Abstract
Abstract:
Rapid sand filters are expected to produce clean, safe water, without interruption, for many years. Unfortunately, filters very often
develop some problems during this time, most of which only become apparent when the damage is already done. Routine
measurement of filter media cleanliness could provide the necessary early warning, which is the reason why the American Water
Works Association (AWWA) suggested a media cleanliness test for inclusion in a structured filter assessment programme at
drinking water treatment plants. After performing such assessments at 3 South African water treatment plants, the authors found
that the results were not consistent, the turbidity could not be measured easily and the guideline values seemed to be excessively
conservative. This led to an investigation to find a method for stripping the filter deposits from the media grains with an easy,
reproducible method, and for characterising the stripped deposits.
After a series of tests on various filter media, using 6 stripping methods, 2 methods were identified that met the criteria in the
first objective. The first, a magnetic stirrer method is a mechanical agitation method and the second, referred to as the cylinder
inversion method, is a manual agitation method. Each of these methods was chosen on the basis of their operator and speed-ofagitation
independence.
The criteria in the second objective were met by an in-depth suspended solids (SS) analysis performed on the filter media
residue, with the total mass of solids removed from the media quantitatively separated on the grounds of acid solubility and
volatility at 550°C. Standard Methods (1985) 209C and 209D were applied to the sample, with and without acid addition, in order
to characterise the total filter media residue using 4 groupings: Soluble, non-volatile; soluble, volatile; non-soluble, non-volatile;
and non-soluble, volatile.
By meeting the criteria of these 2 objectives, the previous non-specific suggestions in the literature have been improved to
suggest the performing of tests in a well-specified, uniform way, the results of which can be internally compared.