After 3 back washes, skim the surface of the filter to remove fines. The process should be repeated until no further accumulation occurs.
Only after this state has been reached should any additional layers be placed. The washing / skimming procedure will need to be repeated with the second( and third) layer.
After washing and skimming, the material level in the filter may require adjustment. It can be quicker to transfer washed material from another filter so that only one bed must be rewashed and skimmed after further topping up. The designated level of the material is attained after washing, when the filter is ready for service. The support layers do not expand and settle.
The final bed should be sampled in depth to ensure that the size grading is as intended. Surface layers will tend to be finer than deeper layers and a composite sample is necessary unless a combined air / water wash is used.
A filter should not be placed on-line within the network until the product water has been tested for compliance with water quality parameters detailed in annexe 1.
Rapid gravity filter
Table 3. Rapid Gravity Filters AFM ® filter bed depths as a percentage for each grade for Horizontal Filters, Bed depth may range from 300mm to 1400mm
20