Research and Development is discussed, focusing on a venture called Pinnacle Biotechnologies International, 
Inc., spun off from the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) to 
commercialize a high solids anaerobic digestion technology developed at NREL over the past 10 years. The 
need for a comparison of soil amendments from anerobic digestate with similar products produced through 
aerobic composting is raised, noting that a protocol for a series of comparative product performance trials 
between aerobic composts and anaerobic digestates is being finalized. 

          When organic materials decompose without oxygen, the anaerobic bacteria that do the work produce methane and carbon dioxide. This naturally occurring bit of microbiological alchemy has long been utilized for energy production on farms in Asia, and as far back as the 1890s, biogas was recovered from sewage treatment facilities and used to fuel street lamps in England. However, only in the last 25 years or so have serious attempts been made to fully develop a technology of anaerobic digestion to treat liquid and solid waste streams from farms, municipalities and industry. 

"The industry has grown tremendously during the past decade," says Philip Lusk of Resource Development Associates in Washington, D.C. "In the area of industrial wastewater treatment alone, more than 600 vendor supplied systems are operating or under construction throughout the world." Reflecting an early research focus in Europe, European plants account for about 44 percent of the installation base with only 14 percent of the systems located in North America. A considerable number also are in India and South America. 

 According to Lusk, who heads the International Energy Agency's (IEA) Bioenergy Anaerobic Digestion Activity, over 35 industries that use digesters for industrial wastewaters have been identified, including processors of chemicals, fiber, food, meat, milk, and pharmaceuticals, among others. "Frequently, AD is used as a pretreatment step to control odors, and reduces the costs of final treatment at a municipal wastewater treatment facility," Lusk notes. Likewise, sewage treatment facilities themselves have been paying increasing attention to AD due to the 50 to 80 percent reduction in biosolids volume that results, along with the production of biogas and a humus rich, biologically stable residual material. 

 "A fairly new application for the technology is the digestion of municipal solid waste (MSW)," says Lusk. "Various systems have been developed; each with their own special benefits (see sidebar). The first proof of concept facility was operated at Pompano Beach, Florida from 1978 through 1985. The last 10 years have seen a variety of pilot scale plants in Europe, with progress being made towards commercialization in the last few years. Over 60 plants are now in operation worldwide, with an additional 70 under construction or in the planning phase." 

 The following sketches of operations utilizing different technologies aims to provide a sense of the industry's present vitality and potential areas of co-utilization with aerobic composting systems. 

 BASSUM, GERMANY  

The anaerobic digestion plant currently under construction in Bassum, represents an "upcoming application for digestion technology," according to Luc DeBaere, general manager of Organic Waste Systems in Ghent, Belgium. Envisioned as a component of an integrated household waste management system in Niedersachsen, Germany, the facility will use a combined anaerobic-aerobic treatment for both "rest waste" (materials remaining after removal of recyclables and biowaste) and municipal biosolids. As DeBaere explains, in many areas of western Europe, collection already is underway for biowaste (e.g. source separated household garbage, plus kitchen organics, grass clippings and some leaves). As in Bassum, this fraction is being composted and finds ready markets. 

On the other hand, the rest waste still contains a large percentage of compostable materials, including items such as soiled paper and diapers. "It's really similar to the mixed waste fraction with a reduced level of organics," says DeBeare. At the Bassum plant, approximately 60,000 metric tons per year of rest waste will be processed to separate out material for aerobic composting (<80 mm) and incineration, as well as small organics (<40 mm) for anaerobic digestion. The small organics (11,000 metric tons per year) will be combined with around 2,500 tons per year of centrifuged municipal biosolids and treated in a  full-scale dry anaerobic composting (DRANCO) digester. 

After raising the temperature of the substrate to 55°C with steam, fermentation takes place in a vertical reactor under thermophilic conditions, without addition of water. Material passes through the reactor from top to bottom where it is removed by an extraction system. Digested residue (about 35 percent solids) will then be mixed with larger organics (<80 mm) and composted in an adjoining composting plant for around eight weeks. Resulting compost -- by then "biologically stabilized" and significantly reduced in volume (but not up to European standards) -- will be landfilled. 
 
 

Figure 1. At facilities in Brecht, Belgium which  uses the DRANCO process, household organics  - yard trimmings and non-recyclable paper -  are fermented in a vertical reactor. About  850 tons of organics are digested monthly  (Photo courtesy OWS)

Organic Waste Systems developed the DRANCO process and began their first pilot plant in 1984. Although generally known as a "high solids" technology, according to DeBeare, it can treat substrates with a solids content ranging from approximately 15 percent to 40 percent and higher. The company presently has two operating facilities -- in Brecht, Belgium (see "Anaerobic Digestion for Household Organics," BioCycle, April, 1995) and Salzburg, Austria (see "Anaerobic-Aerobic System in Salzburg," BioCycle, June, 1994). The Bassum plant is scheduled to start up in spring, 1997. 

WAIMANALO, HAWAII  

On January 1, 1997, a city ordinance goes into effect in Honolulu requiring large restaurants, grocery stores, hotels, hospitals, food courts and food manufacturers and processors to recycle food residuals. A portion of that material will likely go to Unisyn Biowaste Technology, a private firm based in Waimanalo. The company was founded in 1986 and presently is anaerobically converting around 40 tons per day of wet organic residuals such as slurried fish heads, rejected hamburgers, and produce trimmings into biogas, soil amendment, and irrigation water. 

 Incoming materials are reduced in size to one-and-one-half inch cubes and moved to an 83,000 gallon capacity equalization tank (acid digester) where they are chopped and further mixed to produce a homogeneous slurry of 10 to 15 percent total solids. The system also removes grit and other large nondigestibles such as plastic, glass or metal. This resulting slurry is then pumped to methane digesters. 

 Inside the digesters, a patented arrangement of inert plastic attachments help to minimize clogging and provide multiple surfaces that favor the growth of anaerobic bacteria. A pipe routes biogas (55 to 65 percent methane by volume) to a collection header and then on to a biogas engine for electricity production. Digester effluent contains both suspended and dissolved solids. The suspended solids include a large quantity of organic material, as well as an inorganic fraction. After screening, effluent is pumped to a centrifuge for dewatering. The remaining paste is recycled to the equalization tank for further digestion. 

Last fall (1995), Unisyn added an aerobic composting operation to its treatment facility that complements the existing anaerobic component. Yard trimmings and other green materials from the city and county of Honolulu provide the main feedstock, presently between five and 10 tons per day. Incoming materials are placed in aerated static piles for approximately two months and then composted in turned windrows for another month or so. 
 

Figure 2. At the Waimanalo, Hawaii facility, operated  by Unisyn, 40 tons per day of wet organic residuals  are slurried and pumped into digesters (background)  while yard trimmings and other green materials are  placed in aerated static piles (foreground).

A tub grinder (Figure 3) reduces incoming materials  prior to digesting and composting.  (Photo courtesy Unisyn)

"We realized that our anaerobic by-products can enhance a compost," says Matt Lyum, plant manager of the Waimanalo facility. "The fibrous digestate -- which we had been marketing as a soil amendment on its own -- is high in nitrogen, phosphorous, potassium and micronutrients like magnesium and iron. When it's added to compost piles, it accelerates the composting process and produces an end product rich in nutrients. Likewise, our liquid effluent is between 75 percent and 80 percent water and can be used for compost irrigation." Lyum adds that the compost has opened new markets for the facility's end products, which presently is being sold in bulk to farmers, turf farms and landscapers. 

 ST. LOUIS, MISSOURI   

For industrial wastewaters high in organics -- such as those produced by brewing beer -- anaerobic digestion has proven to be a viable treatment alternative to aerobic pollution reduction measures. In May, 1996, the Anheuser-Busch started up six five-story anaerobic digestion tanks at its St. Louis Brewery, the company's sixth installation since 1985. 

 The effluent being treated in St. Louis is 99.9 percent water (both incoming and outgoing) and results from boiling brewing ingredients such as barley, corn, rice and hops, as well as fermentation action brought about by yeast. Previously, it was discharged directly to the Metropolitan Sewer District's (MSD) wastewater treatment system. However, it has been determined that with anaerobic pretreatment the load of organic material from the brewery has been reduced 80 to 90 percent. Additionally, the amount of biosolids produced by MSD from the brewery wastewater has been reduced by more than 50 percent. Treatment fees that Anheuser-Busch must pay also are reduced, and methane from the digesters provides 10 to 15 percent of the boiler fuel used by the brewery. 

The process used in St. Louis is known as an Upflow Fluidized Bed, an improvement in Upflow Anaerobic Sludge Blanket technology (see sidebar), both of which are offered by Biothane Corporation in Camden, New Jersey . Essentially, wastewater flows through screens, which remove solids, and then into large equalization tanks, where the temperature, pH and organic content are moderated before being pumped into the bottom of an enclosed anaerobic chamber. Wastewater then flows upward through a dense bed of bacteria, or "biomass." The bacteria digest the organic material, producing methane which bubbles to the top of the closed reactor for collection. 
 
 

Figure 4. Five story stainless steel tanks contain bacteria that  remove organic materials from brewing wastewater at  Anheuser-Busch facility in St. Louis, Missouri. The process  also recovers methane to provide fuel for boilers.  (Photo courtesy Anheuser-Busch)

 RESEARCH AND DEVELOPMENT  

Although low-solids anaerobic systems for wastewaters are relatively common, high solids processes, as noted above, are less numerous and do not have as long a history. Much of the earlier developmental work was done in Europe where interest in energy conservation and alternative sources of energy has been higher. 

 The National Renewable Energy Laboratory (NREL) of the U.S. Department of Energy (DOE) recently spun off a commercial venture called Pinnacle Biotechnologies International, Inc. to commercialize a high solids anaerobic digestion technology developed at NREL over the past 10 years. A DOE sponsored pilot scale demonstration plant under construction in Stanton, California is scheduled for start-up in December, 1996. Initially, it will be processing approximately two tons per day of tuna cannery sludge mixed with municipal solid waste at up to 40 percent solids. 

In the system developed by Chris Rivard and Brian Duff, Pinnacle's founders, hardware components adapted from the mining and food processing industries are used to shred organic solid wastes and load the high solids bioreactor. A NREL publication explains that "standard" digesters use a cylindrical vessel with a vertical turbine or propeller shaft that rotates at about 100 revolutions per minute. "The NREL high solids digester also uses a cylindrical reactor, but both the reactor and agitator shaft are horizontal. Without the pressure on the material near the bottom of a vertical cylinder exerted by the weight of all the overlying material, mixing dynamics are quite different." 

 Pinnacle has sufficient funding to cover the first two years of operations from contracts to both operate and serve as technical advisor for the demonstration plant. Also referred to as the "Client Development Center," one function of the Stanton facility will be to market the technology to interested industrial and municipal customers. Four bench scale bioreactors, an intermediate scale bioreactor and a complete analytical laboratory will be available. The company intends to particularly focus on small food processors generating up to 40 tons of organics per day, and privately owned material recovery facilities in California. 

 ANAEROBIC COMPOST ?  

Apart from biogas, anaerobic digestion also creates solid and liquid by products which can have value as a fertilizer or soil amendment. "The amount, quality and nature of these products will depend on the quality of the MSW feedstock, the method of digestion (wet or dry) and the extent of the posttreatment refining processes," notes Phil Lusk. As he describes it, the main product of a dry digestion process is a solid digestate which can be matured into a compost product. On the other hand, digestate from wet digestion is generally similar to a concentrated digested sewage sludge or digested animal slurry that reportedly can be spread directly onto farmland or dewatered to provide separate liquid fertilizer and solid compost-like products. 

 How do these soil amendments compare with aerobic compost? James Dickow, industrial microbiologist for Pinnacle, for instance, points out that ammonia is not volatilized in their digestion process. He says the result is a higher nitrogen content in solid end product compared to most aerobic composts. However, the company was still in the process of having its digestate classified and he had no final comparative test figures. 

Recognizing that this type of question needed to be clearly resolved, the IEA Bioenergy Agreement Anaerobic Digestion Activity identified the need to conduct comparative trials. "Likely, there are quality differences between aerobic composts and solid anaerobic digestates," IEA says. "A standardized evaluation of these differences is important so that optimum end uses can be developed for both products. Additionally, there is a large knowledge base on the use of aerobic composts. Where appropriate, the application of this knowledge base to anaerobic digestates would also assist market development." 

A protocol for a series of comparative product performance trials between aerobic composts and anaerobic digestates "that will highlight their similarities and key differences important to the intended use," should be completed by the end of October, according to Lusk. Plans are for researchers in Switzerland to compare solid anaerobic digestate quality from different countries after the protocol is available. Results of the study will presented in a future issue of BioCycle.