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Filtration is allowing water to pass through a thick layer of sand or other filtering media. By doing so, the suspended matter and colloidal matter in water are partially removed, the chemical characteristics of water are changed and the number of bacteria materially reduced.
The phenomenon of filtration though quite complex is generally
explained on the basis of following four actions. i)
MECHANICAL
STRAINING: This is responsible for removing such particles of suspended
matter as are too large to pass through the interstices between the sand
grains. ii)
SEDIMENTATION
& ADSORPTION: They account for the removal of colloids, suspended
and bacterial particles. The interstices between the sand grains act as
minute sedimentation basins in which the suspended particles smaller
than the voids in the filter bed settle upon the sides of the sand
grains. The particles adhere to the grains because of the presence of
gelatinous coating formed on the sand grains by the previously deposited
bacteria and colloidal matter. iii)
BIOLOGICAL
METABOLISM: It is the growth and life process of the living cells. The
surface layer gets coated with a zoological film in which the bacteria
activities are the highest and which feed on organic impurities
converting them by a complex biochemical action into simple, harmless
compound resulting in the purification of water. iv) ELECTROLYTIC ACTION: A certain amount of dissolved and suspended matter in water is ionized i.e. carries charge of one polarity and the particles of sand in filter which are also ionized possess electrical charges of opposite polarity. These neutralize each other and while so doing, change the chemical character of water. TYPES OF FILTERS: (A) Slow sand filter, (B) Rapid sand
filter (A) Slow sand
filter: MECHANISM (PRINCIPLE OF WORKING): While passing water through slow sand filter, the surface gets coated with a skin or layer formed due to the bacterial action of finely suspended matter, plankton and other organic matter present in raw water with algae, bacteria etc. previously coated on the surface of the sand bed by the raw water it self. This layer is called “SCHMUIZECKE”. Thus the principle action in slow sand biter is biological metabolism. “SCHMUIZECKE” completely oxidizes the organic matter, destroys most of the bacteria present and lets only simple and unobjectionable inorganic salts to pass through the filter bed into the effluent.
Fig. PLAN OF SLOW SAND FILTER
CONSTRUCTION: Slow sand filter consists of a water tight tank 2.5 – 3.5m in depth, having a sand bed 0.6 – 0.9m thick, supported on the bed of gravel 0.3 – 0.45m thick laid in 5 – 6 layers, beneath which the under drainage system is laid over a concrete bed sloping towards a central longitudinal drain. The under drainage system consists of open jointed drain of backed clay or concrete pipes. OPERATION (WORKING): The raw water is led gently on the filter bed, and percolation down wards passes through the under drains into an outlet chamber. The outlet chamber is provided with a regulating arrangement consists of a telescopic pipe and an adjustable weir plate in order to keep the rate of filtration constant. It is also equipped with a loss of head gauge operated with a float arrangement to measure the loss of head i.e. the difference in the water level in the filter and in the outlet chamber. The outlet chamber is so arranged as to prevent the possibility of negative pressures and resulting filter damage. CLEANING: For a freshly cleaned filter, the loss of head is 10 –150m. After some use the filter gets clogged, necessitating an increase in the filtration constant. This goes on till the maximum permissible limit of 60 – 90cm has been reached. The filter is now taken out of service. About 13 – 25 mm of the sand surface is carefully scrapped off. (B) Rapid sand
filter: OPERATION (WORKING): The water from coagulation tank enters the
filter unit through inlet pipe and is uniform distributed on the whole
sand bed water after passing through the sand bed is collected through
the under drainage system in the filtered water well. The outer chamber
in this filter is also equipped with filter rate controller. In the
beginning the loss of head is very small. But the bed gets clogged, the
loss of head increases and the filter bed requires its washing. Then the
filter is taken out of service for cleaning
(washing). CLEANING (WASHING OF FILTER): Filter the water is drained out from the filter leaving a few centimeters depth of water on the top of sand bed. The compressed air is passed through sand bed and stirs it well causing the loosening of the dirt, clay etc; inside the sand bed. Now the wash water from wash water tank rises through the channels, which are kept for this purpose. This washing process is continued till the sand bed appears clearly. EXAMPLE PROBLEM:
Obtain dimensions of rapid sand filter and slow sand filter to purify 5 MLD of water? I. Slow sand filter
design: i)
Rate of filtration
= R = 100 to 200 liters per m2 per hour
ii)
Length (L) / width
(B) = 1.25 to 1.33 being maximum site 12 m X 9 m. Given discharge (Q) = 5 MLD =
5X106 lit/day = (5X106/24)
lit/hour Assume rate of filtration R = 100 lit /
m2 / hour Filter area required = (Discharge / Rate of
filtration) = (5X106 / 24X100)
= 2083.34 m2 Assume L/B =1.25 ÞL=1.25 B \1.25 B x B = 2083.34
m2 B=40m But maximum allowable B =
9m. Hence provide 25 units. \Filter are per unit = (2083.34 /25) = 83.34
m2 Now 1.25 B x B = 83.34
m2 ÞB =8.16 m ÞL = 10.2 m \Provide 25 units of size 10.2 m x 8.2
m. II. Rapid sand filter design
criteria: i)
Rate of filtration
= R = 100 liters per m2 per min. ii)
Length (L) / width
(B) = 1.25 to 1.33 being maximum site 12 m X 9 m. Given discharge (Q) = 5 MLD =
5X106 lit/day = [5X106/(24x60)]
lit/min.
= 3,472.2 lit / min. Required Filter area = (Discharge / Rate of
filtration) = (3,472.2 X100)
= 34.72 m2 Assume L/B =1.25 ÞL=1.25 B 1.25 B x B = surface area = 34.72
m2 B = 5.27 m = 5.3 m L =1.25 x 5.3 = 6.625 = 6.7
m Provided one unit of 6.7m x 5.3 m COMPARISION OF SLOW
SAND AND RAPID SAND FILTER:
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