Page 169 - Soil Degradation, Conservation and Remediation

158 6 Soil Pollution

Table 6.1 Acceptable amounts of heavy metals in sewage sludge for agricultural use (Chefetz
et al. 1996 ; Siuta 1999 ; Moreno et al. 1999 )
Soil fertilization, land reclamation
Metals Agricultural exploitation Nonagricultural exploitation
Lead (Pb) 500 1,000
Cadmium (Cd) 10 25
Chromium (Cr) 500 1,000
Copper (Cu) 800 1,200
Nickel (Ni) 100 200
Mercury (Hg) 5 10
Zinc (Zn) 2,500 3,500

On the other hand, repeated applications of compost from sewage sludge on agricultural

soil have significant effects on the physical and chemical properties of soil (Reddy
and Overcash 1981 ; Okereke 1985 ). A considerable amount of research has been
carried out in Europe in order to evaluate the effects of compost from sewage sludge
on soil properties (Williams and Goh 1982 ). The acceptable limits of some heavy
metals in sludge for agricultural use are given in Table 6.1 .
It has been shown in some studies that through application of appropriate rates of
sewage sludge, plant growth and physical properties of soil improve substantially
and their reusable nutritional values increase (Reed et al. 1991 ). The addition of sewage
sludge gives a slight increase in Cu content of soils and Zn content of plants, which
suggests that application of sewage sludge on agricultural lands should be carried
out without surpassing the toxic limits set by the relevant organizations (Saruhan
et al. 2010 ). Since sewage sludge contains high concentrations of potentially toxic
elements such as Zn, Ni, Cd, and Cu, problems may arise when sludge is applied
to an agricultural soil (Sanchez-Monedore et al. 2004 ; Madyiwa et al. 2002 ) and
heavy metal accumulation also occurs in the plant tissues.
Biosolids, the product generated from tertiary treatment of waste activated

sludge, and composted sludge have been used in agricultural fields of Europe and
China for about a century. Substantial increase in crop yield due to sewage sludge
application has been observed. But for heavy metal contamination of soil and public
outrage, Austria, Switzerland, Sweden, and some other European countries have
imposed a ban on the use of sewage sludge as a fertilizer.

6.2.3 Contaminants in Garden Composts

Contaminants in feedstock can impact the quality, marketability, and use of fi nished
composts. Overuse and persistence of some insecticides could result in contamination
of yard waste and compost. Since composting is a biologically mediated process,
there is a considerable interest in knowing if the intense biological activity during