**6. Summary and conclusions**

The expected increase in the generation of sewage sludge in Brazil should intensify its disposal on land. Although the sludge is rich in organic matter and plant nutrients, it has heavy metals in its composition. Due to the presence of potentially toxic metals, application of sludge to land requires a risk assessment of soil-plant system contamination. Knowledge of the sludge composition in terms of heavy metals is the first step in assessing risks of contamination.

We have shown in this work that concentrations of heavy metals in the sludge generated in Brazilian WTPs can vary widely depending on the metal. Cd concentration is extremely variable, while Zn concentration varies relatively little. High variability of Cd concentration may hamper the planning sludge application by the need for additional adjustment in order to avoid excessive application of Cd in soil. Zn concentrations found in the sludges analyzed were very high. Thus, the risk of contamination by Zn can be considered high as well.

Soil contamination by Cu, Ni and Zn, as determined by the total concentration, can be rapid, since high rates of sludge are applied to land. In contrast, low rates of sludge have shown low potential to contaminate soil with Cu, Ni, Pb and Zn. However, available concentrations can indicate contamination by Cu and Zn even with application of low rates of sludge. Further‐ more, they can detect changes in the solubility of metals due to liming.

Nevertheless, the available concentrations depend on the efficiency of the extractant. DTPA, 0.1 mol L–1 HCl, Mehlich 1 and Mehlich 3 have not been as efficient as needed. DTPA has shown best performance among these extractants. DTPA and Mehlich 1 extractants can also be used to monitor the leaching of heavy metals such as Cu, Ni, Pb and Zn in soils amended with sewage sludge, whose importance is to assess the soil contamination at depth and the potential for contamination of groundwater. The fractionation and speciation data have confirmed that Zn has a potential to be leached in soil amended with sludge, which increases the risk of groundwater contamination. Regard to contamination of the plant, the results have shown that the application of sewage sludge has low potential to contaminate maize and sugarcane with Cu, Ni, Pb and Zn.

Despite the apparent low risk of contamination of the soil-plant system by heavy metals due to the application of sewage sludge to land, this review has shown that (i) the results from the field experiments are limited to a restricted number of potentially toxic metals, (ii) the chemical extractantsaregenerally inefficient to estimate the available concentrations for these metals in a range of situations and (iii) the studies on fractionation and speciation of these hazardous elements in soils are still incipient. Furthermore, studies have not used an integrated approach involving total concentration, available concentration, chemical fractions and chemical species of heavy metals to assess more reliably the risk of phytotoxicity and contamination of the food chain and groundwater. Thus, this scenario suggests that the application of sewage sludge to land should be as restrictive as possible in Brazil.
