## 3 Results and Discussion
### Agoronomic Potential
#### 3.1.2 Macronutrient in Biosolids and the Amount Required by Different Agricultural Crops  
One can observe in Figure 2A, that the N content of IBRS WWTP (of the present research) (33.64 \(\pm\) 6.08 kgN ton\({}^{-1}\)) met 32% for corn demand, 33% for rice, 60% for wheat, forage grasses was >100%, for sugar came 22%, for beans 31%, 21% for coffee, and 37% for cotton. Ref. [45] explained that generally most of N is found in its organic form in biosolids and, therefore, is readily available for plants after the degradation of organic matter. Concerning the P content of IBRS WWTP (20.55 \(\pm\) 3 kg ton\({}^{-1}\)) (Figure 2B), the forage grasses P demand was met at >100%, corn 40%, rice 53%, wheat 98%, sugar came 45%, beans 93%, coffee 89%, and cotton 64%. The relevance of the P found in the biosolids is in its bioavailability, which provides its use by the plants, and in its nutritional function [46], especially in tropical soils suffering from very low levels of P [47]. Instead of what occurs naturally in the soil, where P has low bioavailability and is strongly absorbed in the clay minerals in tropical soil, most of this element in the biosolids is readily available for plants in the first months of the production cycle [10]. Ref. [48] documented that the use of biosolids during ten years in tropical soils, provides adequate amounts of N and P for the corn crop (_Zea mays_ L.), sunflower (_Helianthus annuus_ L.), and cortalaria (_Crotalaria juncea_ L.). Figure 2: Percentage meeting macronutrients in corn, rice, wheat, forage grasses, sugar cane, beans, coffee, and cotton crops per 1 ha and 1 ton ha\({}^{-1}\) application rate. N: nitrogen (**A**); P: phosphorus (**B**); K: Potassium (**C**); Ca: calcium (**D**); Mg: magnesium (**E**); and S: sulfur (**F**). The concentrations of N and P were a positive result, meeting a relevant percentage of the needs of crops, especially for the forage grasses culture (>100%). This result corroborates the previous study with another WWTP from the same municipality (Campo Grande, Brazil), where the agronomic potential of biosolids coming from anaerobic digestion has been found for the soy, orange, eucalyptus, and tomato crops [15]. Ref. [15] showed in their study that biosolids were as efficient as conventional fertilizers in increasing the production of wood in the eucalyptus plantation. Ref. [49] documented that after 10 and 17 years of the first and only application of biosolids, organic matter content in the soils, total N and P, and cation exchange capacity were generally more elevated in soils treated with biosolids. That being so, after a single application of biosolids, it is possible to detect a beneficial residual effect on the soil, even after 17 years. The recycling of P through biosolids has enormous potential and avoids this byproduct from being sent to landfills, which does not add value to the production of food, especially due to the imminent scarcity of this element. Moreover, this process can be optimized by way of the biosolids pyrolysis (biochar), which provides it with a considerable amount of P in its soluble form, which in turn increases the bioavailability for the plants' consumption [50]. Ref. [24] used acids for the precipitation of P, such as struvite, hydroxyapatites, and other calcium phosphates; however, the conjoint precipitation of metals was a negative aspect of this method and subsequent procedures are needed for its extraction, which has a high cost. Ref. [51], focusing on solving this negative effect, obtained positive results with the alkaline leaching of biosolids. Ref. [52] sought to extract P from incinerated biosolid and documented that they obtained a high-purity P extract, by the method developed by the researchers (two-step extraction), in which they used ethylenediaminetetraacetate, followed by sulfuric acid. The K concentration in the WWTP of the present research (1.16 \(\pm\) 1.11 kg K ton\({}^{-1}\)) was low in the reference crops (assessed in this study), with the highest percentage at 4%, for forage grasses. Ref.