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Purification of Water on a Large Scale Filtration

Filtration
Filtration is the second stage in the purificatian of water and quite an important stage because 98 to. 99% of the bacteria are removed by filtration apart from other impurities.

The process of filtration may be of two types: (a) biological or slaw sand filter and (b) mechanical or rapid sand filter.

Slow sand or biological filters Slow sand filters for water treatment are in use since the 19th century. The elements of slow sand filter are depicted in Fig. 29.1 and include supernatant (raw) water, a bed of graded sand, an under-drainage system and a system of filter control valves.

elements-slow-sand-filter

1. Supernatant water (raw water) The supernatant water above the sand bed, whose depth varies from 1-1.5 m serves two important purposes. It provides a constant head of water so as to overcome the resistance of the filter bed and thereby promote the downward flow of water through the sand bed. Secondly, it provides waiting period of some hours (3-12 hours) depending upon the filtration velocity) for the raw water to undergo partial purification by sedimentation, oxidation and particle agglomeration. The level of supernatant water is always kept constant.

2. Sand bed/filter bed The most important part of the filter is the sand bed. The thickness of the sand bed is about 1.2 m. The sand grains are carefully chosen so that they are preferably rounded and have an' effective diameter' (0.15-0.35 mm). The sand should be clean and free from clay and organic matter. The sand bed is supported by a layer of graded gravel which also prevents the fine grains being carried into the drainage pipes.

The sand bed presents a vast surface area. Water percolates through the sand bed very slowly (taking 2 hours or more) and as it does so, it is subjected to a number of purification processes ¾ mechanical straining, sedimentation, adsorption, oxidation and bacterial action, all playing their part.

The advantages of a slow sand filter are:
Simple to construct and operate
.The cost of construction is cheaper than that of rapid sand filters
.The physical, chemical and bacteriological quality of filtered water is very high.
It has been shown to reduce total bacterial counts by 99.9-99.99% and E.coli by 99 to 99.9%

Rapid sand or mechanical filters It is popular in industrialised countries. Rapid sand filters are of two types, the gravity type an
d the pressure type. Both the types are in use.
schematic-representation

The following steps are involved in the purification of water by rapid sand filters.
1. Coagulation The raw water is first treated with a chemical coagulant such as alum, the dose of which varies from 5-40 mg/mole/litre depending upon the turbidity and colour, temperature and the pH value of water.

2. Rapid mixing The treated water is then subjected to violent agitation in a "mixing chamber" for a few minutes. This allows a quick and thorough dissemination of alum throughout the bulk of the water, which is very necessary.

3. Flocculation The next phase involves a slow and gentle stirring of the treated water in a 'flocculation chamber' for about 30 minutes. The mechanical type of flocculator is the most widely used. It consists of a number of paddles which rotate at 2-4 rpm. The paddles rotate with the help of motors. This slow and gentle stirring results in the formation of a thick, copious white flocculant precipitate of aluminium hydroxide. The thicker the precipitate or flock diameter, the greater the settling velocity.

4. Sedimentation The coagulated water is now led into sedimentation tanks where it is detained for periods varying from 2-6 hours when the flocculant precipitate together with impurities and bacteria settle down in the tank. At least 95 % of the flocculant precipitate needs to be removed before the water is admitted in the rapid sand filters. 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; otherwise they may become a breeding ground for molluscs and sponges.

5. Filtration: The partly cleared water is now subjected to rapid sand filtration.

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 be s occupy less space.

.Filtration is rapid, 40-50 times that of a slow sand filter.

.The washing of the filter is easy.
.There is more flexibility in operation.

 
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