COMPOSTING: New Process Aids Solid Waste Digestion

Bill Siuru

Low-solids anaerobic digestion, used primarily in wastewater and sewage treatment plants, may be floating toward a new application: solid waste disposal.

An adapted version of anaerobic digestion - a process where bacterial and fungal microorganisms convert organic materials that are suspended or dissolved in the liquid waste into biomass and biogas - has been developed by Pinnacle Biotechnologies International Inc., Golden, Colo.

The new process is known as anaerobic composting, or High Solids Anaerobic Digestion (HSAD).

Between early 1997 and spring 1998, Pinnacle operated a HSAD Pilot Demonstration Unit (PDU) in Stanton, Calif., sponsored by the U.S. Department of Energy (DOE), Washington, D.C. The HSAD technology originally was developed by the DOE's National Renewable Energy Laboratory (NREL) in Golden. This facility processed three tons per day of municipal solid waste and organic sludge produced by the pet food industry, turning it into methane and compost. The methane was burned in an internal combustion engine to produce 35 kilowatts (kW) of electricity - enough to supply electricity to approximately 35 homes.

Prior to Pinnacle's developments, solid feedstocks were not amenable to treatment with anaerobic digestion unless large quantities of water were added to dilute the waste. But adding extra water had economic and logistic disadvantages, thus preventing its widespread application in solid waste disposal.

Pinnacle's bioreactor, which is designed for processing high solids feedstocks, solved the problem using a thermophilic, or high-temperature biological process, to convert biodegradable organic wastes into three salable products:

* Medium-Btu biogas;

* A mixture of 60 percent methane and 40 percent carbon dioxide; and

* A nitrogen-enriched compost and liquid fertilizer.

Methane can replace natural gas as a source to be burned in boilers and heating systems, or used in turbine-powered generators to produce electricity for transportation fuel and in fuel cells.

In the HSAD process, solid and semi-solid waste feedstocks are shredded and mixed to create a consistent "feed" for the bioreactor. The feed, which can contain as much as 45 percent total solids, is continuously introduced into the bioreactor where anaerobic bacteria convert the carbon present into biogas.

The bioreactor can process a broad range of solid and wet organic wastes and feedstocks - food residuals, yard and other green wastes, municipal solid waste, and dissolved air flotation and other industrial sludges. Virtually any biodegradable waste can be recycled using HSAD.

HSAD is completely enclosed and converts up to 95 percent of available carbon to biogas with no hazardous byproducts. This closed process also combats global climate change by recycling methane and containing 100 percent of VOC (volatile organic carbons) emissions. The HSAD facilities can be located close to waste sources, and the process reduces the residual volume of solid waste by up to 85 percent.

Aside from the proprietary bioreactor and process control system, the process equipment is readily assembled from standard equipment in the wastewater treatment industry. The layout of the nearly completely automated plant is designed around the process areas - feedstock delivery and preparation, feedstock mixing, the digester area, gas collection, effluent processing and process control.

The PDU pilot, which operated in a light industrial area, consistently produced high-quality fuel gas, and by implementing operating and safety procedures, the facility had few unexpected complications. It also received virtually no complaints regarding odor, noise or vehicle traffic.

DOE's NREL estimates that many of the 5,000 municipal sewage treatment plants in the United States could use the HSAD technology, with potential customers also including food processors and waste haulers who must now transport waste to landfills or other disposal sites.

For copies of the report, "Recycling and Energy Recovery Pilot Project - Project Report and Future Efforts [SR-570-26158]," contact the NREL's Document Distribution Service at (303) 275-4363.