Filtration, as applied to water treatment, is the process of removing Suspended Particles like Particulate matter, Colloidal particles and other impurities from Water by passing water through Granular material. In most cases it is the last stage in the pre treatment of water for removal of undissolved impurities prior to treatment by Ion Exchange or by Reverse Osmosis. Suspended solids also include very finely divided particles of colloidal dimensions (of size less than 1 micron. These particles result in haziness and make the water turbid. While large particles would settle down easily, the finely divided particles do not settle .Finely divided particles carry a negative charge on their surface and as like charges repel each other, the particles do not collide with each other and therefore do not settle. Coagulation, flocculation and clarification precede filtration. Suspended solids tend to clog Equipment and impair the efficiency of downstream units like Ion Exchange or Reverse Osmosis. The materials used in filters is normally a bed of Sand, Coal, or Other granular substance.
Classification of Filters
Based on the filter media
- Sand filters, e.g. Natural Silica Sand
- Anthracite filters, e.g. Crushed Anthracitic coal
- Diatomaceous Earth Filters, e.g. Diatomaceous earth
- Metal fabric filters (micro strainers), e.g. stainless steel fabric filter.
Based on the Depth of Filter Media
- Surface Filters – Single Media Sand Filter
- Depth Filters, – Dual‐Media and Multi‐Media (combination of two or more media)
Based on the rate of filtration
- Gravity Filters
- Rapid Sand Filters
- High Rate Filters
- -Dual media
- Pressure Filters
- -Sand or Multi-media
The actual removal mechanisms are interrelated and rather complex, but removal of colour and turbidity is based on the following factors:
- chemical characteristics of the water being treated
- nature of suspension (Physical and Chemical characteristics of Particulates suspended in the water)
- Type and Degree of Pre-Treatment (Coagulation, Flocculation, and Clarification)
- Filter type and Operation.
The removal mechanisms include the following processes:
- Sedimentation on media (sieve effect);
- Biological action
Types of Filter
Pressure Sand filters
A typical Pressure Sand Filter consists of a Pressure Vessel-this could be either vertical or horizontal-fitted with a set of frontal pipe work and valves, graded sand supported by layers of graded under bed consisting of pebbles and silex, a top distributor to distribute the incoming water uniformly throughout the cross section of the filter, and an under drain system to collect filtered water.
Raw water flows down wards through the filter bed and as the suspended matter- which has usually been treated by addition of a coagulant like alum- is retained on the sand surface and between the sand grains immediately below the surface. There is steady rise in the loss of head as the filtration process continues and the flow reduces once the pressure drop across the filter is excessive.
Multigrade filter is a depth filter that makes use of coarse and fine media mixed together in a fixed proportion. This arrangement produces a filter bed with adequate pore dimensions for retaining both large and small suspended particles. This filter performs at a substantially higher specific flow rate than conventional filters.
Dual Media filters
Anthracite is used in combination with sand. In this way higher filtration rates in excess of 10m/h are achieved in typical dual media filter consists of a layer of anthracite resting on a layer of fine sand. Anthracite is coarse material and has more dirt holding capacity as compared to sand.
Specific flow rates of 15 – 30 m/h have been successfully obtained for treating waters containing 25 – 50 ppm suspended solids respectively to produce filtrate with less than 5 ppm in both Multigrade Filters and Dual Media Filters
Gravity Sand Filters
Conventional Gravity Filters are units, which are used for filtering gravity flows of water, and which incorporate a nest of valves for the operation of the different functions of filtering, backwashing and rinsing. These units may be made of concrete, steel, or wood, but concrete is the material that is almost universally employed. Also, the rectangular form is the one that is most widely used, because, in that way, the walls of the inner units in any one battery can be made common to one another. Their application is generally limited to the municipal treatment field where high flow rates of water must be treated. The filter effluent water in such a case is usually Pre treated in a coagulation and/or settling basin.