Filtration removes fine particles, turbidity, colloidal matter, dissolved minerals, and microorganisms.
Types of Filters:-
A. Slow Sand Filter
- Generally designed rectangular having L/B ratio of 1.5 to 3
- Depth of filter is 2.5 to 3.5 m
- Plane area of the filter is 100 to 2000 m2 having its’ size of the order of 30m *60m
- 2 ≤ 1 of sand ≤ 4mm
- Coefficient of uniformity D60/D10= 3 to 5
- Design period =10 years
- Depth of sand is 90 to 110 cm
- Below the sand gravel-based of 30 to 75cm thick is used.
- The frequency of cleaning is 1 to 3 months.
- Rate of filtration =very slow 100 to 200lit/hr/m2
- The efficiency of bacteria removal = 98 to 99%
- It can be removed turbidity up to 50 ppm.
- Cleaning of the filter is done manually once in 1-3 months in which top 1.5 cm to 3 cm layer is scrapped and washed manually.
- It is designed for maximum daily demand.
- The number of filter beds required to depend on the total filter area but usually 1 filter bed is provided at the stand by
B. Rapid Sand Filter
- The number of Filter unit is computed using as N = 1.222√Q
- Water to be treated must be Coagulated and Sedimented
- Sand layer depth is about 75 cm.
- Below the sand gravel-based of 60 to 90cm thick is used.
- 1 of sand is 0.35 to 0.55 mm.
- Depth of tank = 2.5 m to 3.5 m.
- Area = 10 to 80 m2 each unit.
- The rate of washing is 15 to 90 cm rise/ minute.
- Rate of filtration 3000 to 6000 lit/hr/ m2 approx 30 times of slow sand filter.
- Less efficient in the removal of bacteria (80- 90%)
C) Pressure Filter
Pressure filters are just like small rapid gravity filters placed in closed vassals, and through which water to be treated is passed under pressure.
Rate of filtration – 6,000 to 15, 000 liter/hr/m2
is less efficient than the rapid gravity filters, in removing bacteria and turbidities.