The phenomena on which filtration process removes bacteria, colour, taste, odours, iron, manganese and make water sparkling can be explained on the basis of following four actions: 1. Mechanical Straining2. Sedimentation 3. Biological Action 4. Electrolytic Action.
1. Mechanical Straining:
Sand consists of small pores, therefore suspended particles which are larger in size cannot pass through sand bed and are removed. Small particles of suspended impurities move through the pores in the sand, come in contact with sand surfaces and adhere causing further reduction in the pore size.
This increases the straining action. Unsettled floe from coagulation tank and settled particles form a mat on the top of sand bed which further arrests very fine suspended particles and removes them from the water.
2. Sedimentation:
The voids between the sand bed act as small settling basins. Very small particles of suspended matter, colloidal particles and some bacteria settle in these small settling basins and adhere to the sand particles due to the physical attraction between two particles of matter and because of the presence of gelatinous coating formed on the sand grains by previously deposited suspended matter.
3. Biological Action:
ADVERTISEMENTS:
Suspended impurities contain some portion of organic impurities such as algae, plankton etc. which are food of various types of micro-organisms. These organisms act on the organic matter and cause chemical and biological change in the water. These organic impurities form a layer on the top of sand bed which is known as ‘Schmutzdecke’ or ‘dirty skin. Micro-organisms live in this dirty skin and act on the organic impurities of water.
4. Electrolytic Action:
The sand particles of filter media and ionized matter in the water carry electrical charges of opposite nature; therefore they attract each other and neutralize the charge of each other. While doing so the chemical constituents of the water are altered. After long use the electric charge of filter sand is exhausted, which is renewed by washing the filter bed.
The top portion of sand bed is responsible for the filtration of water. If the water is more turbid and complete floe is not settled in coagulation basins, the surface of sand will be clogged in very short time by the formation of dirty skin.
The efficiency of filtration depends on the bacteria present below the dirty skin. Up to a certain limit dirty skin increases the efficiency of the filtration, but when thickness of it becomes too much it tends to form ”mud balls’ and considerably reduces the rate of filtration and also makes the work of washing the filter bed difficult.
ADVERTISEMENTS:
Sometimes it is asked that when only top portion of the sand bed takes active part in the process of filtration, what are the advantages of providing more thickness. One advantage of providing more than 50 cm thickness is that, it checks the passing of very finely livider particles in the initial stage of filtration. Second reason is that more thickness of sand bed results in uniform rate of flow. Third reason is that it provides a good support for the topmost layer of sand bed.