Application of a 2-step process for the biological treatment of sulfidic spent caustics

This research demonstrates the feasibility and advantages of a 2-step process for the biological treatment of sulfidic spent caustics under halo-alkaline conditions (i.e. pH 9.5; Na+ = 0.8 M). Experiments with synthetically prepared solutions were performed in a continuously fed system consisting of two gas-lift reactors in series operated at aerobic conditions at 35 °C. The detoxification of sulfide to thiosulfate in the first step allowed the successful biological treatment of total-S loading rates up to 33 mmol L−1 day−1. In the second, biological step, the remaining sulfide and thiosulfate was completely converted to sulfate by haloalkaliphilic sulfide oxidizing bacteria. Mathematical modeling of the 2-step process shows that under the prevailing conditions an optimal reactor configuration consists of 40% ‘abiotic’ and 60% ‘biological’ volume, whilst the total reactor volume is 22% smaller than for the 1-step process.

► In this study a 2-step process for the treatment of spent caustics has been successfully tested.
► The 2-step process comprises of an “abiotic” first step and a “biological” second step.
► Haloalkaliphilic sulfide oxidizing bacteria converted sulfide and thiosulfate to sulfate.
► Experimental data were used to validate a mathematical model that was developed for the 2-step process.
► Modeling shows that smaller total reactor volumes are feasible compared to a 1-step process.