## 3 Results and Discussion
### AD Performance and Wastewater Treatment Efficiency
In variant 4, COD removal, biogas, and methane yield significantly decreased; thus, in the second stage of the study, the experiments were conducted at a constant OLR of 10 g COD/L-d to enhance anaerobic digestion by recirculation. The study was performed for four variants (Table 3). The highest COD removal of 89.2 \(\pm\) 0.4% was achieved in variant 2 with 100% recirculation, which was 10.3% higher than that in experiments without recirculation (Table 5, Figure 5). In variants 1 and 3, the COD removal also exceeded 80% (Table 5, Figure 5). The recirculation degree of 200% in the last variant deteriorated the removal of organic compounds under the level achieved in stage 2 (Figure 5, Tables 4 and 5). The highest organic load removal of 8.92 \(\pm\) 0.04 g COD/L-d was achieved in variant 2 and was 1.03 g COD/L-d higher than that in experiments without recirculation (Tables 4 and 5). The pH values in all experimental variants were close to neutral, ranging from pH 6.83 \(\pm\) 0.06 in variant 4 to 7.06 \(\pm\) 0.04 in variant 2, which was within the optimum range for methanogens [49; 50]. Recirculation provided a better reduction in volatile fatty acid (VFA) concentration in the liquid fraction of digestate, thus avoiding the accumulation of VFAs, increasing pH and alkalinity, and resulting in better process stability [51; 52]. During the whole experiment, the FOS/TAC ratio was within the range of 0.35-0.4, indicating the presence of a proper buffering capacity in the methanogenesis tank [41; 42]. Figure 5: The efficiency of anaerobic dairy treatment in stage 2: (**a**) organic compound removal as COD with a standard deviation in experimental variants V1–V5; (**b**) CH\({}_{4}\) concentration in biogas with a standard deviation in experimental variants V1–V5; (**c**) nutrient removal (nitrogen and phosphorus) with a standard deviation in experimental variants V1–V5. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Variant**} & \multirow{2}{*}{**OLR**} & \multicolumn{3}{c}{**COD**} & \multicolumn{3}{c}{**Biogas Production**} & \multicolumn{3}{c}{**CH\({}_{4}\) Production**} \\ \cline{3-10}  & **(g CODL-d)** & **Removal (\%)** & **Load Removal**  
In stage 2, the most effective biogas and methane production of 156.0 \(\pm\) 6.2 L biogas/d and 109.2 \(\pm\) 3.6 L CH\({}_{4}\)/d, respectively, were obtained in variant 2 with 100% recirculation and were respectively 33.5% and 45.2% more compared to the achievements without recirculation (Tables 4 and 5). A 200% degree of recirculation negatively affected the production of biogas (which reached 124.8 \(\pm\) 7.3 L/d) and methane (78.0 \(\pm\) 2.7 m\({}^{3}\)/d), which were below the levels achieved in stage 1 (Table 5). During the anaerobic digestion of dairy wastewater, the biogas and methane yields were the highest when the recirculation degree ranged from 50% to 100% (Table 5). The maximum methane yield of 0.20 \(\pm\) 0.01 L/g COD removed was obtained with 100% recirculation, which was 25% higher than the production obtained in stage 1 at the OLR of 10 g COD/L-d (Table 5). The concentration of methane in biogas ranged from 60.4 \(\pm\) 1.4% in variant 4 to 70.0 \(\pm\) 0.4% in variant 2 (Figure 5). In the whole experiment, nitrogen removal remained at about 6-7%, while phosphorus removal was at about 3% (Figure 5). The study showed that the new construction of the anaerobic reactor and 100% degree of recirculation of the digestate liquid fraction significantly enhanced the anaerobic digestion of dairy wastewater. [PERSON] and [PERSON] [53] described an anaerobic reactor that was similar in design assumptions. A baffled anaerobic reactor was designed as a series of baffles forcing wastewater flow through the series of compartments from inlet to outlet.