Optimizing manganese removal and washwater recovery at a direct filtration plant in Brazil

Inadequate removal of turbidity, manganese and iron at the Sera Azul direct filtration works was investigated by extensive laboratory and pilot-scale tests of different pretreatment process, filter media and washwater recovery systems. An inverted sequence of adding chlorine, then ferric chloride and finally lime for adjustment to pH 9.5 provided optimal conditions for manganese oxidation. Using polyelectrolytes and recycling washwater had no significant effect on filtered water quality. Replacing the existing deep filters with shallower beds and coarser media and increasing the wash rate facilitated adequate cleaning. Manganese deposits on the sand grains were reduced by soaking in sulphuric acid. Using an anionic polyelectrolyte increased the sludge settling rate almost 3 times and reduced specific resistance by 85 per cent, but did not affect the final filter cake concentration. A filter washwater recovery system consisting of 2 equalization /sedimentation tanks and 3 sludge lagoons was recommended.