Filter media expansion during backwash: The effect of biological activity

Close observation at a number of South African water treatment plants has shown that media losses during backwashing are excessive – much higher than anticipated. The only likely reasons for this phenomenon are either that insufficient freeboard was provided by the designer or that the mechanical behaviour of the media gradually changes after being placed in the filters. A number of media tests confirmed that the biological fraction of the specific deposit on the filter media (after backwashing) is relatively high – about 50% of the total specific deposit. This led to the hypothesis that the combination of high nutrient concentrations in surface waters, coupled with elevated water temperatures, stimulate biofilm formation on the media grains. These films, in turn, somehow affect the mechanical behaviour of the media bed expansion and backwash. This paper reviews the Dharmarajah bed expansion model (as the most advanced model for media expansion to date) and presents evidence that it predicts the expansion of clean, ovendried media reasonably well. It further shows that media from filters which have been in operation for a while, expand significantly more than predicted by the Dharmarajah model. This finding has major implications for filter design, and suggestions are made on how to adapt design procedures for what is now believed to be the formation of biofilm on media grains.