Biogasification of organic solid wastes

Developed at the University of CaliforniaDavis, new AD system will be used commercially at a 1,900-head thoroughbred horse training farm in Florida - turning 80 tpd of stable wastes and food residuals into biogas and soil amendment.

A NEW advanced anaerobic digestion technology called Anaerobic Phased Solids Digester (APS-Digester) has been developed at the University of California (UC), Davis. The APS-Digester is specially designed for biogasification of organic solid wastes that are normally difficult to process using conventional anaerobic digesters. A variety of organic materials may be used as feedstock for the APS-Digester, including crop residues, animal manures, food processing residuals, paper sludge, and municipal solid waste. A U.S. patent has been approved for this new technology. The University of California has licensed the APS-Digester technology exclusively to Onsite Power Systems, a California-based company specializing in renewable energy generation.

The first commercial A'b-Digester system will be built on a new 1,900-head thoroughbred horse training farm in Boynton Beach, Florida, to convert about 80 tons (dry weight) of horse stable wastes each day into biogas and soil amendment product. The first phase is expected to be operational by fall of 2002. The expected benefits that the APS-Digester technology offers such an animal facility include renewable energy production, odor control, pathogen and insect control, truck traffic reduction, and production of high-quality soil amendment.

Several food processors in California also have expressed interest in using the APSDigester for treating their wastes, which have been traditionally used as low-value cattle feed or disposed of in landfills. With more stringent environmental regulations on waste management and increasing demand for alternative fuels, advanced bioconversion technologies such as the APS-Digester will allow agricultural producers and food processors to use waste by-products effectively and increase the profitability and sustainability of their operations.

HOW THE DIGESTER WORKS

The APS-Digester system combines the favorable features of both batch and continuous operations into one biological system. Solids to be digested are handled in batches, while the biogas production is continuous. This allows the solids to be loaded and unloaded without disrupting the anaerobic environment for the bacteria. A typical APSDigester system consists of four hydrolysis reactors and one biogasification reactor. Liquid is recirculated intermittently between each hydrolysis reactor and the biogasification reactor. The solids being digested are housed in the hydrolysis reactors, while most of the bacteria, especially methanogens, are housed in the biogasification reactor.

In each hydrolysis reactor, as the solids are broken down and liquefied, soluble compounds, which are mainly organic acids, are collected and transferred to the biogasification reactor for conversion into biogas. The hydrolysis reactors are operated on different batch schedules so that the biogasification reactor is fed at a constant organic loading rate and therefore produces biogas constantly. The biogasification reactor is specially designed to maintain a high density of bacteria in the reactor for achieving efficient conversion.

Each hydrolysis reactor is loaded from the top and emptied from the bottom. Liquid that carries soluble compounds is collected at three locations along the depth of each hydrolysis reactor through filters installed in the reactor and transferred into the biogasification reactor every four hours. The return liquid is distributed over the top of the hydrolysis reactor. The biogasification reactor is fed at the bottom and decanted from the top; its contents are mixed after each liquid recirculation. Biogas is collected from all the reactors and piped into an engine-generator for cogeneration of electricity and heat.

The APS-Digester operates at a thermophilic temperature of 135 deg F to achieve a high rate of solids conversion. The solids retention time in the hydrolysis reactor ranges from three to 14 days, depending on the degradation rate of the solids. Food waste normally degrades very quickly and therefore requires a three to five day retention time. Crop residues such as rice straw are relatively slow to degrade and require a longer retention time of up to 14 days. The digested solid residue from the hydrolysis reactor can be dewatered and further processed into high quality soil amendment.

The APS-Digester has been technically validated in the laboratory for effectively digesting rice straw, animal manure, paper sludge, and a variety of food processing wastes. With the sponsorship from Onsite Power Systems and the University of California Biotechnology Program, a pilot APSDigester facility has been developed at UC Davis to allow researchers to determine the material handling requirement and optimal parameters for the engineering and operation of full-scale systems for a wide range of feedstocks. The pilot facility consists of two heated and insulated anaerobic reactors (one hydrolysis reactor and one biogasification reactor), a chopper pump, a mixer, and equipment for liquid transfer, gas collection and metering, and computer control and automation. Each reactor has 10,000 gallons of working volume. Except for loading and unloading, the digester system is fully automated and can be monitored both on-site and remotely via the Internet.

COMMERCIALIZING THE SYSTEM

After about five years of laboratory research and development, the APS-Digester technology is being scaled up for commercial installation by Onsite Power Systems. The company will design, construct, and operate commercial APS-Digester systems. Several projects are currently in various design stages with full-scale operation expected in 2002. One of the first is the thoroughbred horse-training farm in Boynton Beach Florida.

Biomass facilities are in the planning stages for two California State University facilities located in southern and north-central California. These facilities will incorporate local municipal yard trimmings and agricultural residuals as feedstock for the anaerobic digesters. One of the major advantages of the APS-digester is that the system can be scaled in size to meet the facility's supply of biomass. This scalability results in project capital costs ranging from $500,000 to $20 million. Based on projected financial returns, which are calculated from the value of electricity and heat generated, cost savings for disposing of the waste materials, and value of soil amendment produced, the payback of the investment for the biomass plants is estimated to be three to seven years.

The biomass plant in Boynton Beach will convert all the horse stable wastes and food residuals generated on the farm. The biogas, which contains about 60 percent methane, will be used for electricity and hot water generation. The design capacity of engine generators is 1.5 MW. The stable waste contains 80 percent wheat straw, which is used as bedding, and 20 percent horse manure and has a moisture content of about 80 percent. The anaerobic digestion system consists of ten sealed digester tanks (eight hydrolysis reactors and two biogasification reactors) of 325,000 gallons each.

All the tanks will be built above ground, preventing any impact on the ground water. This enclosed digester system will eliminate open storage of manure and bedding materials, greatly reducing odors, and insect and pest generation. Each digester tank will hold a two-day quantity of the waste material. Once filled, the tank will remain sealed for 14 days and operate at 135 deg F to allow anaerobic digestion to occur. During the 14 days of retention, a volumetric reduction of up to 70 percent of the waste solids occurs. Liquid recovered from the reduction process is useable for irrigation or wash-down. The recovered solids will be essentially pathogen-free and dried and processed into high quality soil amendment both for use on the farm and for sale to the local nurseries. The whole farm will be managed to have zero discharge.

BUILDING A BIOENERGY BASE

Anaerobic digestion is an important environmental biotechnology that can play a significant role in building the new national bioenergy and biobased economy and in the mitigation of greenhouse gas emissions, odors and public health concerns associated with conventional waste disposal systems. In California, the electrical power produced from anaerobic digestion systems is eligible for a 1.5 cents/kWh green supplementary payment from a renewable energy program run by the California Energy Commission.

The California State Legislature recently passed Senate Bill 5X, which allocated $10 million to provide cost sharing for dairy farmers to build anaerobic digesters to convert animal manure to electrical power. These government assistance programs are certainly helpful in encouraging agricultural producers, industrial processors and urban communities to take on the new anaerobic digestion technologies for renewable energy generation. Anaerobic digestion systems that are high-rate, simple to operate and easy to maintain, and have reliable performance, are needed to make anaerobic digestion of organic wastes a mainstream wasteto-bioenergy conversion technology.

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SAVINGS ON GREEN POWER INVESTMENT

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IN an article on methane generators in California Agriculture, published by the University of California's (UC) Divi

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sion of Agriculture and Natural Resources, author John Stumbos describes how the APS-Digester "is helping to fulfill a brighter outlook for converting organic residuals into energy." As noted in the accompanying report by Dr. Zhang, the technology has been licensed exclusively to Onsite Power Systems of Fresno. Dave Konwinski, co-owner of Onsite, expects that full-scale commercial systems such as the one being installed at the thoroughbred training facility in Florida will generate cost savings in as little as five years.

A generator proposed for the Los Angeles Turf Club racetrack, based on Stumbos' interview with Konwinski, would yield estimated annual savings on electricity and hot water (calculated at 11 cents/ kilowatt-hour) of $2.1 million. The ability to generate pow

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er on-site will also eliminate lost revenue and equipment damage from power-grid outages and poor power quality, estimated at $1.1 million annually. Another $720,000 savings are projected by eliminating the $25/ton cost of hauling stable waste to the landfill. The cost for the racetrack digester system is estimated at about $20 million.

"Environmental laws have become one of the driving forces behind this," Konwinski says, citing recent California regulations pertaining to groundwater and odor containment, as well as landfill reductions. The APS-Digester also will generate a soil amendment, which is being analyzed by UC Cooperative Extension horticulturist Richard Evans.

Zhang's "green power" system could also be tapped for fueling internal combustion engines, gas turbines or fuel cells for electricity production at mills, processing plants and refrigeration facilities with a consistent demand for power, notes Stumbos.

[Author Affiliation]

Ruihong Zhang is an associate professor in the Biological and Agricultural Engineering Department at the University of California, Davis. E-mail: rhzhang@ucdavis.edu. She will be presenting a talk on this anaerobic digestion technology at the BioCycle West Coast Conference in San Francisco, March 4-6, 2002.