Purification of Water - Community Medicine

By Sanjay George

PURIFICATION OF WATER

INTRODUCTION• Purpose of purification of water is to produce water that is safe and wholesome.

• Purification of water on large scale

• Purification of water on small scale

PURIFICATION OF WATER ON LARGE SCALE• The method of treatment to be employed depends upon nature of raw water and desired

standards of quality.

• Components of water purification system

- Storage

- Filtration

- Disinfection

STORAGE• Water is drawn out from the source and impounded in natural or artifical reservoirs.

• Optimum period of storage : 10 – 14 days

Advantages

• Storage provides a reserve of water from which further polution is excluded.

• By mere storage about 90% of suspended impurities settle down in 24 hours by gravity, the water becomes clear allowing penetration of light.

• Aerobic bacteria oxidize organic matter present in the water with the aid of dissolved oxygen. Free ammonia and nitrates drop.

• Pathogenic organisms eventually die out. Decrease in bacterial count by 90% within 5 – 7 days.

• If water is stored for long periods there is likelihood of development of algae which imparts foul odor and color to water.

FILTRATION – SLOW SAND FILTERS• Comprises of :

-Supernatant (raw) water

-A bed of graded sand

-An under-drainage system

-A system of filter control valves

SUPERNATANT WATER• Depth : 1 – 1.5 meter

• Provides constant head of water so as to overcome the resistance of the filter bed and thereby promote down flow.

• Provides waiting period (3- 12 hours) for raw water to undergo purification by sedimentation, oxidation and particle agglomeration.

• Level of supernatant kept constant.

SAND BED• Thickness: 1 meter

• Sand grains : Preferably rounded and have an effective diameter between 0.2 to 0.3 mm. - Should be clean

• Sand bed supported by layer of graded gravel. (30 – 40 cm deep)

• Water percolates slowly.

• Purification processes : mechanical straining, sedimetation, adsorption, oxidation, bacterial action.

• Rate of filtration: 0.1 – 0.4 m3/hour/sq. meter of sand bed surface.

Vital Layer:

• Schmutzdecke, zoogleal layer or biological layer

• Contains threadlike algae and numerous lifeforms like plankton, diatoms and bacteria.

• Process of formation called ripening of the filter.

• Takes several days to form

• Extends 2 – 3 cm into top portion of sand bed.

• Removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen into nitrates.

UNDER-DRAINAGE SYSTEM• Located at bottom of filter bed

• Consists of porous or perforated pipes.

• Serves dual purpose of providing outlet for filtered water and supporting filter medium above.

FILTER BOX• Rectangular open box 2.5 to 4m deep built wholly or partly below the grond.

• Supernatant water 1.5m

• Sand Bed 1.2m

• Gravel support 0.30m

• Filter Bottom 0.16m

FILTER CONTROL• Venturi meter

• Measures bed resistance or “loss of head”

• When loss of head exceeds 1.3 meter its uneconomical to run the filter.

ADVANTAGES OF SLOW SAND FILTER• Simple to construct and operate

• Cost of operation is cheap

• Quality of filtered water is high.

• Known to reduce bacterial counts by 99.9 – 99.99%

RAPID SAND FILTER

River

Mixing Chamber

Flocculation chamber

Sedimentation Tank

Filters

Clear water storage

Consumption

Alum

Chlorine

Coagulation

ADVANTAGES OF RAPID SAND FILTERS• Can deal with raw water directly no preliminary storage required

• Filter beds occupy less space

• Filtration is rapid 40 – 50 times that of a slow sand filter

• Washing of filter is easy

• More flexibility in the operation

DISINFECTIONCriteria for disinfectant in water supplies:

• It should be capable of destroying pathogenic organisms present within contact time available and should not be influenced by constituents present in water.

• Should not leave products of reaction which render water toxic or impart color.

• Should have ready and dependable availability at reasonable cost.

• Possess the property of leaving residual concentration to deal with possible recontamination

• Be amenable to detection

CHLORINATION• Kills pathogenic bacteria, no effect on spores and certain viruses.

• Also oxidizes iron, manganese and hydrogen sulphide, destroys some taste and odor producing constituents

• Also controls growth of algae and slime organisms and aids coagulation.

PRINCIPLES OF CHLORINATION• Water to be chlorinated should be free of turbidity

• Chlorine demand should be estimated:

Chlorine demand = Amount of chlorine added – residual chlorine

Point at which chlorine demand is met is called break point chlorination

• Contact period of at least one hour

• Minimum recommended concentration of free chlorine : 0.5mg/L for one hour

• The sum of chlorine demand of specific water plus the free residual chlorine of 0.5mg/L constitutes the correct dose of chlorine to be added.

METHODS OF CHLORINATION• Chlorine Gas

• Chloramine

• Perchloron

ADVANTAGESAdvantages

• Low cost

• Ease of application

• Kills almost all bacterial contaminants

Disadvantages

• Formation of halogenated compounds which are suspected carcinogens.

ALTERNATIVES• Ozonation

• UV irradiation

PURIFICATION OF WATER ON SMALL SCALE• Boiling

• Chemical disinfection

Bleaching powder

Chlorine solution

High test hypochlorite

Chlorine tablets

Iodine

Potassium permanganate

• Filtration

Pasteur Chamberland filter, Berkefeld filter, Katadyn filter

DISINFECTION OF WELLS

• Wells are main source of water supply in rural areas

• Need arises to disinfect them during epidemics of cholera, gastroenteritis etc.

• Most effective method is by bleaching powder.

Thank You