17
Horizontal Pressure filter, media ratios
Horizontal pressure filters provide more filter bed surface area at a lower cost than vertical pressure filters. However the
bed depth is usually lower, and because the bed depth varies across the diameter of the filter, there is a variable water
pressure gradient across the bed. The distance water travels through the filter bed from the outside is greater than the
distance from the centre of the filter to the under drains. The greater the distance the water travels, the greater the
pressure drop. This means the flow rate down through the centre of the filter bed will be faster when compared to the side
of the filter bed.
The differential flow of water through a horizontal filter leads to rapid biofouling of sand down the sides of the filter. When
the filter is placed in backwash the same situation happens in reverse, preferentially most of the water goes up through the
centre of the filter bed. This in turns means that more organic matter and food for bacteria remain in the sand bed down
the sides, which servers to accelerate the rate of biofouling and bed compaction.
Horizontal sand filters have the advantage that you get a lower cost filter for the surface area, but when used with sand,
performance may be seriously sacrificed. When AFM® is used in horizontal pressure filters, the biofouling, bed compaction
and channelling problems are solved, because AFM® resists biofouling and is much easier to backwash than sand.
Table 2. Horizontal pressure filters; AFM® filter bed depths as a percentage for each grade for
Horizontal Filters. Bed depth may range from 300mm to 1400mm
Grade Grade Grade Grade anthracite Backwash
3
2
1
0
velocity
With or without flocculation
lateral under-drain filter
with nozzle plate
lateral under-drain filter
20%
0%
20%
10%
10%
10%
70%
90%
10%
0%
0%
40%
20%
>45m/hr
>45m/hr
>20m/hr
0.6 to
1.18mm
nozzle plate filters
0%
10%
10%
50%
20%
> 20m/hr
0.6 to
1.18mm
lateral under-drain filter
20%
10%
50%
0%
20%
>45m/hr
1.18 to
2.5mm
nozzle plate filters
0%
10%
60%
0%
20%
>45m/hr
1.18 to
2.5mm
20%
0%
10%
10%
50%
70%
20%
20%
>45m/hr
>45m/hr
lateral under-drain filter 20% 10% 10% 60% >20m/hr
nozzle plate filters 0% 10% 10% 80% >20m/hr
Without flocculation
lateral under-drain filter.
nozzle plate filters
Grade 0 optimised filters