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Rapid Gravity filters media ratios
How to fill an RGF filter with AFM®
Prior to placement of the filter tests for water retention must have been completed satisfactorily. Backwash pump functional
tests should have been completed. If this is not possible before one filter is operational then it is preferable that only one
filter be charged with media. However, circumstances may not always permit such an ideal sequence.
The filter and associated filtered water ducts and channels, pipes and clean wash water tank must be physically clean and
free from loose dirt and other extraneous matter especially polystyrene from form work and plastic wrapping materials.
Ideally, the filter should be vacuum cleaned especially if the discharge arrangement from the filter has fine slots or nozzles.
The filter bed is prepared in the same way as if sand was being used as the filter and / or support material. However, less
AFM ® Grade 1 (by weight) is required compared to sand due to the lower bulk density of AFM ® Grade 1.
AFM ® Grade 1 (0.40 – 1.00mm) is used to replace 16/30 sand.
The walls of the filter should be marked with the levels of each layer and on larger filters suitable gauge sticks may be
erected. The expected quantities of material for each layer should be calculated and included in the method statement.
Table 3 provides the recommend percentages of the different grades of media. The various layers will be levelled off with
levelling boards or raked level to levelling strings. The levels should be marked in a waterproof medium to withstand
submergence during the first backwashes.
Each support layer should be protected after placing and walking boards used for access. Footprints cause compaction of
the support layers and can cause mal-distribution during backwashing. Local humps act as trigger sites for boiling and
spouting.
Support layers should preferably be lowered on to a board within the filter and then spread. They should not be tipped over
the side, particularly on exposed lateral floors where the laterals can be displaced. Tipping over the side can also displace
previously installed layers. The support layers do not expand during a back-wash and should be laid to design depth.
The working layer (grade 1 or grade 0) should also be placed carefully to avoid displacement of the top support layer. If the
working layer is to be delivered by hose (only with water) the hose should be kept off the top of the previous layer. After
this layer, has been placed the situation is less critical. Walking on the working layer after 150mm has been placed is of less
concern if the bed is dry. The working layers need not be levelled accurately. The backwash procedure will perform this task.
The working layers should be left short by 20% of the design depth to accommodate room for bed expansion during back-
wash.
Do not charge second or third layers in multimedia filters until the lower layers have been washed and skimmed.
RGF Bed Conditioning Procedures.
Do not air scour AFM® rapid gravity filters! Air will displace the support grades and will not be reclassified during a
back-wash.
The first step, unless the material was placed into 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 over a period of 60 to 90 seconds, and continue until the water clears.
Skim off any extraneous material. The wash can then be repeated with air scour as per the intended procedure.
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.