Purification of Water

By

Dr. Mrs. O.K. Israel

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Outline

• Introduction

• Water Purification on Small scale

• Water Purification on Large scale

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Introduction

• Water purification or treatment is the transition of dirty harmful (Contaminated) water into clean safe water

• The purpose of water purification or treatment is to provide safe & wholesome water

• Drinking water should be totally clean and free of disease causing organisms

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Introduction

• The degree of water purification or treatment will depend on :

– The source of water

– The quantity of water &

– The population it will serve

• Purification of water can be on a

– Small scale or

– Large scale4

Water Purification on Small scale

• Household purification of water

• Disinfection of wells - By adding bleaching powder (chlorine powder)

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Household Purification of Water

The methods used are:

– Boiling

– Use of chemical disinfectants

– Filtration

• These methods can be used singly or in combination

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Household Purification of Water

• Boiling :

• Simplest, cheapest & safest way

• Done at boiling temperature for at least 10-15min

• Kills all bacteria, spores, cysts, ova & harmful pathogens

• It also removes temporary hardness

• Water should be boiled preferably in the same container in which it is stored to avoid contamination

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Use of chemical disinfectants

• Chemical used are:– Chlorine

– Iodine

– Potassium permanganate

• Iodine & Potassium permanganate are incapable of killing all pathogens

• Iodine & Potassium permanganate are no longer used because they alter colour, smell & taste of water

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Use of chemical disinfectants

• Chlorination

• It is an effective way of water purification

• Chlorine is added as bleaching powder, chlorine solution, hypochlorides & chlorine tablets

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Filtration

• Water can be filtered

– Using ceramic filters

– Modern filters or

– Thick white linning/linen – Monofilament filter

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Disinfection of wells

• Mostly done by adding chlorine powder

• Chlorine is normally used as the disinfecting agent since it destroys bacteria contained in the water

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Water Purification on Large scale

Stages of Large scale water purification

• Protection of the source

• Storage

• Aeration

• Coagulation & Flocculation

• Sedimentation

• Filtration

• Disinfection

• Water analysis & PH control

• Distribution12

Protection of water source

• The source of water should be protected from pollution by:

– Human faeces (a major source of pathogenic organisms) and

– Other human contact which could lead to contamination, e.g. guineaworm.

• Man and animals should be excluded from surface water

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Storage

• Water from different sources are stored in natural or artificial reservoirs

• Storage prevent water from further pollution /contamination

• As a result of storage, a very considerable amount of purification takes place

• This is natural purification

– Physical

– Chemical

– Biological14

Storage

• Physical purification

• By mere storage, the quality of water improves

• About 90% of suspended impurities settle down in 24hours by gravity

• The water becomes clearer

• This allows penetration of light which further improves water quality

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Storage

• Chemical purification• The aerobic bacteria oxidize the organic matter

present in the water with the aid of dissolved oxygen

• As a result, the content of free ammonia is reduced and a rise in nitrates occurs

• Biological purification• A tremendous drop takes place in bacterial count

during storage• The pathogenic organisms gradually die out

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Aeration

• Aeration brings water & air in close contact

– This leads to removal of dissolved gases e.gcarbondioxide

– It also leads to oxidation of dissolved metals e.g. iron, hydrogen sulphide & volatile chemicals

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Coagulation & Flocculation

• Coagulation– Addition of Aluminum sulphate or Ferric chloride to

allow solid matter to coagulate

• Flocculation– Is a slow & gentle stirring of water to allow particles

formed to coagulate so as to settle down

– This results in the formation of a thick, copius, white flocculent precipitate of aluminium hydroxide

– The thicker the precipitate or flock diameter, the greater the settling velocity

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Sedimentation

• This process allows suspended particulate matter to sink & settle out of suspension

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Filtration

• The process of filtration will remove pathogenic bacteria, protozoan cyst & large viruses

• 98-99% of bacteria are removed by filtration, apart from other impurities

• Two types of filters in use are:– Slow sand or Biological filters

– Rapid sand or Mechanical filters

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Filtration

• Slow sand or Biological filters

• It involves passage of water through sand & stones of graded size with fine sand at the top and large stones at the bottom

• This is to remove particulate matter & pathogenic organisms

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Comparison of rapid & slow sand filter

Rapid sand filter Slow sand filter

Space Need very little space Need large space

Cost Expensive to construct Cheap to construct

Storage Does not need preliminary storage Needs storage

Pre-treatment Usually necessary e.g coagulation, floculation, sedimentation

Not necessary

Filtration Very rapid Very slow

Washing of Filter

Back washing is an easy process Scraping of sand bed which is not an easy process

Operation Sophisticated & Complicated Very simple

Bacterial removal

Good Very good

Removal of Colour

Good Better

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Disinfection

Criteria of useful chemical agent as a disinfectant in water supplies:

• It should be capable of destroying pathogenic organisms present, within the contact time available

• It should not unduly influenced the physical and chemical properties of water e.g. temperature, pH and mineral constituents

• It should possess the properties of leaving residual constituent in water to deal with possible recontamination

• It should be available at reasonable cost permitting convenience, safe and accurate application to water

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Disinfection

• In water works practice, the term disinfection is synonymous with chlorination

Chlorination

• It is supplement, not a substitute to sand filtration

• Chlorine kills pathogenic bacteria but has no effect on spores and certain viruses (e.g., polio, viral hepatitis) except in high doses

• It oxidizes iron, manganese and hydrogen sulphide;

• It destroys some taste and odour-producing constituents;

• It controls algae and slime organisms; and

• Aids coagulation24

Disinfection• For disinfecting large bodies of water, chlorine

is applied as

– Chlorine gas

– Chloramine or

– Perchloron or high test hypochlorite

• Residual chlorine is left in water to deal with additional contamination that takes place as the water leaves the treatment point to the point of consumption

• The normal residual chlorine level is 0.2-0.5mg/l 25

Disinfection

Break Point Chlorination

• This is the point at which the residual chlorine appears in water, when all combined chlorines (i.e. those combined with contaminants) have been completely destroyed

• The corresponding dosage is the breakpoint dosage

• The breakpoint dosage is the dosage of chlorine that is added to water (for disinfection) and produces a residual chlorine, after all the combined chlorine have been destroyed

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DisinfectionSuperchlorination:

• Superchlorination is the addition of large doses of chlorine to water, and removal of excess chlorine after disinfection

• In super chlorination, chlorine is applied beyond break point level

• It is done after filtration with contact time of 30 to 60 mins

• This method is applicable to heavily polluted waters whose quality fluctuates greatly

• It is used during epidemic of water borne diseases e.g. cholera

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Disinfection

De Chlorination

• Process of removing excess chlorine from water is known as De-Chlorination

• This can be achieved by aeration or using chemicals such as sodium thiosulphate , sodium bi sulphate, sodium sulphite, Activated carbon & potassium permanganate

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Disinfection

• Other disinfectants that can be used in water purification include:

• Bromine

• Iodine

• Ozone

• Ultra-violet radiation

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Water analysis & PH control

• Is the assessment of water quality before distribution for consumption

• This entails the overall process of evaluating the physical, chemical and biological nature of water

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Distribution

• Once it is ensured that the treated water is of good quality, normal PH of drinking water, & its fit for human consumption

• Water is then distributed for consumption through:

– Individual family connections or

– Water public points

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Stages of Large scale water purification

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Thank you for Listening

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