33
The above is a generalisation, the flow hydraulics, bio-coagulation and compaction will vary between different filter designs.
However, for all filter designs and types of media, if the filter is not back-washed properly filter performance will always be
compromised. It is important to use a high-quality filter media such as AFM®. It is equally important to use a properly
designed filter vessel and to operate the equipment properly.
Time in seconds to 100%, maximum back-wash water flow
Type of filter
German DIN standard vertical filter
30
with nozzle plate
Vertical filter with standard lateral
60
arrangement
Horizontal filters with nozzle plates
90
or laterals
Back-wash water flow to achieve 10% to 20% bed expansion
The minimum back-wash velocity should expand the bed by 10%, but 20% or more bed expansion is recommended
especially for multimedia beds. If activated carbon is used, because it has a lower density than sand, the 20% bed expansion
may result in a lower flowrate than the figures given below which equate with 20% expansion for a single media.
The back-wash velocity depends on several factors, including;
•
•
•
•
•
Particle size distribution
Uniformity coefficient
Density of the media
Water temperature
TDS / salinity of the water
Filter media
AFM® grade 0 (0.25 to
0.50mm)
AFM® grade 1 (0.4 to
1.0mm)
Silica Sand 16 x 30 grade
10%
8
10 deg C
20%
12
20 deg C
30 deg c
10%
9 20%
14 10%
10 20%
16
31 45 36 55 42 57
39 50 46 58 53 66
Water flow velocity m/hr for 10% and 20% bed expansion at different water
temperature