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

Abstract

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 5–9, 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 5–9 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.