А moving bed biofilm reactor (MBBR) was studied in order improve the stability and efficiency of treatment at changing physicochemical parameters of wastewater. Laboratory experiments were conducted on two model of reactors: one operated with activated sludge only, while the other operated with activated sludge and MBBR biofilm carriers. The sludge volume index, hygroscopic moisture, ash content, pH, Eh, and BOD5 were measured. Microscopic analysis showed morphological changes in microbial communities under natural and artificial lighting, allowing to assess the role of light regime, aeration, and carrier surface in biofilm development. The results showed that the presence of MBBR carriers provides more stable pH and Eh values within the structure, promotes the formation of a structured and stable biofilm, and increases the overall efficiency of organic pollutant degradation. Systems with MBBR maintain higher microbiological activity and biomass retention even under limited aeration and reduced illumination. The results demonstrate the possibility of integrating MBBR with activated sludge to ensure stable and efficient wastewater treatment and serve as a basis for further optimization of design and operational parameters.