**3. Sewage sludge**

Sewage sludge is generally defined as a semi-solid or liquid organic residue generated during the treatment of domestic sewage. He has also been called biosolids, but this term is more appropriate for the sludge that was conveniently selected and treated to allow its application to land (i.e., agricultural recycling) [6, 66]. Although the use of the term biosolids is becoming common, sewage sludge was used in this text because it encompasses sludges with different types and degrees of treatment and is the term adopted by the Brazilian legislation [67].

**Heavy metal**

Humans Yes

‡Yes: essentiality recognized. No: essentiality unrecognized.

**Table 1.** Essentiality and toxicity of heavy metals for different organisms.

§Most common manifestations, symptoms and consequences of heavy metal toxic effects.

†In case of animals, they are livestock animals.

878 Environmental Risk Assessment of Soil Contamination

heavy metals should not be neglected.

effects of other metal in the organism [65].

**3. Sewage sludge**

**(Symbol) Organism† Essentiality‡ Toxicity§ Reference**

The relative importance of heavy metals toxicity was addressed by McLaughlin et al. [10] in terms of food chain contamination. According to these authors, Cd is the metal with greatest potential to contaminate plants and subsequently to be transferred to animals and humans that eat these contaminated plants or part of them. This statement is based on the fact that (i) Cd poses animal and human health risks in plant tissue concentrations that are not generally phytotoxic and (ii) Cd concentrations in agricultural soils are increasing in many parts of world due to Cd inadvertent additions through the use of fertilizers, sewage sludge and soil amend‐ ments. Due to the high risk of contaminating the food chain, the risk of Cd to cause toxicity is considered to be high as well. Despite increased concern with Cd, the toxicity risk of other

The toxicity of heavy metals in living organisms is a phenomenon somewhat complex. Toxic effects of a metal depend on a number of factors that often include (i) rate, (ii) exposure time, (iii) tolerance of the organism and (iv) environmental conditions. In recent years, the effect of the interaction between heavy metals on the expression of toxicity has been considered very intensely. As a result of the interaction, a given metal may increase or decrease the negative

Despite the complexity, the toxicity of heavy metals in plants and in animals and humans that eat contaminated plants is primarily associated with previous environmental contamination. Soils may be contaminated with such hazardous elements by the use of sewage sludge. High concentrations of metals in the sludge increase the risks of contamination and therefore

Sewage sludge is generally defined as a semi-solid or liquid organic residue generated during the treatment of domestic sewage. He has also been called biosolids, but this term is more appropriate for the sludge that was conveniently selected and treated to allow its application

toxicity. Thus, it is important to know the chemical composition of sewage sludge.

Animals Yes Diarrhea, anorexia, jaundice, kidney and abomasum

and decreased HDL cholesterol.

damage, arthritis and weight loss. [9, 63]

[9, 64]

Diarrhea, nausea, vomiting, epigastric pain, lethargy, anemia, neutropenia, impaired immune function

The chemical composition of sewage sludge is highly variable, particularly in terms of heavy metals. Such variability can be seen in Table 2 for sludge generated at WTPs in Brazil. The coefficients of variation (CVs) of heavy metal concentrations are somewhat high. Cd, Co, Cr and Ni concentrations had the highest CVs, above 80 %, and Zn the lowest, 43 %. Intermediate variability was observed for Cu, Fe, Mn, Mo and Pb. It was not possible to calculate CV for As, Hg and Se because of the paucity of data. The high variability is explained primarily by the origin of the sludges, since they were obtained in WTP from different cities, each with particular characteristics of generation and wastewater treat‐ ment as well as treatment of generated sludge. However, there was also considerable variation for a same city, which indicates the occurrence of temporal variability (e.g., seasonal, annual etc.) for concentration.

Despite the high variability, the concentrations of heavy metals in sewage sludge for land application may not exceed limits established by environmental institutions. In Table 2 two limits for each metal are shown, one from the Environmental Agency of the State of São Paulo (CETESB) and other from the Brazilian National Environment Council (CONAMA). There is agreement for the limits between the institutions only for the Se. For all elements, CONAMA limits are lower. As CONAMA rule is federal and more restrictive, it must prevail over the rule of CETESB, which is state and less restrictive.

Considering Resolution # 375/2006 of CONAMA, some sewage sludges presented in Table 2 had heavy metal concentrations above the established limits. Seven, six, four and three sludges were with concentrations of Zn, Ni, Cr and Pb, respectively, higher than the maximum allowable concentrations. Generally, more than one metal was in excess, but not necessarily. Improper sludges were generated in the cities of Barueri, Franca and São Paulo, however, Franca produced sludges less contaminated with heavy metals, which probably reflects its lower level of industrialization in relation to the other two cities.

The concentrations of Cd and Cu in sewage sludges were below the limits of CONAMA (Table 2). Similarly, the concentrations of As, Hg, Mo and Se were also below, but there were few values determined, making this finding unrepresentative. However, the high number of cases in which these elements were below the limit of detection (LOD) of the analytical method suggests low concentrations, probably far from reaching the maximum allowable concentra‐ tions. Although they are not legally restrictive, Co, Fe and Mn were in relatively high concen‐ trations in the sludges analyzed.

The finding that the Cd did not exceed the limit established by CONAMA is the most positive result of this survey of sewage sludge used in experiments conducted in Brazil. This result indicates that the sludge has low potential for soil contamination with this heavy metal. Soil contaminated with Cd is a constant concern throughout the world, because this metal can easily enter the food chain and impose risks to human health [10]. On the other hand, the sludge seems to be a considerable source of other heavy metals such as Cr and Pb. Considering that


the sludge can introduce these metals in excess, soils amended with sewage sludge should be carefully evaluated for accumulation and availability of Cu, Cr, Mn, Pb and Zn. Additionally, the Cd should also be monitored because it is highly dangerous.
