5.4.
Filtration performance and grade of filter media
The standard grade of filter media used in most pressure and RGF filter is a sand of 16 x 30 mesh size,
AFM® grade 1 is equivalent to this grade of sand. However, because AFM® does not biofoul this gives
the opportunity to use a much finer grade of media, grade 0 which has a particle size distribution 0.25 to
0.50mm. This is half the size of 16 x 30 sand and grade 1 AFM®.
Particle size removal preformance of AFM® at 20m/hr no coagulation or
flocculation
100.0
95.0
90.0
AFM® 0
85.0
AFM® 1
80.0
Sand
75.0
70.0
1
3
4
5
10
15
20
25
30
35
40
Size of particles in Microns
Figure 3: Filtration performance of AFM® and sand at 20m/hr
Dryden Aqua Ltd
Butlerfield-Bonnyrigg,
Edinburgh EH19 3JQ Scotland
Page 19 of 44
Refer to Table 1, AFM® grade 0 may be used as a 20% layer on top of grade 1, or as a 70% layer. AFM®
grade 0 is used when filtration down to 1 micron is required without the use of coagulation and
flocculation chemicals. AFM® grade 0 may be used with electrolytes, but when used with polyelectrolytes
there is a risk of blinding the filter bed. AFM® grade 0 is therefore suitable for pre-treatment prior to
membranes or when high quality water is required.