The implementation of a Organics-to-Energy facility and the change in commercial collection to a “wet/dry” system has more than tripled the commercial recycling rate in the City of San Jose to over 70 percent, resulting in huge reductions in greenhouse gas emissions and other environmental impacts.
Jim Miller
It is the world’s largest and first of its kind in North America. These were only two of the challenges facing Zero Waste Energy Development Company’s (ZWEDC) Organics-to-Energy Facility in San Jose, California. The first phase of this unique facility was successfully completed in November 2013. This phase processes 90,000 tons per year (TPY) of organic waste, produces 30,000 TPY of compost, and will generate 1.6 Megawatts (MW) of renewable energy using a high-solids dry-fermentation type anaerobic digestion (AD) technology first developed and proven in Germany.
The ZWEDC facility is part of the City of San Jose’s Commercial Franchising Program and Green Vision that is taking organics recovery to the next level. All “wet” commercial waste within the City is taken to Republic Services’ Newby Island Resource Recovery Park in Milpitas, CA, where it is sorted to remove marketable commodities and contaminants. The remaining organic fraction of the waste is delivered to the ZWEDC facility and used as feedstock for the AD process. Source Separated Commercial Organics (SSCO) are also collected within the City and delivered directly to the facility.
Designed and permitted for development in three phases, the ultimate ZWEDC facility will receive and process as much as 270,000 TPY of organic wastes and produce of 4.8MW of renewable energy.
Being the world’s largest and first of its kind in North America weren’t the only ambitious goals or significant accomplishments for this pioneering facility. Another significant challenge was the development of this facility through a public/private partnership between ZWEDC and the City of San Jose. The resulting facility is now a primary contributor to the City’s 2020 Green Vision goals of achieving 100 percent diversion and converting waste-to-energy. Vital team member contributors for the first phase of the project included ZWEDC, the project owner and vision behind the project:
- Zero Waste Energy, LLC (ZWE), the owner and exclusive licensee of the Kompoferm and SmartFerm technology. ZWE provided front-end engineering and process design of the facility and the technology and process equipment package.
- R. Miller & Associates (JRMA), an architectural and engineering firm which designed the balance of plant.
The ZWE/JRMA Team adapted and enhanced the AD processing system, as well as the remainder of the facility from the basic German design in order to comply with U.S. building and fire codes, safety standards and construction means – which was no small task and vital to the project’s success.
The Process
Anaerobic digestion (AD) is a process that occurs in nature when specific bacterial microbes are introduced to organic materials within an ideal temperature and moisture range with minimal oxygen present. The most similar natural process takes place in a cow’s stomach. The basic goal of an AD process is to create and maintain ideal conditions for the bacteria to thrive, to best prepare the organic materials, and to introduce the bacteria on the feedstock in an effective manner.
The specific bacteria essential to the AD process require a high-level of moisture to thrive and for mobility. One of the most significant challenges for a dry-fermentation system is to take fairly dry material, which may have moisture content in the range of 40 to 60 percent and increase the moisture content to an effective level to promote the digestion process, approximately 70 percent.
The ZWE system consists of concrete digesters; each 20’ wide x 15’ high, and for the ZWEDC project, 97’ long. Organic feedstock is both loaded and removed through a single gas-tight hatch that is latched and locked during the operation. After sealing the hatch and prior to starting the anaerobic process, air is introduced through nozzles in the floor to initiate aerobic decomposition. The aerobic process is exothermic and heats the digester and contents to temperatures that are suitable for the introduction of the bacteria.
The target temperature is reached after a few hours, and at this point, the airflow is stopped and percolate is introduced to the organic feedstock through an overhead pipe with spray nozzles, similar to an overhead fire sprinkler system. The percolate saturates the organic materials, and the resulting anaerobic digestion process produces biogas and consumes all oxygen from the digester.
Biogas continues to accumulate until it creates sufficient pressure (approximately 0.145 psi) to force it through a duct and into the biogas circulation and mixing system. All biogas movement through the system is due to the naturally-occurring overpressure from the anaerobic digestion process, thus saving energy.
Biogas is mixed in the circulation system and flexible roof domes and supplied directly to the Combined Heat and Power System (CHPs) for generation of electricity and recovery of heat for process loads. The priorities for using this renewable electricity are: 1) onsite to power all facets of the facility except rolling stock; 2) at the adjacent Zanker Corporation operations to power equipment; and 3) delivery of excess power to the grid of the local utility, PG&E.
After approximately 21 days, biogas production begins to decrease and the AD process is terminated by introducing air into the digester. Once biogas is completely purged from the digester, and the atmosphere within the digester returns to ambient temperature, pressure levels and methane concentrations have reduced to safe levels, the hatch can be opened and the digestate removed.
The resulting digestate is then processed though a de-compactor to prepare for processing through the In-Vessel Composting (IVC) tunnel process. The five-day IVC operation transforms digestate into early-stage compost and strips ammonia, the primary cause of odor. By the end of the five-day cycle, the compost can be placed in outdoor windrows with minimal odor impacts. The compost stabilizes after approximately six weeks in the windrow phase.
Engineering and Design
While replicating a cow’s stomach provides a reasonable model for visualizing an AD process, the ZWEDC facility is very large scale and requires a high level of process engineering and system telemetry to monitor and control all facility functions. The technology was selected because of its long-term success in facilitating all of the natural processes in a controlled manner and providing extensive ability to analyze and control those processes. The primary challenges in the design of this facility were to accommodate and complement all of the basic design requirements, adapt and improve the design for development in the U.S. and address other significant challenges related to the project location.
The ZWEDC facility is situated on a 22.45 acre portion of the former Nine-Par Landfill property owned by the City of San Jose, adjacent to the Don Edwards National Wildlife Refuge in San Jose, CA. Designing and constructing this massive facility on this site presented another difficult challenge for the development team, requiring a methodical and strategic approach.
The site is located in one of the U.S.’s most active seismic zones. In order to address seismic and personnel safety requirements of the California Building Code (CBC), many features were modified from the original German design, including structural components, access and alarm systems. This required extensive coordination with the German Technology developer, including various meetings in Germany, frequent web conferences and extensive exchange of drawings.
Additionally, the site is underlain by an abandoned landfill that was not permitted and not subjected to proper closure and post-closure procedures. During the geotechnical assessment, it was determined that disturbance of the landfill could led to significant air and ground water impacts. To avoid this potential problem, a shallow mat foundation was selected instead of a pile foundation system. This design required pre-loading of the soil for an eight-month period. Additionally, a landfill gas barrier and venting system were required. Key process components include:
- A 12,785 s.f. organics receiving bay.
- Space for a future 30 ton-per-hour integrated mechanical processing/materials recovery system.
- 16 dry-fermentation anaerobic digesters, each with an average 300-ton capacity.
- Two Combined Heat and Power (CHP) units, each with a generating capacity of 800 kW.
- Mechanical preparation system to enrich digestate for composting.
- Four in-vessel compost (IVC) tunnels, each with 400-ton capacity.
Additional significant supporting facility design features included:
- A fully-enclosed 77,040 s.f. building to house all operations.
- Two 340,000 gal. subterranean storage reservoirs for percolate, the inoculant that is introduced to the feedstock to facilitate biogas production.
- Two 23,660 cf roof-mounted flexible biogas storage domes.
Due to the unique and specific processing requirements of this facility, there are many design aspects that were not fully addressed in the CBC, and as a result, had not been previously encountered by the City of San Jose Building and Fire Code officials. To address these design aspects, City officials were consulted periodically throughout the design phase, issues and concerns were discussed, and solutions were identified. These solutions then became requirements that were incorporated as the design progressed. In addition, several fire and life safety aspects were addressed through an Alternate Means and Methods process, requiring approval by a special City committee.
Environmental Impacts and Compliance
One of the primary goals for this facility was to be a good neighbor to the surrounding Bay Area community. This goal was established by the City, presented to the public and embraced by ZWEDC, the developer. There are several aspects of this facility that make it a good neighbor, including:
- Aesthetics—The architecture and landscaping design are consistent with other modern industrial facilities in Silicon Valley. Exterior materials and finishes were chosen so the facility will retain its aesthetic appeal.
- Dust and odor control—All operations at the ZWEDC facility are conducted within the fully-enclosed building which minimizes dust, odor and noise emissions. The exhaust air system is designed to draw air into the facility through louvers or open doors, and exhaust all non-process air through a treatment system that includes two acid scrubbers and four bio filters. This system meets strict Bay Area Air Quality Management District regulations.
- Noise control—Both the building and rolling stock are designed to minimize noise.
- Air Emissions—The exhaust system, the CHPs and the lean-gas flare all comply with Bay Area Air Quality Management District regulations.
- Safety—All operating functions are controlled and monitored by the “Fail-Safe” operating system, which also detects methane and the opening and closing of hatches.
- Education—The facility will serve as part of City of San Jose’s environmental awareness programs.
- Renewable Energy—The facility will produce green energy for many years, helping to reduce our use of fossil fuels and foreign dependence.
Perspectives
Organic material comprises the largest portion of materials buried in landfills today. The organic materials processed through ZWEDC’s AD facility diverts from landfills, where they would normally degrade and release methane into the atmosphere for as long as 30 years. Methane is an approximately 20 times more potent greenhouse gas than carbon dioxide, and landfills contribute significantly to global climate change and related negative environmental effects.
In contrast, the ZWE technology captures and contains nearly all of the biogas containing methane over a 21-day period from the organic waste it processes. Processing this organic material also conserves capacity and extends the life of landfills, thereby reducing the need for new disposal sites.
Consuming the generated electricity onsite further reduces greenhouse gas emissions due to energy offsets. Additionally, the implementation of this facility and the change in commercial collection to a “wet/dry” system has more than tripled the commercial recycling rate in the City of San Jose to over 70 percent, resulting in huge reductions in greenhouse gas emissions and other environmental impacts.
Jim Miller is CEO of J.R. Miller & Associates (Brea, CA). He has been a leading design professional and technical advisor in the solid waste management industry for more than 29 years. His work demonstrates a focus on integrating process and non-process requirements with overall plant layout and site configuration. He brings a wealth of expertise in designing facilities to maximize functionality and efficiency to minimize lifecycle costs. Jim can be reached at (714) 524-1870 or visit www.jrma.com.