Design principles of slow and rapid sand filters

FILTRATION

In order to remove very fine suspended particles like flocculated particles (due to coagulation) and bacteria the water after undergoing the processes of sedimentation and flocculation is filtered through beds of fine granular material such as sand. The process of passing the water through beds of such granular material is known as filtration. Filtration may help in removing 

• Colour
• Odour
• Turbidity
• Some pathogenic bacteria from water

Theory of filtration
Filters  purify water using the below listed processes.

• Mechanical straining Suspended particles in water that are bigger than the voids in the filter media get arrested. Most particles get arrested in the upper sand layers and form a mat on top of the bed which further helps in straining out impurities
• Flocculation and sedimentation Void spaces in the media act like tiny coagulation-sedimentation tanks. The colloidal matter
• Biological metabolism
• Electrolytic changes 

The two types of filters commonly used to treat municipal water supplies are

• Slow sand gravity filters and
• Rapid sand gravity filters

• Slow sand filters have a very slow rate of filtration, require a large area and are expensive. Due to all these disadvantages, slow sand filters are slowly becoming obsolete. 
• Slow sand filters utilise effluents from plain sedimentation tanks and are used for relatively clearer waters
• A slow sand filter consists of the following parts
1. Enclosure tank is an open water-tight rectangular tank made of masonry or concrete with the bed slope about 1 in 100 towards the central drain. Depth varies from 2.5m to 3.5m 
2. Filter media consists of sand layers about 90cm to 110cm in depth. It is placed over a gravel support. Effective size of sand varies from 0.2mm to 0.4mm with a uniformity coefficient of 1.8 to 3. The top 15cm of sand is of finer variety than the rest, which is of uniform grain size. In case, sand of different grades is used, the sand with fine grain is used at the top with coarser grains towards the bottom. Very fine sand produces purer water
3. Base material is gravel which supports sand. It is made up of 30cm to 75cm thick gravel of different sizes and placed in 15cm to 20cm deep layers with coarsest gravel at the bottom and fineest gravel at the topmost layer. The intermediate layers are made of gravel size varying between 20mm and 40mm. The top-most layer has sand of size 3mm to 6mm.
4. Under-drainage system The gravel support is laid on top of an under-drainage system which consists of a central drain and lateral drains. The laterals are open jointed pipe drains placed 3m to 5m apart and sloping towards a central drain. The laterals collect the filtered water and discharge it into the main drain. 
5. Inlet and outlet arrangements An inlet chamber is constructed for allowing the effluent from the sedimentation tank without disturbing the sand layers of the filter and distributing the water uniformly over the filter bed. A filtered water well is constructed on the outlet side in order to collect the filtered water. Inlets and outlets are governed by automatic valves
6. Other appurtenances such as vertical air pipe, gauge, meter, etc are used efficient functioning of filters. The vertical air pipe helps in proper functioning of filtering layers. Meter is used to measure the flow and gauge is used to measure the loss of head. When the loss of head becomes high, the filter unit must be put out of service and be cleaned.

 The treated water from sedimentation tank enters the inlet chamber of filter unit and gets uniformly distributed over the filter bed. The water percoletes through the filter media and gets purified during the process of filtration. The water then percolates through the gravel layers and comes out as filtered water. The filtered water passes through open joints and discharges into the main drain. The rate of filtration is kept constant by arrangements such as telescopic tube.

• The water entering the slow sand filter SHOULD NOT  be treated by coagulants
• The depth of water on filter should be approximately equal to depth of filter sand

 The loss of head called filter head or filtering head or filteration head is limited to (0.7 to 0.8 times depth of filter sand) a maximum of 0.7m to 1.2m, the filter unit must be put out of service and the filter be cleaned

The filter should be cleaned by scrapping and removing 1.5cm to 3cm of top sand layer. The top surface is raked, roughened, cleaned and washed with good water. Cleaning is repeated as often as necessary until the sand depth is reduced to almost 40 cm. More sand is added after this.

After each cleaning, filter is used and raw water is admitted. Water obtained in the first 24 to 36 hours is not used until a film called schmutzdecke consisting of impurities is formed. The filtering action of slow sand filters depends on the formation of this film.

The interval between two successive cleanings depends on nature of impurities and size of sand used as filter media. This interval normally ranges between one to three months.

The rate of filtration obtained from slow sand filters is small and ranges between 100 to 200 litres/hour/sq.m of filter area.

Slow sand filters are highly effective in removing bscteria and suspended solids from raw waters. Bacteria removal is upto 98 to 99% or more. These filters remove odour and taste caused by presence of organic impurities. However, they are not very effective in removal of colour and odour.

Due to low rate of filtration, they require large areas and large volume of filtering material making them costly and uneconomical, especially for treating large volumes of water. They may be preferred for smaller plants for village supplies for under-developed countries


Rapid sand filters or Rapid gravity filters

• These filters employ coarse sand with effective size of 0.5mm or more. These filters may yield as high as 30 times the yield given by slow sand filters. Waters from coagulation-sedimentation tanks are used in these filters and filtered water is treated with disinfectants to obtain potable water supplies.
• The various units of the rapid sand filter are discussed below
1. Enclosure tank consists of a rectangular tank made of masonry or concrete. The depth varies from 2.5m to 3.5m. In order to achieve uniform distribution of water, the area of the filter unit should be about 10 to 80m2.
2. The filtering media consists of sand layers about 60 to 90cm depth and placed over gravel support. The size of sand varies from 0.35mm to 0.55mm. The sand is laid in layers with fine sand at the top and coarser variety towards the bottom
3. The base material is gravel in both slow and rapid sand filters. However in rapid sand filters, in addition to supporting sand, the gravel distributes the wash water. It consists of 60cm to 90cm thick gravel, placed in layers. Five to six layers of 10cm to 15cm in depth are used. The most coarse gravel (40mm size) is used in the bottom most layer and the finest gravel (3mm size) is used in the top-most layer. Size of gravel in the bottom most layer is generally 20mm to 40mm. The intermediate layer has gravel of size 12mm to 20mm and the 6mm to 12mm if two intermediate layers are used. The size of gravel in the top-most layer is 3mm to 6mm.
4. Under-drainage system has two important purposes
1. To receive and collect the filtered water
2. To allow back-washing for cleaning the filter