Bioflocculation of photo-fermentative bacteria induced by calcium ion for enhancing hydrogen production

•Ca2+ greatly decreased interaction energy barrier of PFB based on DLVO theory.•Average floc size of PFB achived maximum value 30.07 μm at 4 mmol/l of Ca2+.•Bioflocculation induced by Ca2+ effectively resisted biomass washout from reactor.•Bioflocculation resulted in better H2 production than free cells in continuous operation.

The applications of photo-fermentative bacteria (PFB) for continuous hydrogen production are generally subjected to a serious biomass washout from photobioreactor, resulting from poor flocculation of PFB. In this study, through reducing the absolute ζ-potentials of PFB, Ca2+ greatly decreased total interaction energy barrier of PFB based on DLVO theory, thus promoted the bioflocculation of Rhodopseudomonas faecalis RLD-53. Average floc size of PFB increased with the Ca2+ concentration, and reached maximum 30.07 μm at 4 mmol/l. Consequently, biomass retention capacity of photobioreactor significantly enhanced after 30 min settling with half working volume discharge. In the continuous photo-fermentative sequencing batch reactor, compared with the free cell culture, bioflocculation reached a higher steady-state hydrogen production rate of 879 ml H2/l/d and hydrogen yield of 2.64 mol H2/mol acetate, respectively. Therefore, bioflocculation promoted by calcium ion was an effective strategy for retaining PFB in photobioreactor to produce hydrogen continuously.