VWS Westgarth Office Contact

Orbital House
3 Redwood Crescent
East Kilbride
G74 5PR

+44 (0)1355 588 000
Fax: +44 (0)1355 588 001


Veolia Water Oil & Gas Angola Contact:

Viana Park
Pavilhão nº1 Quarteirão 9
Estrada do Calumbo
Pólo Industrial de Viana
República de Angola

Tel: +244 940 120 949
Tel: +244 949 229 399


Multi Media Filtration

Dual media filters

The process of multi media filtration is well established and is the most commonly used pre-filtration system. Filtration is achieved by passing the feed water through a filter bed comprised of granular material. Dosing of coagulant/flocculent upstream of the filter is usually required to maintain filter efficiency.

The media used to effect the filtration can be pumice, sand, anthracite or garnet depending upon the quality of filtrate required.

Regular backwashing of the media is essential to ensure consistent filtrate quality. Backwashing is carried out either on a timer (usually every 24 hours) or on high differential pressure (circa 1 bar). During backwashing, the filter is closed off from forward flow and the liquid drained down to level with the top of the media. There then follows a series of flows of water and air up through the media fluidising the media and releasing the trapped solids. The backwashed fluid is routed to drain throughout this procedure. Before recommencing filtration the media has to be settled back into place, this is carried out by forward flow through the media for a period of up to 1 hour. During the backwashing cycle the remaining filter compartments process the entire forward flow thus preventing any reduction in filtrate.

As can be expected from the above description multi media filters are heavy and have a large footprint.


Typically the onshore multi media filters are cylindrical vessels situated in a horizontal orientation. The use of horizontal vessels allows the cylindrical section to be compartmentalised i.e. contain 2 or more discrete filter beds. The benefit of these compartments is that they significantly reduce the water volume and flowrate required for backwashing.

The backwash fluid is typically filtrate that has been diverted into a dedicated backwash tank prior to being used for backwashing. To minimise total water consumption it is VWS Westgarth practice to utilise some of the fluid from the rinse cycle of the backwash to fill the backwash tank.


Typically the offshore multi media filters are cylindrical vessels situated in a vertical orientation. As a result they do not lend themselves to multiple compartments.

To avoid the installation of backwash tanks and pumps the source of the backwash water is usually the unfiltered water, taken from a tapping upstream of the multi media filters. Whilst this does have an impact upon the water quality it is generally accepted in the trade-off of CAPEX vs. OPEX (relating to the cartridge filters downstream and the chemical cleaning of the SRP membranes).