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Purification of Water
Facilitator:
Dr. NAVPREETAssistant Professor, Department of Community MedicineGovt. Medical College & Hospital, Chandigarh.
Specific Learning Objectives
• At the end of session, the learner shall be able to:1.
Know the aim of water purification.
2.
Describe
the
purification
of
water
on
a
large scale.
3.
Differentiate
between
slow
sand
filter
&
rapid sand filter
4.
Describe various methods of disinfection.5.
Describe
the
purification
of
water
on
a
small
scale.6.
Enumerate water quality –
criteria & standards.
Purification of water
• The
aim
of
water
purification
is
to
produce and maintain water that is:
hygienically safe, aesthetically attractive and palatable, in an economical manner.
• On a large scale• On a small scale
Purification of water on a large scale
• The
components
of
a
typical
water
purification system
comprise
one
or
all
of
the
following
measures:
Pre‐treatment,
Filtration, and
Disinfection
Pre‐treatment
• The sub‐steps included in pre‐treatment are:
Storage,
Coagulation,
Rapid Mixing,
Flocculation, and
Sedimentation.
Storage
• Natural purification:Physical
Chemical
Biological
Filtration
• Slow sand or Biological filter• Rapid sand or Mechanical filter
SLOW SAND FILTER
• First
used
for
treatment
of
water
in
1804
in Scotland and subsequently in London.
• The elements of slow sand filter are:
Supernatant (raw) water
A bed of graded sand
An under‐drainage system
A system of filter control valves.
Supernatant (raw) water
• 1 ‐
1.5 metre
• Purposes:To overcome the resistance of filter bed
To provide waiting period (3 to 12 hours)
• Level
of
supernatant
water
is
always
kept constant.
Sand bed
• Most important part of the filter.
• 1 ‐
1.2 meter.
• Effective diameter of sand grains: 0.2‐0.3 mm.
• It
is
supported
by
layer
of
graded
gravels
(30‐ 40 cm)
• Rate
of
filtration:
2‐3
m.g.a.d.
(million
gallons per acre per day)
• Vital layer
Vital layer
• Schmutzdecke
layer,
Zoogleal
layer
or
Biological layer.
• ‘Ripening’
of the filter.
• ‘Heart’
of slow sand filter.
Under‐drainage system
• Porous or perforated pipes• Purpose:Provide an outlet for filtered water
Support the filter medium above
Filter box
• From top to bottom:
Supernatant water 1‐1.5 meter
Sand bed 1.2 meter
Gravel support 0.3‐0.4 meter
Filter bottom 0.16 meter
Filter control
• To maintain a constant rate of filtration.
• ‘Venturi meter’
• ‘Loss of head’
: if > 1.3 meter, it is uneconomical.
• Filter cleaning:• Scrapping off the top portion of sand layer• Manually or by mechanical equipment.
Advantages of slow sand filter
• Simple to construct and operate.
• Cost of construction is cheaper.• Quality of filtered water is high.
RAPID SAND FILTER
• The first rapid sand filter was installed in the USA in 1885.
• Two types:Gravity type (Paterson’s filter)
Pressure type (Candy’s filter)
Steps in purification of water by Rapid Sand Filter
• Coagulation• Rapid mixing
• Flocculation• Sedimentation
• Filtration
Mixing chamberFlocculation
chamberFilterSedimentation
tank
Alum
Filter beds
• Sand is the filtering medium
• Effective size of sand particle: 0.4‐0.7 mm.
• Depth of sand bed: 1 meter.
• Rate of filtration: 200 m.g.a.d.
• Gravel supports the sand.• Under drains at the bottom of the filter beds.
Filtration
• Formation of slimy layer by ‘alum‐floc’
which are still not removed
• “Backwashing”
Advantages
• It can deal with raw water directly.• It occupies less space.• Filtration is rapid (40‐50 times that of a slow
sand filter)
• More flexibility in its operation.
BUZZ!
• Slow sand filter vs. Rapid sand filter
DISINFECTION
• Chlorination• Ozonation• Membrane processes
Chlorination
• Principles• Breakpoint chlorination• Orthotolidine Test
Chlorine added ‐‐‐‐‐‐‐‐‐‐
>
Chlorine
residua
l ‐‐‐‐‐‐‐‐‐‐>
Chlorine destroyed by reducing
compound
Formation of chloroganics &
chloramines
Chlororganics & Chloramines partly
destroyed
Free available residual formed
Combined residual
Free available residual
Purification of water on a small scale
• Boiling• Chemical disinfection:Bleaching powder
Chlorine solution
High test hypochlorite
Chlorine tablets
Iodine
Potassium permanganate
• Filtration• Ultraviolet irradiation• Multi‐stage reverse osmosis
Water Quality‐
Criteria & Standards
• Acceptability aspectsPhysical parameters
Inorganic constituents
• Microbiological aspects
• Chemical aspectsInorganic constituents
Organic constituents
• Radiological aspects
Microbiological aspects
• Bacteriological indicatorsColiform organisms
Faecal streptococci
Clostridium perfringens
• Virological aspects• Biological aspectsProtozoa
Helminths
Surveillance of Drinking Water Quality
• Sanitary survey• Sampling
• Bacteriological surveillance• Biological examination
• Chemical surveillance
Bacteriological surveillance
• Presumptive coliform testMultiple tube method
Membrane filtration technique
• Detection
of
Faecal
Streptococci
&
Cl. Perfringens
• Colony count
Take home message
• Purification
of
water
is essential.
• Rapid
sand
filters
are preferred
but,
slow
sand
filters are also in use.• Chlorination
of
water
is
effective
and
cheap method for disinfection.
• Everyone can play his/her role
in
purification
of
water e.g. at your home.