Source of Water

Sources of Water

• Rain– Surface Water– Groundwater

• Aquifer

• Hydrological Cycle

- Rain – Surface runoff – water storage - infiltration – ground water – evaporation – transpiration -

Choice of Source

• Location of source

• Quantity of water available

• Quality of water

• Cost of the entire scheme

Surface water

• Surface water source may be further classified as:

• -         lakes• -         Impounding reservoirs• -         River, streams, irrigation canals• The area where water is collected for water

supply purpose is called catchment or watershed.

Ground Water

• Aquifer• An aquifer is a layer of pervious material beneath the

ground surface and underlied by impervious stratum.• Water table• The free surface of water in the aquifer is called

water table• Sources - Spring - Wells

Characteristics of Ground Water

• Generally ground water is clear and colorless. When water seeps down into ground, it dissolves inorganic salts.

• Therefore, sometimes harmful

Sources of Water in Hong Kong• natural catchment

• Guangdong Province of China.

• Administration by Water Authority

• Responsible for supply, treatment and distribution

Quality of Water

Quality Monitoring

• conform both chemically and bacteriologically to the Guidelines for Drinking Water Quality (WHO, 1984).

• Parameters:– Physical– Chemical : a) inorganics – b) organic– Biological– Bacterialogical – Radiological

• Physical : e.g. pH, conductivity, turbidity, colour, etc.

• Chemical : – a) inorganics e.g. nurients, ions, heavy metals etc.

– b) organic e.g. trihalomethane, pesticides etc.

• Biological : e.g. algae, zooplankton, macroinvertebrate, fish,

• Bacterialogical : total coliform, E. Coliform, plate count

• Radiological : e.g. gamma emitters.

Consumer Complaints

• Consumers can file complaints regarding the quality of water. Typical complaints on the quality of water in Hong Kong are :– discoloured water– tastes and odours– cloudy, milky or chalky water

Introduction to Water Treatment

Why water need treatment

• The water is polluted by the people inhabiting the catchment.

• Water carries bacteria, some of which are pathogenic, and can cause water-borne diseases.

• Factories let off water into rivers and pollute the water.

• Surface run-off dissolve and minerals.

Impurities

- Physical impurities- Chemical impurities- Bacteriological impurities

• Physical impurities can affect the color, taste, odour and turbidity to the water.

• Chemical impurities cause hardness in water. Excess quantities of metals and dissolved gases cause corrosion to pipes and fittings.

• Bacteriological impurities are due to pathogenic bacteria which spread diseases.

Objectives of Water Treatment

To remove colour, objectionable taste and odour To remove dissolved gases, dissolved and

suspended impurities and harmful minerals To remove suspended and dissolves organic

impurities To remove harmful bacteria To make the water safe and attractive for

drinking and domestic purposes

Components of a Treatment Plant

• Intake work including pumping plant Plain sedimentation tank Clairifier-flocculator: (coagulation & flocculation) Filter Disinfecting plant Water storage reservoir. Pumping plant Distribution system Other processes Hardness (Colour, Iron and

manganese, Fluoride, )

Treatment processes

• Source

• Pre-chlorination

• Flocculation

• Sedimentation

• Filtration

• Post-chlorination

• Storage and pumping

• Distribution system

Plain Sedimentation

• Suspended and colloidal impurities that are settleable are separated in the sedimentation tanks by gravitation.

Principle of Sedimentation

• The main principle of sedimentation is to allow water to rest or flow at a very slow velocity so that the heavier particles settle down due to gravity.

Types of Sedimentation Tanks

• Sedimentation tanks can be

• Horizontal flow tanks

˙ Vertical flow tanks

Horizontal Flow Tanks

• The direction of flow in the tank is essentially horizontal.

• (a) Rectangular tanks

• (b) Circular tanks– Radial flow tanks– Circumferential flow tanks

Vertical Flow Tanks

Sedimentation with coagulation

• Colloidal particles which are fine particles of size finer than 0.0001 mm carry electrical charges on them. These are continuously in motion and will never settle down under gravity.

Coagulation

• The process of adding certain chemicals to water in order to form an insoluble, gelatinous, flocculent precipitate for absorbing and entraining suspended and colloidal matter is called coagulation.

Purpose of Coagulation

• The fine particles are removed rapidly and turbidity is reduced to about 20 ppm. The bacterial load also is reduced by 5%.

Principle of Coagulation

Floc Formation Electric Charges Floc Formation

– forming of a thick gelatinous precipitate which is insoluble in water and settle.

• Electric Charges– Colloidal particle possess negatively charged

ions. The floc thus attracts the colloidal particles and makes settle down.

Coagulants

• The common coagulants used are:

• Salts of aluminium: alum (明礬 ), filter alum, Sodium aluminate

• Salts of iron: Ferrous sulphate, Ferric sulphate, Ferric chloride

Chemistry of Coagulation

• When alum is added to water, Al3+ ion will hydrolyze to different charged hydrolysis products depending on the pH :

• Al(H2O)6 3+ < == > [Al(H2O)5OH] 2+ < == > [Al(H2O)3 (OH)3] (s) < ==> Al(H2O)

2(OH)4]-

Polyelectrolytes

Polyelectrolytes are increasingly used in water treatment as primary coagulant, flocculant coagulant aid or filter aid.

They are synthetic long-chain organic compounds which contain different monomers.

They can exhibit either positive (cationic), negative (anionic) or nonionic charge.

Advantages of polyelectrolytes

increase process stability and efficiency improve water quality improve sludge quality and reduce sludge

quantity

Limitations of polyelectrolytes

expensive products require careful control improper dosing can cause serious filter

problems filter blockage require efficient mixing

Process of Coagulation and Sedimentation

• Feeding

• Mixing

• Flocculation

• Clarification.

Flocculation

• Agglomeration of particles involve 3 separate mechanisms :

• 1. Brownian motion

• 2. stirring/agitating

• 3. Differential settling

Clarifying

• After flocculation, the floc is allowed to settle down in basins called clarifiers. The clear water is drained out for further purification. The detention period is about 1*1/2 - 3 hours.

Filtration

• The process of filtration involves allowing the water to pass through a layers of sand called media which acts as strainers. The water is then collected by means of drains placed under the sand bed.

Objectives of Filtration

• Remove colloidal and suspended matter remaining after sedimentation

• Remove bacterial load

Theory of Filtration

• Mechanical straining

• Sedimentation or adsorption

• Biological Metabolism

• Electrolytic changes

Filter Sand

The characteristics of the sand• Effective size : The effective size of a sand is the

size of sieve, in mm, through which 10% of the sand (by weight) passes. It is usually denoted by D10.

• Uniformity coefficient : This is the ratio of the size of sieve through which 60% of the sample of sand passes to the effective size of sand.

Classification of Filters

• Gravity filters

• Slow sand filters

• Rapid sand filters

• Pressure filters

 

Types of Filter Media configuration in Hong Kong

In Hong Kong, only rapid sand filter is used. Filtering materials can be single media filter or dual media filter.

 

Disinfecting

•  The process by which harmful bacteria are destroyed to make it safe for drinking is called Disinfecting.

• Chemicals used for this purpose are called disinfectants.

 

Criteria for a Good Disinfectant

• Capable of destroying pathogenic organisms within the contact time available.

Should NOT change the physical and chemical characteristics

• Should not be toxic • Available at reasonable cost Should have residual concentration• Detectable in the water by simple tests• Easy in transportation and application

Methods of Disinfecting

• Boiling water

• Ozone gas treatment

• Excess lime treatment

• Iodine and bromine treatment

• Ultraviolet rays treatment

• Potassium permanganate treatment

• Chlorination 

Chlorination• The process is economical and cheap • It is harmless to human beings• It is reliable and effective Residual chlorine can be maintained in the water• Easy to apply and ease to measure Disadvantages of Chlorination• Formation of trihalomethanes (THMs) such as carcinogen

chloroform (CHCl3)• To avoid the formation of THMs, water should be removed

of organic impurities as much as possible before chorination.

Chemistry of Chlorination• Cl2 + H2O < == > HOCL + H+ + Cl- • The hypochlorous acid is the prime

disinfecting agent. • Depending on the pH, the following

reaction can also take place• HOCl < == > H+ + OCl-

• Together, HOCl and OCl- are called free available chlorine.

Chlorine Demand• Chlorine and its compounds are consumed by a

variety of organic and inorganic materials present in water due to its oxidising power before disinfecting is achieved.

• The difference between the amount of chlorine added to the water and the residual chlorine is called chlorine demand.

• This depends on the amount of chlorine added, time of contact, pH and temperature.

Breakpoint Chlorination• Chlorine acts as oxidising agent to remove

bacterial and organic matters• When chlorine is applied, it first kills the bacteria

Any further addition will appear as residual chlorine increases.

• After a certain point, the residual chlorine suddenly decreases with emanation of bad smell and objectionable taste. This indicates that chlorine is being used for oxidising the organic impurities.

Breakpoint Chlorination• Again, after some time, there is a sudden increase

in residual chlorine indicating that oxidation of impurities is over.

• The point at which both the demands are satisfied and residual chlorine increases is called breakpoint.

• Beyond the breakpoint, any further dose of chlorine only gets accumulated. The process of adding chlorine beyond the breakpoint is called breakpoint chlorination.

• Hardness of Water

Problem with Hardness in water

˙ Corrosion and incrustations in pipes˙ Consumption of more soap˙ Scales in boiler˙ Bad taste in water˙ Difficulties in dyeing industry˙Creates bubbles in the manufacturing posses

of paper making, textile finishing, ice making, etc.

Types of Hardness

˙Temporary hardness

˙Permanent hardness

Temporary hardness

˙is caused by the presence of bicarbonates of calcium and magnesium.

Permanent hardness

is caused by the presence of sulphates,

chlorides and nitrates of calcium and

magnesium

Removal of Hardness

Temporary hardness is removed by:

˙Boiling the water

˙Adding lime to the water

 

Removal of Hardness

Permanent Hardnes

˙Lime-soda process

Zeolite process

 

Lime-soda Process

Ca(HCO3) 2+Ca(OH)2 2CaCO3 + 2 H2O

Zeolite Process

• Ca(HCO3) 2 + Na2Z CaZ + 2NaHCO3

• Reverse Process

• CaZ + 2NaCl Na2Z + CaCl

Miscellaneous Treatments

• Removal of Iron and Manganese

• Removal of Fluorides

• Taste and Odour

• Colour