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