At many South African water treatment 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:
moisture content correction where moisture content of filters is dependent on the
time elapsed between draining and sampling and is highly variable (5 to
28%);
agitation by cylinder inversion vs. vigorous shaking where tests determined that
this method yielded lower values but resulted in more reproducible results;
gravimetric vs. nephelometric measurement of deposits where the former is
less dependent on the nature of the particles in suspension;
splitting deposits in terms of acid solubility and volatility which indicates the
nature of the deposits (biological, chemical or inorganic); and,
measurement of the elemental makeup of deposits where the suspension is
dissolved in acid and its elemental composition is measured.
The suggested refinements were put to the test during a survey of eight South
African plants. Large differences in overall media cleanliness were found,
ranging from 1 to 20 mg of deposits/g of media. It was, however, by splitting
the deposits into different categories (acid-soluble, volatile and inert) where
the improved procedures came into their own. The volatile fraction (biological)
ranged from 10 to 60% of the total solids, loosely corresponding to the
eutrophic status of the raw water. The acid-soluble fraction (chemical
precipitates) ranged from 1 to 79% suggesting improper pretreatment and/or
poor backwashing. Where the elemental analyses showed significant iron and
manganese concentrations (in addition to the ubiquitous presence of calcium
and magnesium), it could directly be related to the raw water problems.
The refined floc retention test proved to be a significant improvement to the
old, assisting operators in the detection of potential problems, understanding
their nature, and suggesting focused solutions. Includes 5 references, tables, figure.
| Edition : | Vol. - No. |
| File Size : | 1
file |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 12 |
| Published : | 11/15/2004 |
