médiazur BV and BG dual-media filters

Reading time: 12 minutes

These filters are loaded with two layers of different materials (e.g. sand-anthracite), with a high water head and a filtration rate of between 7 and 20 m · h^–1. Dualmedia filters are usually air-scoured and then washed with water; however, a simultaneous air scour and backwash water is possible providing the filters are adapted.

The letters BV and BG respectively indicate :

B : dual-media

V : V shaped crosswash channel

G: Channels

The Médiazur BV is the degremont® standard for the dual-media in an open filter with a V shaped crosswash channels.

The Médiazur BG is the degremont® standard for the dual-media in an open filter with suspended channels.

Their design is identical to that of the Aquazur V filters with the exception of the backwash and feed mode.

The Médiazur BG and BV share the following features :

a high water head over the filter bed, equal to at least 1 m and, in most cases, 1.20 m.
total depth of several media usually between 0.9 and 1.6 m.
one or more valves or air plug valves totally cutting off the supply of water to be filtered during the drainage and air scour phases. In the case of the Médiazur BV, the supply is only partially cut off during the rinse, in order to create a limited velocity surface crosswash;
a high backwash water flow rate, depending on the materials constituting the filter bed; this water is used to fluidise and grade these materials.

The following constitutes the normal wash (air followed by water) sequence :

drainage down to the filtering media level;
the air blanket is formed;
air scour only (55 to 70 m³·h^–1·m^–2);
air blanket blowdown;
water only rinse, at high speed, designed to expand the filter bed, to flush out the impurities dispersed throughout its depth using scour air, and to regrade the media; in the case of the Médiazur BV filters, this wash phase takes place as water to be filtered is fed through the feed channels for the purpose of crosswashing the surface, thereby accelerating the removal of impurities.

Médiazur BG filters, depending on their width and on rinsing velocity, are equipped with several transverse troughs in order to avoid any overspeed at the weir crest and to prevent any media from being drawn away into the drains during the high speed rinse sequence.

Médiazur BV backwashing systems may include a simultaneous air scour and water wash phase by adapting the filter design. The following constitutes the backwash sequence :

drainage down to the filtering media level;
the air blanket is formed;
scouring by simultaneously injecting air (55 to 70 m³·h^–1·m^–2) and water (7 to 15 m³·h^–1·m^–2) which is switched off before the water reaches the level of the weir (scouring without overflow);
air blanket blowdown followed by high speed water rinse, without surface crosswash, in order to expand the filter bed and discharge the air before any overflow takes place at the weir;
high speed water only rinse to maintain filter bed expansion and regrade the medias, with surface crosswash (5 to 7 m³·h^–1·m^–2) to accelerate the removal of impurities.