8. How to back-wash a filter
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8. How to back-wash a filter
Back-washing an RGF( rapid gravity filter) or a pressure sand filter is a very simple procedure, however it must be conducted properly to ensure that the filter will have optimal performance during the run phase. This applies to all media bed filters, irrespective of the media contained within the filter vessel. The process has 6 stages.
1. Air purge 2. At the start of water back-wash, slowly wind-up water flow to 100 % 3. Back-wash water flow to achieve 10 % to 20 % bed expansion for single bed and multi media beds 4. Back-wash duration to insure completion of the back-wash process 5. Rinse phase to prevent solids entering the product water 6. Run phase
Air purge
Air purging a filter bed will help to scrub and clean the filter media. This is particularly important for filter media that proviode a good substrate for bacterial growth such as;
• Sand
• Most mineral based filter media
• Zeolites
• Activated carbon
Air purging was developed as a means or minimising the amount of back-wash water. For example, in the water industry a typical 16 x 30 grade Rapid Gravity sand filter, will normally be air purged at an air flow of approx; 60m ³/ hr of air per 1 m ² of filter bed surface area for a period of 5 to 10 minutes. The filter will then be back-washed at a flow velocity of 30m / hr( 30m 3 / hr per 1 m 2 of filter bed surface area) for up to 10 minutes.
A flow velocity of 30m / hr is insufficient to expand the bed, but it is sufficient to remove most of the solids that were trapped in the sand and lifted to the surface of the sand bed by the air purge. The operative word is“ most”, but not all the solids will be removed from the sand bed, some will remain and act as a food source for bacteria. Also, if the filter bed has a graded size range of media, then the air purge will serve to mix the grades, and because the water flow does not expand the bed, the grades will not re-classify. The net results, is that all the grades get mixed together in a filter bed that is not back-washed properly. Performance of the filter will therefore be compromised.
It is our opinion that an air purge should only be applied when filter media can be backwashed at a water flow that can expand the bed by at least 15 %, which is sufficient to re-classify the media.
An air purge is neither required nor recommended for AFM ®
At the start of a water back-wash, slowly wind-up to 100 % water flow
If 100 % water flow is immediately applied to a filter, there may be a water-hammer which could damage the pipework or the filter internals. The fluid hydraulics are never perfect, especially with filters using laterals in the“ spokes of wheel, radial” configuration, or with most horizontal filters.
Taking horizontal filters; at the start of a back-wash, water will rush into the filter towards the back-end and proportionally there will be a slightly greater flow velocity at the end of the filter and laterals. The water will therefore kick up the media at the end of the filter and at the edges. The pressure drop is reduced and more water will pass through the ends and sides of the filter vessel.
The filter now goes onto the run phase, and most of the water will initially go through the ends and sides of the filter, because it has been back-washed more effectively due to the high flow velocity. As the sides and ends block the water flow moves towards the centre of the filter. Next time the filter is back-washed slightly less water will go through the sides and ends, because there was more bio-coagulation due to earlier solids / organic penetration at the start of the run-phase. Gradually the flow pattern will shift towards the centre of the filter and the ends and edges will become compacted, coagulated and eventually blocked.