Outlook

Anaerobic digestion of agricultural wastes is a mature technology with numerous full-scale digesters located all over the world. Noteworthy is the recent interest in digesting or co­digesting of bioenergy crops in the EU because of the relative high-energy efficiency of methanogenic food webs (no side product formation and product inhibition). Even though the level of maturity is high, research on reactor stability is necessary, especially when new applications are pursued. It is generally accepted by scientists and engineers that new prob­lems and unanticipated challenges keep coming up. The complex microbial community and food web is the culprit, with more powerful techniques, such as metagenomics and stable — i sotope probing, starting to shed light on the required mechanistic understanding of the microbial community and interactions (Lfibken et al. 2007; Schlfifer et al. 2008; Li et al. 2009 ).

For wastes from agriculture with complex nutrient and water cycles, anaerobic digestion should be seen in the larger context of an integrated system in which nutrients and water from digester effluent are continuously recycled. During the treatment of animal wastes with anaerobic digesters, for example, nutrients, such as ammonia and phosphate, are freed from biomass and are accumulated in solution. These nutrients must be recycled back to agricul­tural production in an environmentally friendly and sustainable way. In North Carolina (United States) an integrated system that recycled digester effluent to a greenhouse for tomato production (Cheng et al. 2004) was studied. Others are optimizing struvite precipitation to recover N and P (Borgerding 1972; Ohlinger et al. 1998; Schuiling and Andrade 1999; Battistoni et al. 2000; Mfinch and Barr 2001) . In addition, upgrading the energy carrier methane to more valuable products may be necessary to guarantee economical viability. In Utah (United States) biogas from a swine waste digester was cleaned, steam reformed into synthesis gas (i. e., syngas), converted into methanol through a thermochemical process, and combined with triglyceride to produce biodiesel through a trans-esterification reaction process (Dugba 2003).

With the need for co-digestion, an opportunity exists to link agriculture, rural communi­ties, and industry for sustainable rural community development. A U. S. example for this system approach is BioTown in Richmond, Indiana, where the goal is to create an energy self-sufficient community of about 500 persons using an anaerobic digester as an integrated technology to create biogas from animal manures, food wastes, organic municipal wastes, and crop residues (BioTown 2008). Communities such as these illustrate that anaerobic digestion is a significant and practical technology with relatively high — energy efficiencies and that agricultural wastes play an essential role.

Acknowledgements

LTA was supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number 2004-35504-14896. In addition, The New York State Energy Research and Development Authority is gratefully acknowl­edged for partial support of anaerobic digestion studies at Cornell University. Finally, we thank Rodrigo Labatut and Nick Scalfone for generating Figures 4.1 and 4.2, respectively.