## 3 Results and Discussion  
The obtained results indicate that in the constantly aerated reactor, aerobic granule formation was favored, whereas in the intermittently aerated reactor, a mixture of aerobic granules and activated sludge flocks was obtained. The average biomass concentration in R\({}_{1}\) was \(4.6\pm 0.9\) g MLSS L\({}^{-1}\) and that in R\({}_{2}\) was \(3.4\pm 1.5\) g MLSS L\({}^{-1}\), but the concentration of TSS in the effluent was nearly 2 times higher in R\({}_{2}\) than in R\({}_{1}\) (0.20 g L\({}^{-1}\) TSS vs. 0.11 g L\({}^{-1}\) TSS). The higher concentration of biomass in R\({}_{2}\) may have resulted from organics being used, not for biomass growth, but for more efficient denitrification and phosphorus removal than in R\({}_{1}\). The SVI\({}_{30}\) was higher in R\({}_{2}\) (65 mL g\({}^{-1}\) MLSS) than in R\({}_{1}\) (44 mL g\({}^{-1}\) MLSS). Aerobic granules exhibit low SVI values (\(<\)50 mL/g MLSS), which is in sharp contrast to activated sludge, in which SVI values exceed 120 mL/g MLSS [9]. In the biomass from R\({}_{1}\), granules with sizes from 90 to 125 \(\upmu\)m predominated, constituting about 25.1% of the biomass, and granules with diameters over 1 mm were observed (Figure 2). In R\({}_{2}\), about 50.0% of the biomass consisted of granules smaller than 90 \(\upmu\)m. Both the superficial air velocity and the oxygen concentration affect aerobic granulation [26]. [PERSON] et al. [27], reported that a higher shear force resulting from a higher superficial air velocity supported formation of more compact, denser, and stronger aerobic granules. The relatively low air velocity in the present experiment (0.5 cm s\({}^{-1}\)) and the reduced oxygen concentration in R\({}_{2}\) worsened the settling properties of the biomass and precluded full granulation of the biomass. The size and formation of stable aerobic granules correlates with the efficiency of pollutant removal because low air velocity favors selection of slow-growing organisms [28]. In a study involving the treatment of slaughterhouse wastewater, the size of granules ranged from 0.6 to 1.2 mm, which favored a high abundance of ammonium-oxidizing bacteria [13]. In the present study, the larger granule diameters in R\({}_{1}\) explain the greater abundance of nitrifiers in this biomass, as indicated by \(amoA\) gene analysis, and the higher nitrification rate. A study by [PERSON] et al. [29], revealed that larger granules possess a higher microbial biodiversity than smaller granules. On the one hand, increasing granule size decreases the surface/volume ratio, resulting in a higher surface loading of pollutants, which may affect aerobic transformations in the biomass. In larger granules, the depth to which oxygen permeates will be less than in smaller granules, and larger anaerobic/anoxic zones will be present, which provide spaces for the growth of denitrifying bacteria. In contrast, in smaller granules, which have a larger surface/volume ratio, oxygen can permeate deeper than in larger granules and provide more aerobic zones for the growth of ammonia-oxidizing bacteria (AOB). In the present study, large changes in the structure of microorganisms colonizing the inoculum and biomass (Figure 3) were noted throughout the experiment. [PERSON] et al. [30] also observed that during the granulation of activated sludge, the percentage of _Competibacter_ sp. increased, and bacteria of the _Xanthomonadaceae_, _Rhodospirillaceae_, and _Aminobacter_ families appeared. According to those authors, the occurrence of bacteria such as _Zoogloea_ sp., _Xanthomonadaceae_, _Sphingomonadales_, _and Rhizobiales_ promotes the formation of EPS, which are responsible for biomass granulation. In the present study, _Dokdonella_ sp. were found to be responsible for the formation of EPS and denitrification in biomass. The presence of microorganisms from the above-mentioned families promotes granulation and affects the strength of the resulting granules. Figure 2: The distribution of granule particle sizes in the batch reactors.