Environmental Engineering Reference
In-Depth Information
Table 13.3 Log mean bacteria concentrations in four farm digesters co-
digesting dairy manure and waste grease
E. coli
(log CFU/100 ml)
Salmonella
(log CFU/100 ml)
Enterococci
(log CFU/100 ml)
Feedstock mixture
(dairy manure + grease)
7.5 ± 0.6
5.6 ± 0.8
7.3 ± 0.4
Digestate
5.0 ± 0.9
4.3 ± 0.5
5.0 ± 0.6
Log reduction
2.5
1.3
2.3
Note: Log mean concentration reported on wet mass basis ± log standard
deviation.
Source: Data collected by University of Guelph.
cattle manure at 35
C, whereas a 5-log reduction in Shigella was observed
under the same conditions. Harikishan and Sung (2003) presented similar
findings with 5-log and 3-log reductions in fecal coliforms and Salmonella
respectively during temperature-phased anaerobic digestion (TPAD) of
dairy manure. The two-stage TPAD system consists of an acid-producing
thermophilic (55
8
8
C) stage followed by a methane-producing mesophilic
(35
C) stage of digestion. The authors suggested pathogen destruction was
not only attributed to the high operating temperatures but also to the
accumulating volatile fatty acids (VFAs) in the first stage. Table 13.3
presents the log mean reductions in E. coli, Salmonella and Enterococci
bacteria in four farm mesophilic (40
8
C) digesters in Ontario, Canada.
Smith et al. (2005) suggest a 2-log reduction in E. coli is perhaps the
highest level of bacterial inactivation that can be anticipated from a
mesophilic digester. This finding is confirmed by Wright et al. (2004) as well
as the bacterial log reductions presented in Table 13.3. It was also suggested
by Smith et al. (2005) that efficient mixing in the digester and organic matter
stabilization (long retention times) were also important factors for bacterial
inactivation in mesophilic digesters.
8
13.2.3 Methods of land application
Raw manure or digestate can be land applied using various types of
equipment. Solid digestate that is greater than 18% total solids (TS) can be
surface broadcast. Digestate that is less than 18% TS, but more typically
12% TS, can either be surface applied or subsurface injected (OMAFRA,
2005; AARD, 2007). Surface applied digestate should be soil incorporated
to avoid odour emissions and loss of nutrients through runoff and
volatilization. Figure 13.1 shows the surface application of liquid digestate
at University of Guelph experimental plots using a tanker spreader with
splash plate (a) followed by soil incorporation using a cultivator (b). The
expected amount of NH
3
losses within the first 24 hours after surface
Search WWH ::
Custom Search