Media properties and their effect on filter performance and backwashing

Granular filter media used in water treatment have a range of physical properties. Media are usually selected on the basis of size, but bed behaviour is also affected by density and voidage of the media, particularly during backwashing. The fluidisation and backwashing bed behaviour of a number of filter media have been investigated. It has been found the fluidisation is affected by the bed packing arrangement which is determined by previous treatment. Filter media may be subject to attrition during backwashing which will affect the grain size and voidage. Combined air and water flow rates for collapse-pulsing are also affected by the media characteristics. When collapse-pulsing is used to clean triple media filters, complete mixing of the layers occurs, but restratification is possible using a high rate fluidising water wash. Backwashing is an important part of the filtration process so understanding the effect of media properties on the process is essential for optimisation.