Regulating biohydrogen production from wastewater by applying organic load-shock: Change in the microbial community structure and bio-electrochemical behavior over long-term operation

Detailed experiments were designed to evaluate the function of load-shock treatment strategy (50 g COD/l; 3 days) for selective enrichment of acidogenic hydrogen (H₂) producing consortia in comparison with untreated anaerobic consortia. Experiments performed in suspended-batch mode bioreactors for 520 days illustrated the relative efficiency of load-shock treated consortia in enhancing H₂ production (16.64 mol/kg COD_R) compared to untreated-parent consortia (3.31 mol/kg COD_R). On the contrary, substrate degradation was higher with control operation (ξ_COD, 62.86%; substrate degradation rate (SDR), 1.10 kg COD_R/m³-day) compared to load-shock culture (52.33%; 0.78 kg COD_R/m³-day). Fatty acid composition documented a shift in the metabolic pathway towards acetate formation after applying load-shock, which manifests higher H₂ production. Microbial profiling documented a significant alteration in species composition of microbial communities after repeated load-shock applications specific to enrichment of Firmicutes which are favourable for H₂ production. Dehydrogenase activity was stabilized with each re-treatment, signifying the adaptation inclination of the biocatalyst towards increased proton shuttling between metabolic intermediates, leading to higher H₂ production. Voltammograms of load-shock treated cultures showed a marked shift in oxidation and reduction catalytic currents towards more positive and negative values respectively with increasing scan rate evidencing simultaneous redox-conversion reactions, facilitating proton gradient in the cell towards increased H₂ production. Load-shock treatment facilitates direct cultivation of inoculums at higher substrate load without any chemical pretreatment. This study documented the feasibility of controlling microbial metabolic function by application of load-shock treatment either for preparing inoculum for startup of the reactor or to the reactor resident microflora (in situ) during operation whenever required to regain the process performance.