-fuel sources to see if they are the right choice for our plants in the U.S. and across the globe, to provide environmental and societal benefits.”