The filter has certain valves and devices, incorporated in the outlet-pipe system. The purpose of these devices is to maintain a constant rate of filtration. An important component of the regulation system is the “Venturi meter” which measures the bed resistance or ‘loss of head’. When the resistance builds up, the operator opens the regulating valve so as to maintain a steady rate of filtration. When the ‘loss of head’ exceeds 1.3 metre it is uneconomical to run the filter.
Filter cleaning: Normally cleaning is not required for weeks or even months. When the bed resistance increases to such an extent that the regulating valve has to be kept fully open, it is time to clean the filter bed, since any further increase in resistance is bound to reduce the filtration rate. For cleaning, the supernatant water is drained off, and the sand bed is cleaned by "scraping" off the top portion of the sand layer to a depth of 1 or 2 cm by an unskilled labour. After 20 or 30 scrapings, when the thickness of the sand bed reduces to about 0.5 to 0.8 meter, the plant is closed down and a new bed is constructed.
The advantages of a slow sand filter are:
- simple to construct and operate
- construction cost is less than that of rapid sand filters
- the physical, chemical and bacteriological quality of filtered water is very high.
When working ideally, slow sand filters have been shown to reduce total bacterial counts by 99.9 to 99.99 per cent and E. coli by 99 to 99.9 per cent.
Rapid sand or mechanical filters
Rapid sand filters have gained considerable popularity in highly industrialized countries (Fig 3). Rapid sand filters are of two types, the gravity type and the pressure type.
Steps involved in the purification of water by rapid sand filters
|Fig 3 : View of Rapid Sand Filter
- Coagulation: The raw water is first treated with a chemical coagulant such as alum. Based on the turbidity, colour, temperature and the pH of the water the dose of alum is decided (5-40 mg/litre).
- Rapid mixing: The treated water is subjected to violent agitation in a "mixing chamber" for a few minutes to allow thorough dissemination of alum throughout.
- Flocculation: The treated water is stirred slowly and gently in a "flocculation- chamber" for about 30 minutes. The mechanical type of flocculator is most widely used. It consists of a number of paddles which rotate at the speed of 2 to 4 rpm with the help of motors. This slow and gentle stirring results in the formation of a thick, copious, white flocculent precipitate of aluminium hydroxide. Thicker the precipitate or flock diameter, greater the settling velocity.
- Sedimentation: The coagulated water is allowed into sedimentation tanks where it is detained for 2-6 hours when the flocculent precipitate together with impurities and bacteria settle down. At least 95 per cent of the flocculent precipitate needs to be removed before the water is admitted into the rapid sand bed filter. The precipitate or sludge which settles at the bottom is removed from time to time without disturbing the operation of the tank. For proper maintenance, the tanks should be cleaned regularly from time to time to avoid becoming a breeding place for molluscs and sponges.
- Filtration: The partly clarified water is now subjected to rapid sand filtration.
Characteristics of Filter beds:
- Surface area - 80 to 90 m2 (about 900 sq ft).
- Filtering medium - sand
- Size of sand particles - 0.4-0.7 mm
- Depth of sand bed - 1 metre (2 ½-3 ft)
- Graded gravel - 30-40cm (1-1½ ft) deep
The gravel supports the sand bed and permits the filtered water to move freely towards under-drains. The depth of the water on the top of the sand bed is 1.0 to 1.5 m (5-6 ft). The under-drains at the bottom of the filter beds collect the filtered water. The rate of filtration is 5-15 m3/m2/hour.
Filtration: As filtration proceeds, the "alum-floc" gets collected on the sand bed and forms a slimy layer similar to zoogleal layer in the slow sand filters. It adsorbs bacteria from water, effects purification and oxidizes ammonia. As filtration proceeds, the suspended impurities and bacteria clog the filters and may become dirty and inefficient. When the "loss of head" approaches 7-8 feet, filtration is stopped and the filters are subjected back-washing.
Back-washing: Rapid sand filters need frequent washing daily or weekly, depending upon the loss of head. Reversing the flow of water through the sand bed is called back-washing. Back-washing dislodges the impurities and cleans up the sand bed. The washing continued till clear sand is visible and the wash water is sufficiently clear. The whole process of washing takes about 15 minutes. In some rapid sand filters compressed air is used as part of back-washing processes.
The advantages of a rapid sand filter over the slow sand filter are:
- Rapid sand filter can deal with raw water directly. No preliminary storage is needed
- The filter beds occupy less space
- Filtration is rapid 40-50 times that of slow sand filter
- The washing of the filter is easy
- There is more flexibility in operation.