Water purification

2. The steps in a typical water purification
system comprise one or more of the
following measures1. Storage2.
Filtration3. Disinfection• The actual steps
would depend upon1. the source of the
raw water and2. the desired standards of
water quality• E.g. ground water (like
springs, deep well) may need only
disinfection• On the other hand, surface
water tend to be turbid and polluted (e.g.
river water) and requires extensive
treatment Storage• Water is drawn out
from the source and stored in reservoir•
The reservoir may be natural or artificial•
Storage helps in two ways:a) Provides a
reserve of water from which further
pollution is excludedb) Considerable
amount of purification occurs as a result
of storing waterc) This purification is a
natural processd) This purification occurs
in 3 aspects i.e. physical, chemical and
biological• Physical quality
improvement:By mere storage, 90% of
suspended impurities settle down in 24
hours due to gravityWater becomes
clearer physicallyThis allows penetration
of light which further improves water
quality (hence reduces the load on filter)•
Chemical:
Certain chemical changes occur during storageThe aerobic bacteria oxidize the
organic matter present in the water with the aid of dissolved oxygenThis results in
reduction of free ammonia and Rise in Nitrates• Biological changesThis is one of
the greatest benefits of storageA tremendous drop in bacterial countThe pathogenic
organisms gradually die outIt has been seen that when river water is stored the total
bacterial count drops by as much as 90% in 5 – 7 daysThe optimum period of
storage of river water is about 10 – 14 daysIf the water is stored for longer periods,
algae might grow which imparts a bad smell and color to the water
2. FiltrationIn this stage, 98 – 99 % of the remaining bacteria get removedOther
impurities are removed tooOne of the two types of filters are may be useda.
Biological or the SLOW sand filter andb. Mechanical or the RAPID sand filter
DisinfectionThe final step in water purification, is disinfection of the filtered
waterMost practical disinfection at the large scale is chemical disinfectionChlorine is
the most widely used disinfectant at the large level