VPE 511: Environment and Environmental Hygiene (2+1)

FILTRATION

• Filtration is normally carried out by allowing the water flowing from the reservoir to percolate through sand, and two types of sand filters are recognized slow sand filters and rapid sand filters.
• The slow sand filters when first put into use act simply as strainers removing solid matter in suspension without retaining bacteria but after a time the sand grains become coated with a film of organisms which are effective in biologically purifying the water as it passes through the filter.
• Slow sand filters which have reached this condition are said to have “ripened” and such filters require careful supervision to maintain them in this ripened condition out of doors .
• Slow sand filters function efficiently , but as they only pass about 120 litres per square metre per hour they suffer from the obvious drawback that they need to be very extensive to cope with a large volume of water.
• In rapid sand filters the water drawn from the reservoir is forced through the filter under pressure, either by means of a pump or by maintaining a head of water above the filter.
• Suspended particles are removed but biological purification does not takes place. Purification is effected in this case by introducing a coagulant before filtration or by sterilizing with chlorine afterwards.
• Coagulants in the form of freshly precipitated hydroxides of many metals possess the property of adsorbing considerable quantities of organic matter and this property is made use of in removing organic contaminants from water, the metallic hydroxide in common use is aluminium hydroxide.
• In practice the aluminium hydroxide precipitate is formed in the water by adding to it either aluminium sulphate or sodium aluminate. In both cases the aluminium salt is hydrolysed to aluminium hydroxide which settles rapidly, dragging with it at the same time the organic matter dissolved or suspended in the water.
• Bacterial contamination is largely removal by this procedure and the removal of filter blocking particles permits of a more rapid and efficient filtration subsequently.
• Impure water when it has filtered is improved is appearance , colour, taste and odour. Its organic matter is reduced and its bacterial content may be reduced 99 per cent.
• The microorganisms that are pathogenic to man are more easily killed in the film than are the harmless ones and coliform bacilli which are always found in sewage are taken as the index of the efficiency of the filtration because if they are absent form filtered water it may rightly be presumed that pathogenic organisms are absent.
• If water containing much suspended solid matter or muddy water is habitually drunk by animals it leaves an even increasing deposit of silica, mica in the alimentary tract.
• This is a cause of colic, constipation and in the case of mica which forms a coating on the mucous membrane, malnutrition. Water of this nature can be roughly filtered by passing it through a gravel and sand bed or in case of emergency, through coarse canvas.