## 3 Results
### Experiment on DF of CW in Continuous Mode in Two Types of Reactors  
Heat-pretreated mesophilic anaerobic sludge (Inoculum 2) was used as an inoculum for the experiment in continuous mode for the following reasons:  
* Thermal inactivation was chosen primarily because of its relative simplicity both in laboratory conditions and in the full-scale implementation of the technology [7];
* Mesophilic sludge was chosen since it contains a microbial community naturally adapted to mesophilic conditions. In turn, the mesophilic mode of operation of AF and AFB was chosen because is considered more stable than thermophilic.  
During the experiment in continuous mode, HRT was decreased and OLR was increased in steps. At the initial stage (S-I Table 1) at HRT of 14.5 days, the hydrogen content was 15.9% in AF and 11.4% in AFB, there was no methane in the biogas. As HRT decreased, the hydrogen content in the biogas increased, reaching a maximum value of 54% (AF) and 53% (AFB) at HRT of 4.5 days (S-IV Table 1), besides, a small amount of methane (up to 1.55%) was found in the biogas (Figure 4).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Inoculum** & **Final pH** & **H\({}_{2}\) in Biogas Max, \%** & **CH\({}_{4}\) in Biogas Max, \%** & **COD Removal, \%** \\ \hline Inoculum 1 untreated (Control 1) & 6.67 & 0 & 16.5 (5) * & 71.4 \\ Inoculum 1 after acid pretreatment & 3.94 & 33.3 (1) * & 0 & 65.2 \\ Inoculum 1 after thermal pretreatment & 6.37 & 8.0 (2) * & 7.5 (8) * & 46.2 \\ Inoculum 2 untreated (Control 2) & 5.4 & 9.3 (1) * & 20.1(1) * & 66.7 \\ Inoculum 2 after acid pretreatment & 4.04 & 0.8 (1) * & 17.0 (1) * & 63.8 \\ Inoculum 2 after thermal pretreatment & 3.92 & 10.7 (1) * & 0 & 43.8 \\ \hline \end{tabular}
* Day of the experiment, COD—chemical oxygen demand.  
\end{table}
Table 2: Characteristics of batch dark fermentation of cheese whey using acid and heat pretreated inoculums.  
With a further decrease in HRT to 2.02 days and an increase in OLR, the amount of hydrogen in the biogas began to decrease to 41% (AF) and 44% (AFB). It was found that for both reactors, the hydrogen content in the biogas differed by no more than 3%, while the methane content in AFB was lower than in AF throughout the experiment. It should also be noted that the volumetric hydrogen production rate (VHPR) for AFB was higher throughout the experiment than for AF, which was especially noticeable at HRT = 2.02 days, at which time the VHPR was 1.3 and 1.9 NL/(L day), respectively for AF and AFB (Figure 5).  
The pH did not drop below 5.29 in both reactors. It was found that with a gradual decrease in HRT and, accordingly, an increase in OLR in both reactors, there was a slight decrease in the pH value, followed by a gradual increase. In AFB the pH value was always higher than in AF. According to experimental data, the highest hydrogen yield of 178 mL/g COD (AFB) and 149 mL/g COD (AF) was obtained at HRT of 4.5 days and OLR of 6.61 kg COD/(m\({}^{3}\) day) (Figure 6).  
Figure 4: The concentration of hydrogen and methane in the biogas depending on hydraulic retention time (HRT).  
Figure 5: Change in the volumetric hydrogen production rate (VHPR) and pH in the anaerobic filter (AF) and anaerobic fluidized bed (AFB) depending on hydraulic retention time (HRT).