470 Organic Waste Recycling: Technology and Management
other heavy metals. The presence of copper somewhat inhibits zinc transport
through the plant.
There is some uncertainty at the present time concerning safe levels of many
elements accumulating in plants. Little is known concerning the long-range
effects of toxic elements applied to agricultural lands through the continuous use
of wastewater and sludge. The guidelines given in Tables 9.10-9.13 have been
proposed to minimize the health impact resulting from the heavy metals present
in sludge. These guidelines should be followed wherever possible in the design
and operation of land treatment of sludge. At present, polychlorinated biphenyls
(PCBs) are the only group of toxic organic compounds addressed in the joint
U.S. EPA/FDA/U.S.DA guidelines for sludge application for fruit and vegetable
production (Table 9.12). Sludge containing more than 10 mg PCB/kg must be
soil incorporated, and sludges containing more than 50 PCB/kg may not be land
applied at any rate, not mixed with less contaminated sludges to lower the
mixture’s PCB content (O’Conner et al. 1990).
The principal problem arising from PCBs is direct ingestion by animals
grazing on forages treated with surface-applied sludge. Dairy cattle are most
susceptible to PCB contamination of forages, because PCBs in the diet are
readily partitioned into milk fat. Several studies have shown that essentially
PCBs do not adsorb onto the surface of root crops such as carrots, hence they
are not a major concern for this kind of crops (U.S. EPA 1983)
9.5 Microbiological aspects of sludge application on land
With the practice of land disposal of sludge becoming more popular and
widespread, the survival of pathogenic organisms in the soil, water, and on
crops grown in sludge assumes increasing importance. Pathogens in soil are
destroyed by the natural environmental conditions that favor native soil
organisms. Some pathogens are entrapped and adsorbed at the soil surface and
undergoing rapid die-off in the soil matrix.
In general, microorganisms may survive in the soil for a period varying from
a few hours to several months, depending on the type of factors such as: (1) type
of organism; (2) temperature: lower temperature increases survival time; (3)
moisture: longevity is greater in moistened soils than in dry soils; (4) type of soil:
neutral pH, high-moisture-holding soils favor survival; (5) organic matter: the type
and amount of organic matter present may serve as a food or energy to sustain the
microorganisms; and (6) the presence of other micro-organisms can have an
antagonistic effect on the pathogens. Pathogens do not generally survive as long on
vegetation as they do in soil because they are exposed to adverse environmental