Land application of digestate - The Biogas: Science, Production and Applications - page 309

emissions under various climatic and soil conditions, thereby helping to
refine NH 3 volatilization models such as AGRIN (Beuning et al., 2008).

13.3.3 Odour emissions

As the urban fringe encroaches onto agricultural lands and livestock
operations intensify, there are growing complaints concerning odours
associated with agricultural activities. Odours from farms can be generated
from barns that house animals and feed, the manure storage structure and
the land application of manure (Powers, 1999). The odorous compounds,
which are generated by numerous bacterial species, include volatile fatty
acids (VFAs), aromatic compounds (e.g. phenols), nitrogen-containing
compounds (e.g. NH 3 ) and sulphur-containing compounds (e.g. H 2 S)
(Rappert and Mu¨ ller, 2005).
Odorous compounds have been shown to be reduced during the anaerobic
digestion process. Powers et al. (1999) demonstrated more than 99%
reduction in total VFAs and 60% reduction in phenols during a lab-scale
digestion of dairy manure. The authors stated that, although NH 3
concentrations increased in the digestate, NH 3 was not a major contributor
to odour. Crolla and Kinsley (2008) also reported significantly lower VFA
concentrations in the digestate from two farm-scale mesophilic (40

C)
digesters (in Ontario, Canada) co-digesting dairy manure and waste grease,
with average reductions in total VFAs greater than 95%.
A quantitative study was conducted by the University of Guelph
(Ontario, Canada) and Ontario Ministry of Agriculture, Food and Rural
Affairs (OMAFRA) to determine whether the decreased odour compounds
in digestate would translate into reduced odours during land application.
Lab simulation studies were conducted to determine surface odour
concentrations expressed as odour units per cubic metre of applied substrate
(OU/m 3 ). Standard flux chamber and wind tunnel protocols and odour flux
measurements were applied. The odour studies were conducted using raw
dairy manure and digestate from the same two farm digesters studied by
Crolla and Kinsley (2008). Table 13.6 summarizes the odour measurements
taken using flux chamber and wind tunnel methods. Odour emissions were
significantly lower with the application of digestate, which demonstrates
that anaerobic digestion is an important tool in reducing odours.

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13.3.4 Crop yields
Digestate typically has higher NH 4 + -N concentrations than raw manure (as
illustrated in Table 13.2) and it is anticipated this readily available form of
N can result in increased crop productivity. However, the potential loss of N
from NH 3 volatilization during land application should be considered. As