Co-digestion of high-strength organics

California agencies leading the way toward best practices, increased biogas production. By Jim Force

As co-digestion begins to attract the attention of the U.S. wastewater sector, a number of California treatment plants are already digesting high strength organics with their biosolids and learning valuable lessons from the experience.


Greg Kester, Directory of Renewable Resource Programs, CASA

According to Greg Kester, Director of Renewable Resource programs with the California Association of Sanitation Agencies, nearly one in five of the state’s treatment facilities are practicing some form of co-digestion, with the trend accelerating.

“There has been a steady increase in co-digestion, especially with FOG, and in some cases, food wastes,” says Kester. “CASA is tracking the trend. We are at 17 percent of plants using co-digestion right now, and that number is increasing almost daily.”

Lauren Fillmore, senior program director at the Water Environment Research Foundation in Alexandria, Virginia, adds that the trend toward co-digestion is up, nationwide. “More and more of our subscribers are looking into it,” she says.

“Five to 10 years from now co-digestion will likely be a standard practice,” Kester predicts.

Many of these practices will likely result from the experience being gained by plants like Fresno and East Bay Municipal Utility District, among others, which have been co-digesting food wastes with biosolids for several years; and the Los Angeles County Sanitation Districts Joint Water Pollution Control Plant at Carson, where a full scale demonstration project is currently underway.

These installations are developing recommendations for such tasks as pre-treating and removing contaminants from food wastes, unloading tankers, controlling odors, blending wastes with biosolids, managing and training staff, and more. CASA recently completed an evaluation of excess capacity for co-digestion at California wastewater treatment plants and estimates that up to 75% of food waste currently landfilled could be accepted. Kester notes that the estimate is dependent on a variety of factors including operational limits, incentives for infrastructure needs, flow demands of connected users, the market and ability to beneficially use the increased methane and biosolids.

A first at East Bay MUD

Jackie Zipkin, Environmental Services Division Manager, East Bay MUD

The 60 mgd East Bay MUD Main Wastewater Treatment Plant was one of the first municipal wastewater utilities in the U.S. to take in trucked food processing waste, says Jackie Zipkin, environmental services division manager.

“When our wastewater plant was constructed,” she says, “there were a lot of food processors in our area—canneries, yeast operations, etc.—and they were delivering heavy (wastewater) loads to the treatment plant.”

In the years since, many of these food processors moved away, she notes, replaced by commercial-residential developments that provided lower loadings to the treatment plant. “We had excess capacity, especially in the solids processing area, so in 2002, we started taking chicken blood waste from Foster Farms, a chicken processor.

“Since then, we’ve grown to the point where we’re taking over 100 trucks a day of a variety of wastes. Last year, our revenue from trucked-in wastes was $11 million—a significant benefit to our rate-payers.”

Zipkin says the vast majority of waste co-digested at East Bay MUD is liquid. The waste is taken in at three
locations—the head of the plant for lower strength wastes such as septage, and two high strength receiving facilities connected to the digesters via blend tanks.

“Trucks connect by hose to the receiving tanks, and the liquid waste is blended with our sludge,” she says. “We employ a rock removal step but there is very little pre-processing required with the liquid wastes.”

In addition, the plant receives an average of eight tons per day (tpd) of post-consumer solid food waste from RecycleSmart the Central Contra Costa Solid Waste Authority. It is tipped into a slurry tank and a rock trap and paddle finisher remove plastics and other material, according to Zipkin. The waste is then blended with sludge in the same blend tanks as the liquid waste.

Fresno taking FOG

In May 2011, Fresno completed construction of its co-digestion project at the 80 mgd Fresno Regional Wastewater Reclamation Facility, installing three 16,000 gallon receiving tanks. Each tank contains a dedicated chopper pump and rock trap.  Progressive cavity pumps move the tank contents directly into the plant’s primary sludge lines which feed the digesters.

“We have three off-loading stations, each with separate truck connections and FOG tank. We can handle three trucks at a time,” says Kevin Norgaard, supervising professional engineer with the city’s wastewater division. “Some weeks, we can average six trucks a day.”

The project included a new electrical building with controls. The off-loading stations have a lighted signal system– green for ready, red for in discharging into primary sludge line, and yellow for truck unloading.

“The drivers activate the pump automatically when they are ready to unload,” explains Norgaard. “Using an electronic badge system, they deactivate the pump when their truck is empty.”

The load is measured automatically and the FOG is mixed with an equal volume of primary sludge, and the blend is fed to four of the plant’s 13 anaerobic digesters via the primary sludge loop.

“Eventually, with enough load, we would feed all 13 digesters,” says Norgaard. “We have excess digester capacity. Our required detention time is 15 days and averaging in the low to mid 20s.” Norgaard explains that mixing the FOG with primary sludge provides heat to the FOG to keep it from clogging the pipes. Plus, the gritty material in the sludge helps scour the pipe walls and prevent grease buildup.

“When we investigated co-digestion with FOG, we talked with other plants who stated they heated their lines to prevent the FOG from clogging the pipes,” he says. “We do have provisions to heat the FOG tanks with hot water and heat exchangers if necessary, but we haven’t had to do that.”

In addition to FOG, Fresno accepts turkey blood and syrupy-type food wastes from area food processors, but no actual post-consumer food wastes yet. “We are open to more volumes and streams, and periodically we discuss it with area companies,” says Norgaard. “In order for the city to accept those types of waste, they would have to add water to create the appropriate slurry.”

Demonstration project at LACSD


David Czerniak, Senior Engineer, LACSD

The Los Angeles County Sanitation Districts are operating a full-scale co-digestion demonstration project at its 280 mgd Carson facility. “We started back in 2013, after signing an agreement with Waste Management to partner with us on the demo,” says David Czerniak, Senior Engineer. “Waste Management brings in food waste as a slurry and we pump it into a full-sized digester for co-digestion. While our target is 85 tons per day of food wastes, we are currently receiving approximately 28 tpd, delivered to the plant in 5,000-gallon tankers.”

The waste is offloaded at the receiving station, stored in receiving tanks, and then metered to the test digester.

Waste Management solicits grocery stores, food processors and restaurants, who source separate the food
wastes. “That’s their job,” says Czerniak. “Then they process the wastes at their processing facility, sorting, and bio-separating, taking out the inerts and contaminants. They add water to make the material pumpable at about 13-14 percent solids.”

The Carson facility has 24 active digesters, each holding about 4 million gallons apiece, and each receiving about 200,000 gallons of biosolids daily,” Czerniak explains. “On the one digester we are testing, we are adding an additional 9 percent (by volume) of food waste.”

Lessons learned

As the California plants advance the practice of co-digestion, important lessons and best practices are emerging, especially in the area of handling and processing food wastes, odor control, digestion, as well as staff management.

Removing contaminants from the incoming food waste stream is a leading concern.

“When we started, we had a minor issue with a small amount of contaminants (shredded plastic bags, and food containers like ketchup packets) getting into the digester and causing a thick mat to form on top of the digester material,” says Czerniak of LACSD. “The concern was that this would impede mixing and add excessive build-up in the digester.  Waste Management took that seriously and made modifications to their bio-separator. It’s under control now.”

Adds Zipkin: “While processing solid waste onsite is trickier than liquid waste, we’re focusing on solid waste because the waste is local and is always going to be here. Contamination is the biggest issue; we’re trying all sorts of things – screens and cyclones, for example—to remove grit, and plastics and particles.”

Quality control is also important. “You need to pay attention to what they are bringing you,” says Norgaard at the Fresno operation. “We always test every load (using the in-plant laboratory). Every new hauler brings us a sample first, so we can make sure it’s worth it for us to take it. We were asked to take cheese whey once, but it was high in EC (electrical conductivity).”

Another issue is the type of tanker being used, says Czerniak. “To begin with, Waste Management was using flat tankers and the food waste had to flow all the way to the back to be off-loaded. They were trying to keep the solids content as high as possible, since it costs money to haul water. Ultimately they used a new tanker truck with a sloped belly, so the material only has to flow half way—from the front to the middle, or the back to the middle.

“It’s much faster now,” Czerniak adds. “Before it might have taken an hour and a half to pump out a truck and wash it out. Now we’re down to 20 minutes.”

Truck unloading has been an issue elsewhere. “If we did it again,” Norgaard says, “we would change the level in the loading area. Ideally, if we had a 10 percent slope there, our trucks could unload faster.”

Fresno has also learned something about feeding food wastes into the digestion process. “One thing we looked at was how fast we could dump FOG into the digester,” says Norgaard. “Originally we thought 50 gpm might be too fast. But we’ve learned we can’t do it too fast (minus the gas production spikes). Even at 200 gpm, it doesn’t seem to hurt anything. As soon as we get it, we dump it right into the digester. Also, we find the FOG improves the health of all the digesters. Rather than using just one dedicated digester, we use our existing primary sludge line as our delivery system (to four digesters).”

Norgaard also advocates having a redundant system in case the line plugs due to grease buildup. “In our case,” he says, “we could still use our old primary sludge line if the other line plugged. But our highly automated process doesn’t allow the FOG to congeal. We mix it and inject it. If you didn’t mix it with primary like we do, you could heat it (to prevent congealing).”

He also recommends PVC or glass lined pipe as opposed to ductile iron. “PVC is cheap and it is similarly smooth like glass,” he notes, “so grease doesn’t build up as easily as it would with ductile.”

At LACSD, odors have required special attention. “We have a lot of businesses and homes close to the Carson plant,” says Czerniak. “Food waste doesn’t smell very good. It comes in an enclosed tanker but when we opened the lid, it created a vacuum and we’d get a puff, a plume of odors coming out of there.

“Now we hook a four-inch hose up to our odor control system, which consists of passive carbon filters. The air and headspace in the tanker pushes the odorous air through the canisters and discharges it to the atmosphere. It’s completely sealed. We’ve had no complaints.

“Our research group does all the lab work and reports results. It’s a team effort among several departments and Waste Management.”

Gas production.

So what happens to biogas production when you feed food waste into the digester?

“We have some numbers,” says Czerniak. “When we add 205,000 gallons of sludge to a 4 million gallon digester, we get approximately 320,000 cubic feet of gas per day. By adding 7,000 gallons of food waste, we get roughly an additional 90,000 – 100,000 cf of gas a day.”

LACSD is seeking ways to use the extra gas. “Exporting power is one option,” Czerniak says. “Pipeline injection is another. If we could upgrade and condition the biogas to natural gas standards, we could inject the gas into the natural gas transmission lines and enter into vehicle fuel contracts. Right now we have a CNG fueling station at the plant that is open to the public. We could use biogas instead….and go green.”

Drying biosolids would be still another possibility at LACSD. “Right now our centrifuges dewater the cake to 29 percent solids,” explains Czerniak. “We still have 70 percent water. Drying could produce a 90 percent solids material (or more) and reduce our hauling costs. That’s currently being evaluated at the plant.”

At Fresno, Norgaard says the impact of food wastes is instantaneous and dramatic. “Some loads are hotter than others,” he says. “We get a spike in gas production. That can have an impact on our gas conditioning system and our turbines. Right now we are working on holding FOG deliveries and injecting them over a longer period of time to help smooth out the spikes in gas production.”

Increased gas production is part of the reason East Bay MUD is taking food wastes, according to Zipkin. “With biogas, we are generating about 135 percent of our energy needs and selling the excess back to the grid.

“As we scale up with food wastes, we are paying attention to sulfur content. Another thing is the gas conditioning system. At present it is a bottleneck, and we are looking to expand that.”

East Bay MUD is also developing a gas refining system to produce CNG transportation fuel. “We will wholesale it in canisters and deliver it to a fueling facility, says Zipkin. “In the future, there’s potential for the trucks to come in to the facility, drop their wastes, and fuel up.”

Increased gas production can have an impact on costs.

“Co-digestion will probably be cost-effective in the long run,” says Norgaard at Fresno, “but we did it for other reasons. We are working on the ability to provide energy to people needing renewable energy sources.

“We haven’t had to add extra staff with co-digestion,”he adds.  “Our original cost analysis predicted a 10-20 year payback.”

Kester agrees that co-digestion could ultimately wind up being cost positive, “New costs for co-digestion of FOG are pretty minimal, generally requiring holding tanks and mixing, and a rock trap or some screening device,” he says. “Heating of the material may or may not be required.”

On the other hand, co-digestion of food waste can be more involved, according to Kester, because of pre-processing, screening out of organic masses, and paddle finishers to get the material to a pumpable slurry.

“But through the use of existing digesters,” he says, “new anaerobic digesters generally don’t have to be built, and there are new options like converting the biogas to transportation fuel (that may help make co-digestion cost-positive).”

The value of the biogas could be improved even further if alternatives such as direct injection into the common carrier pipelines owned by the investor owned utilities  were cost-effective. “The cost to interconnect is a formidable hurdle today, as well as the pretreatment requirements for the biogas,” Kester says. “Discussions are ongoing at the State level and financial incentives may ultimately be
available to help make co-digestion a cost-effective option.”

The future.

East Bay MUD, LACSD and Fresno all may increase co-digestion in the future.

New on-site pre-processing facilities and new equipment being planned now will enable East Bay MUD to accept as much as 190 tpd of commercial source-separated food wastes—up from 8 tpd currently. The new facility will be for commercial source-separated food waste, similar to what is received now, and will have a bag breaker and screens and additional pre-processing equipment to remove contaminants like forks and plates, as well as grit and small pieces of ceramics and glass. “We’re in the process of selecting the equipment,” says Zipkin.

“In addition, we are planning a separate facility that will allow us to take food waste that ends up in the trash bin (the black bin, not the green bin),” she explains. “In partnership with San Francisco’s waste hauler—Recology—we will accept material that has been pre-processed through an extrusion press that separates the organics from trash. Recology will deliver the output of the press —an organic paste material —to East Bay MUD for further polishing to remove remaining grit and plastics. This waste stream will also be slurried and pumped to digesters.

“We will start piloting this winter and if it works we plan to expand the program. We are focusing on multi-family apartment buildings.”

Zipkin says East Bay MUD will be dedicating several of its digesters to food wastes instead of co-digesting with biosolids. “We have 11 existing digesters and probably will dedicate two to food wastes only,” she says. “It depends on how much food waste we can take in. The dedication of food waste digesters allows us to isolate any potential operational impacts associated with remaining contamination in the food, and it provides for a broader range of high value end uses for the food waste digestate.”

Czerniak notes that a new bill in the California Assembly requiring more recycling of food wastes will expand the opportunities for co-digestion at LACSD. “We’re preparing for that,” he says. “At our plant, we may have the capacity to accept 200 to 500 tpd of food waste depending on various factors.”

Statewide, however, Czerniak suggests there may be  enough excess capacity to handle the food waste that needs to be diverted from landfills, but high quality, clean food waste, a necessity for co-digestion, is expensive and may limit what’s available for use at a WWTP “At some point we will have to build dedicated digesters for varying qualities of food wastes to reach the goals that the state has proposed,” he says. “We will face competition from private companies, but the municipal plants (to co-digest the material) are here now.”

There are technical and regulatory issues that may impact the expansion of co-digestion in California, but the outlook appears favorable.

In ozone non-attainment areas of the state, emission limits on internal combustion engines using methane gas present a technical challenge. “We are looking at ways of meeting those limits,” says Kester. “I am hopeful. Some of the technology looks very promising.”

Another issue pertains to injection of biogas into common carrier pipelines. “There are stringent requirements that the gas have a 990 (Btu per cfm) heating value, where wastewater biogas is usually around 950 or 960,” Kester explains. “That’s not easy or cheap to attain. Right now the solution is to do multiple passes, and that increases costs.”

Determining the impact of high strength organic wastes on digester performance is also critical, according
Fillmore at WERF. “(The impact) can be beneficial by producing more biogas, or negative with unintended side effects,” she says. “WERF is currently funding several projects to better understand how to safely add high strength organics and reduce the potential for unintended effects.”

She adds that transportation of material to the treatment facility, or the proximity of sources, are also important considerations. “The ability to establish partnerships to ensure delivery and also prepare or blend materials can have an impact on the implementability of co-digestion.

Meantime, most regulatory hurdles have been overcome, says Kester, “at least for anaerobically digestible FOG, food wastes and vegetative food wastes.” For these, he explains, solid waste permits are no longer needed at wastewater treatment plants as long as they are in compliance with water board permit conditions.

Some regulatory questions remain. An example is stand-alone digestion, Kester notes, where digesters are dedicated solely to fats, oil and grease or to food wastes, with no sewage sludge.

Commonly Used Co-Digestion Terms:

  • Fats, Oils & Grease (FOG/yellow grease/ brown grease/grease trap waste)

  • Brewery waste

  • Food processing waste (expired yogurt/soup/cola syrup)

  • Food waste

  • Dairy waste (expired milk/whey)

  • Green waste (grass clippings/tree branches)

  • Biodiesel waste

Co-digestion is a process whereby energy rich organic waste materials (e.g. Fats, Oils, and Grease (FOG) and/or food scraps) are added to dairy or wastewater digesters with excess capacity. In addition to diverting food waste and FOG from land fills and the public sewer lines, these high-energy materials have at least three times the methane production potential (e.g. biogas) of biosolids and manure.

A primary benefit of co-digestion is that it uses existing infrastructure and expertise to divert food waste and FOG for the purpose of biogas production. In California alone there are almost 140 wastewater treatment facilities that utilize anaerobic digesters, with an estimated excess capacity of 15-30%. Other benefits include greenhouse gas emission reductions, economic benefits and diversion opportunities. Source EPA

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Megan Barillo

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