Composting 1293.7 Public health aspects of composting
The composting of human or animal wastes involves the following disease risks:
- From the pathogens normally present in the raw wastes (called
primary pathogens), and - From fungi and actinomycetes that grow during composting (called
secondary pathogens).
The primary pathogens such as bacteria, viruses, protozoa and helminthes
can initiate an infection in healthy individuals, while secondary pathogens
usually infect people with debilitated immune systems, such as those weakened
by primary infections or respiratory diseases (U.S. EPA 1981). Examples of
primary and secondary pathogens are listed in Table 3.5; more examples of
these pathogens can be found in Chapter 2.
3.7.1 Die-offs of primary pathogens
Time and temperature are the two most important parameters responsible for
pathogen die-offs during composting. It should be noted that a complete
inactivation of pathogens in a compost pile is rarely achieved. This is due to
many reasons such as:
- The heterogeneous character of the compost materials, which may
form clumps with the pathogens and protect them from being fully
exposed to thermophilic temperatures. - The uneven temperature distribution in the compost piles. Unless
completely mixed continuously, the outer surfaces of a compost
heap normally have a lower temperature than the inside, causing a
lower efficiency of pathogen kill (see Figure 3.16). - Partial inactivation of pathogens. Many pathogens such as spore-
forming bacteria, cysts and helminthes ova are only partially
inactivated during composting. They can re-grow and become
infective again if exposed to a favorable environment such as under
moistened conditions in crop fields.
A number of researchers (Kawata et al. 1977; and Cooper and Golueke 1979)
reported several orders of magnitude reduction of bacteria (total coliforms, fecal
coliforms and fecal streptococci) and viruses (poliovirus and coliphages) during
composting. Better inactivation rates were observed with the BARC system in
which these bacteria including Salmonella were non-detectable after 10 days of
composting (Figure 3.20) and for F 2 bacteriophages about 15-20 days (Figure
3.21). The thermal death points of some common pathogens and parasites are