Land application of digestate - The Biogas: Science, Production and Applications - page 301

Table 13.2 Mean nutrient concentrations from four farm digesters co-digesting
dairy manure and waste grease

NH 4 + -N
(g/kg)

PO 4 3 -P
(g/kg)

Total solids
(g/kg)

Total N
(g/kg)

Total P
(g/kg)

Feedstock mixture
(dairy manure + grease)

131 ± 22 3.5 ± 0.6 1.4 ± 0.3 0.5 ± 0.1 0.3 ± 0.0

Digestate 57 ± 5 3.6 ± 0.4 1.9 ± 0.5 0.5 ± 0.0 0.4 ± 0.1

Change 56.4% None +35.7% None + 33.3%

Note: Mean concentrations reported on wet mass basis ± standard deviation.
Source: Data collected by University of Guelph.

contaminants are present in the digestate, local regulations stipulating the
legal limits for the land application of the contaminants must be respected.

13.2.2 Transformation of nutrients and pathogen die-off
during digestion

During anaerobic digestion, organic matter is degraded, with organic C
converted to CH 4 and CO 2 . As well, nutrients in organic matter are
transformed into their inorganic forms; for example, organic N is converted
to NH 4 + -N and organic P is converted to PO 4 3 -P (Gerardi, 2003). It
should be noted the total nutrient content remains the same during
digestion; only their respective forms are changed. Table 13.2 summarizes
the mean change in digestate characteristics from four farm mesophilic
(40

C) digesters in Ontario, Canada, co-digesting liquid dairy manure with
waste grease.
The increased concentration of inorganic nutrients, particularly
NH 4 + -N, can contribute to increased crop yields as mineral nutrients are
readily available for crop uptake soon after land application. However, if
the pH of the digestate is high enough (pH

8

7.2) and climate conditions are
suitable, increased quantities of unionized ammonia (NH 3 ) may volatilize
upon surface application. The impacts associated with the land application
of digestate compared to raw manure on crop productivity and on the
environment are addressed later in the chapter.
The die-off of pathogens in anaerobic digesters has always been of
importance in the treatment of sewage sludge and is now of greater interest
in the treatment of agriculture residues, as it may lead to reduced pathogen
migration to soil subsurface drainage tiles. Pathogens can be inactivated
during exposure to heat above their optimum growth temperature. The
period of exposure is dependent on the temperature and on the species of the
organism. A lab study conducted by Kumar et al. (1999) demonstrated a 3-
log reduction in E. coli and Salmonella after 10 days of batch digestion of

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