Agriculture Reference
In-Depth Information
The most impactful mechanism of pathogen inactivation during composting is
through temperature elevation caused by metabolically active microorganisms,
although ammonia gas (Himathongkham and others 1999; Nicholson and others
2005), desiccation (Redlinger and others 2001), and microbial antagonism (Ichida and
others 2001) are additional factors that contribute to pathogen reduction during com-
posting. Table 8.2 summarizes some recent studies on pathogen inactivation during
composting.
Several studies have revealed that composting can effectively inactivate
E. coli
O157 or
S
. Enteritidis in manure ranging from 3 to less than 14 days when the initial
pathogen populations were ca. 10
7
CFU/g, provided the composting temperatures were
maintained for at least 45 °C (Lung and others 2001; Jiang and others 2003a; Hess
and others 2004). D-values for
E. coli
O157:H7 in dairy compost at typical compost-
ing temperatures of 50, 55, and 60 °C were 135, 35.4, and 3.9 min, respectively (Jiang
and others 2003b). Therefore, composting temperatures of 50, 55, and 60 °C for at
least 14 h, 4 h, and 24 min, respectively, should be suffi cient to provide a 6-D reduction
of
E. coli
O157:H7 during composting.
Composting is an outdoor process for which there are many variables affecting the
fate of pathogens. Field studies under different environmental conditions should be
used to validate the results generated in laboratories. Recently, in three separate fi eld
composting studies,
E. coli
O157:H7, non - O157 STEC
, Salmonella,
and
Listeria
survived for from 8 to less than 14, 9-20, 4, and 4 days, respectively, in dairy manure
compost (Nicholson and others 2005; Fremaux and others 2007a; Shepherd and others
2007). Both Fremaux and others (2007a) and Shepherd and others (2007) have shown
the uneven pathogen inactivation throughout the compost heaps with longer survival
on the surface or around the periphery of heaps due to lower temperatures there. The
results from these studies indicate that the enteric pathogens at a population of
10
7
CFU/g or less inside the active composting heap should be inactivated rapidly
within 3 weeks during composting in fi eld. However, many variables, such as the
types of animal manure, C : N ratios, pH, moisture content of compost mix, degrada-
tion extent of agricultural wastes, digestibility, size of heaps, ambient temperature,
frequency of turning, initial cell numbers of target pathogens, strain variation, season
of the year, and geographical locations can affect the exact length of time required
for pathogen inactivation in compost.
Identifying Improper Composting Practices That Allow the Extended Survival
of Pathogens
Composting techniques may range from “passive,” with little or no input, to a windrow
system that is turned and watered routinely with expensive, specialized equipment.
Large-scale, properly managed, on-farm composting has a proven track record of
consistently producing high-quality compost that went through an adequate thermo-
philic phase during the composting process. However, for small farmers and organic
growers, some composting practices are less effective than others, in part because
compost piles are infrequently turned and the C : N ratio, moisture content, or pH of
compost materials are inadequate for optimal microbial activity within the compost
heaps (Granberry and others 2003). Such practices may cause slow heat-up of the
compost pile, which can enable low populations of pathogens to become acclimatized
