Granular Media Pressure Filtration

Granular media pressure filters uses granular media to remove contaminants from water. Depending on the type of filter media used, granular media filtration has a variety of applications including:

filtration of water after chemical dosing to remove colloidal haze and turbidity

pre-filtration to protect membrane processes from fouling

filtration of cooling water to prevent heat exchanger fouling

filtration of borehole water to remove iron and manganese

removal of chlorine from water to protect ion exchange resins or membranes

increasing the pH of acidic water to prevent corrosion

feed water for drip irrigation systems

Process

Fig. 1 - Druck™ Granular Media Pressure Filters

In a granular media filter, the filter media is contained in a pressure vessel of glass reinforced plastic (GRP) up to about 1500mm diameter or steel for larger units (Fig. 1). The filter bed is typically 1 - 1.5m deep. The filter vessel usually has about 30% freeboard to allow for expansion of the filter bed during backwashing.

The usual filter medium is sand but other materials - anthracite, garnet, manganese dioxide, granular activated carbon, dolomite and others - are used for special applications. Water enters the top of the vessel and flows downwards through the bed of media and leaves through a collector system at the bottom. Contaminants are removed through straining and/or adsorption depending on the chosen media. Fig. 2 shows a schematic of a typical granular media pressure filter.

Water is passed through the filter until either the head loss across it rises to a preset value, or contaminant breakthrough is detected. It is then taken out of service and cleaned by backwashing with clean water. The backwash water enters the bottom of the filter vessel and flows upwards through the media bed, which starts to fluidize, that is the media grains begin to separate from each other and to float freely. The fluidization of the media releases the retained contaminants, a process which may be assisted by bubbling air through the bed (air scouring) which causes the sand grains to rub against each other dislodging the retained contaminants from the surface. The contaminants are then washed from the bed and out of the filter to waste.

Fig. 2 - Filter Schematic

The most commonly used filter media is 0.5-1.0mm sand but, where there is a high suspended solids load, a layer of coarse (1.2-2.5mm) anthracite can be added above the sand. The anthracite is less dense than the sand and remains as a separate layer during backwashing. This multimedia filtration (Fig. 3) approach can also be used if very fine filtration is needed by adding a layer of fine (0.3-0.6mm) garnet placed below the sand. Sometimes two stages of filtration are used - often anthracite and sand in the first vessel with sand and garnet as the second. This allows chemicals like polyelectrolytes to be added between the stages to improve filtration efficiency.

Granular activated carbon is widely used to remove chlorine from mains water supplies prior to treatment by ion exchange or reverse osmosis where it could cause damage to the resins or membranes. Dechlorination by activated carbon filtration also removes the taste associated with chlorine. Activated carbon filtration is also used to remove a wide range of organic chemicals including chlorinated hydrocarbons, pesticides and the species that cause taste, odor and color.

Fig. 3 -Multimedia Filters from Veolia Water Solutions & Technologies

Manganese dioxide is used as a catalytic filter bed to remove iron and manganese from borehole water for a range of industrial applications.

Dolomite or limestone chips are occasionally used in filters, either alone or as a separate layer above sand, to neutralize low pH waters. The media slowly dissolves and has to be replaced at regular intervals.

Because filtration is a batch process, filters are usually built in batteries consisting of two or more filters so that filtration to service can continue while one filter is being backwashed.

Performance

Influent suspended solids to granular media filters should not exceed about 20mg/l and preferably should be less than 10mg/l.

Granular media filters using a 1m deep bed of 0.5-1.0mm sand are usually specified to remove >98% of all particles larger than 2µm, but will usually achieve better performance than this. Typical filtered water suspended solids will be less than 2mg/l and turbidity will normally be less than 1NTU.

Depending on the particular application, filter runs are typically around 12 - 24 hours and backwashing takes about ½ -¾ hour.

The head loss through a filter depends on the rate of flow (filtration velocity), the size of the media grains and the depth of the bed.

Glossary

Granular media - The material used to effect filtration.

Backwash - Cleaning a granular media bed by means of an upward flow of clean water

Air scour - The use of air either alone or in combination with backwash to enhance filter cleaning

Filter run - The time of filter operation between backwashes

Granular Media Pressure Filters from Veolia Water Solutions & Technologies

Solution	Flow Rates	Water Treatment

Druck^TM ZMA Range	0.8 - 2.3 m³/hr	Iron and manganese
Druck^TM M Range	0.4 - 10 m³/hr	Suspended solids, iron, manganese, hydrogen sulfide, chlorine, phenol and organics
Druck^TM KF Range	4.2 - 94 m³/hr	Suspended solids, iron, manganese, hydrogen sulfide, chlorine, phenol and organics
Multimedia Filters	1.5 - 141.4 m³/hr	Suspended solids, iron, manganese, hydrogen sulfide, chlorine, phenol, arsenic and organics
Double Filtration in Series	1.5 - 141.4 m³/hr	Suspended solids, iron, manganese, hydrogen sulfide, chlorine, phenol, arsenic and organics