Destruction of pathogenic organisms is a most important aspect-and a problem- of compost containing sewage, or other highly contaminated materials. Experiments have shown that aerobic composting at high temperatures is effective in destroying pathogenic organisms. The absence of health hazards is characteristic of well-managed composting operations in many parts of the world. This is significant evidence of the effectiveness of thermophilic composting.

An analysis of the typical temperature and of thermal death points of a number of pathogenic microorganisms, parasites, and parasite ova, indicates the unlikelihood of survival of some of the common disease-bearing organisms. The highest thermal death points are appreciably lower than the maximum temperatures found inside the composting pile or bin. The magnitude and duration of the high temperatures, as well as the antibiosis which is characteristic of a mixed population of microorganisms, provide a sound basis for believing that no pathogens, parasites, or parasite ova survive the aerobic composting process.

The high temperature zone usually extends only to within 4 to 8 inches of the surface. Therefore, turning is necessary, quite apart from its function in aerating the mass, for ensuring pathogen and parasite destruction, particularly if a composting period under six months is used. The compost temperature curves and thermal-death-point values may indicate that one turning will be sufficient eliminate the pathogens and parasites if all of the surface material is completely turned to the inside, thus exposing any organisms present to lethal internal temperatures. But, although this may be true in many cases, as a safety factor, and to guard against failure to turn all of the material to the inside, at least two turns are required, and at least three for maximum assurance of complete destruction. Three turns would also be the normal practice for aeration purposes when rapid composting is done in stacks or piles on the ground surface.

In some composting operations the material is turned only once or not at all. A thermophilic temperature is developed after the initial aerobic stacking. This is considered to be sufficient to destroy pathogens and parasites. It is doubtful this practice is sufficiently safe when contaminated material is composted, since some pathogen and parasites may escape destruction in the cooler side and top layers.

Anaerobic composting in the mesophilic temperature range does not affect good destruction of parasites in an anaerobic environment. The biological antagonisms will eventually eliminate them, but this will generally take at least 6 months.

Unless six months or more can be allowed to elapse before the compost is used, anaerobic composting should be preceded by aerobic conditions and thermophilic temperatures for at least a week with at least one turning, in order to ensure the desired destruction of pathogens.

General cleanliness and systematic attention to the details of operation around the compost site, is necessary and particularly important when contaminated material is used.