## 4 Discussion
### DF of CW in Continuous Mode in Two Types of Reactors  
Summarizing the results obtained, it can be noted that the most optimal mode for obtaining the maximum specific yield of hydrogen, amounting to 178 mL/g COD and 149 mL/g COD for AFB and AF, respectively, was obtained at HRT of 4.5 days and OLR of 6.61 kg COD/(m\({}^{3}\) day). At the same time, for the maximum hydrogen production rate of 1.28 and 1.9 NL/(L day) for AF and AFB, respectively, it was necessary to maintain the minimum HRT of 2.02 days (Figure 5), at which OLR was maximal (Figure 6). In the only work, similar polyurethane foam was used as a carrier for obtaining biohydrogen from CW [35]. A packed bed reactor was used, which is identical in principle to the AF (Figure 1b), since the recirculation rate in the reactor was relatively low, not allowing the carrier to be kept in fluidized form. Under semi-continuous operation the maximum hydrogen yield (HY) achieved was 48.4 mL H\({}_{2}\)/g COD when using synthetic whey as substrate at an HRT of 2 days, which was 3 times lower than the HY observed in our work, probably due to less complete transformation of lactose. At the same time, when using real CW, the hydrogen yield was significantly lower (up to 25 mL H\({}_{2}\)/g COD). The maximum hydrogen concentration in biogas was 26.3% at OLR of 12.5 kg COD/(m\({}^{3}\) d) and HRT of 2 days. The authors of the work note that an increase in OLR up to 20 kg COD/(m\({}^{3}\) d) led to a significant decrease of hydrogen in biogas [39]. When using polyurethane foam, which is 90% air, as a carrier, we recorded a maximum hydrogen concentration in biogas of 54% (AF) and 53% (AFB) at HRT = 4.54 d and OLR = 6.61 kg COD/(m\({}^{3}\) d). Three different organic loading rates of 22, 33 and 37 kg COD/(m\({}^{3}\) d), which were much higher than in our work, were used at a fixed HRT of 24 h [19]. The increase of the OLR from the initial value of 22 kg COD/(m\({}^{3}\) d) to a value of 37 kg COD/(m\({}^{3}\) d) and the adjustment of the pH to values greater than pH 5 had a positive effect on the hydrogen production and values of up to 1 NL H\({}_{2}\)/(L d) were achieved, which is lower than the maximum values that were obtained in our work. VHPR data similar to our data was obtained in [40], which, when fermenting CW in the anaerobic structured-bed reactor (ASTBR) at 25 \({}^{\circ}\)C, OLR of 24 kg COD/(m\({}^{3}\) d) and HRT of 24 h, reached VHPR of 1.6 \(\pm\) 0.7 NL H\({}_{2}\)/(L d). At the same time, in a number of studies, extremely high VHPRs were observed. For example, when using cheese whey powder (CWP) as a substrate in a batch experiment, a maximum VHPR of 4.35 NLH\({}_{2}\)/(L d) (or 8.1 mmol H\({}_{2}\)/(L h)) was attained at pH 7.5 and 25 g CWP/L [41]. It should be noted that in our work AFB performed better than AF. We attribute this to the fact that the AFB configuration provides better mass transfer and the absence of stagnant zones within the reactor, which may have contributed to the improved results in terms of biohydrogen production compared to conventional packed bed reactors. [PERSON] et al. [42] and [PERSON] et al. [43] observed that reactors with a higher porosity layer (more than 90%) promote the periodic removal of suspended biomass while allowing immobilized biomass to grow, resulting in increased biohydrogen production. If methane is generated during continuous operation of the DF process, HRT control is recommended to suppress methanogenic activity and increase biohydrogen yield. [PERSON] et al. showed that with a decrease in HRT from 24 h to 12 h for 13-22 days in a CSTR reactor fermenting molasses wastewater, the hydrogen content can be increased from 5.9% to 12.1% [44]. Our results indicated an increase in the amount of methane generated in the reactor with a decrease in HRT from 9.08 days to 5.58 days for AFB and to 4.54 days for AF within 5-9 days and an increase in OLR load from 3.31 to 6.61 kg COD/(m\({}^{3}\) d) (Figure 4). With a subsequent decrease in HRT, a decrease in the methane content in the biogas was observed.