Filter Materials

The ideal filter medium should have a size and of such material that it provide satisfactory effluent, retain a maximum quantity of solids and be readily cleaned with minimum of wash water. The commonly used filter materials are

(a) Sand
(b) Anthracite
(c) Garnet sand
(d) Other locally available material

• Sand: If sand is to be used as a filter material, then it should have the following properties:
  (i) It should be free from dirt and other impurities.
  (ii) It should be uniform in nature and size.
  (iii) It should be hard and resistant.
  (iv) It should be such as not to loose more than 5% of its weight after being placed in hydrochloric acid for 24 hours.

Size of sand is measured and expressed by term called effective size or D₁₀. If sand is used as a filter medium, then the selection of correct effective size is very important because

• • Too smaller size of sand will lead to frequent clogging of filters and will given very low filtration rates.
  • Too large size of sand will permit the suspended particles and bacteria to pass thought it, without being removed.
• Anthracite: Crushed anthracite has been successfully used as filter medium in some filters. Although it is costlier than sand, but it has greater volume per unit weight and lower transportation cost per unit volume may make it more economical at some location.
• Garnet sand: It has higher specific gravity due to which it is a very dense material. However, due to its high cost, it is used in conjugation with other filter materials.
• Other materials: Other locally available materials, such as shredded coconut husks, burned rice husks, crushed glass, slag, metallic ores etc. have been used as a filter material in past.

Design Criteria of Slow Sand Filter

• It is designed for maximum daily demand.
• Minimum sand depth is kept as 0.4 m. However, preferred depth is 1 meter.
• Depth of filter tank = 2.5 – 3.5 m.
• Sand characteristics
  (a) Coefficient of uniformity, Cu = 3 – 5.
  (b) D₁₀ = 0.25 – 0.35 mm.
  (c) C_c = 1 – 3
• Rate of filtration = 100 – 200 l/hr/m².
• Free board = 0.2 – 0.3 m.
• Minimum units required = 2 (one working and one stand by)
• Number of beds of filter required depends on area as given in table.

Back Washing in Rapid Sand Gravity Filter

When sand becomes dirty, as indicated by excessive loss of head, the filter must be cleaned and washed. For cleaning, the raw supplies as well as filtered supplies have to be cutoff, bed drained down, and wash water sent back upwards through the filter bed. This forced upward movement of wash water and compressed air (if used) will agitate the sand particles and thus the suspended impurities from it will be removed. This is done in following steps:

(a) Valves 1 and 4 are closed and valves 5 and 6 are opened. The wash water and compressed air are thus forced from under-drainage through the gravel and sand beds.
(b) Valve 5 is closed now after supplying the required amount of air. The dirty water resulting from washing, overflows into wash water troughs and is removed by opening valve no. 2 through inlet chamber into wash water gutter. This complete process takes about 3 to 5 minutes.
(c) After washing of filter has been completed, valves 2 and 6 will be closed and valves 1 and 3 are opened. It restores the inlet supplies through valve 1, but filtered water in beginning is not collected and washed for a few minutes through valve 3 to gutter. It is necessary because remains of wash water must be removed from voids of filter and a surface mat must be allowed to be formed on sand. Finally, valve 3 is closed and valve 4 is opened to get filtered supplies again.

The entire process of backwashing the filters and re-maintain filtered supplies, takes about 15 minutes and filter units remain out of operation for this much of time. The amount of water required for washing a rapid gravity filter may vary from 2 to 5 percent of total amount of water filtered. The rapid gravity filters get clogged very frequently and have to be washed every 24 to 48 hours.
A figure of 45 cm/minutes is acceptable as rate of backwashing. A total of about 30 minutes are accepted to be consumed in backwashing. The rate of application of wash water may range from 200 to 600 litres/minutes sq.m.