## 3 Results and Discussion
The quality of the treated wastewater in the present study indicates that systems with aerobic granules can successfully treat TSS-rich wastewater from the meat industry. Organic compounds were removed with a very high efficiency in both reactors (above 94%). This indicates that periodic decreases in oxygen concentration during the reactor cycle, which resulted from intermittent aeration, did not reduce the efficiency of organics removal. The average COD concentrations were 91.6 \(\pm\) 13.5 mg L\({}^{-1}\) and 115.6 \(\pm\) 43.6 mg L\({}^{-1}\) in the outflows from R\({}_{1}\) and R\({}_{2}\), respectively, but COD removal was more stable in R\({}_{1}\) (Figure 4). The COD removal rate was similar in both reactors (197.1 mg (L h\({}^{-1}\)) vs. 194.6 mg (L h\({}^{-1}\)) in R\({}_{1}\) and R\({}_{2}\), respectively). The carbon source that is available in wastewater affects the composition of the microbiological community. [PERSON] et al. [31], observed that in a reactor to which acetate was dosed, microorganisms from the genera _Flavobacterium_ (30%), _Thauera_ (15%) and _Aquimonas_ (12%) dominated. In a reactor to which ethanol was dosed, _Paracooccus_ sp. (36%), _Fluvicicola_ sp. (20%) and _Sediminibacterium_ sp. (9%) dominated, while addition of glucose resulted in the growth of numerous polyphosphate-accumulating organisms (PAOs) belonging to the taxa _Cryomorphaceae_, _Rhodobacteraceae_, _Cytophagaceae_, and _Saprospiraceae_. Acetate is the main source of carbon in wastewater from the meat industry, which may explain the high percentage of _Thauera_ sp. (up to 20.1%) in the granules in the present study. Figure 3: Heatmap of relative abundance of bacterial taxa present in aerobic granules (abundance higher that 0.5%). The average N-NH\({}_{4}\) concentration in each effluent was around 1 mg L\({}^{-1}\) (Figure 4). The efficiencies of ammonium nitrogen removal were 82 and 86% in R\({}_{1}\) and R\({}_{2}\), respectively, which is similar to the efficiency obtained by [PERSON] and [PERSON] [32], in a granular sludge batch reactor fed with wastewater with a COD/N ratio of 15:2 and operated with anoxic phases. The concentration of N-NO\({}_{2}\) in the effluents was low (0.03 \(\pm\) 0.05 mg L\({}^{-1}\) vs. 0.12 \(\pm\) 0.12 mg L\({}^{-1}\) in R\({}_{1}\) and R\({}_{2}\), respectively). The average concentration of N-NO\({}_{3}\) in the outflow from R\({}_{2}\) was significantly lower than that in the outflow from R\({}_{1}\) (3.6 \(\pm\) 1.5 mg L\({}^{-1}\) vs. 7.0 \(\pm\) 3.2 mg L\({}^{-1}\)). Correspondingly, denitrification efficiency was higher in R\({}_{2}\) than in R\({}_{1}\) (68% vs. 43%). The environmental conditions favored the growth of aerobic denitrifiers; for example, _Pseudomonas_ sp. and _Paracooccus_ sp., which can increase the efficiency of denitrification under aerobic conditions, were identified in the analyzed biomass [33]. In the present study, although the overall efficiency of ammonium nitrogen removal was very similar in both reactors, the ammonium removal rate was more than three times higher in the intermittently aerated reactor (9.4 mg/(L h\({}^{-1}\))) than in the constantly aerated one (3.2 mg/(L h\({}^{-1}\)). This indicates that ammonium nitrogen oxidation is stimulated by anoxic conditions. It has been reported that increasing the number of anaerobic phases in the reactor cycle increased the total number of bacteria (as shown by abundance of _16S rRNA_ gene), the number of AOB bacteria involved in nitrification (_amoA_ gene abundance) and the number of complete denitrifiers (_nosZ_ gene abundance) [34]. In the present study, such increases were not observed; however, the share of such taxa as _Thuera_ sp., _Dokdonella ginsengisoli_, and _Lysobacter daejonenensis_, which participate in nitrogen transformations, was higher in the reactor with intermittent aeration than in the one with constant aeration. In both reactors, low concentrations of N-NO\({}_{2}\) (below 1 mg L\({}^{-1}\)) were only observed in the first hour of the cycle.