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At many South African water treatment plants, filters appear to be inadequately cleaned byroutine backwash procedures. The hypothesis is that the root of thisphenomenon lies in the high degree of biological activity within filter beds.This presumably results in a sticky biofilm on the media grains, which isdifficult to remove. The floc retention test proved to be an indispensable toolfor a systematic survey of water filtration plants currently being carried out totest this hypothesis. To improve reproducibility and insight into the reasons formedia fouling, the floc retention test was refined in a number of ways, which isthe main focus of this presentation:moisture content correction where moisture content of filters is dependent on thetime elapsed between draining and sampling and is highly variable (5 to28%);agitation by cylinder inversion vs. vigorous shaking where tests determined thatthis method yielded lower values but resulted in more reproducible results;gravimetric vs. nephelometric measurement of deposits where the former isless dependent on the nature of the particles in suspension;splitting deposits in terms of acid solubility and volatility which indicates thenature of the deposits (biological, chemical or inorganic); and,measurement of the elemental makeup of deposits where the suspension isdissolved in acid and its elemental composition is measured.The suggested refinements were put to the test during a survey of eight SouthAfrican plants. Large differences in overall media cleanliness were found,ranging from 1 to 20 mg of deposits/g of media. It was, however, by splittingthe deposits into different categories (acid-soluble, volatile and inert) wherethe improved procedures came into their own. The volatile fraction (biological)ranged from 10 to 60% of the total solids, loosely corresponding to theeutrophic status of the raw water. The acid-soluble fraction (chemicalprecipitates) ranged from 1 to 79% suggesting improper pretreatment and/orpoor backwashing. Where the elemental analyses showed significant iron andmanganese concentrations (in addition to the ubiquitous presence of calciumand magnesium), it could directly be related to the raw water problems.The refined floc retention test proved to be a significant improvement to theold, assisting operators in the detection of potential problems, understandingtheir nature, and suggesting focused solutions. Includes 5 references, tables, figure.

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Edition: Vol. - No. Published: 11/15/2004 Number of Pages: 12File Size: 1 file , 540 KB