Contact flocculation filtration using natural coagulants for developing countries
2014-12-15T10:36:51Z (GMT) by
Contact flocculation filtration using natural cationic polyelectrolytes extracted from seeds of the tree M.oleifera were found to be effective in the treatment of low turbidity waters. The coagulant was dosed immediately prior to the filter inlet, with subsequent flocculation and deposition occurring in the filter bed. This single stage treatment option was considered appropriate for developing countries, due to observed treatment performance, robustness of operation and reduced treatment costs. This work extends and complements previously successful studies on the treatment of medium to high turbidity raw waters using M.oleifera seed.;Laboratory studies using twin 100 mm diameter filter columns, were undertaken with the following variables: turbidities of 5-75 NTU; filtration rates of 5-20 m/h; filter depths of 70 and 120 cm; dual and single media beds, and media sizes of 0.50-1.00 mm and 0.85-1.70 mm. Deeper beds and smaller media were found to considerably reduce filtrate turbidity when using M.oleifera seed. The consequent headloss increase was only significant with the higher turbidity waters; dual media beds were most effective on such waters. Turbidity removal was reduced at higher filtration rates (10m/h), due to lower retention times in the bed, and increased detachment of retained particles causing early turbidity breakthrough. Higher filtration rates with another natural coagulant, chitosan, increased turbidity removal and prolonged the time to breakthrough, due to the reduction in surface removal in the filter. At lower loading rates (5 NTU at 5 m/h), removal with M.oleifera seed was comparable with chitosan and aluminium sulphate, with the additional advantage of a lower headloss. Optimisation of the hydraulic variables for a specific coagulant was considered necessary to ensure maximum output and filtrate quality.;Field trials on a low turbidity natural raw water indicated that M.oleifera reduced the turbidity by >95% and bacterial numbers by 100% at the optimum dose. Despite the rise in organic matter in the final water, trihalomethane levels were not excessive.