The document discusses the significance of water purification and the standards for drinking water, emphasizing the importance of accessible, safe, and non-contaminated water for health in developing countries. It outlines large-scale and small-scale purification methods, including storage, filtration, and disinfection processes, while detailing specific techniques such as slow sand filters and chlorination. Additionally, it highlights water quality criteria set by WHO to minimize health hazards associated with contaminated water.

1. Water Purification and
Standards for Drinking Water
Dr. Khem R. Sharma,
Associate Professor

2. Water
• Much of the ill-health which affects developing countries can be traced to
lack of safe and wholesome water supply.
• Water should be easily accessible, adequate in quantity, free from
contamination, safe and readily available throughout the year.
• There are three main sources of water:
1) RAIN
2) SURFACE WATER
• Impounding reservoirs
• Rivers and streams
• Tanks , ponds and lakes
3) GROUND WATER
• Shallow wells
• Deep wells
• Springs

3. Safe and wholesome water
• Water that is:
a) free from pathogenic agents
b) free from harmful chemical substances
c) pleasant to the taste (free from colour/odour)
d) usable for domestic purposes.
• Water is said to be polluted or contaminated when it does not fulfil the
above criteria and needs to be purified.
• Purification of water has great importance for improving health of the
community and can be considered under two headings :
1) Purification on a large scale. 2) Purification on a small scale.

4. LARGE SCALE WATER PURIFICATION
• To produce safe and wholesome water, treatment is undertaken and
the method employed depends upon the nature of raw water and the
desired standards of water quality.
• Ground water (wells and springs) may need no treatment other than
disinfection while surface water (river water) tends to be turbid and
polluted and requires extensive treatment.
• A typical water purification system utilize these measures :
I. Storage II. Filtration III. Disinfection

5. I. Storage
• Provides a reserve for water drawn out from the source and
impounded in natural/artificial reservoirs to prevent its pollution.
• This is natural process where considerable amount of purification
takes place by the following processes.
1) Physical:
- 90 % of suspended
impurities settle down
in 24 hours by gravity,
allowing light to
penetrate
2) Chemical:
- Aerobic bacteria oxidize the
organic matter with the aid
of dissolved 02 resulting in
reduction in free ammonia
and rise in nitrates
3) Biological:
- Total bacterial count
drops by as much as 90 per
cent in the first 5- 7 days.
- Algae growths may impart
a bad smell and colour

6. II. Filtration
• Second but quite an important stage in the purification of water (98-
99 % of bacteria are removed by filtration, apart from impurities).
• Two types of filters are in use:
A. Biological/Slow sand filters
B. Mechanical/Rapid sand filters

7. A. SLOW SAND / BIOLOGICAL FILTERS
• Documented first use for water treatment in 1804 in Scotland.
• Generally accepted as the standard method of water purification.
• Elements of a slow sand filter
1) Supernatant (raw) water;
2) Bed of graded sand;
3) Under-drainage system; and
4) System of filter control valves.

8. (1) Supernatant water
• The supernatant water above the sand bed is 1 - 1.5 meter deep and
serves two important purposes:
1) provides a constant head of water to overcome the resistance of
the filter bed and thereby promote the downward flow of water
through the sand bed;
2) it provides waiting period for the raw water to undergo partial
purification by sedimentation (3 to 12 hours depending upon the
filtration velocity).

9. (2) Sand bed
• Most important part of the filter, about 1 meter thick
with carefully chosen round sand grains with
effective diameter between 0.2 and 0.3 mm.
• Sand bed represents a vast surface area for filtration
which should be clean and free from clay & organic
matter and is supported by a layer of graded gravel
(30-40 cm deep) which prevents sand entering into the drainage pipes.
• Water percolates through the sand bed very slowly (0.1 - 0.4 m3/hour /m2
of sand bed surface) and is subjected to a number of purification steps like
mechanical straining, sedimentation, adsorption, oxidation & bacterial
action, all playing their part.

10. Vital/zoogleal/biological layer/Schmutzdecke
• When the filter is newly laid, it acts merely as a mechanical strainer,
and cannot truly be considered as "biological".
• The surface of the sand bed gets covered with a slimy growth of algae
plankton, diatoms and bacteria over time that extends for 2 to 3 cm
into the top portion of the sand bed when fully formed.
• The "heart" of the slow sand filter and removes organic matter, holds
back bacteria and oxidizes ammoniacal nitrogen into nitrates and
helps in yielding a bacteria-free water.

11. (3) Under-drainage system
• It consists of porous or perforated pipes at the bottom of the filter
bed, that serve the dual purpose of providing an outlet for filtered
water, and supporting the filter medium above.
• Filter Box: It consists of the first 3 elements of slow sand filters, the
supernatant water, sand bed and under-drainage system.
• These are contained in an open box 2.5 to 4 m deep built wholly or
partly below ground with walls of stone, brick or cement.

12. (4) Filter control
• Valves and devices are incorporated in the outlet-pipe system to
maintain a constant rate of filtration .
• An important component of the regulation system is the Venturi
meter which measures the filter bed resistance or "loss of head".
• When resistance builds up in filter bed, the regulating valve is opened
to maintain a steady rate of filtration, until it has to be kept fully open
indicating the time to clean the filter bed.
• The supernatant water is drained off and 1 - 2 cm of the sand bed is
"scraped" off as it is not economical to run the filter when the "loss of
head" exceeds 1.3m.

13. B. RAPID SAND / MECHANICAL FILTERS
• First used in 1885 in USA
• Steps in the purification of water by rapid sand filters:
1) Coagulation
2) Rapid mixing
3) Flocculation
4) Sedimentation
5) Filtration
Rapid Sand Filtration plant

14. Steps in the purification of water by rapid sand filters
1) Coagulation: The raw water is first treated with a chemical
coagulant such as alum (5 to 40 mg or more/l) depending upon its
turbidity, colour, temperature and pH value.
2) Rapid mixing: The treated water is subjected to violent agitation in a
"mixing chamber" for a few minutes which allows the alum to
quickly and thoroughly disseminate, which is an important step.
3) Flocculation: The next phase involves a slow and gentle stirring of
the treated water in a "flocculation chamber" for about 30 minutes
that results in the formation of a thick, copious, white flocculant
precipitate of aluminium hydroxide .

15. Steps in the purification of water by rapid sand filters…
4) Sedimentation: The coagulated water is detained in sedimentation
tanks for 2-6 hours when the flocculent precipitate together with
impurities and the bacteria settle down in the tank.
- At least 95 % of the precipitate needs to be removed before the
water is admitted into the rapid sand filters. -
The sludge that settles at the bottom is removed from time to time
without disturbing the operation of the tank
5) Filtration: The partly clarified water is now subjected to rapid sand
filtration.

16. Filter beds
• Each unit of filter bed has a surface of
about 80 -90 m2, depth of 1m and sand
as the filtering medium with particles
between 0.4-0.7mm.
• Below the sand bed is a layer of graded gravel (30 - 40 cm deep) that
supports the sand bed and permits the filtered water to move freely
towards the under-drains.
• The under-drains at the bottom of the filter beds collect the filtered
water and the rate of filtration is 5-15 m3/m2/hour.
rapid sand filter

17. Filter beds…
Filtration: As filtration proceeds, the "alum-floc" not removed by
sedimentation is held back on the sand bed and forms a slimy layer
(comparable to zoogleal layer) that adsorbs bacteria from the water.
• Filtration is stopped when the "loss of head" approaches 7- 8 feet as the
suspended impurities and bacteria clog the filters, and the filters are
subjected to backwashing.
Backwashing: Reversing the flow of water through the sand bed dislodges
the impurities and cleans up the sand bed that takes about 15 minutes.
The washing is stopped when clear sand is visible and the wash water is
sufficiently clear, while compressed air is used as part of the backwashing
processes in some rapid sand filters.

18. SLOW versus RAPID SAND FILTERS
RAPID SAND FILTERS SLOW SAND FILTERS
Space Occupies very little space Occupies large area
Rate of Filtration 200 m.g.a.d* 2-3 m.g.a.d.
Effective size of sand 0.4- 0.7 mm 0.2- 0.3 mm
Preliminary treatment Chemical coagulation and sedimentation Plain sedimentation
Washing By back-washing By scraping the sand bed
Operation Highly skilled Less skilled
Loss of head allowed 6-8 feet (2- 2.5 m) 4 feet (1.5 m)
Removal of Turbidity Good Good
Removal of Colour Good Fair
Removal of Bacteria 98-99 per cent 99.9- 99.99 per cent
* - million gallons per acre per day

19. III. Disinfection
• Disinfection of water supplies requires that the disinfectant agent
fulfill the following criteria :
a) should be capable of destroying the pathogenic organisms present,
within the contact time available and not unduly be influenced by
the range of physical and chemical properties of water encountered
(temperature, pH and mineral constituents);
b) should not leave products of reaction which render the water toxic
or impart colour or otherwise make it unpotable;

20. III. Disinfection…
c) have ready and dependable availability at reasonable cost
permitting convenient, safe and accurate application to water;
d) possess the property of leaving residual concentration to deal with
small possible recontamination
e) be amenable to detection by practical, rapid and simple analytical
techniques in the small concentration ranges to permit the control
of the efficiency of the disinfection process.
• In water works practice, the term disinfection is synonymous with
chlorination.

21. CHLORINATION
• One of the greatest advances in water purification and is a supplement,
but not a substitute to sand filtration.
• Chlorine has a germicidal effect & kills pathogenic bacteria along with
oxidizing iron, manganese and hydrogen sulphide, destroying some taste
and odour producing constituents and control algae, slime organisms and
aids coagulation.
• H2O + Cl2 →HCI (neutrlized by water alkalinity) + HOCl (hypochlorous acid)
• HOCI →H + OCI (hypochlorite ions)
• The disinfecting action of chlorine is best at a pH of around 7 which
results in a predominance of hypochlorous acid which is the main
disinfectant, with only a small role played by hypochlorite ions.

22. Principles of chlorination
• Measures undertaken to ensure proper chlorination
1) The water to be chlorinated should be clear and free from turbidity as it
impedes efficient chlorination
2) Chlorine demand of the water: The difference between amount of
chlorine added and residual chlorine remaining after the contact period
(60 minutes), at a given temperature and pH of the water.
- Amount of chlorine needed to destroy
bacteria and oxidize all organic matter & ammoniacal substances present
in water.
- Break-point: Point at which the chlorine demand of the water is met and
further addition produces free chlorine in water (HOCl and OCl).

23. Principles of chlorination…
3) The presence of free residual chlorine for a contact period of at
least one hour is essential to kill bacteria and viruses.
4) The minimum recommended concentration of free chlorine is 0.5
mg/L for one hour which provides a margin of safety against
subsequent microbial contamination during storage and
distribution.
5) The sum of the chlorine demand of the specific water plus the free
residual chlorine of 0.5 mg/L constitutes the correct dose of
chlorine to be applied.

24. Chlorination methods to disinfect large water bodies
• Chlorine is applied as (1) chlorine gas (2) chloramine or (3) perchloron
1) Chlorine gas - the first choice as it is cheap, acts quickly, efficiently and easy to
apply but it is poisonous and irritates the eyes, so, a special equipment -
chlorinating equipment is required.
2) Chloramines - (loose compounds of chlorine and ammonia) give a more persistent
type of residual chlorine but are slow to act and are not being used to any great
extent in water treatment.
3) Perchloron or high test hypochlorite (H.T.H.) is a calcium compound which carries
60- 70 per cent of available chlorine.
Super-chlorination followed by de-chlorination is applicable to heavily polluted waters
and comprises the addition of large doses of chlorine to the water with removal of
excess of chlorine after disinfection.

25. Assessment of Chlorination
• ORTHOTOLIDINE (OT) TEST: Developed in 1918, it determines with speed and
accuracy the free and combined chlorine in water.
• Analytical grade Orthotolidine dissolved in 10 % solution of HCl added to water
containing chlorine, turns it yellow and the intensity of the colour varies with
the concentration of the gas.
• It is essential to take the reading within 10 seconds after the addition of the
reagent to estimate free chlorine in water and the yellow colour produced is
matched against suitable standards/colour discs.
• ORTHOTOLIDINE-ARSENITE (OTA) TEST: modification of the OT test to determine
the free and combined chlorine residuals separately without interference from
substances such as nitrites, iron and manganese which also give a yellow colour.

26. Other agents
• Although chlorine continues to be the most commonly used sterilizing
agent (germicidal properties, low cost and ease of application), its
alternatives are being sought because chlorination of water can lead
to the formation halogenated compounds which are known or
suspected carcinogens.
• Ozonation: Ozone gas (O3), a powerful oxidant, is formed by passing
dry air/O2 through a high-voltage electric field with the resultant
ozone-enriched air dosed directly into the water providing a 10-20
minutes of contact time.

27. Other agents…
• Ozone reacts with natural organics and increases their biodegradability
(measured as assimilable organic carbon), but ozonation is followed by
biological filtration/granular activated carbon (subsequent treatment) to
remove biodegradable organics and introduce a chlorine residual, as ozone
does not provide a disinfectant residual.
• Membrane Processes: Reverse osmosis, ultrafiltration, microfiltration and
nanofiltration are processes that have been applied traditionally to the
production of water for industrial/pharmaceutical applications, but are now
being applied to the treatment of drinking-water.
1) High-pressure processes: Reverse osmosis, nanofiltration
2) Lower-pressure processes: ultrafiltration, microfiltration

28. SMALL SCALE WATER PURIFICATION
1) Household purification of water:
a) BOILING: "rolling boil " for 10 to 20 minutes kills all bacteria, spores, cysts and ova and
yields sterilized water.
b) FILTRATION: ceramic filters such as Pasteur Chamberland filter, Berkefeld filter and
"Katadyn" filter
c) ULTRAVIOLET IRRADIATION: UV rays have germicidal properties that are effective against
most pathogens known to contaminate water. -Exposures
are short but it has no residual effect in the treated water.
d) MULTI-STAGE REVERSE OSMOSIS: used to make water both chemically and
microbiologically potable
- Clarity cartridge removes the suspended particles (dust/mud/sand)
- Reverse osmosis cartridge reduces the total dissolved solids,
hardness, heavy metals (like arsenic, lead, mercury) and eliminates micro-organisms.

29. SMALL SCALE WATER PURIFICATION…
e) CHEMICAL DISINFECTION:
i. Bleaching powder (CaOCl2): Chlorinated lime when freshly made contains
about 33 % available chlorine but is an unstable compound that rapidly loses its
chlorine content on exposure to air, light and moisture.
ii. Chlorine solution: Also subject to losses on exposure to light/prolonged storage.
iii. High test hypochlorite (HTH): Perchloron is more stable
iv. Chlorine tablets: Single tablet (0.5 g) sufficient to disinfect 20 litres of water but
costly.
v. Iodine: Used for emergency disinfection, two drops of 2 % ethanol solution of
iodine will suffice for one litre of clear water with a contact time of 20-30m.
vi. Potassium permanganate: no longer recommended for water disinfection

30. SMALL SCALE WATER PURIFICATION…
2) Disinfection of wells: The main source of water supply in the rural
areas needs disinfection by bleaching powder. Steps include:
a) Find the volume of water in a well
b) Find the amount of bleaching powder required for disinfection
c) Dissolve bleaching powder in water bucket
d) Delivery of chlorine solution into the well
e) Contact period
f) Orthotolidine arsenite test

31. SMALL SCALE WATER PURIFICATION…
THE DOUBLE POT METHOD: During an emergency/epidemics, it is
desirable to ensure a constant dosage of chlorine to well water.
• Two cylindrical pots, one placed inside the other.
• The inner pot is filled with mixture of 1 kg of
bleaching powder and 2 kg of coarse sand.
• The inner pot is introduced into the outer one,
and the mouth closed with polyethylene foil.
• Lowered into the well and immersed at least 1 m
below the water level.

32. WATER QUALITY- CRITERIA AND STANDARDS
• Standards are set to provide a basis for judging the quality of water based
on exposure limits for bacteriological, viral, chemical and physical agents.
• It’s purpose is to minimize all the known health hazards, since it is obviously
impossible to prevent all pollution.
• The guidelines for drinking water quality recommended by WHO (2011)
relate to following variables:
I. Acceptability aspects
II. Microbiological aspects
III. Chemical aspects and
IV. Radiological aspects.

33. WATER QUALITY-CRITERIA AND STANDARDS…
I. ACCEPTABILITY ASPECTS
a) Physical parameters: Turbidity, Color, Taste and Odor, Temperature
b) Inorganic constituents: Chlorides, Hardness, Ammonia, pH, Hydrogen sulphide,
Iron, Sodium, Sulphate, Total dissolved solids, Zinc, Manganese, Dissolved
oxygen, Copper, Aluminium
II. MICROBIOLOGICAL ASPECTS
a) Bacteriological indicators: i) Coliform organisms (is constantly present in the
human intestine & presence in water is proof of faecal contamination, easily
detected by culture methods, survive longer and have greater resistance to the
forces of natural purification than other pathogens.
ii) Faecal streptococci iii) Cl. Perfringens

34. WATER QUALITY-CRITERIA AND STANDARDS…
b) Virological aspects: Drinking- water should be free from virus
c) Biological aspects: Protozoa, Helminths, Free-living organisms
III. CHEMICAL ASPECTS: problem associated with chemical constituents of
drinking water arise primarily from their ability to cause adverse health effects
after prolonged periods of exposure
a) Inorganic constituents: Arsenic, cadmium, chromium, cyanide, fluoride , lead,
mercury, nickel, nitrate, selenium etc.
b) Organic constituents: Polynuclear aromatic hydrocarbons, Pesticides
IV. RADIOLOGICAL ASPECTS: The effects of radiation exposure are called
"somatic" if they become manifest in the exposed individual, and "hereditary"
if they affect the descendants.

35. SURVEILLANCE OF DRINKING WATER QUALITY
• Surveillance of drinking water is essentially a health measure intended to
protect the public from waterborne diseases.
• The elements of a surveillance program are:
1) Sanitary survey: On-the-spot inspection and evaluation of the entire water
supply system by a qualified person to identify and correct faults and
deficiencies.
2) Sampling: Carried out by competent and trained personnel in strict
accordance with the accepted methods and guidelines.
3) Biological examination: Microscopic organisms such as algae, fungi, yeast,
protozoa, rotifers, crustaceans, minute worms collectively called plankton
that are an index of water pollution.

36. SURVEILLANCE OF DRINKING WATER QUALITY…
4) Bacteriological surveillance:
A. PRESUMPTIVE COLIFORM TEST:
i. Multiple tube method to estimate the most probable number (MPN) of
coliform organisms in 100 ml of water verified with confirmatory tests
ii. Membrane filtration technique: measured volume of sample is passed
through special membrane (cellulose ester) → transfer to culture media
B. DETECTION OF FAECAL STREPTOCOCCI AND Cl. PERFRINGENS
C. COLONY COUNT
5) Chemical surveillance: Basic tests are for pH, colour, turbidity, chlorides,
ammonia, chlorine demand and residual chlorine.

37. Best Wishes