A California city gears up for a biogas production project that will convert food waste from schools and businesses into clean gas for vehicles.
Removing food from solid waste streams to preserve landfill space is nothing new, but one utility in California has plans to turn that waste into a big cost benefit.
In a few years, if plans in Manteca bear fruit, city trucks will be essentially running on food — more specifically, methane generated from anaerobic digestion of food waste.
The project is the result of two sets of state regulations. One is the standard to reduce food waste entering landfills. The other is the set of air pollution rules from the California Air Resources Board that provide an incentive to leave diesel engines behind in the quest for cleaner air.
Although the California food recycling law is just beginning to take effect, the city of Manteca plans to have a food waste collection and composting system up and running for all of its large food waste producers by the end of summer 2016.
For 2016, the state says that any entity producing 8 cubic yards or more of food waste per week must recycle it. That drops to 4 or more cubic yards in 2017, and may decrease to 2 cubic yards in 2020 if the state determines there is less than 50 percent recycling. “Rather than waiting to get to that point, we decided to come up with a plan now,” says Jeremy Kline, the city’s solid waste supervisor.
Creating the plan
The food waste collection will focus on businesses and schools. “If you look at a map of Manteca, it’s broken up into four or five major shopping areas,” Kline says. There are some small independent restaurants in-between, but the chain restaurants in the large shopping centers account for most of the food waste. Some of the city’s independent restaurants are small enough that their waste volume may not meet the state threshold.
All participating restaurants receive a 30-gallon orange trash can. As in other municipalities, Manteca’s waste bins are color-coded: brown for trash, blue for recyclable plastics and glass, green for yard waste. “With this system, once we start diverting it to the wastewater treatment plant, the food waste is as pure as you can get,” Kline says.
For the moment, the food waste the city collects is not going to the wastewater treatment plant for methane production. Instead, it is taken to a county-owned waste transfer facility where it is mixed with yard waste and other green materials and then shipped to a composter. Methane generation will happen when the treatment plant is expanded to accommodate more anaerobic digestion and when a new food waste separator is added to the waste transfer station.
The team is looking at a turbo separator (Scott Equipment Company) that will accept even packaged food. Waste coming from restaurants and other sources will dump into a bin, where an auger will push it into the separator. Inside is a shaft fitted with paddles and rods of various shapes. Some of these break open food containers by pinching them against the separator housing; others squeeze the material through screens. The resulting slurry drops out the bottom of the separator. Pieces of plastic and metal fall onto a conveyor that deposits them in another bin for shipment to a materials recycler.
When the treatment plant expansion is complete, the food slurry will be trucked the 5 miles from the transfer station. “Solid waste and wastewater are now partners in this project, and we are working on the facilities we’ll need,” says Heather Grove, wastewater systems superintendent. The plan is to construct two receiving stations, one for the food slurry and another for fats, oils and grease. There will be a new control building, two new digesters, gas compressors and fueling stations. The estimated cost for the projects, including the separator, is $29 million.
The city has discussed allowing other haulers to use the turbo separator at the county transfer facility. “That is a long-term vision, something we’ll look at once we have the process functioning with Manteca-only material,” Kline says. “At that point we would work out a price structure.”
The project also includes rehabilitation of the plant’s two existing digesters. “They’re aging. They need new mechanicals inside and new lids,” says Grove. “The existing lids are concrete, and they’re cracked. Even without this project, we would have to put new lids on.” The new digesters will be 65 feet in diameter, versus 60 feet now, with a capacity of about 750,000 gallons each.
At the treatment plant receiving station, food slurry will be mixed with sludges and fed to the digesters. The city now takes in about 36 cubic yards of food waste per week.
When collection expands to the entire city, trucks will bring in about 1,500 wet tons per week.
No fracking, no drilling
In the digesters this additional feedstock will produce nearly 18,000 cubic feet of methane per day. Gas coming out of the digesters is processed through a scrubber to remove water and contaminants and then through a SulfaTreat (Schlumberger) system to remove hydrogen disulfide. There has been talk about selling the resulting compressed natural gas to the public, Kline says. Waste management in nearby Stockton is converting its fleet to CNG, but there is only one fueling station in that city.
“That provides an opportunity for us,” says Kline. “Being on the edge of the San Francisco Bay Area, clean energy is an emphasis here, so more people have natural gas vehicles.” CNG sales would be primarily to fleet or transit vehicles because most average motorists do not have CNG autos.
The primary gas user will be the city’s own fleet of solid waste trucks. The city now owns four natural-gas-fueled trucks: one for the wastewater department and three for solid waste. Beginning in 2017, the city will replace four diesel-fueled trucks with CNG equivalents each year, Kline says. This falls within the normal replacement cycle, but is also driven by state air pollution laws. When the change is complete, about 20 city trucks will run on CNG.
Included in the CNG expansion plans are 22 fueling stations at the treatment plant. Two will have high-pressure pumps and lines to fill tanks within minutes. These may be available to the public, and they will be able to quickly top off the supply for any of the city’s trucks. The other 20 will be low-pressure stations where trucks can be plugged in to fill overnight. If plans and funding come together as expected, construction will end in November 2017.
The waste process
The Manteca Wastewater Quality Control Facility uses activated sludge treatment followed by Aqua-Disk cloth-media filters (Aqua-Aerobic Systems) for tertiary treatment and UV disinfection (WEDECO - a Xylem Brand). Waste activated sludge thickened with dissolved air flotation moves to the two existing anaerobic digesters. A centrifuge (Centrisys) dewaters the material to produce cake. Biosolids are sent to a landfill to be used as alternate daily cover.
The plant also has several treatment ponds. The largest holds secondary effluent only. Next to it is a small pond that takes wastewater directly from a food processor on the east edge of the city; this pond is aerated with rotary brushes. Water from the two ponds is blended to irrigate alfalfa and other forage crops on 200 acres surrounding the plant. A third pond equalizes flow through the UV system, followed by discharge into the San Joaquin River. Chlorination of a portion of this water qualifies it as recycled water. At the moment, Manteca’s only permitted use for this water is in dust control, but an upgrade will allow more uses.
Running the wastewater operation are: Dustin Valiquette, chief plant operator; Jonathan Clark, William Jenkins, Justin Nave, Cody Robinson, Chris Rudolff and Josh Zamora, wastewater operator III; and Robert Bennett and Kevin Mello-Hall, wastewater operator II.
Spending means saving
The Manteca team hopes to offset the cost of the plant expansion and biogas project in three ways. First is a $1.8 million grant the utility received through the San Joaquin Air Pollution Control District, and that is enough to build some of the CNG refueling stations.
Next are diesel fuel savings once the biogas operation is producing at full capacity and the city is running trucks on its own fuel supply. Last is a rate hike. The city is completing a rate study now; the price structure has been the same since 2003. “This project aside, it’s generally time for us to consider rate increases,” Kline says.
At a time when food represents about 15 percent of all municipal solid waste but only 2 percent of waste recovery, the Manteca waste team is setting itself apart and ahead with its efforts. “It’s like the ounce of prevention,” Kline says. “It made more sense to start now and break the project into bite-sized pieces than to wait and be overwhelmed in 2017 or 2018. Now we have cities around us calling and asking how to do this.”
Laying the groundwork
Although the biogas project at the Manteca Wastewater Quality Control Facility has not begun, work with the community is in progress. The city handles all food waste collection, domestic and commercial. That means picking up waste from the city’s four large shopping areas and from 17 elementary schools and two high schools.
Since December 2015, Jeremy Kline, solid waste supervisor, has been meeting with businesses and agencies to explain the food waste recycling system. “When I meet with customers, I explain that if they divert food out of their trash, their bill should get smaller because they’re not throwing away as much,” Kline says. “We’re not charging for the food recycling.”
Charges are based on the size of the waste bin each business uses. “So in theory, if they’re diverting 30 or 40 percent of the material that would go into a bin, they should be able to reduce their trash bills by 30 or 40 percent, too,” Kline says.
The only objection came from custodians at area schools because the food waste bins become another trash container they must look after and manage. But here the city took a step to ease the load on the school staff. “We enlisted the help of the fifth- and sixth-graders to help manage that recycling,” says Kline. “We educate the kids about how much food they throw away, but the kids are also helping with separation.”
Students used to dump all their trash into a single container. Now they approach a series of bins where the fifth- and sixth-graders direct the younger students where to put waste food, waste paper and recyclable foam trays.