Article Sidebar
Published:
Aug 3, 2004Keywords:
Filter deposits Filter residue Filter media Cleanliness Floc retention
Article Details
Issue
Section
Articles
Copyright for articles published in this journal is retained by the Water Research Commission.
Main Article Content
A standard test for filter media cleanliness
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-of agitation 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 550oC. 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.
Water SA Vol.30(1): 81-88
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-of agitation 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 550oC. 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.
Water SA Vol.30(1): 81-88
