Environmental Engineering Reference
In-Depth Information
sewage sludge from purely domestic origin, toxic metals concentrations in the sludge are
usually low, then the limiting parameter might become nitrogen, which is easily
transported to ground water and might lead to the pollution of ground water resources.
The recommended criteria vary considerably in the different countries, with USA
recommendations being more relaxed, while EC recommendations are more stringent. In
South Africa, the criteria given in Table 8.1 have been recommended with respect to
toxic elements (WRC 1997).
It should be emphasized, that guidelines and regulatory instruments usually prescribe
the maximum permissible concentrations of selected toxic elements in the soils.
Therefore, decisions regarding the clos-ing of a given land disposal site should be based
on such criteria after a thorough monitoring program has shown that the prescribed
values are exceeded.
The Zimbabwean regulations (WWEDR 2000) treat sludge land disposal on land as a
specific case of effluent discharge and requires a permit for the activity. Permit
limitations and corresponding charges are based on two major criteria - the annual
application rates with respect to cadmium and nitrogen. The maximum allowable annual
application rates are 4 kg Cd/ha and 600 kg nitrogen/ha, which present high
environmental hazard risk. The maximum allowable soil concentration of Cd is 10 mg/kg
dry soil.
In the case of wet sludge used for irrigation the pollutant loading rates are dependent
on the pollutant concentrations in the sludge. Guidelines regarding these criteria are
presented in more detail in Chapter 10.
3.3
Safety requirements
The risks, which are associated with the reuse of wastewater and sludge for irrigation,
could be classified into three categories:
Table 8.1. Recommended criteria for sewage sludge
land application in SA.
Metal
Maximum Permissible
Content in Soil
(mg/kg)
Total Load in kg/ha
for 25 years of
operation
Main reason why
elements are limited
Phytotoxicity
Zoo-
toxicity
Cadmium (Cd)
2.0
3 140
x
Cobalt (Co)
20.0
20 000
x
Chromium
(Cr)
80.0
350 000
x
Copper (Cu)
6.6
10 100
x
Mercury (Hg)
0.5
2 000
x
Molybdenum
(Mo)
2.3
5 000
x
