Co-Digestion of Organic Wastes at Central Marin Sanitation Agency

Resource Recovery, Technology and Innovation

Coverage of biosolids and co-digestion supported by Anaergia

Jason Dow, CMSA General Manager

Already a leader in co-digestion of organic food wastes and FOG, the Central Marin Sanitation Agency (CMSA) is launching a new study to determine optimum feed rates of organic wastes to its anaerobic digesters.

“The generally understood ratio of FOG to wastewater solids is around 30 to 35%, but our study has shown that the digestion process can be stabilized at 40% with a FOG and food waste slurry, and hopefully higher,” says Jason Dow, CMSA General Manager.

Results like that would help CMSA expand its organic waste and co-digestion programs, which dates to 2014 and now accepts as much as eight tons a day of food wastes through a public-private partnership with Marin Sanitary Service (MSS), the area resource hauler MSS picks up wastes from its service area restaurants and grocery stores and processes the material through a new organic waste processing facility at its main transfer station.

In addition, CMSA accepted about 3.1 million gallons of FOG in 2019 from\ haulers on a non-contract basis.

Dow reported on the program in a workshop at the 2019 Water Environment Federation Annual Conference in Chicago last September.


The CMSA plant is a regional advanced secondary treatment facility, serving approximately 105,000 people – including the prison population at San Quentin. CMSA is a Joint Powers Agency (JPA), with five collections satellites, three of which are members of the JPA. Flow can vary from a million gallons per day (MGD) average dry weather flow, to 121 MGD during major wet weather events.

The plant is equipped with two anaerobic digesters. During dry weather, dewatered biosolids are spread on farmland or converted to bio fertilizer by a private firm. During the rainy season, they are used as alternate daily cover at the Redwood Landfill.

Digester biogas is conditioned to remove H2S, moisture, and siloxanes, and routed to a cogeneration unit.

The food-to-energy project was born out of plans to rehabilitate the anaerobic digesters and new California initiatives to reduce greenhouse gas emissions.

“PGE – our power supplier – was offering grant funds to study greenhouse gas reduction alternatives, in line with the Global Warming Solutions Act of 2006,” Dow says.“Anaerobic digestion was on the list.

“We met with MSS, PG&E, and local city managers to talk about the grant and how much potential food waste could be collected. MSS said they could collect up to 15 tons per day, based on a recent Waste Characterization Study, and after completion of a feasibility study we determined we had digester capacity for significantly more than that. It looked like a great way to reduce landfill GHG emissions and produce more energy at the plant.”

The food to energy program was supported by the city and town councils in the CMSA service area, the Marin County Board of Supervisors, and local environmental groups, and was approved by the CMSA Board of Commissioners. It was then permitted. by CalRecycle and the SF Bay Regional Water Board. With SOPs in place, the system went online in early 2014 and processed 1,165 tons of material from 70 customers that first year.


MSS collects the organic wastes from over 200 restaurants and markets throughout its Marin County service area. At its transfer station, food collection truck contents are dumped onto a tipping floor, where contaminants are removed, such as bones, plastics and metal items. A magnet removes ferrous materials, then the material is cleaned and placed onto the sorting belt. Two workers manually sort the waste and remove any remaining unwanted items. MSS project manager Kim Scheibly says contaminants include aluminum, which is not removed by the magnet.

“We have zero tolerance for contaminants,” she says.

Following sorting, the material is shredded and the debris is smashed into smaller than one-inch pieces by a rock-trap grinder. Then, a conveyor loads a truck which transports the material to the CMSA treatment facility.

At the plant, the food wastes enter the organic waste receiving facility, which includes a storage tank, grinder, and mixing pumps to ensure the plant’s primary and thickened waste activated biosolids are thoroughly blended with the incoming organic wastes. The blended slurry is then pumped to a screening system to remove small materials and contaminants before the slurry is pumped to the digesters.

The digester biogas is cleaned and dried and then routed to the cogeneration unit, where it fuels the cogen engine – usually providing enough electricity to power the treatment plant and Agency buildings. At times, there is enough to sell to the grid. Heat exchangers capture waste heat, which is used to heat the digesters.

Since the advent of the food-to-energy program, gas production at CMSA has increased from around 100,000 cubic feet per day to over 325,000.

The plant accepts the material from 6 a.m. to 12 noon, six days a week, and runs the digesters daily from 6 to 10 p.m. In 2019, the feed to the digesters consists of 41% primary biosolids, 35% thickened waste activated biosolids, and 24% food and organic wastes.


Moving the project from concept to reality required hard work and investment by both CMSA and MSS.

CMSA conducted extensive public outreach, talking to municipalities about how the plan would work. In his presentation, Dow promoted it as “a local renewable energy project that reduces landfilling of food waste and increases the treatment plant’s energy self-sufficiency – in line with the state’s renewable energy generation goals.”

Another potential benefit is the ability to deliver energy to the local utility grid in the future.

MSS first had to recruit participants: both large and small generators, including restaurants, grocery stores, cafeterias, delis, and other commercial customers.

Most of the waste is ‘pre-consumer’ – leftover food or spoiled meat or produce. Only a few clients deliver ‘post-consumer’ organics – scraps left from cooking meals or plate scrapings.

The equipment was another issue for MSS.

“No manufacturer has the equipment just for food wastes,” Scheibly says. “We actually considered building some of the equipment ourselves.”

The company invested approximately $530,000 to tailor a system capable of meeting the requirements for delivering organic waste to the treatment plant.

One refinement involved creating a ‘stop belt’ feature on the conveyor system, as well as a method for controlling the belt speed. Trucks and carts had to be modified. Scheibly says a special manual rear-loading truck design was required so the driver could get out of the truck and ‘flip the lid’ to inspect the dumpster contents before loading them. With the automated front-loading trucks common today, such load inspection would not be possible and loads might contain unwanted material.

“The manual operation also lets us communicate with customers who may need more information,” she says. Dow reports CMSA spent about $2 million to install the organic waste receiving facilities, which was incorporated into the digester rehabilitation project in 2011. The digesters received new membrane covers, the old iron sponge digester gas cleaning system was replaced with activated carbon system to reduce H2S content, and a pump mixing system replaced the original gas mixing system.

To deliver power to the grid, CMSA had to obtain a new interconnection agreement from PG&E. That required PG&E to evaluate its local electrical system and CMSA to design and install several on-site electrical improvements.

CMSA has plans for a new 995 kW co-generation unit and will use the old 750 kW unit as backup. The move will make the Agency energy self-sufficient and increase power sales to the grid. Design will be completed by the summer, followed by public bidding for the construction work, and the unit is expected to go online sometime in 2021.


Dow says the organic waste program requires that pumps, bins, and the grinders be cleaned regularly. Quarterly, the plant staff cleans and inspects the paddle finisher, pipes, storage tanks, and protective coatings. The only unplanned maintenance has been ruptures in the hoses on the paddle finisher feed pumps, caused by metal debris that makes it through the MSS and CMSA screening systems.

“It happens four to six times a year,” he says.

In Scheibly’s view, contamination is the key issue. “While we’re diverting organics from our landfills,” she says, “we can’t receive just any food waste without cleaning it up.” Both Scheibly and Dow agree on the need for continuous public education. “You need stakeholder support,” says Dow. “And make sure to keep costs reasonable.”

“You need to have clear communication with all stakeholders,” says Scheibly. “Our program has been successful from the get-go because everyone was involved. We have a successful public-private partnership and a rate model that works for customers, the city, and the hauler.”