Purification of Water on Large Scale

The purpose of large-scale water treatment is to produce water that is safe and wholesome. Surface water (e.g., river water) which is turbid and polluted requires extensive treatment. The process involves three main stages:

I. Storage

Water from the source is impounded in natural or artificial reservoirs. This achieves natural purification in three ways:

(a) Physical: About 90% of suspended impurities settle down by gravity within 24 hours. The water becomes clearer, allowing light penetration and reducing the load on filters.
(b) Chemical: Aerobic bacteria oxidize organic matter using dissolved oxygen. Free ammonia content decreases and nitrate levels rise.
(c) Biological: Bacterial count drops by up to 90% in the first 5-7 days. Pathogenic organisms gradually die out. The optimum storage period for river water is 10-14 days. Prolonged storage may encourage algal growth, causing bad odour and colour.

II. Filtration

Filtration is the second stage and removes 98-99% of bacteria, along with other impurities. Two types of filters are used:

(a) Slow Sand (Biological) Filters

First used in 1804 in Scotland.
Work through a biological film (Schmutzdecke) - a gelatinous layer of algae, diatoms, and bacteria that forms on top of the sand bed.
This layer physically strains particles and biologically oxidizes impurities.
Rate of filtration: 0.1-0.4 m/hour (very slow).
Remove 99% of bacteria; highly effective but require large land area.

(b) Rapid Sand (Mechanical) Filters

Work at a much faster rate: 5-15 m/hour.
Require prior coagulation - raw water is treated with a chemical coagulant such as alum (aluminium sulphate), dose 5-40 mg/L.
Alum reacts with water to form aluminium hydroxide floc [Al(OH)₃], which traps colloidal particles, bacteria, and suspended matter and settles them out (sedimentation/flocculation).
Water then passes through sand and gravel layers; filters are backwashed every 24-48 hours.
Remove about 98-99% of bacteria.

III. Disinfection (Chlorination)

After filtration, the water is disinfected to kill remaining pathogenic organisms. Chlorination is the most widely used method:

Chlorine is added as chlorine gas, bleaching powder [Ca(OCl)Cl], or sodium hypochlorite.
A residual chlorine level of 0.2 mg/L (0.2 ppm) after 30 minutes contact time ensures safety (WHO standard).
Chlorine acts by releasing nascent oxygen which kills bacteria and other pathogens.
Excess chlorine is removed before distribution (dechlorination).
Other disinfection methods include ozonation and UV radiation.

Summary Table

Stage	Process	Result
Storage	Natural sedimentation, oxidation	90% bacteria removed
Filtration	Slow/rapid sand filters	98-99% bacteria removed
Disinfection	Chlorination	Remaining pathogens killed

Key point: Ground water (wells, springs) may only need disinfection, while surface water (rivers) requires all three stages.

(Source: Park's Textbook of Preventive and Social Medicine)

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