## 3 Results and Discussion
### Inorganic Substances (Metals)  
The IBRS WWTP biosolids content showed potential for meeting the need for macronutrients of crops used as a reference for agronomic potential in the present research. This was a positive aspect in the matter of agricultural recycling of biosolids in tropical soils. Yet, bearing in mind the application of biosolids in the soil, it is important to know what are the potentially toxic substances that can hit the soil, the food, and groundwaters, in view of the health and safety of the population. Therefore, identifying and quantifying these substances are necessary because they are considered heavy metals and are, too, micronutrients essential to the plants, in adequate amounts, beyond being scarce in tropical soils. Thus, as far as the use of biosolids in Brazilian territory is concerned, it is necessary to meet the requirements proposed by the Conama Resolution 498/20 [27], including the concentration of inorganic contaminants (metals). On a global scale, it is important to verify compliance with foreign legislation. In this case, Norm 503 [28] and the 86/278 European Community's Directive [30] were chosen. It is possible to observe in Table 4, a comparison of the concentration of samples of biosolids relative to the limits established by the Brazilian (class 1) [27], American [28], and European [30] legislation. The average concentration of metals in the WWTP of the present research is within limits established by Brazilian, American, and European norms. Ref. [9] reviewed cases of application of biosolids in agricultural soils that followed the criteria and procedures of local legislation and identified that when the concentrations followed the quality standards of the legislation, they would not cause negative effects on the population and the environment. In Brazil, Ref. [34] documented that only three from 19 batches of biosolids generated in 19 WWTPs were considered inadequate for direct application in agricultural soils due to the high Zn concentration, of \(>\)2.800 (mg Zn kg\({}^{-1}\)), and one batch for excess Ni (\(>\)420 mg Ni kg\({}^{-1}\)), as per legislation [27]. However, it is known that tropical soils are deficient in these elements which are essential micronutrients for diverse agricultural crops [49]. Ref. [34] reported that biosolids that were considered inadequate could be recurrent in sewage networks with clandestine discharge from industries, which the authors report being a common practice in the state of Sao Paulo (Brazil), where they did their investigation, and different from the Brazilian reality in general. Zn is an element common in several industrial products and processes and in the human diet, therefore, among the metals, it is one of the most abundant elements in sewage [60]. Some studies reported that the application of biosolids with high Zn concentrations may occur, as long as with a low application frequency, in that the soils obtained \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**(1) Conama**} & \multicolumn{2}{c}{**(3) Directive**} \\
**Parameter** & **Results** & **Resolution** & **(2) Norm 503** & **86/278 EEC** \\
**Cr** & 33.44 \(\pm\) 43.53 & 1000 & NS & NS & Fulfills \\
**Hg** & 0.46 \(\pm\) 1.29 & 17 & 57 & 25 & Fulfills \\
**Mo** & 4.83 \(\pm\) 9.86 & 50 & 75 & NS & Fulfills \\
**Ni** & 17.11 \(\pm\) 21.28 & 420 & 420 & 400 & Fulfills \\
**Se** & \(<\)1 \(\pm\) 0 & 36 & 100 & NS & Fulfills \\
**Zn** & 349.23 \(\pm\) 319.92 & 2800 & 7500 & 4000 & Fulfills \\ \hline \hline \end{tabular}  
* Fulfills: meets all requirements of the analyzed legislation; BR-US-EU: [1][27], [28], [3][3]; NS: The maximum limit is not specified by legislation.  
\end{table} 
Table 4: Concentration of metals (mg kg\({}^{-1}\)) in the IBRS WWTP biosolids in comparison with legislation from Brazil, USA, and European Union. the same benefits without a strong significant change in the quantity of Zn [61].