Abstract
Abstract:
A standard “floc retention test” was made available by the American Water
Works Association1 to provide a routine measure of filter media cleanliness.
This would allow early detection of the potential for serious problems. The test
is performed by preparing a 50g media sample and shaking it vigorously in
100ml of water. The resultant suspension is then decanted and, after five
repetitions, the turbidity of the combined suspension is measured, doubled
and reported as NTU (nephelometric turbidity units)/100g of sand. The
turbidity, according to AWWA guidelines, ranges from < 60 (clean filters) to
300 NTU/100g (filters with a possible mudball problem)2.
At many South African plants, filters appear to be inadequately cleaned by
routine backwash procedures. The hypothesis is that the root of this
phenomenon lies in the high degree of biological activity within filter beds.
This presumably results in a sticky biofilm on the media grains, which is
difficult to remove. The floc retention test proved to be an indispensable tool
for a systematic survey of water filtration plants currently being carried out to
test this hypothesis. To improve reproducibility and insight into the reasons for
media fouling, the floc retention test was refined in a number of ways, which is
the main focus of this presentation.