Various water treatment method psm

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Water Treatment Methods - PSM (Park's Textbook)

Source: Park's Textbook of Preventive and Social Medicine

Overview

Water purification in community medicine is classified into two broad categories:

Purification on a large scale (municipal/community level)
Purification on a small scale (household/emergency level)

LARGE-SCALE PURIFICATION

The method depends on the nature of raw water. Ground water (wells, springs) may need only disinfection, while surface water (river water) - which is turbid and polluted - requires extensive treatment.

The components of a typical water purification system are:

I. STORAGE

Water is impounded in natural or artificial reservoirs. Three types of purification occur naturally during storage:

(a) Physical: About 90% of suspended impurities settle by gravity within 24 hours. The water clarifies, allowing light penetration and reducing the workload on filters.
(b) Chemical: Aerobic bacteria oxidize organic matter using dissolved oxygen. Free ammonia decreases and nitrates rise.
(c) Biological: Bacterial count drops by up to 90% in the first 5-7 days. Pathogenic organisms gradually die out.

Optimum storage period for river water: 10-14 days. Excessive storage risks algal growth (bad smell and colour).

II. FILTRATION

Filtration removes 98-99% of bacteria, apart from other impurities. Two types are used:

A. Slow Sand (Biological) Filters

First used in 1804 in Scotland; still accepted as the standard method of water purification.

Elements:

Supernatant (raw) water - depth 1 to 1.5 m; provides constant head of pressure and acts as a settling/oxidation chamber (3-12 hours waiting)
Sand bed - about 1 metre thick; sand grains with effective diameter 0.2-0.3 mm (rounded, clean, free from clay)
Gravel support layer - 30-40 cm deep, prevents fine sand entering drainage pipes
Under-drainage system
Filter control valves

The Schmutzdecke (Biological layer):

A slimy, gelatinous layer forms on top of the sand within 8-10 days
Contains algae, diatoms, bacteria, and protozoa
This is the most important part of the filter - it is responsible for the removal of bacteria
Biological action by oxidation removes organic matter

Performance:

Parameter	Result
Rate of filtration	0.1-0.4 m/hour
Bacteria removal	Up to 99.9%
Turbidity of raw water limit	Should not exceed 50 mg/L (60 NTU)
Cleaning cycle	Every 1-3 months (scraping top 2 cm of sand)
Area of filter beds needed	Large (e.g., 1 acre for 4 million litres/day)

Advantages:

Simple construction and operation
Highly efficient bactericidal action
Low running costs
No chemicals required

Disadvantages:

Requires large land area
Cannot handle highly turbid water (>50 mg/L)
Slow rate of filtration
Lengthy and expensive cleaning process

B. Rapid Sand (Mechanical) Filters

Key differences from slow sand filters:

Feature	Slow Sand	Rapid Sand
Rate of filtration	0.1-0.4 m/hr	5-15 m/hr (~40x faster)
Sand grain size	0.2-0.3 mm	0.4-0.7 mm
Pre-treatment needed	Sedimentation only	Coagulation + sedimentation
Schmutzdecke	Present (essential)	Absent
Bacteria removal	Mainly biological	Mainly physical/chemical
Cleaning	Scraping top sand	Backwashing with water under pressure
Area required	Very large	Much smaller
Chemical use	None	Coagulants used

Coagulation (used before rapid filtration):

Coagulants (usually alum - aluminium sulphate) are added to water
These cause colloidal particles to aggregate into larger flocs (flocculation) that settle rapidly
Other coagulants: ferric sulphate, ferric chloride, sodium aluminate
After coagulation and sedimentation, water passes through the rapid sand filter

III. DISINFECTION (Chlorination)

"In water works practice, the term disinfection is synonymous with chlorination."

Chlorination is one of the greatest advances in water purification. It is a supplement, not a substitute to sand filtration.

Action of Chlorine:

H₂O + Cl₂ → HCl + HOCl
HOCl → H⁺ + OCl⁻

The disinfecting action is mainly due to hypochlorous acid (HOCl)
HOCl is 70-80 times more effective than the hypochlorite ion (OCl⁻)
Works best when pH is around 7 (HOCl predominates)
Unreliable at pH > 8.5 (90% ionized to hypochlorite)

Chlorine kills: Pathogenic bacteria, cholera, typhoid organisms Does NOT kill: Spores, certain viruses (polio, viral hepatitis) except in high doses

Secondary properties of chlorine:

Oxidizes iron, manganese, and hydrogen sulphide
Destroys taste and odour-producing substances
Controls algae and slime organisms
Aids coagulation

Principles of chlorination:

Water must be clear and free from turbidity first (turbidity impedes chlorination)
"Chlorine demand" must be met - chlorine used up by organic matter, bacteria, and chemical reactions
A free residual chlorine of 0.5 mg/litre must remain at the end of 1 hour contact

Methods of Chlorination:

Method	Details
Plain chlorination	Chlorine gas added at 0.5 ppm; used for relatively clean water
Superchlorination	Large doses (5-15 ppm) followed by dechlorination with sodium thiosulphate or activated carbon
Double chlorination	Chlorine added at intake AND after filtration
Break-point chlorination	Chlorine added until "break-point" is reached where all chlorine demand is satisfied; beyond this, free residual chlorine appears

SMALL-SCALE PURIFICATION

For individual households or emergency use:

(a) Boiling

Most reliable method of water purification
5-10 minutes of vigorous boiling kills all pathogens including cysts and spores
Kills all pathogenic organisms
Does not remove chemical impurities
Practical limitation: fuel cost, time

(b) Chemical Disinfection

(1) Bleaching Powder (CaOCl₂)

White amorphous powder with chlorine smell
Freshly made: ~33% available chlorine
Unstable - loses chlorine on exposure to air, light, moisture
Should be stored in dark, cool, dry, closed containers
Stabilized bleach = bleaching powder + excess lime

(2) Chlorine Solution

Prepared from bleaching powder: 4 kg (25% available Cl) + 20 L water = 5% chlorine solution
Subject to loss on exposure to light or prolonged storage

(3) High Test Hypochlorite (HTH / Perchloron)

Calcium compound with 60-70% available chlorine
More stable than bleaching powder

(4) Chlorine Tablets (e.g., Halazone tablets)

Convenient for small quantities
New NEERI (Nagpur) formulation: 0.5 g tablet disinfects 20 litres of water
15x better than ordinary halogen tablets

(5) Iodine

Used for emergency disinfection
2 drops of 2% ethanol-iodine per litre of clear water
Contact time: 20-30 minutes
Active over wide pH range; persists longer than chlorine
Disadvantages: high cost; physiologically active (affects thyroid)

(6) Potassium Permanganate

Previously widely used; no longer recommended
Powerful oxidizing agent but not satisfactory for disinfection
May kill cholera vibrios but ineffective against others
Alters colour, smell, and taste of water

(c) Filtration (Small Scale)

Water is filtered through ceramic filters:

Pasteur Chamberland filter - unglazed porcelain candle with pores 0.2-1 micron; removes bacteria
Berkefeld filter - kieselguhr (diatomaceous earth) candle
Katadyn filter - silver-impregnated ceramic; bactericidal action
The essential part is the "candle" through which water is forced under pressure or gravity

(d) Other Household Methods

Solar disinfection (SODIS): Clear PET bottles exposed to sunlight for 6-8 hours; UV + thermal inactivation
Alum (Phitkari): Traditional coagulant for removing turbidity before disinfection

Summary Table

Method	Scale	Removes	Does NOT Remove
Storage	Large	90% bacteria, turbidity	Chemical pollutants
Slow sand filtration	Large	99.9% bacteria, turbidity	Viruses (most), dissolved chemicals
Rapid sand filtration	Large	Turbidity, bacteria	Viruses, dissolved chemicals
Chlorination	Both	Bacteria, some viruses	Spores, polio, hepatitis viruses
Boiling	Small	ALL pathogens, cysts	Chemical impurities
Ceramic filters	Small	Bacteria	Viruses, chemicals
Iodine/HTH	Small/Emergency	Bacteria, most pathogens	Spores, cysts (variable)

Park's Textbook of Preventive and Social Medicine, Chapter on Environmental Health - Water

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