5. Operational Routines & criteria
5.1.
Turning off and starting up a media bed filter
AFM® media bed filters should be operated continuously; they must never be stopped or allowed to go
anaerobic. If the filters have to be turned off for a protracted period of time the following procedure
should be used prior to start up.
5.2.
Decommissioning and commissioning a filter
Prior to turning off an AFM® filter it should be given a standard backwash with clean water. After the
backwash the filter should be drained of as much water as possible.
Prior to going back-online, backwash the filter for a period of 5 minutes at a water flow rate that gives a
20% bed expansion followed by a rinse phase for 10 minutes. Repeat the backwash and rinse phase for a
2 nd and 3 rd time prior to going back on-line.
5.3.
Run Phase
The filtration performance of any media bed varies inversely in proportion to the velocity of water passed
through the filter. It is always best to operate the filter at the slowest possible flow rate in order to
maximise performance. This is particularly important in regions where there may be a high concentration
of cryptosporidium oocysts.
Different filtration media and sand from different countries / deposits will have a different performance
this is a function of particle size distribution, sphericity, chemical composition and uniformity coefficient.
Typically RGF fitters operate at 6m/hr and pressure filters at 12m/hr. AFM® filters should be operated at
exactly the same water flow rates.
Dryden Aqua Ltd
Butlerfield-Bonnyrigg,
Edinburgh EH19 3JQ Scotland
Page 16 of 44
The flow rate for an AFM® filter depends upon the design of the filter and the application. For most RGF
and pressure filter applications the filtration velocity should be below 12m/hr. This equates to a water
flow rate of 12m 3 /hr of water, for every 1m 2 of filter bed surface area. RGF filters will be operated at a
slower flow rate due to pressure head limitations, typi