The United States Environmental Protection Agency defines municipal solid waste (MSW) as everyday items, including product packaging, grass clippings, furniture, clothing, bottles, food scraps, newspapers, appliances, and batteries. Over two-thirds of the 254 million tons of MSW produced in 2007 was biodegradable. However, as a practical matter, only about one-third of the biodegradable material would have been amenable to anaerobic digestion, in part because much of the paper was usefully recycled and yard trimmings were composted. Otherwise, it is the extreme heterogeneity of MSW and the difficulties of separating its components that are key obstacles to energy recovery from this waste stream via microbiological processing (Fig. 1).

Source-separation programs targeting food waste and other organics are gaining popularity in the U.S. Compared to mixed MSW, such waste streams need less facility-level removal of non-biodegradable inclusions. Organics-rich waste streams are currently composted, but could be anaerobically digested with the recovery of energy.

Because solid wastes are complex mixtures, anaerobic digestion facilities necessarily include three major components: a mechanical step to recover recyclable materials and remove nonprocessible inclusions for disposal; an anaerobic digestion step to transform the organics to biogas (methane, carbon dioxide, and trace contaminants), leaving slow-to-degrade residual material usable as compost and liberated water; and a third step to separate residual solids and water and to condition the biogas as necessary. After contaminant removal, the biogas can be used directly to generate electricity on site, or the carbon dioxide can be removed and the methane can be injected into pipelines or used as vehicular fuel.

In France, Germany, and Spain, anaerobic digestion has found a role in the management of MSW. In the United States, some cities and counties have sponsored studies of alternatives to the dominant practices of incineration and landfilling. Although the reports identify anaerobic digestion as a top candidate, no plant has been built. Moreover, technical evaluations focus on mechanics instead of facing more fundamental microbial-level process design and control issues. Because there are so many ways to configure anaerobic digestion technologies, commercial development awaits decision-making based on meaningful microbiology-based analysis.

Methanogenesis Is a Key Part of Global Carbon Cycle

The degradative part of the global carbon cycle in anaerobic environments generates methane, as can be demonstrated by the "Volta experiment" (http://www.youtube.com/watch?v__e2Gz-h35HCE ; Volta experiment courtesy of Craig Phelps). Thus methanogenesis occurs spontaneously in, among other places, organicsrich sediments, landfills, and anaerobic digestion plants whether or not they are rationally designed. However, inefficient, empirical practices are not sufficient for large-scale waste processsing.

How well an anaerobic digestion system performs is determined by the rate and extent of degradation, reflected in the amount of methane produced and the stability of the residual digestate. The amount of methane is inversely related to the amount of digestate but directly related to the digestate's stability. Thus degradative rate and extent need to be taken into account in conjunction with a variety of other factors, including odor prevention, the area that a treatment facility occupies, reactor height and diameter, economic feasibility, public acceptability, and costs of construction and operation. All depend, ultimately, on how the microbes are managed.

Anaerobic digestion has long been used in treating sewage sludge, where process design and control are stubbornly empirical and tradition-bound. Moreover, owing to pilot programs in which food waste is injected into sewage sludge digesters with excess capacity, outmoded practices may spill over into the solid waste domain.

In contrast, a generic reactor design and informed process control strategy is widely used for treating aqueous industrial wastewaters, including those from breweries. These systems reflect a sound understanding of microbial methanogenesis. While not directly applicable to treating solid wastes or waste streams laden with particulate solids, this generic approach provides amodel for the more problematic solid waste domain.

Factors in Achieving Fast Microbial Methanogenesis

Methanogenesis is one of the most ancient biological functions on earth, and one of the best understood. Diverse microorganisms partly digest a range of complex solid materials, leading toward the final steps in which methane is generated (Fig. 2). Microorganisms in nature typically degrade relatively low concentrations of organic carbon compounds at a relaxed pace. However, the idea behind anaerobic digestion- harnessing methanogenesis in engineered reactors-is to intensively transform concentrated wastes as rapidly as possible into methane and degraded residual. The goal is to get the most bang for the volumetric buck by accelerating the process.

Several factors help to explain why particular approaches to reactor design and control might achieve high degradation rates and why others will not. First, acidogens are fast growers. However, some of them, particularly those that transform butyrate and propionate to hydrogen, carbon dioxide, and acetate, are subject to feedback inhibition when hydrogen gas accumulates to even very low levels. In contrast, methanogens are intrinsically slow growers, and are inhibited at acidic pH values. Thus, to achieve overall high rates, both the production and consumption of hydrogen and acids should be in balance, with production matched by consumption. An imbalance causes a bottleneck that reverberates though the metabolic web, slowing overall degradation of waste.

Second, microbial methanogenesis depends on syntrophy in which one species consumes the products of another; interspecies hydrogen transfer, the particular form of syntrophy in which acetogens and methanogens produce and consume hydrogen and acetate; and consortiums, in which acetogens and methanogens form close associations leading to methane production. The more intimate the association the shorter the diffusive distance, making the syntrophy more efficient.

Third, there are technological hurdles to overcome before methane-producing consortia can be optimally harnessed to degrade wastes on an industrial scale. Process design and control imperatives include the promotion of consortia development and the protection of their integrity. Also, the consortia need to be retained in the reactor, as in the form of an expanded bed or blanket, and the reactor should be designed to allow high organic loading rates, albeit not so high as to upset the balance between acidogens and methanogens.

Diverse Designs Are Being Used or Proposed to Treat Solid Wastes

Several anaerobic systems are in use or projected to treat solid wastes, and they embody critical differences (Table 1). Upflow anaerobic sludge blanket digestion (UASB) is a rational technology in that it is based on the science of microbial methanogenesis and informed by factors needed to achieve high rates. It fosters the development and retention of methanogenic consortia, which self-organize into granules slightly denser than water, forming a powerfully methanogenic suspended bed through which aqueous waste flows in its upward transit through the reactor. UASB was developed specifically for treating strong industrial wastewaters, such as those generated at breweries. Its limitation is that particulate solids in the waste stream tend to upset the system through poorly understood mechanisms.

Owing to this limitation, in treating solid wastes via UASB, the solids must first be converted to an aqueous wastewater stream. This conversion starts prior to biological treatment, with extensive mechanical disruption, size reduction, and successive screenings. Biological processing is in two stages; first an acidogenic stage performed in a continuously stirred tank reactor (CSTR), followed by a UASB methanogenic stage. Additional screening is needed between stages to remove recalcitrant particulates. While UASB is the gold standard in the wastewater domain, its application to solid wastes is extremely awkward.

Like UASB, induced bed reactor (IBR) is a rational technology informed by the science of microbial methanogenesis and the factors needed to achieve high rates. It, too, fosters development and retention of methanogenic consortia in the form of a suspended bed. IBR was initially developed for treating dairy manure flush water laden with as high as 10% particulate solids, without the necessity of their prior removal. That need is eliminated through more effective retention of culture by means of an active three-phase separator. Acidogenesis and methanogenesis are thus accomplished in a single reactor, with multiple reactors operated in parallel.

IBR is an example of a technology originally intended for a narrow application that has much wider utility. As applied to solid waste, while mechanical separation work prior to biological processing is not entirely eliminated, it is greatly reduced because IBR thrives on particulate-laden waste streams. IBR has the advantages of UASB without its intolerance of particulates.

Continuous stirring of anaerobically digesting liquor in sealed tanks is a historic transplant from aerobic sewage treatment, where vigorous mechanical agitation was used in open tanks to keep particulates in suspension and introduce oxygen into solution. Continuously stirred tank reactor (CSTR) was adapted to anaerobic digestion of primary settled sewage and secondary waste activated sludge before the profound difference between aerobic and anaerobic treatment was understood. But stirring disrupts consortia and is inconsistent with the disparate growth rates and environmental requirements of acidogens and methanogens within the anaerobic system. Nonetheless, CSTR anaerobic digestion remains a standard practice for U.S. municipal wastewater treatment facilities, and its fundamental deficiency of disrupting, rather than fostering, consortia is not considered in professional organization guidance documents and wastewater engineering textbooks. Richard Speece of the Vanderbilt University Department of Civil & Environmental Engineering, Nashville, Tenn., has reviewed recent research on this issue.

The leach bed approach is a transplant from aerobic composting. Fresh solid waste is heavily inoculated by mixing with previously processed material at high recycle ratios. Within a sealed vault, the new batch is irrigated with leachate amended with lime to counteract the decline in pH that would otherwise occur through acidogenesis getting ahead of methanogenesis. Gas is conveyed to storage. Material is moved in and out of the vault via wheeled loaders. A large component of a leach bed facility is after-the-fact aerobic composting, reflecting incomplete anaerobic digestion with much of the potential for methane production unexpressed.

The tall silo method involves very high recycle ratios. Successive batches are lifted to the top of a column to fill the void left as material exits from the bottom of the silo. The moisture content of the mixture must be high enough for it to slip through the silo. Gas is collected from a capped space at the top. At tall silo facilities, finishing through aerobic composting is a major operation.

In recent decades, great strides have been made in the basic science of microbial methanogenesis. Application of this progress to anaerobic digestion would advance the practice of solid waste management.

SUGGESTED READING

Dustin, J. S. 2010. Fundamentals of operation of the induced bed reactor (IBR) anaerobic digester. Ph.D. dissertation. Utah State University Logan.

Finstein, M. S., F. C. Miller, and P. F. Strom. l986. Waste treatment composting as a controlled system, p. 363-398. In H.-J. Rehm and G. Reed (ed.), Biotechnology, vol. 8, microbial degradations. VCH Verlagsgesellshaft (German Chemical Society), Weinheim, Germany.

Gale, J. A., P. E. Zempke, B. D. Wood, S. Sunderland, and S. A. Weeks. 2009. Comparison of induced sludge bed (blanket) anaerobic reactor to vertical plug flow technology for animal manure digestion. Western Nutrient Management Conference (Salt Lake City, Utah), 8:40-44.

Lettinga, G. 1995. Anaerobic digestion and wastewater treatment systems. Antonie van Leeuwenhoek 67:3-28.

Silvey, P., P. C. Pullammanappallil, L. Blackall, and P. Nichols. 2000. Microbial ecology of the leach bed anaerobic digestion of unsorted municipal solid waste. Water Sci. Technol. 41:9-16.

Speece, R. E., S. Boonyakitsombut, M. Kim, N. Azbar, and P. Ursillo. 2006. Overview of anaerobic treatment: thermophilic and propionate implications. Water Environ. Res. 78:460-473.

Thiele, J. H., M. Chartrain, and J. G. Zeikus. 1988. Control of interspecies electron flow during anaerobic digestion: role of floc formation in syntrophic methanogenesis. Appl. Environ. Microbiol. 54:10-19.

United States Environmental Protection Agency. 2008. Municipal solid waste in the United States: 2007 facts and figures. United States Environmental Protection Agency, Office of Solid Waste (5306P), EPA 530-R-08-010, p. 177. November 2008. www.EPA.Gov/osw.

Wall, J. D., C. S. Harwood, and A. Demain (ed.). 2008. Bioenergy. ASM Press.

Zinder, S. H. 1993. Physiological ecology of methanogens, p. 128-206. In J. G. Ferry (ed.), Methanogenesis: ecology, physiology, biochemistry & genetics. Chapman & Hall, New York, London.