Zhongtang Yu and Floyd L. Schanbacher

Abstract Anaerobic digestion (AD) is a biotechnology by which biomass is con­verted by microbes to methane (CH4) biogas, which can then be utilized as a renewable fuel to generate heat and electricity. A genetically and metabolically diverse community of microbes (mainly bacteria and methanogens) drives the AD process through a series of complex microbiological processes in the absence of oxygen. During AD, bacteria hydrolyze the polymeric components (e. g., polysac­charides, proteins, and lipids) present in the feedstock and further ferment the resulting hydrolysis products to short chain fatty acids (SCFA), H2 and CO2, which are ultimately converted to methane biogas (a mixture of CH4 and CO2) by archaeal methanogens. Various biomass wastes (e. g., livestock manure, crop residues, food wastes, food-processing wastes, municipal sludge, and municipal solid wastes) are especially suitable for AD. As one of the few technologies that can both cost — effectively generate bioenergy and reduce environmental pollution, AD has been increasingly implemented in different sectors to convert otherwise wasted biomass to bioenergy. AD technologies can be categorized in many different ways. Each AD technology has its own advantages and disadvantages that make it suitable for particular feedstocks or objectives (i. e., production of energy or stabilization and treatment of wastewaters). Both drivers and barriers exist for commercial imple­mentation of AD projects, with the former stimulating, enabling, or facilitating AD implementation, while the latter function in opposite direction. This chapter will provide an overview of the microbiology underpinning the AD process, and discuss the characteristics of the biomass wastes suitable for AD and the AD technologies appropriate for each type of these feedstocks. The drivers and barriers for AD as well as the AD technology gaps and future research needs will also be discussed.

Z. Yu (b)

Department of Animal Sciences and Environmental Science Graduate Program,

The Ohio Agricultural Research and Development Center, The Ohio State University, Columbus, OH 43210, USA e-mail: yu.226@osu. edu

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_6, © Springer Science+Business Media B. V. 2010

Keywords Anaerobic digestion ■ Biomethanation ■ Methanogens ■ Methane biogas ■ Digesters ■ Biomass wastes ■ Feedstocks

1 Introduction

Anaerobic digestion (AD) is underpinned by a series of bioconversion processes that transform organic compounds, especially biomass wastes, to methane biogas (a mixture of approx. 60% CH4 and 40% CO2). Although it has been used for more than a century in treatment of municipal sludge and high-strength organic wastew­aters from industries, the main objectives have been to stabilize and sanitize the sludge and to remove the organic pollutants from the influents, with relatively little focus on biogas production. Recently, AD received tremendous renewed interest as the demand for and price of fuels continue to rise. AD is looked upon to be an impor­tant biotechnology to help build a sustainable society by simultaneously producing

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renewable bioenergy and protecting the environment. Indeed, a diverse range of feedstocks (e. g., municipal sludge, food-processing wastes and wastewaters, live­stock manures, the organic fraction of municipal solid wastes (OFMSW), crop residues, and some energy crops) are being diverted to AD for increasing biogas production [4]. Although AD is a relatively slow process and its operation and per­formance are sometimes unstable, the methane biogas derived from biomass wastes has become competitive, in both efficiency and cost, with heat (via burning), steam, and ethanol production [31]. In this chapter, the microbiological underpinning of the AD process as well as the recent understanding of the microbial communities driving AD will be discussed from a biotechnological perspective. This chapter will also provide an overview of the common characteristics of feedstocks that have great biogas potentials and the AD technologies suitable for each of these types of feed­stocks. The drivers and barriers for commercial AD implementation as well as the AD technology gaps and the research needs will also be discussed.