because the bed will usually expand by that amount as it stratifies. This is not the case with combined air
and water unless a high rate rinse is included.
Do not charge second or third layers in multimedia filters until the lower layers have been washed and
skimmed.
4.5. RGF Bed Conditioning Procedures
Do not air scour a freshly installed bed.
The first step, unless the material was placed in to a flooded filter, is to gradually initiate a backwash. If
the media shows signs of floating allow it to stand for a few hours or overnight with the water level below
the wash out cill.
Raise the backwash rate to the maximum design value and continue until the water clears. Skim off any
extraneous material. The wash can then be repeated with air scour according to the intended procedure.
The same is true for combined air water washing.
In cases where there is no water available until the first filter has been commissioned then the first filter in
the freshly charged condition may be allowed to filter slowly before the first wash but this is a last resort
dispensation and may cause some penetration of fines in to the under drain. Temporary arrangements to
fill the clean wash water tanks are recommended in this case.
After 3 back washes, skim the surface of the filter to remove fines. The process should be repeated until
no further accumulation occurs. It is not necessary to skim AFM® beds, but with new installations there
may be other debris in the system that needs to be removed.
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. Such skimming is less important
with combined air and water washing unless high rate re-grading washes are employed. The air tends to
prevent stratification and also carries fines over in to the washout channel (launder).
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 has to 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.
Dryden Aqua Ltd
Butlerfield-Bonnyrigg,
Edinburgh EH19 3JQ Scotland
Page 15 of 44
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.