Water purification

Water Purification
Dr.Rajkumar Patil A V Medical College & Hospital,Pondicherry

PURIFICATION OF WATER
LARGE SCALE
Storage Filtration Disinfection
SMALL SCALE (DOMESTIC)
A. Household purification
– Boiling
– Chemical disinfection :
Bleaching powder, Chlorine solution , High test hypochlorite(HTH),
Chlorine tablets , Iodine, Potassium permagnate
– Household filtration :
Ceramic filters
B. Disinfection of well
a. By adding bleaching powder
b. Double pot method

STORAGE
In Natural / Artificial Reservoirs
Effects of storage:
Physical: Gravity – 90%suspended impurities settle down in one day
Oxidizing action
Chemical: Organic matter Nitrates, Free Ammonia
Aerobic bacteria, dissolved O2
Biological: Only 10% bacteria remains at the end of 1 week
Optimum period of storage: 2 weeks

FILTRATION
Water pass through porous media
SLOW SAND / BIOLOGICAL FILTERS
Used first in 19 th century in scotland
Elements of slow sand filter
Filter Box:
a. Supernatant water
b. Sand bed
c. Under drainage system
Filter control valves

GRAVEL IN SLOW SAND FILTER

FILTER BOX
A. Supernatant water
depth: 1-1.5 m
promotes downward flow of water through the sand bed
provide waiting time of 3-12 hours for raw water to undergo partial purification by sedimentation and oxidation
B. Sand bed supported by gravel:
Depth: 1 m (sand with 0.2-0.3 mm diameter) ,
0.3m (gravel with 0.2-1 cm diameter)
Vital/Biological/Zoogleal layer/Schumtzdecke:
Slimy,gelatinous layer over sand bed containing threadlike algae,bacteria and diatoms.
Heart of the slow sand filter.
Ripening of filter : Formation of vital layer

Mechanism of action of slow sand filter: 1.Sedimentation: The supernatant water acts as a settling reservoir. Settleable particles sink to the sand surface. 2. Mechanical straining: Particles too big to pass through the interstices between the sand grains are retained. 3. Adhesion: The suspended particles that come in contact with the surface of the sand grains are retained by adhesion to the biological layer (Schmutzdecke) 4. Biochemical processes in the biological layer: Removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen in to nitrates Conversion of soluble iron and manganese compounds into insoluble hydroxides which attach themselves to the sand surfaces

C. UNDER DRAINAGE SYSTEM Depth: 0.15 m At the bottom of filter bed Porous pipes: Outlet for filtered water as well as support to the filter media above FILTER CONTROL VALVES To regulate the flow of water in and out FILTER CLEANING Increased bed resistance Necessary to open the regulating valves fully Scrapping top portion of sand bed up to 2 cm depth Time for cleaning the filter After 3-4 years new filter bed is constructed

REMOVAL OF VITAL LAYER

CLEAN SLOW SAND FILTER WITHOUT WATER

RAPID SAND FILTRATION First in 1885 in USA 1.Gravity type(Open)/Paterson’s 2. Pressure type (Closed)/Candy’s River In Mixing Chamber Coagulation by Alum (5-40 mg/litre) Violent mixing of alum (minutes) In Flocculation Chamber Slow stirring of water by paddles(30 minutes) Flocculent ppt. of Aluminium Hydroxide entangles all particulate ,suspended matter along with bacteria In Sedimentation Chamber Flocculent ppt. settle down (removal is done time to time) Clear water above goes for filtration

Rapid Sand Filter
Remaining alum floc—floc layer over sand bed, it holds back bacterias,oxidize organic matter
Back washing-by air bubbles or water when floc layer becomes very thick

Paddles in a flocculation chamber

Properties Rapid sand filter Slow sand filter Area Small area Large area Rate of filtration(L/m2/hr) 4000-7500 100-400 Sand size (diameter) 0.4-0.7 mm 0.2-0.3 mm Pretreatment Coagulation and sedimentation Sedimentation Filter cleaning Backwashing Scraping Operation More skilled Less skilled Removal of colour Good Better Removal of bacteria 98-99% 99.9%-99.99% Prior water storage Storage needed No need

DISINFECTION Criteria for satisfactory disinfectant: 1. Destroy the pathogenic organism without being influenced from properties of water (pH, temp. etc.) within a time period 2. Should not be toxic and colour imparting or leave the water impotable 3. Available, cheap,easy to use 4. Leave the residual concentration to deal with recontamination 5. Detectable by rapid,simple techniques in small concentration ranges to permit the control of disinfection process

Chlorination METHOD OF CHLORINATION Chlorinating equipment (Paterson’s chloronome) for adding gaseous chlorine Action: Kills pathogenic bacteria (no effect on spores and viruses) Oxidize iron, manganese and hydrogen sulphide Reduces taste and odours Controls algae Maintains residual disinfection Mechanism of action: 1. H 2 O+Cl 2 (at pH 7) ------- HCl + HOCl (main disinfectant) HOCl (at pH > 8.5) -------- H + + OCl - (minor action) 2. NH 3 + Cl 2 -------- NH 2 Cl/NHCl 2 /NCl 3 + H 2 O (Mono, Di ,Tri Chloramines)

PRINCIPLES OF CHLORINATION 1. Water should be clear,free from turbidity 2. Chlorine demand : Chlorine needed to destroy bacteria, to oxidize organic matter and to neutralize the ammonia in water 3.Free residual chlorine for a contact period of 1 hour is essential 4. Breakpoint : P oint when chlorine demand of water is met and free residual chlorine appears 5. Breakpoint chlorination: Chlorination beyond the breakpoint . The principle of break point chlorination is to add sufficient chlorine so that 0.5 mg/L free residual chlorine is present in the water after one hour of contact time 4. Dose of Chlorine = Chlorine demand + Free residual chlorine

Tests to measure Residual Chlorine
OT Test- Yellow colour
( In 10 seconds-free chlorine,
In 15 min-both free and combined chlorine)
Ortho Tolidine Arsenite (OTA) Test-Yellow colour
Tests both free and combined chlorine separately
(Yellow colour due to nitrites,iron,mangenese
are overcome by OTA test)

Super chlorination
method of choice for highly polluted waters
high dose of chlorine is added
after 20 minutes of contact, dechlorination
is done with sodium sulphate / sodium thiosulphate to reduce the taste of excess chlorine

OTHER DISINFECTANTS
OZONE
Used in Europe and Canada
Strong oxidizing agent
Strong Virucidal
No residual effect
Should be used with chlorination
UV RAYS
Used in UK
Water should be clear
No residual effect
Expensive
CHLORAMINE
Chlorine + Ammonia — Chloramine
Less effective than chlorine

PURIFICATION OF WATER ON SMALL SCALE
A . HOUSEHOLD PURIFICATION:
1.BOILING
Rolling boil for 10 min.
Kills all bacteria,spores,cysts,ova,remives temporary hardness
No residual protection
2.CHEMICAL DISINFECTION
Bleaching powder: White powder,pungent smell
33% available chlorine
Unstable
Storage in dark,cool,dry place in a closed container
Prepared by passing chlorine gas over lime (CaO)
Chlorinated lime(CaOCl2)-another name
Stabilized bleach-Bleaching powder mixed with excess of lime
Chlorine solution:
200 gm bleaching powder (25% available chlorine) + 1 litre water = 5% solution
1 drop of this solution---For disinfection of 1 L water

b. HTH/Perchloron: High strength Ca Hypochlorite 70% available chlorine 1 gm/L of water c. Chlorine tablets: NEERI developed cheaper good tablets 1 tab(0.5g) for 20 L water d. Iodine : 2 drops (2% solution in alcohol) for 1 L water Contact time 30 minutes Used only in emergency situation e. Potassium permangnate: Expensive,unreliable,not recommended f. Alum : For turbidity reduction: 0.4-1.6 g/20 L water

CALCIUM HYPOCHLORITE
0.5% Ca hypochlorite solution produced by Environment & Public health
Organization in Nepal
3 drops/ litre water ,use water after 30 minutes

3. FILTRATION AT HOUSEHOLD LEVEL
1.FOUR GHARA METHOD: Top: Muddy water, II: Sand,III: Charcoal,
(MSCE) IV: Empty ghara to collect filtered water
2. CERAMIC FILTERS
Water pass through micro-pores of candle placed inside the water container.
Bacteria: Unable to pass through the pores,
Virus: Can pass through the pores
PASTEUR CHAMBERLAND FILTER:
porcelain candles, used in labs & dispensaries.
BERKEFELD FILTER:
infusorial earth (keiselgurh) candles, less reliable

KATADYN FILTER:
katadyn: activated form of silver
filter coated with silver catalyst (oligodynamic action)
oligodynamic action: certain metals in very small doses act as powerful germicide
Cleaning of filter:
candles scrubbing with a hard brush - every 3 days,
boiling of candles-once in a week

DISINFECTION OF WELL 1. By adding bleaching powder: Find volume of water in well (V in Litres) = 3.14 x h x d 2 x 1000 4 Find the amount of bleaching powder needed by Horrock’s apparatus Bleaching powder (max.100 g) mixed with water in a bucket (water up to 3/4 th of bucket) Allow 10 minutes sedimentation Transfer supernatant chlorine solution to another bucket Deliver chlorine solution in to the well Allow 1 hour contact Do OTA test to measure free residual chlorine If free residual chlorine < 0.5 mg/L Add more bleaching powder

2 . DOUBLE POT METHOD/DOUBLE JAR DIFFUSER
By NEERI,Nagpur,India
Placed 1 meter below the water level
Effective for chlorination of well (4500 L water)
with a daily withdrawal of 450 L water ---2-3 weeks
Inner pot (28 cm ht) Hole (1 cm diam) Polyethylene foil Hole 1 cm diam Bleaching powder 1kg + 2 kg coarse sand Moist mixture Outer pot ( 1ft ht) Rope

Solar Disinfection (SODIS) ,Nepal

Tube well

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Water purification

by Rajkumar Patil on Aug 24, 2009

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