Retention of Bentonite in Granular Natural Pozzolan: Implications for Water Filtration

Slow sand filtration is wide spread as a rural drinking water treatment process in most of the developing countries, and is well known to become effective after a long period of filtration. This study evaluates the efficiency of granular pozzolans from Djoungo and Koutaba (Cameroon), at the beginning of a slow filtration process, using bentonite as a model colloid, under different physicochemical conditions. Experiments were conducted at various pH 5-95, 6, 7, 8, 9, different ionic strengths (10-4-3 · 10-2 M KCl) and various flow velocities up to 0.24 mm/s, with a filter grain size of 400-500 µm. The results show that the pH variation 5-95, 6, 7, 8, 9 has little influence on the retention of bentonite clay when the ionic strength is less than 10-2 M KCl; whereas conditioning the grains at 10-2 M KCl and pH 5 improves the efficiency of retention, increasing flow velocity results in a small decrease in retention efficiency. The three pozzolans tested gave similar retention efficiencies.