## 4 Conclusions  
A new reactor concept was designed to combine the benefits of the anaerobic contact process with the UASB reactor and a settling tank in dairy wastewater treatment. Experimental studies were conducted in the pilot plant-scale hybrid anaerobic labyrinth-flow bioreactor to assess anaerobic digestion performance, biogas productivity, and the structure of microorganisms. The research was carried out in two stages. In the first stage, the anaerobic labyrinth-flow bioreactor was operated with increasing OLR from 4.0 g COD/L-d to 12.0 g COD/L-d \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Stage**} & \multirow{2}{*}{**Variant**} & **Y\({}_{\text{CH4}}\)** & **HV** & **GE** & **P\({}_{\text{P}}\)** & **T\({}_{\text{w}}\)** & **DE** & **NE** \\  & & **(m\({}^{3}\)/d)** & **(kWh/m\({}^{3}\))** & **(kWh/d)** & **(kW)** & **(h/d)** & **(kWh/d)** & **(kWh/d)** \\ \hline \multirow{4}{*}{1} & V1 & 6.8 \(\pm\) 0.6 & & 62.17 & & 0.25 & 3.00 & 59.17 \\  & V2 & 10.3 \(\pm\) 0.6 & & 94.91 & & 0.38 & 4.56 & 90.35 \\  & V3 & 10.3 \(\pm\) 1.0 & 9.17 & 94.54 & 12 & 0.50 & 6.00 & 88.54 \\  & V4 & 9.8 \(\pm\) 0.9 & & 90.05 & & 0.62 & 7.44 & 82.61 \\  & V5 & 9.6 \(\pm\) 0.7 & & 60.52 & & 0.75 & 9.00 & 51.52 \\ \hline \multirow{4}{*}{2} & V1 & 12.4 \(\pm\) 0.6 & & 114.07 & & 0.92 & 11.04 & 103.03 \\  & V2 & 14.3 \(\pm\) 0.6 & & 130.76 & & 1.23 & 14.76 & 116.00 \\ \cline{1-1}  & V3 & 10.3 \(\pm\) 0.7 & & 94.73 & & 1.85 & 22.20 & 72.53 \\ \cline{1-1}  & V4 & 7.6 \(\pm\) 0.5 & & 69.78 & & 2.47 & 29.64 & 40.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Energy balance of the anaerobic digester depending on the stage and variant of the experiment (Y\({}_{\text{CH4}}\)—methane yield, HV—the heating value of methane, P\({}_{\text{P}}\)—power of pumping system, T\({}_{\text{w}}\)—pump operating time, GE—the gross energy, DE—energy demand for pumping and recirculation, NE—net energy output). in five experimental variants. The anaerobic digestion was the most effective at OLRs ranging from 4.0 g COD/L-d to 8.0 g COD/L-d. The highest COD reduction was more than 87%, and biogas and methane yields reached maximum values of 0.32 \(\pm\) 0.02 L/g COD removed and 0.23 \(\pm\) 0.02 L/g COD removed, respectively. The highest methane production reached 101.4 \(\pm\) 1.9 L/d, while methane concentration in biogas ranged from 68.12 \(\pm\) 1.3% to 73.0 \(\pm\) 1.9%. This evenest distribution of phyla at the lowest OLR ensured high biogas production. The determination of the microbial community showed a selection of classes more sensitive to overloading (e.g., Flavibacteria) than others (e.g., Thermotogae). In the second stage, effluent recirculation was used to improve the treatment. The reactor was operated at a constant OLR of 10.0 g COD/L-d and different degrees of internal recirculation. The maximum methane production yield of 0.20 \(\pm\) 0.01 L CH\({}_{4}\)/g COD removed was achieved with 100% recirculation, which was 25% more compared to the achievements without recirculation. This study showed that the new construction of an anaerobic reactor and the use of recirculation significantly enhanced the anaerobic digestion of dairy wastewater. Sludge flotation and biomass washout were avoided during the study, proving that the installation was powerful for treating dairy wastewater rich in fat, oil, and grease. The most favorable technological variant was characterized by a positive energy balance. These findings could play an important role in scaling the new hybrid reactor. Conceptualization, M.Z. and M.D. ([PERSON]); methodology, M.Z., M.D. ([PERSON]) and P.R. ; validation, M.Z. and M.K. ; formal analysis, M.Z., M.K. and M.D. ([PERSON]); investigation, M.Z., M.K., M.D. ([PERSON]) and P.R. ; resources, M.Z., M.D. ([PERSON]), M.K., P.R., A.N. and M.D. ([PERSON]); data curation, M.Z., M.D. ([PERSON]), M.K., P.R., A.N. and M.D. ([PERSON]); writing--original draft preparation, M.K. and M.D. ([PERSON]); writing--review and editing, M.Z., M.D. ([PERSON]), M.K., P.R., A.N. and M.D.