Land treatment of sludge 471conditions like UV radiation, desiccation, and temperature extremes (see Figures
8.13 and 8.14).
There are various forces acting to retain or facilitate the movement of
microorganisms through soil. Filtration by the soil at the soil-water interface is the
primary means of retaining bacteria in the soil, or in some cases in an additional
biological mat formed in the top 0.5 cm of soil. Other mechanisms that retain
bacteria in the top few cm of fine soil are intergrain contacts, sedimentation and
adsorption by soil particles. The soils containing clay remove most microorganisms
through adsorption, while soils containing sand remove them through filtration at
the soil-water interface. The movement of microorganisms through soil relates
directly to the hydraulic infiltration rate and inversely to the size of soil particles
and the concentration of cations in the solute. Microorganisms will travel quickly
through fissured zones, such as limestone, to the groundwater.
The potential for pathogen transmission exists and can cause a public health
problem if land application is carried out improperly. The transmission can occur
through groundwater, surface runoff, aerosols formed during application and direct
contact with the sludge or raw crops from the application site, because bacteria,
viruses and parasites do not enter plant tissue, transmittal of pathogens via crops
grown on the land application site results from contamination of the plant surface.
If contaminated crops are consumed raw, disease transmission is possible;
however, disease transmission due to application of sludge onto farmland is rare.
Reported outbreaks of disease generally have been the result of application of
inadequately treated sludge to gardens or other crops which were eaten raw.
Pathogens in land-applied sludge usually will die rapidly depending on
temperature, moisture and exposure to sunlight. In general, pathogen survival is
shorter on plant surfaces than in the soil. To prevent disease transmission, sludge
should not be applied to land during a year when crops are to be grown that will be
eaten. Where humans have little physical contact, the presence of pathogens may
be of less concern. The soil can filter and inactivate bacteria and viruses. Sludge
application methods and rates should take advantage of the soil to reduce public
health concerns.
The microorganisms present in human/animal wastes and in sludge as described
in Chapter 2 can pose potential health risks when these wastes are applied on land
and agricultural crops. From a review of epidemiological effects of the use of
sludge on land, the transmissions of hookworm, Ascaris, Trichuris and
Schistosoma japonicum infections should be of major concern (Blum and Feachem
1985). The above helminths are present in ova form in sludge, hence their long
survival in soil or on crops and becoming high risks to human health (see Table
2.26). In general, the microbiological guidelines as given in Table 2.27 should be
followed to minimize this public health impact.