## 3 Results and Discussion
### Physicochemical Characterisation of Biosolids  
The physicochemical characteristics of the biosolids at different times of the year are presented in Table 2. The pH values obtained ranged between 6.10 and 6.41, so they were considered slightly acidic These pH values were not significantly different (F = 0.237, \(p\) = 0.869) from each other. The tendency to present a slight acidity may be due to the presence of organic acids derived from the degradation of organic matter present in wastewater. In other studies, in which biosolids have been generated from urban wastewater treatment and used for organic amendments, pH values have been reported in a range of 5.5 to 6.5 [13; 63], similar to those obtained in this study. [PERSON] et al. [64] established that the slightly acidic pH of organic amendments (e.g., biosolids) can be attributed to mineralisation processes of organic matter due to the presence of labile organic compounds available for microbial metabolism. These compounds may be aliphatic organic acids, amino acids and their polymeric derivatives (e.g., proteins and polypeptides), polysaccharides, lipids and other low molecular weight organic compounds. Thus, the slightly acidic pH of the evaluated biosolids may be related to the chemical composition of the organic matter present in the biosolids and their mineralisation. The EC in biosolids evaluated at different times of the year showed significant differences (F = 8.43, \(p\) = 0.007) (Table 2). Values between 1.60 and 5.71 dS m\({}^{-1}\) were observed, which implied an important salt content, characteristic of biosolids from urban wastewater treatment [65; 66]. The highest value was presented in spring (5.71 dS m\({}^{-1}\)) and the lowest in autumn (1.60 dS m\({}^{-1}\)). However, the EC values obtained were within the range of those reported in the literature (0.90 to 12.5 dS m\({}^{-1}\)) for biosolids generated during urban wastewater treatment of aerobic and anaerobic processes [67; 68; 69]. Studies on the application of biosolids and their effect on soil physicochemical properties have reported an increase in EC values, including as a function of the application rates used [19; 70; 71; 72]. On the other hand, [PERSON] et al. [72] evaluated the effect of the application of biosolids coming from aerobic and anaerobic treatments on the chemical and biological properties of the soil. The evaluated soil presented an EC value of 0.48 dS m\({}^{-1}\). After the application of the aerobic and anaerobic biosolids, the EC in the soil increased to 2.0 and 2.8 dS m\({}^{-1}\), respectively, and these values were maintained for the 2 months that the experiment lasted. With this information, the EC and its variation in biosolids generated in urban wastewater treatment plants, both in aerobic and anaerobic processes, should be monitored over time [21; 61], as well as before and after being applied to agricultural soils, to prevent possible salt accumulations causing salinity or salinity-sodicity problems in soils [73]. Under the same context, the benefit of applying biosolids as organic amendments to agricultural soils lies in the presence of labile organic matter, which can constitute an energy reserve in the soil [74]. Therefore, for a better use of biosolids, it is important to know the total organic carbon content (TOC) and its variation over time before its application to agricultural soils. TOC varied in the biosolids sampled at different times of the year in an interval of 36.90 to 57.94% (Table 2). The highest TOC value was in summer (57.94%) and the lowest was in winter (36.90%). The TOC values obtained at different times of the year were significantly different (F = 55.655, \(p\) = 0.000) from each other. With respect to TOC values in biosolids and sewage sludge reported in the literature, these are lower than those obtained in the present study. [PERSON] et al. [75], for the purpose of soil application, evaluated different biosolids from different UWWTP as a result of both aerobic and anaerobic wastewater treatment and digestion processes.