Environmental practicals for MBBS students with diagrams & description from Exam point of view.

1. Environmental Practicals
Dr. Jagannath S Shete
Assistant Professor, Community Medicine,
DYPMC, Kolhapur

2. • Well is artificial pit or hole into earth to reach subsoil water.
• Shallow , deep, artesian well
Shallow well Deep well
Taps water from above 1st
impervious layer of earth
Taps water from below 1st
impervious layer of earth
Hard water Very hard water
Liable for contamination Not liable
Not safe water Safe water
Dries up in summer Doesnot dries up in summer
Easy & cheap to construct Difficult & costly to
construct

3. • SANITARY WELL
• Properly located, well constructed, protected against contamination &
gives safe water
• Criteria-
1. Location- about 60m (not more than 100m) away from human
habitation, preferably in elevated area.
2. Construction-
• Lining- should be done by stones & cement up to depth of 6m (20ft). So
that purer water should come from bottom of well.
• Parapet wall- Lining should be extended 1 m above the ground level to
prevent surface washings from entering well.
• Platform- Concrete wall about 1 m around well, sloping outwards and
connected to drain.
3. Protection-
• Cover- well must be covered to prevent contamination from dust &
animal droppings.
• Hand pump- to lift water
• Cleanliness- maintained by prohibiting washing clothes, utensils,
animals, bathing. There should be common rope and bucket for water
withdrawal.

5. Purification Of Water

6. • Purification of water on Large scale:
• 3 steps - Storage , Filtration & Chlorination
• Storage: Raw water stored in big concrete tanks called as
storage tanks in which heavy particles & suspended impurities
settle down due to gravity so water is purified by physical
method. Aerobic bacteria oxidize organic matter with help of
dissolved oxygen, causes chemical purification. And death of
some pathogens causes biological purification.
• Filtration: To remove pathogens & colloidal particles.
• Types :
• Rapid sand filters
• Slow sand filters

10. • Rapid sand filter is cleaned by reversing flow of current of
water from bottom so that sand bed gets elevated, particles
become loose, impurities dislodged and washed away. This is
called as Back washing.
• Advantages of Rapid sand filters:
• Preliminary storage not necessary, deals with raw water
directly, occupy less space, filtration is continuous & rapid,
cleaning is easy, cheap & efficient filters
• Disadvantages of Rapid sand filters:
• Water requires Preliminary treatment, requires services of
skilled person, filters can get blocked by impurities.

11. • SLOW SAND FILTERS (BIOLOGICAL FILTERS)
• Rectangular 2 tanks constructed below ground depth of 4m.
• Elements are supernatant raw water column , a bed of graded
sand & gravel , under drainage system, filter control valves
• Upper water column 1.5m height, filtering medium is bed of fine
sand 1.2 m height, effective diameter of sand grains 0.15-0.35
mm. , this is supported by bed of course sand which is supported
by graded gravel 0.4m height, at bottom it is supported by
perforated pipes which drains filtered water.
• Physical processes are sedimentation & straining due to constant
pressure over sand bed & straining in interstices of sand particles.
• Filters acts as mechanical strainer, within 2-3 days surface of sand
bed gets covered with slimy, greenish, jelly like layer known as
‘vital layer’(zoogleal/biological/schmutzdecke layer) composed of
algae, fungi, diatoms. This layer purifies water by chemical &
biological processes. Formation of this layer called as ripening of
filter.

12. • The zoogleal layer absorbs colloidal particles, arrests pathogens,
oxidizes organic matter killing pathogens, absorbs carbon
dioxide, phosphates, nitrates. Finally water becomes bacteria
free. This layer is heart of filter.
• Rate of filtration 0.1 – 0.4 m3 /hr/m2 , 99.9 % bacteria removed
• The frictional resistance offered by vital layer & sand bed called
as ‘loss of hydraulic head’ measured by ‘venturimeter’. When it is
1.3m it is time to clean filter bed by scraping top layer of sand
bed. Scarping done once in 6-8 wks. When sand bed reduced
from 1.2 to 0.8 m bed has to be renewed.
• Advantages are its simple to construct, easy to operate, cheaper,
quality of filtered water high (99.9%)
• Disadvantages are it requires large area, rate of filtration slow,
not efficient in removing colloidal matters & colour, long waiting
period for filtration (18hrs)

14. Horrock’s Apparatus & OT test

15. • Chlorination –
• Process of disinfection of water by adding chlorine in gaseous ,
liquid (chloramines) or solid form.
• Solid forms are 1) Monohypochlorite of Ca (CaClOCl)- classical
bleaching powder. It is unstable compound contains 33.3.% of
available chlorine. 2) Dihypochlorite of calcium- stable
compound, contains twice as much available chlorine. 3) High
test hypochlorite- highly stable compound contains 75% of
available chlorine.
• When chlorine is added to water hypochlorous acid and HCL is
formed. Disinfecting action is mainly due to hypochlorous acid.
• H2O + Cl2 <=> HCL + HOCl
• HOCl <=> H + OCl
• Chlorine demand, Breakpoint chlorination, contact period.
• For drinking purpose min conc. of free residual chlorine
0.5mg/lit (0.5ppm)

16. • Horrocks Apparatus :
• It is used to find out the dose of bleaching powder required
for disinfection of water.
• It consists of :
• One black cup (200 ml ) with a marking inside.
• Six white cups (200 ml ).
• Seven glass stirring rods.
• One pipette.
• Two droppers.
• Starch-iodide indicator solution.
• Bleaching powder.
• Two metal spoons which can hold 2 gms of bleaching
powder in each.
• Plastic or wooden box container for all these items.

17. • Procedure :
• Take 1 level spoonful (2 gms) of bleaching powder in the black
cup & make into a thick paste Add water upto circular mark
with vigorous stirring. Allow it to settle. This is Stock Solution.
• Fill all 6 white cups with water to b tested, 1 cm below rim.
• With special pipette add 1 drop of stock solution to 1st cup, 2
drops to 2nd cup, 3 drops to 3rd cup, and so on.
• Stir each cup with separate rod & Wait for ½ hour for action
of chlorine.
• Add 3 drops of starch-iodide solution to each white cup and
stir again.
• Development of blue colour indicates free residual chlorine.
• Note the first cup which shows distinct blue colour. Eg: if 3rd
cup shows colour first, then 3rd level i.e. 6gms of bleaching
powder is required to disinfect 455 litres of water.

18. • O.T. Test : Orthotoluidine Test
• Developed in 1918
• Determines the free residual chlorine present in water.
• Reagent : Ortho- toluidine, dissolved in 10% solution of
hydrochloric acid.
• Procedure : 1 ml of reagent is taken in one cylinder. In another
cylinder 10 ml of water to be tested is taken. The water is
poured over reagent, mixed well and left for 1 min. Colour
changes to yellow (varies with the concentration of the gas).
• The yellow colour produced is matched against suitable
standards / Colour discs. Yellow colour produced by both free
and combined chlorine.
• O.T. reacts instantaneously with free chlorine but slowly with
combined chlorine.
• Thus, reading should be taken within 10 secs, after adding the
reagent, to estimate free chlorine in water.

19. • Yellow colour may also appear due to iron, nitrites impurities in
water. It can be overcome by modified test “Orthotoluidine
Arsenite Test (OTA)”. In this test Orthotoluidine - arsenite reagent
is used.
• Chloroscope :
• It is used to determine the amount of free residual chlorine in
water after chlorination.
• The unit consists of :
– Main Body – for holding the sample tubes and reagent dropper.
– Glass test tubes – two in number.
– Graduated colour disc showing four standards : 0.1, 0.2, 0.5, 1.0
– A dropper for the reagent.
– A bottle for orthotoluidine reagent in unbreakable bottle.
• Method :
• Remove the side cover and fill the right- hand side tube with
sample water up to the mark.

20. • Take the dropper and add 4 drops of reagent in the test tube.
• Shake well and replace the tube in the holder.
• Fill the left-hand side tube with plain water and replace in the
holder.
• Now, hold the disc in day-light and compare and match the
colour of the sample tube with standard disc. Note the reading.

21. Disinfection of well & Filters

22. • WATER SANITATION :
• Purification of water on a small scale.
a) Household methods
- Physical Methods:
Boiling – cheap & best method, destroys all bacteria, viruses,
cysts, ova, also removes temporary hardness of water. Rolling boil
for 5-10 min.
Ozonation, UV irradiation
- Chemical disinfection. Bleaching powder, Chlorine solution, High
test hypochlorite, Chlorine tablets.
Iodine- Lugols Iodine (2% ethanol sol of iodine), contact period 30
min. Used only in emergency coz iodine interferes with thyroid
functions, not much efficient, high cost.
Potassium permanganate- powerful oxidising agent but not
reliable, also alters taste, smell, colour of water.

23. • Mechanical Methods : Filtration-
• Water can be purified on a small scale by filtering through
ceramic filters such as Pasteur Chamberland filter, Berkefeld filter
and “Katadyn” filter.
• The essential part of a filter is the ‘candle’ which is made of
porcelain in the Chamberland type, and of kieselgurh or infusorial
earth in the Berkefeld filter. In the Katadyn filter, the surface of
the filter is coated with a silver catalyst so that bacteria coming in
contact with the surface are killed by the oligo-dynamic action of
the silver ions, which are liberated in water.
• Filter candles remove bacteria found in drinking water, but not
the filter passing viruses.
• Filter candles are liable to be logged with impurities and bacteria.
They should be cleaned by scrubbing with a hard brush under
running water and boiled at least once a week.

25. • Reverse Osmosis Treatment:
Five stage water purification-
• Stage 1. Sediment filter removes sand, dust, rust particles.
• Stage 2. Activated carbon block filter removes organic matters,
colours, chlorine.
• Stage 3. Gag filter removes harmful chemicals & colours, smell
• Stage 4. TF membrane removes germs, bacteria, viruses.
• Stage 5. Silver impregnated activated carbon prevents growth of
bacteria at point of use.
• Demerits: naturally occurring minerals lost, slow process, only
20-30% of water gets purified, costly.
• Chemical purifiers:
• Run on carbon method. Carbon filter cartridge traps
particulates, impurities. Positively charged charcoal captures
negatively charged impurities, pesticides, herbicides. Can not
remove heavy metals.

26. • b) Disinfection of wells:
• Wells are the main source of water supply in the rural areas.
The need often arises to disinfect them, sometimes on a mass
scale, during epidemics of cholera and gastroenteritis. The
most effective and cheapest method of disinfecting wells is by
bleaching powder.

27. • Steps in Well Disinfection :
• 1)Find the volume of water in the well.
• Volume (litres)= 3.14 x d2 x h/4 x 1000
• h- depth of water column (metre),d- diameter of well (metre)
(1 cubic meter = 1000 litres of water )
• 2) Find the amount of bleaching powder required by “Horrocks
Apparatus”
• 3) Dissolve the bleaching powder required for disinfecting the well
into a thin paste in a bucket- add more water till the bucket is ¾
full- Stir well – Allow it to sediment for 5 to 10 mins.
• 4) The supernatant solution is chlorine solution- transfer to
another bucket –> lower it below the water surface – >Stir
violently by moving the bucket in horizontal n vertical movement.
• 5) Contact period of 1 hour is given after which water can be
drawn for use.