4.4. How to fill an RGF filter with AFM ®
Prior to placement of the filter material( s) tests for water retention must have been completed satisfactorily. Air scour testing and backwash pump functional tests should if possible 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 1 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 of the working material( s) should be marked in a waterproof medium to withstand submergence during washing.
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 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( with air or 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 providing that 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 10 % of the design layer depth if fluidising washes are proposed,
Dryden Aqua Ltd Butlerfield-Bonnyrigg,
Edinburgh EH19 3JQ Scotland Page 14 of 44
July 2015