5.5.
How to backwash
Backwash water flow is critical for the continued high performance of AFM®. Backwashing is
accomplished by reversing the water flow through the filter bed to up-flow. The water flow should be
sufficient to fluidise the filter bed by 20% to 30%. The transition from run-phase to up flow on backwash
should be as gentle as possible and over a period of 30 seconds. It is important to lift the bed evenly and
gently for the most effective backwash.
Bed fluidisation should continue for a period of 3 to 5 minutes, or until the backwash water runs clear.
The backwash efficiency data presented results from tests carried out by an accredited laboratory
showing that AFM® has completed a backwash in less than 3 minutes.
5.5.1. Backwash velocity for pressure and RGF filters
The backwash water flow rate is a function of water temperature and water salinity or TDS.
•
Extra headroom is required for bed expansion of grade 0 which may otherwise be lost from the filter during
backwash
.
Table 3: Water type
AFM® backwash velocity, recommended flow rates for 20% bed expansion
Freshwater >25 o C
Freshwater <25 o C
Seawater at 35ppt >30 o C
Seawater at 35ppt 20 to 29 o C
Seawater at 35ppt < 20 o C
Sand 16 x 30
grade
>55m/hr
>50m/hr
>42m/hr
>38m/hr
>35m/hr
AFM® 1
>45 m/hr
>40m/hr
>36 m/hr
>33 m/hr
>30m/hr
*AFM® 1
with 20% AFM® 0
>45 m/hr
>40 m/hr
>36 m/hr
>33 m/hr
>30 m/hr
AFM® 0
(70% in filter)
>25m/hr
>23m/hr
>20m/hr
>18m/hr
>16m/hr
Dryden Aqua Ltd
Butlerfield-Bonnyrigg,
Edinburgh EH19 3JQ Scotland
Page 20 of 44
Seawater filtration is included in this report because filtration of seawater is required prior to desalination
by RO membranes to provide drinking water. The density of water is a function of TDS, the greater the
density the easier it is to fluidise the filter media and as such water flow rates can be reduced as TDS is
increased. The same logic applies to water temperature as warm water is less dense than cold water,
flow rates should be increased as water temperature is increased.