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Anaerobic Digestion Technology: A Primer PREAMBLE This web page is meant as an “introductory overview” to the mechanical and biological elements of an anaerobic digestion facility and to some of the issues involved in such an undertaking. An anaerobic digestion facility would integrate three distinct elements into a holistic system.
To frame the discussion it is first noted that the pre- and post-digestion elements differ fundamentally from the digestion element with respect to the type of information available for their design. Design of the front and back ends is empirical art, whereas the central element, anaerobic digestion per se, is well understood scientifically, hence is amenable to rational design. A word of clarification is in order. Empirically designed technologies are the result of trial and error experience. Throughout almost all of human history, empiricism was the sole means of technological advance. The rise of science, with its insights into basic phenomena made it possible, in some instances, to rationally improve on traditional technologies or processes, or to invent new ones. The point is illustrated in the brewing of beer. Given even marginally suitable temperature and moisture content and only few other factors, grain-derived sugars are spontaneously and rapidly fermented by indigenous yeast into alcohol and carbon dioxide, the essence of beer. Long before yeasts were even known to exist, prehistoric man developed brewing methods empirically. With the discovery of yeasts less than two centuries ago, and the progressively deepening understanding of their environmental and nutritional requirements, it became possible to put beer production on a rational basis, to avoid spoilage and off flavors, to improve predictability, and to realize economies of scale. Today, a commercial brewery ignoring what is understood about the underlying phenomenon and reverting to strictly empirical methods would soon be out of business. Anaerobic digestion is a very different traditional microbiological technology, in use for about a century in the treatment of sewage sludge. Since the responsible organisms are ubiquitous, given suitable conditions organic waste is spontaneously transformed to methane-containing biogas. The system is slow to start up, however, and, if managed improperly, prone to failure; all the more reason to exploit the major recent advances in the understanding of the phenomenon that underlies anaerobic digestion – that of microbial methanogenesis (generation of methane). Yet in the solid waste field, rational methods are slow to replace strictly empirical ones. Having made the distinction between empirical and rational design, the discussion now turns to the front end element of a facility that is centered on anaerobic digestion. Subsequently, discussion of the biology of anaerobic digestion is resumed. THE FRONT END ELEMENT: PRE-DIGESTION The design of the front end element is empirical, being based on trial and error experience. The function of a front end is to separate materials not intended for biological treatment from those so intended. Obviously, non-biodegradable materials should be removed, but also biodegradable ones such as clean paper products (fiber) suitable for recycling as secondary raw materials. The removal is effected using various types of machines exploiting properties such as size, specific gravity, and chemical makeup. The number of machines, their specific function, sequence, and configuration are tailored to the characteristics of the waste stream and the overall processing objectives. The most complex and problematic of waste streams is mixed, unsorted, municipal solid waste (MSW), and the most difficult of objectives is threefold: recovery of recyclable materials (metals, glass, plastic, fiber); removal of non-recyclable and non-processible materials (non-descript items, stones, grit); and preparation of the now-isolated biodegradable materials intended for digestion. The MRF (materials recovery facility) familiar to all in the waste management field focuses on the removal of materials of value from the mixture for recycling purposes. The remainder is often landfilled, with loss of the potential to recover energy from the biodegradable organic fraction. Adding to the MRF’s function the separation of non-processibles from the organics, for purposes of energy production, would make of it a MERF (materials and energy recovery facility). Given the transformation of the organics to methane as the energy product, the MERF could equally be called an anaerobic digestion facility. The complexity of a front-end element to accomplish all three objectives with MSW reflects the complexity of the waste stream, as indicated by the following description:
Various types of machines are employed to accomplish front-end objectives (Table 1). Table 1. Generic machines found in MRFs, hence applicable to the front end of MERFs in which anaerobic digestion is the central element
The machines are listed in no particular order, and each has its range of settings with respect to operational speed or strength of action. The list is neither exhaustive, nor is each type necessarily used in all facilities. Water, not being a machine, is excluded from Table 1. But special purpose water vat systems may be used for fractionation, based on differences in specific gravity (buoyancy) and solubility. Less complex waste streams, such as source separated organics, cafeteria waste, and waste from food processing industries, require less complex front ends. Perhaps the least complicated of applications is on-farm treatment of particulate-laden manure flush water, since relatively little pre-digestion manipulation is needed. Yet even here designing the infrastructure within which the digester system resides is a significant task. The types of machines employed in the MRF/MERF are manufactured by a number of companies. Their web sites may be viewed for descriptions, pictures, and even videos showing equipment in operation. The subject may be explored via computer search engine by entering terms such as recycling industry equipment, or materials recycling facility machines. Today’s showroom is a virtual one! THE CENTRAL ELEMENT: ANAEROBIC DIGESTION The transition from the front end element to anaerobic digestion per se may involve the removal of fugitive non-biodegradables, adjustment of moisture content by addition or removal of water and heating of the feed so that the biological action can proceed at the desired temperature. Operationally, machines such as fine screens, pumps of various types, mixers, and heating devices may be used. Anaerobic digestion occurs spontaneously, but it is slow to start. This is because it involves the sequential action of different microbial types having seemingly contradictory environmental and nutritional requirements and growth rates. The responsible organisms may be broadly labeled as hydrolytic, fermentative, acidogenic, and methanogenic. Once the system matures, over a period of weeks or months, sustained high rates of treatment are possible. But if metabolic balance is not maintained, poor performance or system failure can result. These matters are detailed in the paper "Suitability of Anaerobic Digestion Variants for the Treatment of Municipal Solid Waste" in Microbe magazine, the online peer reviewed publication of the American Society of Microbiology. It comparatively analyzes five different approaches to the design of anaerobic digestion reactors and their associated process control strategies. The factors compared are three: conformance to the contemporary scientific understanding of the microbiological dynamics at work; suitability in the technology for processing solid waste; process performance in terms such as rate of transformation of waste to methane and other practical operational aspects. The five approaches analyzed are: upflow anaerobic sludge blanket (UASB); induced blanket reactor (IBR); continuously stirred tank reactor (CSTR), a.k.a. completely mixed reactor; leach bed reactor; and tall silo reactor. The analysis concludes that the IBR technology is superior with respect to all three factors compared, in that it conforms to scientific understanding (i.e., is a rational process), is applicable to the treatment of solid waste, and outperforms other approaches. THE BACK END ELEMENT: POST-DIGESTION The products of the biological action are liquid water freed from the moisture content of the waste, undigested solids, and biogas. The disposition and/or use of the products is highly circumstance-specific. Like the front end facility element, design of the back end is empirical art. Following digestion, the solid and liquid phases may be separated using devices such as a belt filter press, screw press, or plate and frame press, usually with the aid of a dewatering agent polymer. The water may be discharged to the local sewer or, after treatment, used in irrigation. The solids, or digestate, may be suitable for use as compost (organic soil amendment) or, at the least, as landfill cover material. Storage of the biogas and its conditioning before use may be necessary unless it is flared off (not put to use) or conveyed to a boiler that can utilize it directly. For use in fueling an internal combustion engine to generate electricity, chilling the biogas to condense out the moisture may suffice. Or, the biogas may be upgraded to pipeline quality by removing trace impurities and most of the carbon dioxide. Use as vehicular fuel requires compression and/or liquefaction. While technologies for a wide range of upgrades are available, economical feasibility is circumstance-specific. FINAL COMMMENT Developing an anaerobic digestion facility is no simple task. That it is undertaken reflects the need in the 21st to address the twin problems of diverting wastes from the landfill into useful channels, as in the recovery of both materials for reuse and the production of "green" energy.
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Municipal Solid Waste (MSW): A complex, problematic, mixture. Integration of machine and microbe Front-end configurations Fundamentals of microbial methanogenesis Anaerobic digestion – application of fundamentals Comparative analysis of anaerobic digestion systems IBR facilities and performance
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