Filtration and backwashing studies using crushed palm kernel shells

Abstract

This study is a continuation of the efforts to establish crushed palm kernel shells alongside other locally available materials as a filter media to replace the use of imported sand as the filter medium for filtration. A constant head pilot filter (40 × 20 × 220 cm) was constructed with perforated pipe underdrains and pots for the monitoring of both turbidity and headloss values with depth and time. A 20 cm gravel bed was overlaid with a 55 cm bed of crushed shells made up of sizes 0.45 – 0.71 mm (65% by volume) and 0.71 – 0.85 mm (35% by volume). Two runs were carried out, each run having 5 sub runs, 1A to 1B and 2A to 2B. The filter was rested after each sub run and backwashed after Run 1B. A solution of Chlorine was added during the period of rest that followed runs 2C and 2D to verify if biological activities went on during the period of rest. It was found that the filter produced clean and odour free effluent of acceptable turbidity (0.5 NTU average), could operate up to 6 gpm/ft² but was limited to 2.8 gpm/ft² and developed headloss slowly. At the 40 h of filtration, the headloss was 43 cm of water which indicates that the length of filter run could exceed the 72 h upper limit in conventional rapid sand filters. Backwashing with 1.2% of filtered water at 15.8 gpm/ft² for 4.5 minutes produced a clean filter. The addition of Chlorine destroyed the gain of headloss that resting brought but created additional headloss which went down as filtration occurred.