## 3 Results and Discussion
The abundance of bacteria with the ammonium monooxygenase (_amoA_) gene in the inoculum was \(4.46\times 10^{2}\) copies/5 ng DNA. A rapid increase in the number of copies of the _amoA_ gene was observed in R\({}_{1}\) at the beginning of the experiment, and during the experiment the number of copies remained in the range of \(3.00\)-\(3.66\times 10^{3}\) copies/5 ng DNA. In R\({}_{2}\), the highest number of the investigated genes (\(2.49\times 10^{3}\) copies/5 ng DNA) was recorded in 56 th cycle of experiment. At the end of the experiment, the number of ammonium monooxygenase gene in biomass from R\({}_{1}\) was 2.4 times higher than in R\({}_{2}\). The copy number of the nitrous oxide reductase (_nosZ_) gene in DNA isolated from the inoculum was \(1.48\times 10^{4}\) copies/5 ng DNA. In R\({}_{1}\), the number of compiled denitrifying bacteria increased in the 56 th cycle of reactor operation to \(3.17\times 10^{4}\) copies/5 ng DNA which could improve the _nosZ_ gene expression and lead to higher emission of N\({}_{2}\)[46]. After that, this value slowly decreased to \(1.35\times 10^{4}\) copies/5 ng DNA (164 th cycle). In R\({}_{2}\), the number of copies of the _nosZ_ gene increased to \(5.41\times 10^{4}\) copies/5 ng DNA in the 56 th cycle of the experiment thus the number of denitrification bacteria was 41% higher in the R\({}_{2}\) than in R\({}_{1}\). After this time, a reverse tendency was observed and at the end of the experiment the copy number of the investigated gene in biomass was about 1.4-time higher in R\({}_{1}\) than R\({}_{2}\). [PERSON] et al. [50], observed that the highest number of nitrifiers was recorded in the reactor with constant aeration but the introduction of a 60-min anaerobic phase into the working cycle lowered the number of nitrifiers while the highest number of denitrifying bacteria (_nosZ_ gene analysis) characterized biomass from a reactor with a short 20-min anaerobic phase in the cycle. The highest efficiency of nitrogen removal from wastewater was obtained in the reactor with constant aeration (89%), while the lowest in the reactor with the initial 60-min anaerobic phase (58%). In the presented study, a lower number of denitrification bacteria in R\({}_{2}\) was observed despite a higher denitrification efficiency. This may be because denitrification numbers were estimated based on the _nosZ_ gene analysis. Abundance of complete denitrifiers possessing _nosZ_ gene in biomass is favorable because ensures full conversion of nitrogen compounds to N\({}_{2}\) and reduction in greenhouse gas emissions such as NO and N\({}_{2}\)O [51]. Complete denitrification can be carried out by bacteria from the genera _Dokdonella, Flavobacterium, Acietobacter, Pseudomonas, Arcobacter_, and _Comamonas_, while microorganisms carrying out partial denitrification belong to genera _Diaphorobacter_, _Thuera_ and _Zoogloea_[52]. In the present study, in the biomass treating wastewater from the meat industry bacteria from genera _Dokdonella_ (_Dokdonella ginsengoli_) and _Thuera_ (_Thuera_ sp.) predominated. Microorganisms from the genera _Pseudomonas, Burkholderia_ (_Burkholderia sp._), _Geobacter_, and _Rhizobium_ were also found in the biomass from R\({}_{2}\). These microorganisms have _norB_ gene in their genomes, which product is responsible for the conversion of nitric oxide to nitrous oxide and can support the denitrification process. An increase in the occurrence of _Paracoccus_ sp. was also noted at the end of the experiment in both reactors. _Paracoccus_ sp. uses methanol as the carbon source and can denitrify at a high rate (250 mg NO\({}_{3}\)/g TSS h\({}^{-1}\)) [53; 54]. The high percentage of unclassified microorganisms in R\({}_{2}\) indicates that more variable microenvironments created in biomass by intermittent aeration favored growth of yet not classified bacteria and resulted in higher microbial biodiversity. Bacterial consortia in aerobic granules were analyzed using NGS to draw conclusion about the relationships between their structure and aeration mode in the reactor.