## 3 Results and Discussion
### Extracellular Enzyme Profiles per API ZYM(r) Kit for Biosolids
The families of phosphatases, proteases and glycosyl hydrolases had an important activity in summer. Figure 4: Profile of 19 extracellular enzymes determined by the API ZYM® system in the biosolids analysed during one year of UWWTP operation. The shading in the table represents the relative abundance of enzymes. The values are as follows: White, not detected; light grey, low intensity reactions (value = 1); dark grey, moderate intensity reactions (value = 2 and 3); black, high intensity reactions (value = 4 and 5). The number of enzymes showing reactions \(\geq\)1, \(\geq\)2 and \(\geq\)4, as well as the total number of enzymes showing no activity for each month are indicated below each column. In autumn, the enzyme activity of biosolids was moderate to high; however, this was the season of the year with the highest number of enzymes with undetected activity. In September and November there was undetected enzyme activity in 4 enzymes, and in October there was undetected enzyme activity in 6 enzymes. The aminopeptidase family showed no enzyme activity in any of the biosolids samples analysed for the fall. There was low enzyme activity in September and October in the biosolids observed in this phenomenon in 4 enzymes. In November this low activity was only present in 5 enzymes. Most of the low activity enzymes were glycosylhydrolase. With moderate enzyme activity, 5, 2 and 4 enzymes were obtained for the September, October and November biosolids, respectively (Figure 4). The predominant moderate enzymatic activity in the biosolids in the autumn season corresponded to phosphatases, being that this time of the year was the one that had less enzymatic activity for this family of enzymes in the analysed biosolids. Finally, for the biosolids analysed in autumn in the months of September and November, the number of enzymes with high activity was 6 and 7 for the biosolids of October, mainly from the families of proteases and hydrolases of glycosil. In winter, the enzymatic activity of the biosolids analysed in a different season of the year was moderate to high. In the biosolids of December and February, the enzyme \(\alpha\)-Fucosidase showed no activity. Low enzyme activity was detected for 5 enzymes in December and January and 4 enzymes for February, mainly from the aminopeptidase family. Thus, moderate enzyme activity for December biosolids was obtained in 6 enzymes, and for January and February biosolids, 8 enzymes were distributed in the 5 families (Figure 4). Finally, 7 enzymes presented high activity for December and 6 enzymes for January and February, with the proteases and glycosyl hydrolases families standing out. The extracellular enzyme profile (Figure 4) showed that 9 enzymes were similar in \"intensity\" during different season of the year (acid phosphatase, phosphohydrolase, alkaline phosphatase, esterase lipase, trypsin, \(\alpha\)-chemotrypsin, \(\beta\)-glucosidase, \(N\)-acetyl-\(\beta\)-glucosaminidase, \(\alpha\)-fucosidase). On the other hand, 10 enzymes showed differences in intensity during different season of the year (esterase, lipase, leucine arylamidase, valine arylamidase, cysteine arylamidase, \(\alpha\)-galactosidase, \(\beta\)-galactosidase, \(\beta\)-glucuronidase, \(\alpha\)-glucosidase and \(\alpha\)-manosidase). These results demonstrated that biosolids are not only a source of organic matter, but also have an important contribution of hydrolytic enzymes, confirming the importance of evaluating the biological parameters in biosolids prior to their addition to soils. However, the enzymatic machinery of the biosolids varied in the intensity of their activity in more than 50% of the 19 enzymes. Nevertheless, in average for all seasons of the year, 13 enzymes present an activity with a moderate to high intensity, which can positively influence the mineralisation processes of organic matter, mainly in hydrolysis process considered as the limiting stage of the transformation process of organic matter [139].