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Experiments were conducted using a laboratory filter column to establish the best wash regimeand establish the nature, and extent, of filter medium fouling for filter sand taken from a watertreatment plant in London, United Kingdom. A range of different air-water combinations were investigatedstarting from those currently used and spanning a range of collapse-pulsing combinations.Chlorinated backwash water was also used to assess whether this could be effective in removingthe fouling layer. The maximum existing air and water flow rates used at the works were foundto be insufficient to give collapse-pulsing and did not show strong bed agitation or good cleaningin the laboratory. SEM analysis showed a biofilm on the sand grains, with bacteria and EPSmatrix growing over grain surfaces and a lot of bacteria in grain crevices. This biofilm was notbeing removed by the existing backwash regime. Best cleaning in the laboratory was bycollapse-pulsing, with a water velocity at 60% of vmf and an air velocity of 48m/h. This resultedin the cleanest grain surfaces, particularly after 20ppm chlorine was employed in combinationwith the air-water wash. The filter start-up head loss gradient was reduced by 18%. The sanddiscoloration did not change significantly during the experiments, even by employing 50ppmchlorine in the wash water. SEM analysis demonstrated that the dark brown colour of sand wasdue to the biofilm, dirt and some small amounts of manganese. This established biofilm was verystrongly attached to the sand grains. Includes 4 references, figures.

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Edition: Vol. - No. Published: 11/15/2004 Number of Pages: 8File Size: 1 file , 840 KB