San Luis Obispo County, located along the Pacific Ocean in Central California, midway between San Francisco and Los Angeles, is known for its breathtaking seascapes, miles of oak-studded rolling chaparral hills, and prolific wine production—the third largest wine producing county in California. Its population of 283,000 is located amongst small communities scattered along the beaches, coastal hills, and mountains of the California Coast Ranges. Cities like Pismo Beach, Grover Beach, and Morro Bay dot the coast, while slightly inland is the county’s largest community, the city of San Luis Obispo. It is here where California’s most streamlined project is underway for the conversion of organic waste into carbon-neutral biogas and high-grade natural compost, through the process of anaerobic digestion.
Anaerobic Digestion—a Preferred Process for Disposal of Organic Waste in California
Utilizing anaerobic digestion as an alternative to landfills for the disposing of collected source separated organic (SSO) waste is not new to California. Nor is the conversion of organic waste into biogas and compost new to the state. California has 17 organic waste anaerobic digestion plants in operation and another 10 in the approval/building process, including the facility at San Luis Obispo, which is scheduled to begin full production in mid-2018.
Methane emissions resulting from the decomposition of organic waste in landfills are a significant source of greenhouse gas emissions contributing to global climate change. Organic materials account for a significant portion of California’s overall waste stream. Food waste alone represents approximately 17% of total landfill disposal within the state.
California recognizes that expanding anaerobic digestion of organic waste throughout the state will help reduce methane emissions from its landfills and produce biogas that can be used to create electricity or renewable transportation fuels.
Consequently, the California State Legislature and the Governor, Jerry Brown Jr., have enacted a number of stringent laws directed toward the collection and handling of organic municipal solid waste. California Assembly Bill Number 1826, enacted in September 2014, requires businesses that generate a specified amount of organic waste per week to arrange for recycling services for that waste by April 2016 and requires each jurisdiction to implement an organic waste recycling program to divert the waste collected away from landfills. As well, California Assembly Bill Number 1383, enacted in September 2016, establishes targets to achieve a 50% reduction in the level of the statewide disposal of organic waste from the 2014 level by 2020 and a 75% reduction by 2025. Both Assembly Bills 1326 and 1383 build upon California’s leading commitments to reduce greenhouse gas emissions and air pollution statewide.
San Luis Obispo County’s Unique Facility for Source-Separated Organic Waste
The San Luis Obispo County anaerobic digestion plant has been conceived in support of these initiatives. However, it is significantly more unique and streamlined in its scope of operation when compared to California’s other 26 anaerobic digestion facilities.
The plant will process the organic solid waste at a thermophilic temperature of 131°F (55°C), considerably higher than the lower mesophilic temperature range of most of the state’s other anaerobic digestion facilities. The higher processing temperature ensures that spores and bacteria are eliminated, completely sanitizing the organic matter during processing. And equally important, the higher temperature permits the biogas potential to be fully exploited by the time the material comes out of the digester, extracting a higher volume of carbon-neutral biogas.
Additionally, the San Luis Obispo facility is the only dry anaerobic digestion plant in California that is designed, financed, built, owned, and operated by an independent engineering company, separate from a government agency or waste collection contractor. That company is Hitachi Zosen Inova, US, LLC (HZI), an engineering, procurement, and construction (EPC) contractor that delivers complete turnkey plants and system solutions for recycling organic waste. HZI is the market and technology leader worldwide in dry anaerobic digestion processes.
“We started this process 10 years ago, even before the mandates were in place,” says Bill Worrell, general manager, San Luis Obispo County Integrated Waste Management Authority (IWMA). “We recognized that an enclosed anaerobic digestion facility is really the state-of-the-art solution to handling organic MSW. It keeps everything indoors, there is no runoff, and it recovers energy. It is just the best way to compost.”
“As early as 2007, I was visiting anaerobic digestion plants in Germany and Switzerland, then subsequently in Canada and the United States, about a dozen plants in total,” adds Worrell. “We were looking for a long-term solution for our organics waste. So we looked at all the options, and what was available.”
“In 2014, Hitachi Zosen Inova, a Swiss company, began offering its Kompogas anaerobic digestion technology in North America,” continues Worrell. “I had visited three of the company’s plants in Europe earlier so I was familiar with their systems and thought their technology would be a good fit for our needs at IWMA. So this started discussions between HZI and IWMA.”
The San Luis Obispo County IWMA is a joint powers agency that includes San Luis Obispo County and seven cities within the County. The IWMA plans and implements regional solid waste and hazardous waste programs. The IWMA Board of Directors is composed of the five county supervisors, one elected official for each city, and an elected representative from the special districts.
Servicing the waste collection for five cities and much of the County unincorporated area is Waste Connections (TSX/NYSE), an integrated waste services company that provides solid waste collection, transfer, disposal, and recycling services in mostly exclusive and secondary markets. Each of the five cities and the County has its own contract with Waste Connections. Both IWMA and Waste Connections are jointly responsible to comply with California Assembly Bills 1326 and 1383 relating to organic MSW collection and disposal within the county.
“To implement the project, each of the five cities and the County agreed to 20-year waste collection contracts with Waste Connections, who would deliver the organics to the Kompogas facility, which was to be located at the Waste Connections yard near the city of San Luis Obispo,” explains Worrell. “The Kompogas plant would be designed, financed, built, owned, and operated by Hitachi Zosen Inova, with a 20-year commitment from Waste Connections to provide the plant with the organic wastestream.”
With the agreement of all parties now in place, detailed planning, design, and permitting of the new Kompogas plant could move forward.
“We worked closely with IWMA and Waste Connections on our plans, much of which entailed ensuring all participant parties, including the public, were fully informed on the processes and procedures involved with the plant operation,” says William Skinner, West Coast sales manager for Hitachi Zosen Inova. “Much of the process involved securing state permitting requirements. Part of this planning had to do with diverting green waste away from two open-air compost facilities being used by Waste Connections and moving that organic wastestream into the new Kompogas facility, once in operation. This was part of the overall strategy for supplying waste to the new facility.”
Kompogas Continuous Dry Anaerobic Digestion
The Kompogas continuous dry anaerobic digestion process is based on the dry digestion of solid biowastes in an oxygen-free environment. Anaerobic digestion is a multistep biological process with four fundamental steps that include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Throughout this entire process, large organic polymers that make up biomass are broken down into smaller molecules by microorganisms. Upon completion of the anaerobic digestion process, the biomass is converted into biogas, as well as into digestate—liquid and solid.
Biogas is composed primarily of methane (CH4) and carbon dioxide (CO2), with trace amounts of hydrogen sulfide (H2S) and ammonia (NH3). Typically, biogas is saturated with water vapor and may have trace amounts of hydrogen (H2), nitrogen (N2), and oxygen (O2). Biogas can be directly combusted—such as in a combined heat and power (CHP) plant—thereby generating environmentally friendly electricity and heat, which is typically fed into the public power supply or local heating network. Alternatively, the biogas can also be processed and upgraded for use as CO2-neutral vehicle fuel or a natural gas replacement.
Digestate is the remaining solid and/or liquid residual from the anaerobic digestion process. The anaerobic digestion process recovers and recycles the nutrients contained in this organic material. From the produced digestate, top-grade solid compost is produced, which is made available to nurseries, farmers, and market gardens. The process also produces liquid digestate, which is distributed for agricultural use as a certified organic liquid soil conditioner.
“The Kompogas process uses a horizontal plug-flow digester,” explains Skinner. “The organic material is transported inside the digester, with the material moving horizontally through the system by feeding on the inlet side and discharging on the outlet side. A slowly-turning agitator ensures that the substrate is optimally mixed within the digester and the biogas bubbles are permitted to vent for the high-yield formation of methane. This facilitates the biological strength of the Kompogas anaerobic environment, enabling it to make maximum use of the organic waste’s energy potential.”
Uniquely different from other anaerobic digestion processes, the Kompogas digester creates an extremely efficient microorganism environment, which separates and optimizes the different steps of biomass degradation throughout the process much better than conventional anaerobic digestion, making for very stable microbiology inside the reactor.
The process recirculates approximately one-third of the digestate, rich in thermophilic microorganisms, from the output and back up front into the digester to activate and accelerate the anaerobic digestion process of the fresh material fed into the digester. This allows a perfect adjustment of the hydrolysis/acidogenesis rate in the digester feeding section and facilitates high bioprocessing efficiency within the system.
Conversely, with conventional anaerobic digestion, the key steps of hydrolysis, acidogenesis, acetogenesis, and methanogenesis are completely mixed. Each step takes place at the same time and at the same place in the digester. This does not permit optimum conditions for the microorganisms to digest the organic material.
If required, additional process water creates the optimal consistency for decomposition, with humidity residing around 70%. A specially developed heating system regulates the temperature during processing at 131°F (55°C) during a retention period of approximately 14 days. The thermophilic microorganisms decompose the organic matter and produce carbon-neutral biogas. Because the digestate is completely sterilized, the system does not require any upstream pasteurization, as is the case with other anaerobic digestion solutions.
Kompogas Process Steps at the San Luis Obispo Facility
Following are the process steps encompassed in the Kompogas dry anaerobic digestion system being installed at the San Luis Obispo County facility:
- Material Reception, Pre-Treatment, and Feeding
The organic material delivered to the plant is tipped in the reception hall. All handling of organic materials takes place in closed and ventilated rooms, thereby minimizing odor emissions to acceptable levels. Automatic roll doors allow trucks to enter the facility and close immediately upon safe entry. From here, the material is fed into the processing area using a wheel loader. The material is pre-processed through a shredder and followed by a star screen that removes contaminants such as plastic, paper, ferromagnetic particles and other non-organic items. The material is screened to pieces of approximately 2 inches in size. The pre-treated material is then transported to an intermediate storage bunker. A dosing unit, equipped with a scale to monitor the amount of material fed into the digester, is fed by a fully-automated crane. From there, the material is fed to the digester via belt conveyors.
The continuously-fed, horizontal PF1800 plug-flow digester has its nominal capacity of 64,000 cubic feet at a filling level of approximately 85%. The digester is a patented steel structure with inner dimensions of 130 feet long and a 28-foot diameter. Hot water supplied by the CHP (or biogas boiler) is used as the heating media. To minimize heat losses, the steel tank is enclosed by thermal insulation. The entire digester module can be installed outdoors due to its weatherproof housing. The central heat distribution system is installed underneath the digester within the enclosure, accessible by doors from both ends. A slowly turning agitator device results in optimal degasification, while sedimentation of heavy matter in the digestion substrate is prevented due to the special positioning of the agitator paddles.
The digested material remaining after the process is removed out of the digester by the outlet pump and dewatered by screw presses, which separate the digested substrate into solid digestate (press cake) and liquid digestate (press water). The liquid digestate is piped into the liquid digestate collecting tank and treated by advanced mechanical treatment (decanter/centrifuge). A portion of it is recirculated for moistening the input feedstock material.
- Press Water and Loading
The remaining liquid digestate is pumped to one large storage tank outside of the main building. The storage tank is covered by a gas- and odor-tight double membrane roof and equipped with a watertight door. This allows access for periodic removal of sediments. Agriculturists can pick up this liquid fertilizer and fill their trucks directly at the storage tank by means of a digestate loading station.
- Post-Treatment of Solid Digestate—Aerobic Stabilization
Solid digestate is taken from underneath the dewatering presses with a shovel loader and deposited into one of several open aerobization boxes, located in the compost hall. The digestate will be subject to aerobic stabilization and removal of volatile organic compounds. Air is blown for approximately two to three weeks through the material by means of ventilation channels in the floor, therefore allowing a rapid aerobic stabilization. The exhaust air of those boxes, as well as the air of the whole post-treatment hall, is collected and piped to a waste air treatment plant—a system including piping, humidification, and bio-filter.
- Biogas Pre-Treatment
Raw biogas from the digester is first desulfurized and then dewatered to an acceptable level for the following biogas utilization systems. The biogas is analyzed for its content of methane, carbon dioxide, oxygen, and hydrogen sulfide.
One of the major points driving the final decision to move forward with the Kompogas plant was the amount of electricity that is capable of being produced on a continual basis. At full capacity, the plant is designed to produce enough biogas to generate 6.2 million kilowatt-hours annually.
“Biogas can be used either to fuel the waste collection trucks or create electricity,” says Skinner. “What it came down to is, while Waste Connections had some CNG trucks, they did not have enough to take all the biogas that would be produced. So, we went with electricity. That way, the electricity can be used internally to power the plant, with the excess put into the grid.”
The pre-treated biogas is led to a combined heat and power unit. The CHP unit is a completely containerized module with gas controller, gas engine, generator, exhaust funnel, heat recovery, cooling unit, catalyst, and control unit. The CHP is designed to ensure maximum possible electrical efficiency and high availability.
Exhaust Air Collection and Treatment
“A critical factor throughout the entire development of this project was containing the release of contaminants and odors into the surrounding environment,” adds Skinner. “HZI implemented very stringent processes throughout the facility to ensure this measure was intact.”
“The plant also needed to comply with the California Environmental Quality Act [CEQA],” continues Skinner. “This is a statute that requires state and local agencies to identify the significant environmental impacts of their actions and to avoid or mitigate those impacts.”
The digester itself is a completely closed system because the process operates under anaerobic conditions. Therefore, no emissions are released into the surrounding environment by the digestion plant. Exhaust air collected from the various halls is moistened with water by means of a special nozzle system operated with compressed air. Reaching humidity levels of 95% guarantees an optimal operation of the subsequent biofilter.
To lower the total air volume to be treated by the biofilter, the total exhaust air collected in the waste treatment hall is directed to the composting hall as inlet air. Hence, the air from the treatment hall is reused for aeration of the composting hall before it is led to the biofilter for treatment.
The biofilter consists of a large open structure with a permeable floor to allow for airflow and is filled completely with pieces of tree roots. After being shredded and sieved, the wood chunks offer a large surface as a breeding ground for natural microorganisms which absorb the volatile organic compounds contained in the exhaust air. The loosely stacked biofilter results in a minimal pressure drop of the exhaust air stream.
To prevent the air from penetrating into the environment, both the treatment hall and the composting hall are kept at negative pressure.
Waste as an Energy Resource
For the residents of San Luis Obispo County, the implementation of this new anaerobic digestion plant represents a significant step forward towards the reduction of greenhouse gas emissions by diverting organic waste away from open landfills. But it is also a movement away from wasted resources, to the utilization of waste as an energy resource.
The new plant about to be opened in San Luis Obispo County makes a bold statement of environmental consciousness, not just to the residents of the county, but statewide to all Californians, and indeed across the nation. It focuses on maximizing utilization of biomass waste resources and minimizing adverse impacts on the environment.
About San Luis Obispo County Integrated Waste Management Authority (IWMA)—The San Luis Obispo County Integrated Waste Management Authority (IWMA) includes San Luis Obispo County and the Cities of Arroyo Grande, Atascadero, Grover Beach, Morro Bay, Paso Robles, Pismo Beach, and San Luis Obispo, and the Community Service Districts. These jurisdictions formed IWMA in 1994 to plan and implement regional solid waste and hazardous waste programs.
About Hitachi Zosen Inova—Zurich-based Hitachi Zosen Inova (HZI) is a global leader in energy from waste (EfW), operating as part of the Hitachi Zosen Corporation Group. HZI acts as an engineering, procurement, and construction (EPC) contractor delivering complete turnkey plants and system solutions for thermal and biological EfW recovery. Its solutions are based on efficient and environmentally-sound technology, are thoroughly tested, can be flexibly adapted to user requirements, and cover the entire plant life cycle. The company’s customers range from experienced waste management companies to up-and-coming partners in new markets worldwide. HZI’s innovative and reliable waste and flue gas treatment solutions have been part of over 600 reference projects delivered since 1933.
About Waste Connections Inc.—Waste Connections (TSX/NYSE: WCN) is the premier provider of solid waste collection, transfer, recycling, and disposal services in mostly exclusive and secondary markets across the US and Canada. It also provides intermodal services for the rail haul movement of cargo and solid waste containers in the Pacific Northwest.
The San Luis Obispo County Anaerobic Digestion Facility is being funded in part by the California Department of Resources Recycling and Recovery’s (CalRecycle) Organics Grant Program, a subset of California Climate Investments. CalRecycle funds will pay $4,000,000 for the anaerobic digestion facility in San Luis Obispo. California Climate Investments is a statewide program that puts billions of cap-and-trade dollars to work reducing greenhouse gas emissions, strengthening the economy, and improving public health and the environment— particularly in disadvantaged and low-income communities. The Cap-and-Trade program also creates a financial incentive for industries to invest in clean technologies and develop innovative ways to reduce pollution. For more information, visit California Climate Investments.