Land application of digestate - The Biogas: Science, Production and Applications - page 299

the atmosphere, thereby reducing GHG emissions (Venczel and Powers,
2010). The collection and use of CH 4 in the production of electricity and
heat has been estimated by Sommer et al. (2004) to reduce CH 4 emissions by
up to 85-90% when compared with the conventional method of storing
manure in uncovered reservoirs. During the anaerobic digestion process,
manure nutrients are also transformed: organic N into NH 4 + -N and
organic P into PO 4 3 -P (Burke, 2001). Collins et al. (2010) suggest reduced
N 2 O emissions could be expected after the land application of digested
manures due to lower C to N ratios (C:N), resulting in less available organic
C needed for denitrification. Pathogens are also reduced during the
anaerobic digestion process. Crolla and Kinsley (2008) and Monreal et al.
(2012) demonstrated a 1 to 2 log reduction in a suite of pathogen indicators
(E. coli, Salmonella, C. perfringens and Enterococci) after the anaerobic
digestion of liquid dairy manure.
This chapter discusses the potential environmental impacts associated
with the land application of digestate on agricultural fields, when compared
with raw manure and inorganic fertilizer. The transformation of nutrients
and pathogen die-off during the digestion process is discussed. An extensive
examination of the effects of land application of digestate on NH 3 and N 2 O
emissions, odour emissions, crop yield and migration of NO 3 -N and
bacteria to drainage tiles is presented. Experimental field data collected by
the authors supporting the impacts associated with the land application of
digestate are referenced throughout the chapter.
Anaerobic digestion is commonly used in various applications, including
the digestion of agri-food residues, municipal sludge and industrial wastes.
This chapter, however, will be exclusively dedicated to the discussion of
agriculture-based digestates.

13.2 Overview of substrates and land application of
digestate

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13.2.1 Characteristics of manures and co-substrates

During anaerobic digestion, the organic matter in feedstocks is transformed
into biogas and digestate. The feedstocks typically used in agriculture-based
digesters include manures, agriculture crops, agri-food industry wastes and
the organic fraction of household wastes. Manure itself has a relatively low
biogas yield; however, when co-digested with organic-rich substrates, biogas
production is significantly increased. The addition of co-substrates to
manure can also improve the C:N ratio, which results in a stable digestion
process and good digestate fertilizer quality (Braun and Wellinger, 2003).
Table 13.1 describes the characteristics of selected substrates suitable for
anaerobic digestion.