BioCycle November 2011, Vol. 52, No. 11, p. 39
Oakland, California’s wastewater treatment plant’s seven-year-old postconsumer food waste-to-energy program — the first in the United States — is ready to grow.
THE East Bay Municipal Utility District (EBMUD) in Oakland, California has been a pioneer in tapping the energy in food waste to boost biogas production in its wastewater treatment plant (WWTP) digesters. Now, the utility’s food-waste-to-energy program is taking some significant next steps. In the next 24 months, the District will begin a 120 tons/day (tpd) postconsumer food waste contract with an area hauler who is building a preprocessing facility at the WWTP, designate a food waste only anaerobic digester, start up a new 4.5 MW CHP unit that will generate all of the energy the plant needs to operate, and sell excess biogas-generated electricity to the local power grid.
“Nobody’s ever done 120 tpd before,” says EBMUD wastewater plant director David Williams. The treatment plant, which serves 650,000 residents in the East Bay, currently uses seven anaerobic digesters to process about 40 tons (10,000 gallons) of commercial postconsumer food waste each weekday. EBMUD charges a tipping fee to receive the food waste. The plant also receives 240,000 gallons/day of food processing waste for digesting. These materials are mixed with the municipal sludge (700,000 gallons/day are generated) and added to the digesters.
Located at the base of the Bay Bridge in Oakland, the WWTP now generates 90 percent of the 5 MW it needs to operate. It has three 2.1 MW internal combustion engines fueled by its biomethane. Downtime for the 25-plus-year-old engines, both scheduled and unscheduled, currently limits the plant from covering all of its energy needs. Additional electricity is purchased from the grid. Heat recovered from the engines is used to warm the digesters to their thermophilic operating temperature, and to provide heat elsewhere in the plant. When the new turbine goes online in early 2012, the plant will begin to produce more energy than it needs.
EBMUD began experimenting with the addition of food waste to its digesters in 2004 through a collaboration with Recology, a Bay Area resource recovery company. A 2007 bench-scale study by EBMUD, funded in part by an EPA research grant, (see “Green Energy From Food Wastes At Wastewater Treatment Plant,” January 2008) found that food waste generates about three times the biomethane as municipal sludge during anaerobic digestion. The study helped to accurately quantify methane gas production rates, mean cell residence time requirements, and volatile solids reduction values that are specific to food waste digestion. “It’s a really awesome substrate to be digesting,” says EPA scientist Laura Moreno, who worked with EBMUD to develop its program. The study also showed that food waste can be digested at higher concentrations, breaks down more completely, and has a shorter digestion time than sludge.
The Bay Area generates about 2,100 tpd of commercial food waste. Five percent or more of the 2,100 tons, from restaurants and grocery stores in San Francisco, Oakland and some rural neighboring counties, will end up at EBMUD when it completes the next phase of its program.
Recology collects over 500 tons/day of organic material including postconsumer residential and commercial food waste in San Francisco and processes it at its composting sites in the region (including Jepson Prairie Organics and Grover Environmental Products). The contract between Recology and EBMUD is to provide 120 tpd of food waste and process it at a new facility to be constructed at the plant. Preprocessing involves removing coarse contaminants and reducing the size of the material. The facility, which Recology will own and operate, will be a stone’s throw from EBMUD’s anaerobic digesters and food waste receiving area. It will have enough capacity to handle significantly more material than the current contract’s 120 tpd.
Allied Waste Services, a hauler that delivers about 40 tons/week of preprocessed food waste to EBMUD, is building a preprocessing facility in nearby Martinez, which will lower hauling costs. This 40 tons/week is from a commercial food waste collection program initiated by the Central Contra Costa Solid Waste Authority (CCCSWA). The preprocessing facility is on schedule to be completed in early 2012, says Allied Services general manager Tim Argenti, and will be capable of handling 60 tpd. Allied will continue to service CCCSWA’s diversion program. EBMUD is exploring new contracts with other haulers to supply food waste for the plant as well. “Converting food waste to energy is a can’t lose situation for us,” says Paul Morsen, executive director of CCCSWA, which has sent all of its commercial separated food waste to EBMUD since 2008 (see “Pioneering Partnership Optimizes Power Production).
HISTORY AND DESIGN
The EBMUD wastewater treatment plant, built about 60 years ago, incorporated 12 digesters to handle the high-flow, high-strength organic waste from the canning industry that thrived in the East Bay by the 1970s. Peaches, pears, spinach, tomatoes, fish and other foods fed an estimated 72 canneries that dotted Oakland, Berkeley and Emeryville. The last cannery, owned by Del Monte, closed in 1992. Loss of that industry left a lot of excess capacity at the plant.
About 10 years ago EBMUD started looking for ways to utilize the unused digesters. “We thought, why not explore other waste streams,” says Williams. EBMUD started with easily-processed waste like liquid from portable toilets and oils and greases from restaurants. In 2004, it added postconsumer food waste as a feedstock on an experimental basis.
One of the major hurdles to using postconsumer food waste as a feedstock for wet anaerobic digestion is that it must be processed prior to blending with other substrates, including preprocessing to remove contaminants. Unlike food processing waste, postconsumer food scraps can come laced with contaminants like cutlery, plastic, cloth and seafood shells. After experiencing seafood shell-clogged outlet pipes and cutlery-damaged pumps, EBMUD developed its patented food waste processing system for anaerobic digestion.
On the plant’s north end stand 12 white two million gallon anaerobic digesters. Five of them are not being utilized now. About 22 yards away lies the food waste staging area. The District’s food waste processing system works in several stages. The preprocessed postconsumer food waste arrives at the site primarily in tarp-covered transfer trucks that unload into one of three underground slurry tanks through ground-level doors. A heavy-duty rotocutter then grinds and settles out the large, indigestible items like stones, metal and seafood shells left in the mix. Incoming food waste needs to be in the 2- to 3-inch size range.
Next the mixture arrives at the heart of the processing setup, a four-blade paddle finisher that extrudes a ready-to-digest fine pulp through an interchangeable screen. The blades rotate along the interior of the cylindrically-shaped screen, extruding digestible pulp through its holes. Material not pushed through the screen is discharged into a nearby open-top hauler as a finely-shredded, landfill-bound reject. A progressive cavity pump then sends the pulp to the nearby digesters. The system processes about 250 gallons of slurry per minute and retains about 95 percent of the slurry’s biogas-generating potential.
Currently, processed food waste mixes with municipal sludge and is sent to all the active digesters. Food waste codigested with biosolids is a Class B product that is used as alternative landfill cover or applied to agricultural soils. Soon, the processed food waste will remain separate and feed a food waste-only digester. Digested food waste solids, after composting, can be used as certified organic soil amendment.
The dedicated digester will be managed similarly to the municipal sludge codigesters. It will operate at thermophilic temperatures with a minimum of 122 ºF and be initially started up with municipal sludge, then transitioned to food waste only. Higher degradation rates and thus shorter retention time in the digester are anticipated. According to EBMUD’s 2007 bench-scale study, food waste requires about a third less retention time than municipal sludge, can be processed at five times the concentration and produces three times as much methane.