Land in the San Francisco Bay area is at a premium, making it a challenge to site organics processing facilities close to where the materials are generated. So when Doug Button, president of South San Francisco Scavenger Company (SSFSC) and its sister company, Blue Line Transfer, decided to take the organics processing plunge, he sought a facility design that could tuck into the existing footprint at Blue Line’s 11-acre site in South San Francisco that houses a materials recovery facility and waste transfer operations. The result: a fully enclosed one-quarter-acre facility comprised of an aerated receiving bay, 8 modular digesters, and two in-vessel composting (IVC) tunnels that can process 11,200 tons/year of food scraps and yard trimmings. SSFSC/Blue Line Transfer is receiving about 40 tons/day of food scraps and green waste.

SSFSC collects both commercial and residential organics in a service area that includes South San Francisco, Brisbane, Millbrae, Colma, San Francisco International Airport and San Mateo County. Soiled paper and compostable cups and plates are accepted. About 20,000 households in four cities are currently serviced with organics collection; residents can place food scraps in their 96-gallon green waste carts. “When we began the residential program in 2015, all customers received a 2-gallon pail for food scraps,” explains Button. “We started out offering every-other-week collection, but switched the frequency to weekly because of vectors. About 60 percent of households with the service set their carts out weekly. Overall, we’ve seen a 30 percent increase in the amount of organics collected, and about a 7 to 8 percent decrease in the amount of solid waste collected.”
Trucks servicing the residential organics collection routes are weighed coming into Blue Line Transfer and then unload directly at the receiving bay, which is 20-feet wide, 50-feet long and 28-feet high. There is minimal contamination in the residential loads. Trucks servicing the commercial organics collection route unload in the MRF, where a trained employee hand sorts all visible contaminants — primarily plastic bags. Cleaned material is loaded into a drop box, weighed and then moved to the aeration bay, which is about 100 yards away from the MRF. Yard trimmings and food scraps are mixed with a wheeled loader before being put in the bay.
Material stays in the aeration bay for 2 to 3 days prior to being loaded into one of the 8 digesters. The bay is operated under negative pressure; exhaust air is treated in an acid scrubber that uses sulfuric acid to cleanse gases. Gas from the acid scrubber passes through a biofilter made of wood chips and kept in a moist condition. “The digesters and IVC tunnels are less than 100 yards away from a 800,000 sq. ft. complex of Genentech buildings, as well as other neighbors, so odor control is a main priority,” notes Button.
The look and layout of the site also was front and center when building the plant. “We needed to locate in this area, which at one time was very industrialized,” he adds. “Today over 70 biotech firms are located here, where there once were animal slaughter and packing houses, and steel factories.  Our facility, although industrial by nature, could not have an industrial look. Therefore it was developed to a ‘campus standard’, where all of our buildings are more color coordinated and designed so that they blend in rather than stick out. We also installed a walking trail around the facility so that folks can walk or bike on the Bay Trail around the site and enjoy the peacefulness of the setting as opposed to hearing and seeing loud trucks and equipment.”

Digestion And Composting

SSFSC/Blue Line installed the SmartFerm dry fermentation anaerobic digestion and IVC composting system developed by Eggersmann, and supplied by Zero Waste Energy, LLC, the North American licensee. Each digester has capacity for 240 cubic yards of material. Digesters are loaded with an average of 62 tons/batch; design tonnage is 81 tons/batch.
On average, the mix going into the digesters is 60 to 65 percent food waste, and 35 to 40 percent green waste. “We like to keep the amount of food waste as high as possible in order to produce more biogas,” explains Button. “But there is a delta, where if too much food waste is in the mix, it may not all break down and generate the maximum amount of biogas in the 21-day digestion cycle.”
Makeup of the green waste varies by season. “In the first quarter of the year, people are pruning, plus we get Christmas trees,” he says. “In summer, there are a lot of plant cuttings, weeds and grass, so the particle size of the green waste is smaller. Overall, the green waste provides adequate porosity year-round.”
Once inside the digester, material is aerated using an in-floor air supply system until the material reaches 125° to 130°F. Exhaust air from this stage is sent to the acid scrubber and biofilter to remove ammonia, VOCs and particulates. After the desired temperature is achieved, the system switches over to the anaerobic process, with microbes consuming the available oxygen. Material is sprayed with a fine mist of liquid from the percolate tank, located under the digesters, facilitating decomposition and biogas production.
Retention time in the digester is 19 to 21 days. Biogas is produced almost immediately after the system is switched to anaerobic digestion mode. It is pulled into the percolate tank, where biogas from all 8 digesters (all loaded at different times) is mixed and homogenized. “Each digester is making different amounts of gas, so we need to capture it all in the percolate tank,” explains Button. “Biogas then is pulled into a storage bladder located on top of the digesters. Ever few hours, the bladder is drained by a small compressor. The biogas is cleaned up using a BioCNG conditioning system, and then put into storage.”

Once the methane content drops below one percent, and hydrogen sulfide is not detected, the digester can be opened. Material is moved to one of the IVC tunnels from 12:00 am to 4:00 am to minimize odor impacts. “There is a lot of ammonia in the digestate, so the main purpose of the IVC stage is to scrub out the ammonia before the material is transported to the composting facility,” he adds. “It stays in the IVC for 2 to 3 days.” The digestate is very wet at this point, so goes through a curing phase at the composting site for about 45 to 60 days before it is screened. Outbound tonnages from SSFSC/Blue Line are on average 48 percent of the weight of inbound tonnages.
Compostable products break down during digestion, and then are fully disintegrated following the curing phase. Button notes that plastic is the primary contaminant when material is removed from the IVCs.

Fleet Fuel

SSFSC has converted 27 of the 40 trucks in its fleet to run on compressed natural gas (CNG). It uses Cummins Westport CNG engines. The 27 trucks are fueled every night between 10:00 pm and 5:00 am using a slow fill system. The digesters produce 380 to 500 diesel gallon equivalents (DGE)/day of compressed renewable natural gas (RNG), depending on the quantity of food waste in the feedstock mix. Each truck utilizes an average of 30 gallons/day. Instead of fueling about a dozen of its trucks on RNG only, SSFSC uses it in all 27 trucks with CNG engines.

“The trucks are hooked up to the slow fill system in a line,” explains Button. “On week nights, each truck gets a little bit of RNG because that is how it is distributed. Then the tank is topped off with line [natural] gas, for a total of 3,600 lbs/fill. On weekends, however, when the trucks are parked for several nights, we cut the line gas, and are able to fill all 27 trucks with RNG. Otherwise, we would need a lot more biogas storage, or the gas would have to be flared.”
He adds that the RNG has a methane content of 92 to 94 percent, whereas the line gas is slightly lower. “The engines don’t know the difference between the two compressed gases. Both fuels perform equally well.”
The tail gases (lean gas) from the biogas conditioning system, such as propane and butane, are combusted in an on-site boiler. Heat generated by the boiler is used to warm the air for the IVC tunnels and heat the percolate tank. “No gas is being flared,” says Button.
The RNG has a carbon negative value of -23.95, which makes it a valuable commodity in California, which has aggressive greenhouse gas reduction requirements. “This is better than any other technology out there for reducing carbon emissions from fuels,” he notes. In 2015, 828 metric tons of CO2 equivalent were avoided, and about 48,000 gallons of diesel fuel displaced.

Facility Economics

The urban footprint in a nonindustrial area with neighbors in very close proximity resulted in higher than projected construction costs of the AD/composting plant. “As noted, we had to build the facility to campus standards,” explains Button. “One of my biggest lessons learned from this project is having your eyes wide open as to the costs. That said, I wouldn’t change anything we have done, other than wishing we could have built a larger facility here, as it would be great to make more RNG. This technology is a great solution for managing source separated organics.”
SSFSC received about $3 million in state grants and tax credits for facility construction. Solid waste tipping fees in the Bay area are $90 to $100/ton, which contributes to the facility’s revenue stream. Recently, the company sold its first Low Carbon Fuel Standard (LCFS) credits for the RNG, which are trading at $85-$100/credit. The LCFS program is an initiative of the California Air Resources Board, which administers the state’s GHG emissions reduction programs.
So far, notes Button, SSFSC has not been able to tap the federal RIN market, which is part of the U.S. EPA’s Renewable Fuel Standard (RFS) incentive program for alternative fuels such as biogas (classified as an advanced biofuel). “We first have to get certified to sell RINs, which has been an extremely difficult process,” he says. “In the meantime, we’ve essentially been flushing the RINs we could be earning down the toilet, as we don’t get credit for biofuel that has been produced to date. The LCFS credits, on the other hand, are retroactive, so we were able to sell credits generated from the day the facility started.”
Right now, savings from using RNG instead of diesel fuel are not being realized. “As a solid waste and organics collection company, taking this plunge made more sense when diesel fuel was $5/gallon versus $2 to $2.50/gallon,” Button adds. “But that will go up again.”