Measuring the cleanliness of filter media

Abstract: Filter media starts its working life as almost pure silica, freshly crushed, sieved, washed and dried. Upon examination a few years later, we find discoloured, often sticky material hardly recognisable as the original. As long as the media retains its granular character and the filter beds stay smooth and level, this is no cause for great concern. However, when the media forms clumps, when cracks become apparent in the bed or the filtrate quality deteriorates for no apparent reason, the media demands closer attention. Such media investigations have been carried out for more than a decade at the Water Research Group of the University of Johannesburg. Drawing on this reservoir of experience and case studies, this paper reviews the different approaches to measuring and expressing the degree of cleanliness of in situ filter media. A conceptual model of the different types of specific deposit on the media is developed first, classifying the specific deposit into those fractions that are washed out by the treatment plant backwash system, the fraction hat can be additionally washed out by a laboratory column, the fraction that needs mechanical agitation to be stripped off the media, and the fraction that can only be chemically stripped. Typical values for the four fractions, as measured during a comprehensive survey of South African water treatment plants, are presented, together with suggested remedial measures for each of the media fractions