Land treatment of wastewater 439The potential for transport of pathogens by aerosols from land treatment sites
has been the most controversial issue in recent years. The main reason for this
controversial is due to the unawareness of the difference between aerosols and
sprinkler droplets. To avoid bacterial and virus transport through aerosols it is
preferable to locate the treatment site away and down wind from the residential
areas, when sprinklers are used for distribution of wastewater. Depending on the
wind speed and topography, pathogen-carried aerosols can travel several
hundred meters away from the land treatment site.
With respect to epidemiological study of land treatment of wastewater, it
appears from Table 2.26 that the relative health risks due to intestinal nematodes
are high, followed by bacterial and viral infections. The microbiological quality
guidelines (Table 2.27) suggest that the density of viable eggs of intestinal
nematodes (Ascaris, Trichuris and hookworms) be equal to or less than 1 per L
of wastewater to be used for both restricted and unrestricted irrigation. With
respect to fecal coliforms, their density should be less than 1000 no. per 100 mL
of wastewater to be used for unrestricted irrigation. A relaxed standard of fecal
coliform density is applied to wastewater to be used for restricted irrigation.
Figure 8.14 Pathogen survival in soil vs vegetable growth periods in warm climates
(from Strauss 1986; reproduced by the permission of IRCWS, Switzerland)
1) Determined under widely varying conditions
2) Maturation period from transplanting or from sowing if not transplanted1