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The treatments which are generally given to treat raw water supplies, follow the sequence
Most of the big and visible objects, such as trees, branches, sticks, vegetations, fish, animal life, etc., present in raw waters of surface sources can be removed by screening . The coarser suspended materials can than be removed by letting the water settle in sedimentation basins. This process is called plain sedimentation. The finer particles in suspension, which may avoid settling in sedimentation basins even after using chemical coagulation, may then be removed by filtering the water through filters. This process is called filtration. The filtered water which may still contain pathogenic bacteria, is then made bacteria- proof by adding certain chemicals such as chlorine, ozone etc. This process is called disinfection.
The settling velocity of inorganic particles of less than 0.1 mm dia., varies with the dia (d), in proportion to:
Settling velocity for particle size less than 0.1 mm,
Vs in mm/s,
d in mm
T temperature of water in °C,
The settling velocity of inorganic particles, larger than 0.1 mm dia, varies with the dia (d), in proportion to
For d between 0.1 mm to 1 mm,
For d > 1 mm
Sedimentation can remove inorganic particles, having specific gravity more than say:
The common particles present in turbid waters do have a specific gravity ranging between 2.65 for discrete sand particles to about 1.03 for flocculated mud particles. The particles having specific gravity more than 1.2 can settle easily, while the lighter particles do not settle easily.
Particles intended to be removed in a continuous flow water sedimentation tank, should have settling velocity:
In a continuous flow water sedimentation tank water enters from one end and comes out from the other end. The settling velocity more than surface loading rate will ensure that the time taken by sand particle to travel from one end to another is slightly more than the time required for settlement of that particle.
Detention time for a sedimentation tank (continuous flow type), is given for a tank, passing a discharge = Q, and having length = L, width = B, and depth = H, as:
The detention period (t) of a settling tank maybe defined as the average theoretical time required for the water to flow through the tank length. It is that time which would be required by the flow of water to fill the tank, if there were no outflows. In other words, it is the average time for which the water is detained in the tank. Hence, it is the- ratio of the volume of the basin to the rate of flow (or discharge) through the basin
∴ Detention time t for a Rectangular tank
The detention time for a water sedimentation tank may vary between
Detention time varies between 4 to 8 hours for plain sedimentation and from 2 to 4 hrs when coagulants are used.
For a given discharge, the efficiency of a sedimentation tank can be increased by
Increasing the surface area of tank at constant discharge will increase the detention time of settling particles and thus high removal efficiency can be achieved.
The amount of coagulant needed in water treatment, increases with the
Coagulants, used in water treatment, function better when the raw water is: