## 3 Results and Discussion  
### AD Performance and Wastewater Treatment Efficiency  
In stage 1, the AL-FB was operated with increasing OLR from 4.0 g COD/L-d to 12.0 g COD/L-d in five experimental variants (Table 3). The highest COD reduction of over 87% was noted at OLRs between 4.0 g COD/L-d and 8.0 g COD/L-d with an average COD concentration in the effluent of 1279.6 mg O\({}_{2}\)/L (Figure 2, Table 4). The application of a higher load of organic compounds in dairy wastewater influenced the reduction in COD removal to the value of approx. 74% and an average COD concentration in the effluent of 5521.0 mg O\({}_{2}\)/L in V5 (Figure 2, Table 4). However, increasing OLR involved an increase in COD load removal, which was 3.76 \(\pm\) 0.1 g COD/L-d at 4.0 g COD/L-d and rose to 8.89 \(\pm\) 0.1 g COD/L-d at the highest OLR (Table 4). The FOS/TAC ratio was found to be less than 0.4 for OLRs from 4.0 g COD/L-d to 8.0 g COD/L-d, which ensured a proper buffering capacity in the AL-FB chambers (Figure 3). Within this OLR range, the pH in the methanogenesis tank remained at an average level of pH 7.19 \(\pm\) 0.11 (Figure 3). Dosing a larger organic load into the anaerobic reactor resulted in an increase in the FOS/TAC ratio over 0.4 in variant 4 and over 0.5 in variant 5, as well as a decrease in pH to the value of 6.49 at the highest OLR, indicating that the reactor was overfed (Figure 3) [41,42]. Biogas production remained at a high level within OLRs from 6.0 g COD/L-d to 12.0 g COD/L-d (Table 4). The highest average productivity of 121.7 \(\pm\) 2.9 L/d was noted at the highest OLR, but there were no differences in variants 4-5 (\(p\) > 0.05) (Table 4). The highest methane production of 101.4 \(\pm\) 1.9 m\({}^{3}\)/d was achieved at an OLR of 10.0 g COD/L-d (Table 4). Methane concentration in biogas ranged from 73.0 \(\pm\) 1.9% at the OLR of 4.0 g COD/L-d to 59.3 \(\pm\) 3.0% at the OLR of 12.0 g COD/L-d (Figure 4). Biogas and methane yields were the highest at OLRs ranging from 4.0 g COD/L-d to 6.0 g COD/L-d and amounted respectively to 0.32 \(\pm\) 0.02 L/g COD removed and 0.23 \(\pm\) 0.02 L/g COD removed (Table 4). The yields decreased with increasing organic load to the value of 0.23 \(\pm\) 0.01 L biogas/g COD removed and 0.13 \(\pm\) 0.01 L CH\({}_{4}\)/g COD removed in the last variant (Table 4). Similarly, the yields calculated per gram of COD inlet dropped with increasing OLR (Figure 4). Nutrient removal was low and is shown in Figure 3. Phosphorus was not removed effectively regardless of the load, while the highest nitrogen removal of 14.42 \(\pm\) 1.9% was noted in variant 1. A similar COD removal efficiency of about 90% at the OLR of 5 kg COD/m\({}^{3}\) was obtained by [PERSON] [43] in a system of anaerobic reactors with separation of acidogenesis and methanogenesis, treating dairy wastewater. A hybrid UASB reactor in a lab-scale study was used by [PERSON] and co-workers [44] to treat dairy wastewater. The reactor was operated at an OLR varying from 8 to 20 kg COD/m\({}^{3}\)-d for a period of 110 days. The maximum COD removal and biogas production was obtained at the lowest OLR, and when the OLR rose to 19.2 kg COD/m\({}^{3}\)-d, the COD removal decreased to 84%. In turn, [PERSON] and co-workers [45] achieved a poor COD removal of 44.3% in a UASB reactor with modifications and improvements, treating dairy wastewater with inlet COD concentrations ranging from 1090 to 1415 mg O\({}_{2}\)/L. [PERSON] and co-workers [46] used magneto-active packing to enhance the anaerobic digestion of synthetic dairy wastewater in a moving biofilm reactor, in which methanogenesis was separated from the acidogenic phase. At the OLR of 3.9-7.5 kg COD/m\({}^{3}\)-d, the removal of organic substances was 86.6% with the maximum methane yield of 0.23 L/g COD removed. The authors postulated that the appropriate design of the reactor in combination with stimulating factors (e.g., magnetic field) allowed for achieving more biogas productivity and higher COD removal.