## 3 Results and Discussion
### Characteristics of the FTIR Spectrum of Biosolids  
Fourier Transform Infrared Spectrometry (FTIR) was used to identify functional groups present in biosolids. The FTIR spectra for biosolids evaluated at different times of the year were very similar to that shown in Figure 2. The bands observed between 3400 and 3500 cm\({}^{-1}\) were attributed to O-H stretching of hydroxyl groups in carboxylic groups, phenols, polysaccharides, and saturated aliphatic species [89; 90]. In addition, aliphatic methylene groups between 2850 and 2930 cm\({}^{-1}\) were detected in all biosolids samples, mainly related to fats and lipids [33; 91; 92]. Bands found between 1650 and 1750 cm\({}^{-1}\), due to C=O stretching vibrations, indicated the presence of ketones, aldehydes, and carboxylic acid derivatives in biosolids [81; 93]. On the other hand, the 1650 cm\({}^{-1}\) band was attributed to C-N stretching vibrations in the primary amides of proteins [33; 90; 91]. The presence of primary amides in the evaluated biosolids is confirmed by the presence of bands between 1630 and 1640 cm\({}^{-1}\), which corresponds to bending vibrations of N-H [92; 94]. In addition, the absorption bands found that between 1540 and 1560 cm\({}^{-1}\) were attributed to secondary amide bonds of protein origin that probably came from human cells present in biosolids [91; 95]. Reflection spectral of bands observed that between 1430 and 1450 cm\({}^{-1}\) indicated the presence of aliphatic chains, possibly hemicellulose and lignin, due to stretching and bending vibrations C-H in CH\({}_{2}\)/CH\({}_{3}\)[90; 93]. Likewise, the bands found that between 1440 and 1450 cm\({}^{-1}\) have been attributed to aromatic compounds due to stretching vibrations C=C, mainly related to aromatic components of humic acids [81; 96]. In addition, a characteristic band at 1434 cm\({}^{-1}\) has been reported for aldehydes and ketones due to composting vibrations C=O, as a result of the presence of humic and fulvic acids in the stabilisation process of biosolids organic matter because of UWWTP secondary digestion treatments [92]. The bands found that between 1380 and 1400 cm\({}^{-1}\) were attributed to symmetrical COO stretching and CH\({}_{3}\) bending vibrations, related to proteins and lipids [93; 95]. Likewise, bands observed between 1230 and 1250 cm\({}^{-1}\) were associated with tertiary amines due to C-N stretching and N-H bending vibrations [90], as well as antisymmetric PO\({}_{2}\) stretching vibrations related to compounds with phosphodieser bonds, such as phospholipids and nucleic acids: RNA (ribonucleic acid) and DNA (desoxyribonucleic acid) [94]. Finally, absorbance bands between 875 and 1050 cm\({}^{-1}\) corresponded to C-O and C-C stretching vibrations caused by polysaccharides and aromatic compounds [81; 92]. Overall, the band-reflection spectra obtained by FTIR from the evaluated biosolids were like the results reported in the literature on biosolids and sewage sludge [90; 91; 92; 93]. In fact, these molecular analyses showed a similar chemical composition in all the biosolids analysed in the different seasons of the year, presenting mostly the same absorbance bands. Therefore, it was possible to mention that the evaluated biosolids presented the same functional groups indicating that the chemical compounds present in the different seasons of the year were very similar and of low variability. With respect to the changes in the \"absorbance intensity\" of the bands identified in the FTIR reflection spectra, these changes were not significant (see Figure 3; A, F = 1.427, \(p\) = 0.304; B, F = 0.999, \(p\) = 0.441; C, F = 2.846, \(p\) = 0.106; D, F = 5.023, \(p\) = 0.030; E, F = 2.217, \(p\) = 0.122; F, F = 0.013, \(p\) = 0.998; G, F = 0.859, \(p\) = 0.500; H, F = 1.589, \(p\) = 0.267; I, F = 0.751, \(p\) = 0.552) between seasons of the year. It was then determined that the \"functional groups\" identified for each of the analysed bands were similar in all biosolids samples, regardless of the time of year.