## 2 Materials and Methods
### Experimental Station Construction and Exploitation  
The biogas production of dairy wastewater was performed in the AL-FB, in which the construction of the reactor forced a vertical wastewater flow (Figure 1).  
The AL-FB consisted of three coaxially placed tanks with a total working volume of 178 L. The inner tank (working volume of 4 L, working height of 0.51 m, diameter of 0.1 m) was inoculated with ASS and was used as a hydrolyzer. The middle container (working volume of 78 L, working height of 0.51 m, diameter of 0.45 m) served as a methanogenesis tank and was inoculated with AGS, while the sedimentation process and clarification of digestate were in the external tank (working volume of 96 L, working height of 0.51 m, diameter of 0.5 m). Additionally, the final clarification of effluent was in the external effluent/sludge storage tank. The reactor was equipped with a peristaltic pump  
Figure 1: Scheme of anaerobic labyrinth-flow bioreactor.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Variant**} & \multicolumn{4}{c}{**Stage 1**} \\ \cline{2-4}  & **OLR (g COD/L-d)** & **Q (L/d)** & **HRT (d)** \\ \hline V1 & 4.0 & 15 & 5.3 \\ V2 & 6.0 & 23 & 3.5 \\ V3 & 8.0 & 30 & 2.7 \\ V4 & 10.0 & 37 & 2.2 \\ V5 & 12.0 & 45 & 1.8 \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Variant**} & \multicolumn{4}{c}{**Stage 2**} \\ \cline{2-4}  & **OLR (g COD/L-d)** & **Recirculation Degree (\%)** & **Q (L/d)** & **HRT (d)** \\ \hline V1 & & 50 & 55 & 1.5 \\ V2 & & 100 & 74 & 1.1 \\ V3 & & 150 & 111 & 0.7 \\ V4 & & 200 & 148 & 0.5 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The study organization and exploitation parameters in stages 1 and 2(ALLWEILER ASH 15, Radolfzell, Germany) with a capacity of 1 L/min and pressure of 1 bar with an electric motor (0.12 kW, 12 rpm/min, IE1, 400 V, 50 Hz) connected to the inverter with thermistors (SV002 iE5-C, 0.2 kW, 1.4 A).  
Substrate (raw dairy wastewater) was pumped from the bottom of the central part of the hydrolyzer to the top (upward wastewater flow). Then it flowed down to the backflow chamber and up to the methanogenesis tank. Additionally, the effluent from the effluent/sludge storage tank was recirculated to the middle container (internal digestate recirculation). The mixture of digestate and effluent flowed into the second backflow chamber and then into the external tank equipped with a quadruple sludge trap with biogas discharge pipes. Biogas produced was collected in the gas chamber of the reactor and discharged through a pipe in the upper part of the AL-FB dome equipped with a gas flow meter. The clarified solids (biomass) were collected at the bottom of the external tank and pumped to the methanogenesis container or discharged out of the system. The effluent from the AL-FB was directed to the effluent/sludge storage tank through the sawtooth-shaped outlet.  
The temperature inside the methanogenesis tank was maintained at the level of 37 \(\pm\) 1 \({}^{\circ}\)C. To maintain the temperature, the AL-FB was insulated with a 50 mm layer of mineral wool and heated by a water jacked consisting of a hot water storage tank with a capacity of 40 L and an electric heater (2000 W).