Evaluating the use of manganese-oxidizing bacteria in surface water treatment plants

Soluble manganese (Mn) presents a significant treatment challenge to many water utilities, causing aesthetic and operational concerns. Surface water treatment plants using ozonation followed by biofiltration are unable to apply free chlorine across the filter, a method used by many utilities for soluble Mn control. These facilities are vulnerable to periodic problems of elevated Mn in the finished water. Manganese-oxidizing bacteria (MOB) readily oxidize Mn in groundwater treatment applications, which normally involve pH values above 7.0.  However, the ability of MOB to facilitate Mn oxidation under lower pH conditions (6.2--6.3) often employed to optimize organics removal has not been demonstrated. Laboratory-scale biofilters were operated to evaluate the ability of MOB to accomplish soluble Mn control at a range of pH (6.3--7.3).  The biofilters were able to oxidize Mn at a pH as low as 6.3 at greater than 98% removal.  Ozonation by-product removal was also greater than 90% in all filter columns. Stress studies indicated that well-acclimated MOB can withstand variations in Mn concentration, hydraulic loading rate, and temperature typically found at surface water treatment plants, at least for relatively short (1-2 days) periods of time. Pilot studies demonstrated that there are unknown factors that affect MOB acclimation, and MOB may be present in full-scale biofilters already. MOB are difficult to identify with current biological analysis techniques and comprise small percentages of the total microbial population. MOB have demonstrated potential for use in surface water treatment plants, but further research is needed before this application is fully feasible.