“BACKWASHING”-An act of cleaning the filter
Gone are those days when purification of water at large scale needs huge manpower and time. It would take days to clean the old type (slow sand Filters) cleaners/filters in water plants. Since the introduction of rapid sand filters in which chemical coagulants (Alum) are used rather than relying on formation of algae/biological layer over the filter sand bed. New system of rapid filtration works 24/7 without any delay which is very essential for the present fast world and also to meet the high demands of the people..
But filter clogging still become a problem as some amount of dirt remains in the water even after the application of alum which went through sedimentation and flocculation. Old type sand filters were cleaned with men working on the large sand filter bed and “scraping off” the top layer. Then the bed left as such for 1-2 weeks with stagnant water for the new layer of biological layer to get formed.
Here comes the simple and non-laborious process of filter cleaning in rapid sand filters- It is done using air pumps which direct high pressure air in reverse direction of normal flow. So it removes the impurities clogged on the top layer of sand bed. This can be done using air blower and a man to operate the switches and valve- much easier than the older one.
Cleaning the cleaner/filter is a fast paced/less laborious work nowadays without stoppage or delay of water purification system unlike previous slow sand/Biological filters. Newer filters can pump fresh water 10- 100 times faster than the conventional system. when comparing the quality of water the older ,biological method without any chemicals found to be better than the newer method. Rapid sand method has disadvantage of its odor and taste which is not well accepted by the public. Despite these short comings we cannot depend on the older system as our demand is huge and cant be met with the slow sand filters.
REFERENCE-K PARK ;TEXT PBOOK OF PSM;22ND EDITION