Promoting the Biological Oxidation of Reduced Sulphur Compounds by pH Adjustment in a High Temperature Membrane Bioreactor Treating Kraft Pulp Mill Foul Condensate

Over 99% of the reduced sulphur compounds (RSC) contained in a synthetic foul evaporator condensate were removed during treatment using a high temperature membrane bioreactor (MBR). At a neutral pH, the removal of the RSC was entirely due to stripping by the aeration system. It was possible to reduce the amount of RSC that was stripped to the atmosphere by promoting the biological oxidation of RSC through pH adjustment. A pH of less than approximately 4.5 was required to establish biological oxidation of RSC in the MBR. However, even at a pH of 3, which has been reported by others to be the optimal pH for the growth of thermophilic sulphur-oxidizing microorganisms, biological oxidation accounted for only approximately 50% of the RSC removed during treatment. The removal of the remaining 50% of the RSC removed during treatment was still due to stripping by the aeration system. The results further suggested that the long-term stability of a high temperature MBR operated at a low pH is questionable. In addition, the biological oxidation of methanol, which is considered to be the primary contaminant of concern contained in evaporator condensate, was significantly inhibited at a pH of less than approximately 4.5. Consequently, the simultaneous biological removal of methanol and RSC from foul evaporator condensate using a high temperature MBR was concluded to be impractical.