May 18, 2004 | General

Biogas Digesters To Recycle Green Waste

Lyn Corum
BioCycle May 2004, Vol. 45, No. 5, p 54

Two California cities – Los Angeles and Lancaster – finalize contracts to build anaerobic systems for power generation which will be financed by a private company called BioConverter LLC.
Three years of negotiations appear to have paid off for BioConverter LLC when the Los Angeles City Council approved the company’s power purchase contract with the Los Angeles Department of Water and Power (LADWP). That Department has agreed to a 20-year, $320 million contract to buy 40 MW/day of electricity – enough to power 40,000 homes – produced by BioConverter’s 2,700 ton/day biogas power plant once it is built in five years.
In contrast, it took the company just ten weeks to finalize a project with the California city of Lancaster for a 200 ton/day facility. The project will produce compressed natural gas for a public fast-fill (natural) gas station. It should be operational within 24 months. Lancaster’s city manager, Jim Gilley, said the plant “fits well” with the community’s passion for clean air and its Clean Cities designation in the U.S. Department of Energy program that supports public-private partnerships deploying alternative fuel vehicles and building infrastructure.
The two plants will be second-generation versions of two commercial prototypes built by James McElvaney, an engineer with 22 years experience in the field, over the past two decades in Hawaii. He refined a biofilm anaerobic digestion system in which bacteria live on a geotextile surface within the vessel and become acclimated to the feed stock. In this technology, solids are purged at the bottom of the vessel while retaining a large amount of the biological population to which new solids are added. The lack of moving parts is another design innovation. Recycled gas is used as a force to mix the organic waste. McElvaney holds the patent on the current process.
The Los Angeles plant will take five years to design and build at a cost of about $375 million and will process source-separated urban green waste in the high-efficiency patented anaerobic digestion system to produce biogas power for the city’s residents. Solids from the anaerobic process will be turned into organic fertilizers and animal feed.


Gary Petersen, Chairman of the Board of Directors of BioConverter Los Angeles’s parent company, BioConverter LLC, first met Jim McElvaney in 1997 when Petersen walked through the second prototype, a two-ton/day BioConverter facility on Maui. A “Green Team” from Amory Lovin’s Rocky Mountain Institute had visited the facility during an environmental conference earlier and had recommended that Petersen check it out. This meeting led to the partnership that has produced the BioConverter projects in Southern California.
McElvaney had first developed the Unisyn biowaste system in the 1980s. It used a horizontal biofilm form of anaerobic digestion. The 200-ton/day facility, located on Oahu, processed poultry and cattle manure but later converted to food waste processing. A patent was issued for the technology in 1992 but McElvaney had already assigned patent rights to others in 1991 because he wanted to design a better system.
By 1994, McElvaney had built the BioConverter, also a biofilm anaerobic digester technology, but this design was vertical. The two-ton/day prototype, which had no moving parts, operated on 50 percent food waste, 25 percent green waste and 25 percent paper. The waste was converted by the anaerobic process into biogas which was then used to produce power to operate the plant. The remaining organic residues were refined into plant foods, fertilizers and protein feeds.
A company called Sustainable Technologies, Inc. financed BioConverter but in 1997 the company owner died, the estate was sold and since the new owner was no longer interested in the biogas facility, it was shut down. McElvaney left the company in August 1997.
Petersen and McElvaney, as chief operating officer, cofounded BioConverter LLC when they submitted their proposal for a BioConverter facility in Los Angeles in response to LADWP’s request for renewable power proposals in Dec. 2000. The company won rights to negotiate the LADWP power purchase contract in 2001. BioConverter LLC will finance both the Los Angeles and Lancaster plants.
The processing of source-separated green materials in BioConverter’s system will be the same in both the Los Angeles and Lancaster facilities. The green organic wastes (yard clippings, tree and shrub clippings, food waste) are delivered to an enclosed building, processed to remove contaminants and transferred to covered conveyors. The material is moved on the conveyors into pulping tanks where blending and size reduction occurs.
Material is then pumped in batches into sealed, steel cylindrical vessels which contain a heating coil for temperature maintenance, sparging (bubbling) tubes for gas transfer and a draft tube for slurry recirculation. A fiberglass reinforced plastic cap acts as a gas holder to maintain the produced biogas at a slight positive pressure, preserving the anaerobic environment. A differential pressure sensor is used to measure liquid level and a device similar to a thermocoupler measures the liquid temperature.
Bacteria convert the organic material into a medium BTU gas, which passes through scrubbers to remove hydrogen sulfide, then is routed to a combustion turbine or fuel cell to generate electricity for sale and for system operations. The waste heat from the combustion turbines is captured in the form of steam to maintain the condition of the anaerobic process and to refine the organic fertilizer, liquid plant food, organic by-products and recycled water.
Residuals not converted to biogas in the anaerobic process are fibrous, nondigested and nondigestible solids. They are screened, pressed and placed into a dryer for pasteurization and conditioning. With additional drying and extrusion, the product is converted into a poultry and aquaculture feed additive.
The screened effluent also produces Bio-Green, a low analysis liquid fertilizer and plant food. With filtration and distillation, several additional organic plant foods will be produced to be shipped in bulk to buyers.

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