Biomass concentration enhances the stability of the biological desulfurization process through biochemical transformation of (poly)sulfides

The biological desulfurization process utilizes sulfide-oxidizing bacteria (SOB) to transform toxic hydrogen sulfide (H₂S) gas into elemental sulfur that is recovered and reused in various applications. The recent addition of an anaerobic bioreactor enhanced sulfur selectivity by exposing SOB to alternating highly sulfidic conditions followed by micro-oxic conditions. These alternating conditions led to an interest in polysulfide anions (S_x^2-), which are formed as a result of the equilibrium between HS^- and elemental sulfur and enhanced with longer sulfidic retention times. Here, the effect of sulfide concentration was investigated to determine the influence on the uptake of (poly)sulfides in the sulfidic bioreactor and the influence in combination with the total biomass concentration for process stability. For the first time, internal sulfane was found to be stored in SOB from a continuous dual-reactor process. Interestingly, a maximum relative concentration of 0.1 mg-S mg-N⁻¹ was found at the lowest sulfide concentration, indicating the conversion of sulfane into other species rather than storage. The S_x^2- chain length distribution changed the most with both an increased sulfide and biomass concentration, where the relative abundance of chain length 5 increased by ∼10 % and chain length 7 decreased by ∼5 %. Chain length 4 only increased when the sulfide concentration increased. Here, it is proposed that the nutrient dosing and sulfide concentration in the sulfidic bioreactor should be balanced. By balancing the biomass concentration and sulfide concentration in the sulfidic bioreactor, increased process stability can be achieved through the enhancement of (poly)sulfide uptake and prevention of chemical oxidation.