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Back-wash duration to ensure completion of the back-wash process
The back-wash must progress until the solids are removed from the filter bed, and evacuated from the water above the filter bed. To achieve this task, the bed must be expanded by at least 15 % to release the solids, if this is not achieved, the filter bed will never be cleaned, irrespective of how long the bed is back-washed.
A useful technique to evaluate backwash performance is to measure the back-wash profile. This is achieved by taking a sample of water at the very start of a back-wash, and then every 15 seconds until completion of the backwash.
If the filter media is stable and not subjected to compaction, or coagulation by bacterium or chemicals in the water, the back-wash profile will be a smooth curve. If the filter is not stable, as in the sand backwash profile( see right) there will be an irregular back-wash profile that can be attributed to coagulated lumps of media breaking up during the back-wash process. The area under the curve can be measured and is proportional to the total mass of solids discharged in backwash for each media tested; the greater the area, the more solids are discharged. This means that more solids were removed by AFM ® than by sand during the run phase in the above tests.
In most cases the back-wash will be complete within 300 seconds, however if the bed is not fluidised, the curve will be flat and very protracted. If there is deep solids penetration into the bed, or if there is a large head space above the media, then a longer back-wash will be required, not only to clean the media but also to evacuate all the water above the bed.
Percentage of particles eluted during backwash %
100.00 90.00 80.00 70.00 60.00 50.00 40.00 30.00
Back-wash efficiency
IFTS verified data AFM grade 1
AFM grade 0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 Time in minutes for back-wash
Rinse phase to prevent solids entering the product water, and back to Run phase
After the back-wash the filter bed needs to settle and compact slightly. Any dislodged solids near the base of the filter bed also need to be discharged to waste. Normally the rinse phase lasts only a few minutes however for, drinking water treatment to reduce the risk of solids such as Cryptosporidium parasites passing into the product water, or for discharge of solids that might foul or block a membrane, the water from an unstable sand filter needs to be sent to waste for up to 30 minutes before the filter can be placed back online.
If the back-wash profile defines the bed as unstable, then the rinse phase will need to up to 30 minutes in duration. The instantaneous filtration performance can also be measured. The following data from IFTS( Institute of Technical separation), presents measurements for AFM ® grade running at 20m. hr and sand at 20m / hr.
In the following two graphs, note the much higher performance of AFM ® in terms of percentage removal of 5 micron particles. Also note that the smallest particles measured with AFM ® was 2 microns compared to 5 microns with sand.