## 3 Results and Discussion
In R\({}_{1}\), the N-NO\({}_{3}\) concentration increased during the first two hours of the working cycle,  
Figure 4: Changes in COD, nitrogen, and phosphorus concentrations in the effluents from the constantly aerated reactor, R\({}_{1}\) (**A**), and the reactor with intermittent aeration phases, R\({}_{2}\) (**B**). after which it remained at a few mg L\({}^{-1}\). In R\({}_{2}\), its concentration initially increased to 5.86 mg/(L h\({}^{-1}\)), and then it was removed at different rates in subsequent non-aerated phases (22.5 mg (L h\({}^{-1}\)), 15.7 mg (L h\({}^{-1}\)), and 23.6 mg (L h\({}^{-1}\)) in phases 1, 2, and 3, respectively) (Figure 5). Due to intermittent aeration of R\({}_{2}\), the concentration of orthophosphates in the effluent from that reactor was significantly lower than that in the effluent from R\({}_{1}\) (6.5 \(\pm\) 2.9 mg L\({}^{-1}\) vs. 8.6 \(\pm\) 1.7 mg L\({}^{-1}\), Figure 5). As a result, the removal efficiencies were 73% and 65%, respectively. A low oxygen concentration stimulates removal of COD, N and P due to heterotrophic growth inside granules consisting mostly of DPAO (denitrifying PAO) [35]. It has also been reported that lower phosphorus removal performance was often observed at high dissolved oxygen (DO) concentrations of 5.0 mg/L, while DO concentrations of approximately 2.5-3.0 mg/L seemed to be associated with the dominance of PAOs [36]. Taking into account the specific characteristics of wastewater from the meat industry and the lack of strict anaerobic phases, P removal was satisfactory and similar to the efficiencies obtained in other studies conducted in either constantly aerated granular batch reactors (83%) [14]. Presence of high concentrations of volatile fatty acids (VFA) stimulates PAOs to take up VFAs anaerobically and convert them to intracellular poly-\(\beta\)-hydroxyalkanoates (PHAs). PAOs gain the energy and reducing power required for anaerobic VFA uptake and conversion to PHA through the hydrolysis of their intracellularly stored polyphosphate (poly-P) and glycogen [37]. In the present study, PAO belonging to _Tetrasphaera_ sp., _Arenimonas_ sp., and _Pseudomonas_ sp. [38; 39], were identified and their presence was much higher in R\({}_{2}\). In R\({}_{1}\) percentage of _Amaricoccus tamworthensis_ gradually increased. [PERSON] et al. [40], point out that _Amaricoccus_ sp. in pure culture failed to synthesize poly-P aerobically but they could synthesize glycogen aerobically, they did not assimilate either acetate or glucose  
Figure 5: Concentration of ammonium, nitrite and nitrate during the cycles of (**A**) R\({}_{1}\) and (**B**) R\({}_{2}\). Gray shading indicates phases without aeration. anaerobically, and PHAs synthesis occurred aerobically but not anaerobically. Glycogen-accumulating organisms (GAOs) are a bacterial group capable of competing with PAOs for VFA in wastewater. In the present study the most numerous GAO was _Candidatus Competibacter_ that was present in both reactors [41]. The pH of the effluents was 8.4 and 8.2 in R1 and R2, respectively. The wastewater pH is considered to be a critical factor impacting N2O emission. [PERSON] and [PERSON] [42] had pointed out that the maximum emission of N2O was observed at pH 8.5 and decreased with decreasing pH, which can impact of _nosZ_ gene abundance in the present study. Alkaliinity of the effluent from R1 (\(7.2\pm 0.5\) mvaI L\({}^{-1}\)) was significantly lower than from R2 (to \(8.5\pm 0.5\) mvaI L\({}^{-1}\)) pointing to more efficient recovery of alkalinity during denitrification in R2 than in R1. Even though the alkalinity was low, ammonium nitrogen was efficiently oxidized during the reactor cycle. The distribution of EPS in the structure of granules plays an important role in improving the stability of the aerobic granular sludge and affects its SVI [43]. Extracellular polymers (EPS) were isolated from the aerobic granules at the beginning and end of the reactor cycle.