Membrane concept for valorization of bicarbonate in sulfate-reduced effluent

Bicarbonate and toxic hydrogen sulfide are generated in sulfate-reducing bacterial processes in the presence of suitable bacteria and conditions. They are often preferably eliminated from the solutions rather than utilized. However, hydrogen sulfide could be utilized for precipitation of metal products, whereas bicarbonate could be considered as a precursor for a calcium precipitation chemical. This study focused on the valorization of bicarbonate-containing mine water effluent from a sulfate-reducing bacterial process, called biosolution, using membrane concentration as the main technology in the concept. Concentration was found to be an essential step for the utilization of biosolution in calcium precipitation. Good quality water was produced at the same time, for reuse or safe discharge. Biosolutions contained residual concentrations of sparingly soluble sulfides, which caused problems for membrane filtration performance. This study revealed that microfiltration to stop the biological sulfate-reducing process by removing bacteria, aeration to oxidize residual sulfides, and microfiltration to remove suspended solids were needed for sufficient pre-treatment prior to membrane concentration. Both nanofiltration and reverse osmosis worked well as a bicarbonate concentration technology. Concentrated bicarbonate solution was then converted to effective calcium precipitation chemical by adjusting the pH, as a result of which the chemical balance favored the effective carbonate instead of the ineffective bicarbonate. This membrane concept is a step towards the closed-loop processes of the circular economy.