Land treatment of wastewater 4378.7.2 Heavy metals and other toxic organic compounds
The pathways of potential concern are movement of metals and other toxic
organic compounds to drinking water sources and translocation of these
compounds through the food chain to human. The plants grown on a land
treatment site can accumulate toxic compounds in their leaves, stems and fruits.
Removal of metals and toxic compounds is generally effective in all three land
treatment methods with that effectiveness retained for considerable period of
time. Since the problem of heavy metal and toxic compound pollution is more
serious with respect to sludge application to land, this matter will be discussed
more in Chapter 9.
8.7.3 Pathogens
The pathogens of concern in land treatment systems are bacteria, parasites and
viruses. The major pathways of concern are to the groundwater, internal or
external contamination of crops, translocation to grazing animals and human,
and off-site transmission via aerosols or runoff. There is no evidence available
indicating transmission of parasite diseases from application of wastewater in
land treatment systems, because the parasite cysts and eggs will normally settle
out during pre application treatment or in storage ponds. But there are chances
of human infection by parasite diseases if the land treatment area is accessible to
the public. It has been found that Ascaris ova will survive in soil for long period
and investigations on a site in France proved the presence of ascaris ova in soil.
Concerns, with respect to crop contamination, focus mainly on surface
contamination and then persistence of the pathogens until consumed by human
or animals or the internal infection of the plant via the roots. As shown in Figure
8.13, pathogen survival periods on leaves and fruit crops tend to be shorter than
the growth periods of most of these plants (Strauss 1986). In soil, however,
survival of viruses, Salmonellae bacteria, and Ascaris eggs may exceed the
growth period of crops (Figure 8.14). Viable pathogens are therefore more likely
found on root crops and soil than on leaf crops.
Human or animal infection can be reduced by taking preventive methods
such as prohibiting the consumption of raw vegetables grown on land treatment
sites, and allowing a period of two weeks or more after application of
wastewater before animals are allowed to graze. This will allow sunlight to kill
most of the fecal bacteria and viruses. As time and temperatures have significant
effects on pathogen die-off (Figure 3.1), prolonged storage of wastewater or
sludge to be applied to crops will help minimize the public health risks. Strauss
(1986) suggested storage periods for pathogen inactivation in excreta, as shown
in Table 8.12.