## 5 Conclusions  
This work provides preliminary insights into the nature of the acid and heat pretreatments used for the inactivation of methanogenic archeae in anaerobic sludge in order to achieve the higher production of biohydrogen during batch and continuous DF of CW under mesophilic condition. According to the results, acid treatment was effective for thermophilic inoculum to suppress methanogenic activity, and heat treatment was effective for mesophilic inoculum. The composition of the VFA formed during DF of CW using pretreated inoculums suggested that the predominant metabolite during the process was the butyric acid.  
The characteristics of continuous DF process of CW biodegradation under mesophilic conditions in AF and AFB using a polyurethane carrier for immobilization of heat-pretreated mesophilic anaerobic sludge (inoculum) were obtained. With a gradual stepwise decrease in HRT from 14.52 to 2.02 days and a corresponding increase in OLR from 2.07 to 14.88 kg COD/(m\({}^{3}\) day), the optimal mode for obtaining the maximum specific yields of hydrogen, namely 178 mL/g COD and 149 mL/g COD for AFB and AF, respectively, were obtained at HRT of 4.5 days and OLR of 6.61 kg COD/(m\({}^{3}\) day). At the same time, the maximum biohydrogen production rates of 1.28 and 1.9 NL/(L day) for AF and AFB, respectively, were obtained with a minimum HRT of 2.02 days at which the OLR was maximum (14.88 kg COD/(m\({}^{3}\) d)).  
The microbial composition in AF and AFB did not significantly differ from each other. _Firmicutes_ (65% in AF and 60% in AFB) and _Actinobacteria_ (26% in AF and 32% in AFB) were the two most dominant taxa at the phylum level. Representatives of _Lactobacillus_ and _Bifidobacterium_ made up the largest part of the microbial community at the genus level, 43 and 24% in AF, and 40% and 30% in AFB, respectively. At the end of the start-up period, the relative number of methanogenic archaea in AFB (0.1%) was lower than in AF (0.4%) and such elimination of hydrogen scavengers may explain why a higher specific biohydrogen yield and VHPR were obtained in AFB. Better mixing in the reactor increased the efficiency of biohydrogen production from CW, possibly due to better mass transfer, while recirculation negatively influenced on the hydrogen content in the biogas.  
Using cheese whey as a substrate for biohydrogen production, the main technological parameters (HRT, OLR) were determined, which ensure the stable operation of anaerobic reactors (AF and AFB with polyurethane foam as a carrier) at the start-up, which can be used in the practical implementation of this process further on an industrial scale.  
Conceptualization, E.R.M. and I.V.K.; methodology, I.V.K. and A.A.K.; software, V.P. and U.F.; validation, A.N.N.; formal analysis, I.V.K. and A.A.K.; investigation, E.R.M. and I.V.K.; resources, D.A.K.; data curation, Y.V.L.; writing--E.R.M., I.V.K. and Y.V.L.; writing--review and editing, Y.V.L. and A.A.K.; visualization, E.R.M. and I.V.K.; supervision, Y.V.L. and A.A.K.; project administration, E.R.M. and I.V.K.; funding acquisition, D.A.K. All authors have read and agreed to the published version of the manuscript.  
This work was partially supported by the Federal State Budgetary Institution \"Russian Foundation for Basic Research\" as part of scientific project No. 18-29-25042.  
Not applicable.  
The authors declare no conflict of interest.