water treatment 25-01-2012

Water Treatment

M.M. GhangrekarDepartment of Civil Engineering Indian Institute of Technology

Kharagpur

Water Demand

• Various types of water demands for a city

– Domestic water demands

– Commercial & industrial demand

– Fire demand

– Demand for public uses

– Compensate losses demand

• Domestic water demand– It depends on the habits, social status, climatic conditions and customs

of the people.

– The domestic consumption of water under normal conditions is considered to be about 135 LPCD (IS 1172-1971)

– The details of domestic consumption per person are as below:• Drinking 5 L• Cooking 5 L• Bathing 55 L• Cloth washing 20 L• Utensils washing 10 L• House washing 10 L• Flushing of latrines 30 L

» Total 135 L/Capita.day

• For rural water supply with stand post – 40 LPCD• For rural water supply with house connection 70 – 100 LPCD

Water Demand

• Commercial and Industrial Demand– The water demand for commercial and public places may be up to 45

LPCD– Water supply requriements for few public buildings (IS 1172-1963)

• Factories with bathrooms 45 LPCD• Factories without bathrooms 30 LPCD• Hospitals, no of beds < 100 340 LPCD• Hospitals, no. of beds > 100 450 LPCD• Hostels 135 LPCD• Offices 45 LPCD• Restaurants (per seat) 70 • Hotels (per bed) 180• Cinema Hall (per seat) 15• Schools (day schools) 45 LPCD• Boarding schools 135 LPCD

– The water requirement for the industries differs from industry to industry and generally considered as 20 to 25% of the total demand of the city.

Water Demand

• Fire demand

– During fire breakdowns large quantity of the water is required for throwing it over the fire to extinguish it.

– Hence, provision is made in the water works to supply sufficient quantity of water or keep as reserve in the mains for this purpose.

– Fire hydrants are provided on the mains at 100 to 150 m apart for this purpose.

– The quantity of water required for fire fighting is calculated using following empirical formulae:

• Kuichling’s formula : Q = 3182 (P)1/2

• Buston’s formula : Q = 5663 (P)1/2

• CPHEEO formula: Q = 100 (P)1/2 (for P > 50000)

– Where, Q – quantity of water, L/min; P – Population in thousands

Water Demand

• Demand for public use

– Water is required for public utility purposes such as washing and sprinkling on roads, lawns, cleaning sewers, beautification purposes, such as, fountains and impoundments, etc.

– The provision of about 5% of the total consumption is made for this purpose.

• Compensate losses– Loss of water occurs in the distribution system due to faulty

joints, broken pipes or water loss due to open taps, and due to unauthorized and illegal connections.

– Generally allowance of 15% of the total quantity of water is made to compensate for losses, thefts and wastage of water.

Water Demand

• Therefore for average Indian conditions per capita water demand for a town is:– Domestic 135 LPCD

– Industrial 40 LPCD

– Public uses 25 LPCD

– Fire Demand 15 LPCD

– Losses 55 LPCD

• Total 270 LPCD• Hence, total quantity of water for a city = 270 x population

• It varies with population of the city and generally considered in the range of 150 to 270 LPCD.

Water Demand

• Fluctuations in water Demand

– Per capita water demand varies from hour to hour, day to day and season to season.

– Seasonal fluctuation: Water demand will be more in summer and less in winter, except cold countries where taps are kept open not to allow freezing of ice. This seasonal variation may be 15 to 50%.

– Daily and hourly variation: Sunday and holiday more water consumption. Morning and evening hours more water consumption.

– The maximum daily consumption = 1.8 times annual average consumption

– The maximum hourly consumption = 1.5 times average daily consumption.

– The variation in hourly demand is important for designing rate of pumping and determining capacity of reservoir.

Water Demand

Factors affecting water demand• The per capita water demand may vary from 100 to 300 LPCD for different cities

depending upon following factors:

1. Climatic conditions: Hot and dry places require more water – cold countries require less water.

2. Size of the city: Water demand will increase with size of the city due to increased demand for public utilities.

3. Living standard of the people: Water demand increases with increase in living standard.

4. Industrial and commercial activities: Presence of these activities will increase the water demand.

5. Pressure in the distribution system: water consumption and losses increases with increase in pressure of distribution system.

6. System of sanitation: Water carriage system water demand will be more.

7. System of water supply: Intermittent water supply for sufficient time- demand is more; for continuous water supply demand is less, if the system is maintained properly.

8. System of metering and charges: If water cost is more less is the demand. If water supply is metered less demand.

9. Quality of water: For safe and good water quality, water demand will be higher.

Design period

• The number of future years for which the design of the water works have been done is known as design period.

• Water supply projects may be designed to meet the requirements over thirty years after their completion.

• The factors affecting design period are:

– Ease and difficulty of future expansion

– Funds available & rate of interest on loans

– Hydraulic constraints while designing different components

– Anticipated expansion of town

– Life of the material used for construction.

• Design period for different components:

– Storage Dams 50 years

– Infiltration works 30 years

– Pump house 30 years

– Pumps 10 – 15 years

– Water treatment units 15 years

– Raw and clear water conveying mains 30 years

– Clear water reservoir 15 years

– Balancing tanks, service reservoir 15 years

– Distribution system 30 years

Design period

Sources of water

• The sources of water can be broadly classifies as:

(a) Surface water source, (b) Ground water source.

• Surface Water Sources

– Streams: Normally good quality water being flowing in valleys. Source of w/s to villages.

– Lakes: Limited to cities where sufficient storage capacity lakes are available.

– Rivers: For non-perennial river storage arrangement is required.

– impounded reservoirs: To meet the water requirement during hot weather, water is stored by construction of bund, weir or dams across the river. This is not feasible when the annual average inflow is less than demand.

• Criteria for selection of site for impounded reservoirs:

– Narrow width of river with rapidly widening upstream.

– Sufficient quantity of water should be available.

– Site should be near to the point of supply and as far as possible water should flow by gravity.

– Bed of reservoir should be impervious.

– It should not submerged costly land, forests.

– Watershed should be free from swampy areas, soil should not contain soluble salts and minerals, otherwise treatment will be costly.

– Useful construction material should be available nearby.

Sources of water

Water Treatment

• (a) Surface water

• (b) Ground Water

• (c) Ground water

Post-chlorinationStorage

Post-chlorination

Aeration Rapid mixing

Flocculation

Sedim-entation

Rapid sand filter

Post-chlorination

• (d) Surface water

• (e) Surface water

• (f) Ground water

• Sometimes demineralization is carried out after softening and chlorination.

Rapid sand filtration

Softening Post-chlorination

Slow sand filtration

Sedimentation

Post-chlorination

Pre-chlori-nation

AerationRapid mixing

Floccul-ation

Sedimentation

Post-chlorination

Rapid sand filter

Water Treatment

• Conventional water Treatment Plant

ClariflocculatorRapid sand filter

Water Treatment

Water Treatment

Aeration

• It is necessary to promote the exchange of gases between the water and the atmosphere. The aeration is practiced for following purposes in water treatment:

– Expulsion of CO2, H2S, and other volatile substances causing taste and odour.

– To precipitate impurities like iron and manganese in certain forms, e.g. in case of ground water source.

– To add oxygen to water for imparting freshness, e.g., water from underground sources are devoid of oxygen.

• Types of aerators:

– Those forming drops or thin sheets of water exposed to the atmosphere, i.e., more area of water is brought in contact with air.

• Spray aerator

• Multiple tray aerator

• Cascade aerator

• Mechanical aeration (surface aerators)

– Introducing air bubbles in water, i.e., air is brought in contact with water.

• Diffused air aeration

Aeration

• Cascade aerator

– Most commonly used aerator type in water treatment.

– Water is allowed to flow down over the steps spreading more area of water in contact with air.

– Design criteria:

• No. of steps : 4 to 6

• Total head requirement: 0.5 to 3.0 m

• Area requirement: 0.015 to 0.045 m2/m3/hr (0.03).

Aeration

Cascade aerator

Coagulation

• When coagulants are mixed in water they form insoluble, gelatinous, flocculent precipitation.

• This gelatinous precipitation absorb and entangle very fine suspended matter and colloidal impurities.

• Thus fine suspended matter comes together to form bigger size floc.

• These floc can settle effectively in sedimentation tank.

• Coagulation can also remove colour, odour, and taste from water.

• Types of coagulants

• A) Aluminum Sulfate Al2(SO4)3.18H2O

• Commercial alum contain @ 17% aluminum sulfate.

• For best result the pH range is 6.5 – 8.5.

• Al2(SO4)3.18H2O + 3Na2CO3 2Al(OH)3 + 3Na2SO4 + 3CO2+ 18H2O

• Al2(SO4)3.18H2O + 3Ca(OH)2 2Al(OH)3 + 3CaSO4 + 18H2O

• Al2(SO4)3.18H2O + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2 + 18H2O

Coagulation

• Ferric coagulants: Ferric chloride, Ferric sulfate

• 2 FeCl3 + 3 Ca(OH)2 2Fe(OH)3 + 3CaCl2

• Fe2(SO4)3 + 3 Ca(OH)2 2Fe(OH)3 + 3CaSO4

• FeSO4.7H2O + 2 Ca(OH)2 2Fe(OH)2 + CaSO4

• 4Fe(OH)2 + 2 H2O + O2 4 Fe(OH)3

• Good result above pH 8.5

• Good oxidizing agent and hence effective in removing H2S, taste and odour.

Coagulation

• Chlorinated copperas

– Chlorine is added to the solution of Ferrous sulfate

– 6 FeSO4 + 3Cl2 2 Fe2(SO4)3 + 2FeCl3

• Sodium Aluminate (Na2Al2O4)

– Na2Al2O4 + CaSO4 CaAl2O4 + Na2SO4

– Na2Al2O4 + CaCl2 CaAl2O4 + 2 NaCl

– Na2Al2O4 + Ca(HCO3)2 CaAl2O4 + 2 Na2CO3 + CO2 + H2O

– Hardness is not increased

• Polyelectrolyte

Coagulation

Rapid Mixing• Process introduces coagulant chemicals into the treatment

process to stabilize electrostatic charges.

• Most common chemicals include alum or iron salts as primary coagulant and cationic polymer as secondary coagulant.

• Many small systems only use cationic polymers.

• Rapid mixing energy is determined by a factor called G which is a measure of energy applied as HP to viscosity of water expressed as sec -1.

• Temporal mean velocity gradient ‘G’ is defined as relative velocity of two flow lines divided by perpendicular distance between them.

• Alum and iron salts require a high G value of greater than 300.

• Secondary coagulants such as cationic polymer requires a G value of approximately 300.

• Jar testing is most common means of determining chemical dosage requirements.

Jar Testing/Mixing

• Flash Mixer : It is the mechanical type of rapid mixing device

• Design guidelines:– Detention time: 30 to 60 sec

– Speed of rotation of blade = 400 to 1400 rpm

– Velocity gradient G = 300 sec-1

– Power requirement = 1 to 3 W/m3/hr

– Ratio of impeller dia. to tank dia = 0.2 to 0.4

– Tangential velocity at the tip of blade = 3 m/s

– Ratio of tank height to diameter = 1:1 to 3:1

– Vertical baffles are provided projecting 1/10 to 1/12 tank dia. to avoid vortex formation (minimum 4).

Rapid Mixing

Flocculation or slow mixing

• Process designed to increase particle size of colloids to floc for removal.

• Many types of flocculation equipment :– Roughing filter– Paddle wheel (Horizontal & Vertical)– Turbines– Up flow plastic beads– Baffles

• In addition to the intensity of the mixing the time provided for flocculation is important.

• The number of collision of the particles is proportional to ‘G.t’. Where, t is detention time.

• It is useful dimensionless number for design of flocculator (104 to 105).

• Detention time: 15 to 30 min

• G = 10 to 75 sec-1

• For mechanical type flocculator

– Depth of tank = 3 to 4.5 m

– Detention time = 10 to 40 min (30 min)

– Velocity of flow = 0.2 to 0.8 m/s (0.4)

– Total area of the paddles = 10 to 25 % c/s area of the tank

– Peripheral velocity of blades = 0.2 to 0.6 m/s (0.3-0.4)

– Power consumption = 10 to 36 KW/MLD


Clariflocculator

Clarifier

• Design guidelines

• Surface overflow rate = 30 to 40 m3/m2.d

• Detention time = 1.5 to 2.5 h

• Weir loading rate < 300 m3/m.d

• Bottom slope = 1 % for rectangular tank

» 8 – 12 % for circular tank

• Diameter or length = generally less than 40 m.

Filtration

• Filtration is a process of separating suspended and colloidal impurities from water by passing through a porous medium.

• The removal of turbidity is not only essential for aesthetic acceptability but also important for effective disinfection.

• In municipal water supply normally filters used are of two types:

– Rapid Sand Filter

– Slow sand filter

Rapid Sand Filtration

• Coarser and more uniform sand media is used to utilize greater depths of filter media to support higher rate of filtration along with effective removal of turbidity.

• Pretreatment such as coagulation and flocculation is must before RSF.

• The rate of filtration in RSF is 4 to 6 m3/m2.h. (4.8 typical)

• Length to width ratio of each filter bed should be between 1.25 to 1.33

• Sand bed depth generally of 0.6 to 0.75 m. (1.0 m maximum).

• Standing depth of water over filter 1 – 2 m.

• Free board of 0.5 m.

• The sand with effective size of 0.4 to 0.5 mm and uniformity coefficient of 1.4 to 1.7 is used. (D60/D10)

• Graded gravel is placed below the sand to support it (0.3 to 0.6 m).

• The under drainage system of RSF has the following functions:– Supports sand

– It collects the filtrate and carries it to clear water storage.

– It distributes wash water uniformly to the filter bed during backwashing.


Multimedia Filtration

Size of media 2 mm at top and 0.15 mm at bottom.

Specific gravity of the material: Coal = 1.4; sand = 2.65; Garnet = 4.2

• Filter backwashing

• The backwash water without air supplementation is applied at therate of 36 to 56 m/h for 10 min duration from the under drainagesystem.

• In large treatment plant compressed air is used to ensure effective scrubbing for about 5 min. This also reduces the wastage of water.

• The air is applied at the rate of 0.9 to 1.5 m3/m2.min followed by wash water application at the rate of 19 to 29 m/h.

• This removes the turbidity entrapped in the sand pores and restores the filtration rate.

• Wash water troughs are provided above the sand bed to collect this wash water.


• This can be used as single stage treatment for removal of turbidity when raw water turbidity is less than 20 NTU.

• The standing water column height (1 to 1.5 m) above the sand bed provide driving force for water to overcome frictional resistance.

• Sand depth 0.5 to 0.7 m with effective sand size of 0.25 to 0.35 mm and uniformity coefficient of 3 to 5 is used.

• For new filter initial sand depth of about 1.0 m is provided.

• A layer of 10 to 20 mm will be removed every time the filter is cleaned.

• The under drainage system is made up of open jointed bricks, perforated pipes, or un-jointed tiles.

• Graded gravel about 0.2 to 0.3 m is placed above it to support sand and avoid its entry in treated water.


• Purification of water takes place by combination of straining, sedimentation, and biochemical processes.

• After start of filtration a thin slim layer called ‘schmutzdecke’ is formed on the surface of sand.

• The rate of filtration is 100 to 200 L/m2.hr (max. 400), no pretreatment is required.

• Minimum two filters should be used.

• 99% bacteria removal and more than 90% turbidity removal will occur.

• When the filter has reached to maximum permissible head loss, it is taken out of cleaning.

• The top few cm removed sand layer is washed externally dried andreplaced.


Disinfection

• Criteria for good disinfectant:

– It should be capable of destroying pathogenic organisms present within the contact time available.

– Also, the concentration at which it is toxic to microorganisms should be well below the toxic threshold for human and animals.

– It should not unduly influenced by the range of physical and chemical properties of water, such as, pH, temperature, mineral constituents.

– Should not leave products of reaction which render the water toxicor impart colour or otherwise make it unpotable.

– Posses the property of leaving residual concentrations to deal with small possible recontamination.

– It should be amenable to detection by practical, rapid and simple analytical techniques in the small concentration range.

• Mechanism of Disinfection: Killing of the pathogens depends largely on the nature of the disinfectant and on the type of microorganisms.

• The general four mechanism for destruction or inactivation of organisms are:

– Damage to cell wall (causes cell death).

– Alteration of cell permeability (causes outflow of nutrients from the cell).

– Changing the colloidal nature of the cell protoplasm.

– Inactivation of critical enzyme systems responsible for metabolic activities.

Disinfection

• Factors affecting efficiency of disinfection:

– Type, condition, concentration and distribution of organisms to be destroyed.

– Type and concentration of disinfectant.

– Chemical and physical characteristics of water to be treated.

– Contact time available for disinfection.

– Temperature of water.

Disinfection

Chlorination

• Gaseous chlorine is greenish yellow in colour and 2.5 times heavier than air.

• Under pressure it is liquid with an amber colour and oily natureabout 15 times heavier than water.

• Dry chlorine is non-corrosive but highly corrosive in contact with moisture.

• Chlorine gas is harmful to human beings. It is powerful irritant to lungs and eyes.

• For safety limit the permissible concentration in air is 1 ppm for exposure of 8 hr.

• Chlorine react with water to form hypochlorous acid (HOCl) and Hydrochloric acid (HCl).

• The hypochlorous acid dissociate in to (H+) ion and hypochlorite ions (OCl-)

• Free available chlorine is defined as the chlorine existing in water as hypochlorous acid and hypochlorite ions.

• The undissociated HOCl is about 80 to 100 times more potent as a disinfectant than the OCl ions.

• At pH value of 5.5 and below 100% unionized HOCl exists.

• While above pH 9.5, OCl ions are 100%.

• Between pH 6 and 8.0 there occurs very sharp change from undissociated to completely dissociated OCl- form.

• Chlorine concentration of 2 to 3 mg/L with contact time of 30 min is sufficient.

Chlorination

• Points of chlorination– Pre-chlorination: Application prior to any treatment process to:

• control biological growth in raw water conduit, • improve coagulation, • prevent slim formation on filter beds and prevent development of

large population growth within the filter bed;• reduction of taste and odour, and • control undesirable production of gases in the sludge settled in

clarifier.– Post-chlorination: Application of chlorine to water before it

enters the distribution system to maintain required amount of free chlorine.

– Re-chlorination: To maintain minimum residual chlorine of 0.2 mg/L in case of large distribution system, chlorine is added stage wise in distribution system (ESR).

Chlorination

• Break point chlorination– When chlorine is added in water it first react with ammonia and

other organic matter present in water and form chlorine compounds.

– In practice it has been found that many times with the increase of applied chlorine dosage, the residual combine chlorine increases, and then suddenly drops down.

– With further addition of chlorine dose, It increases steadily beyond certain dose in a straight line manner.

– This point where it reaches to the minimum is referred as ‘break point’.

– When chlorine is added beyond this point it is known as break point chlorination.

Chlorination

• When Chlorine is added to water it does not react until certain stage is reached.

• After point ‘C’, it starts reacting and combining with them.

• So the residual chlorine drops down from C to D in spite of increased chlorine dose.

• The portion OC chows formation of chloramines is known as combined residual.

• CD shows the oxidation of chloramines.

• Point ‘D’ is break point, and line E represents free residual chlorine.

Chlorination

Disinfection other than chlorine gas• Chlorine compounds: Bleaching powder, sodium or calcium hypochlorite are

used in small plants.

• Bleaching Powder: Mixture of calcium hydroxide, calcium chloride, and calcium hypochlorite

– When mixed with water calcium hypochlorite breaks into Cl2 and CaCl2.

– Bleaching powder has 20 to 30% (w/w) available chlorine.

– It is unstable compound and looses available chlorine during storage.

• Hypochlorite: Calcium hypochlorite can be fed as solution and sodium hypochlorite is added as solution.

– 1 to 2% of chlorine solution is added.

• Chlorine dioxide: It is unstable gas, but aqueous solution is stable and safe.

– Efficiency is unaffected in the pH range of 6 to 10.

– It does not combine with ammonia and organic compounds before oxidizing them.

Other Disinfectant• Heat

– Boiling of water will disinfect it.

– Can not be used for public water supplies

• Iodine

– It is used for disinfection of swimming pool water and small quantity water treatment.

– It is bluish black solid and its addition to water yields hypoidous acid (HOI) and hypoidodite (IO).

– It react less with organic matter and does not react with ammonia, but oxidizes ammonia.

– Both iodine and hypoidous acid are good disinfectant.

• Other halogens such as bromine and fluorine can not be used as disinfectant.

• Ozone

– It is faint blue gas with pungent odour.

– It break downs to nascent oxygen and it is powerful oxidizing agent.

– Typical dosege range from 1 to 5.3 mg/L

– Cost 2 to 3 times more than chlorine.

– Superior bactericidal than chlorine (99.99% E-coli killing within 100 sec.) and effective in removal of taste, odour, colour, ironand manganese.

– Efficiency is unaffected over wider pH and temperature range.

– The drawback is no residual effect.

Other Disinfectant

• Potassium permanganate

– Effective in removal of taste, odour, iron, and manganese, H2S, apart from disinfection.

– Expensive than chlorine and give colour for some time.

• Ultraviolet radiation

– Exposure of water to sunlight or ultraviolet rays can lead to destruction of microorganisms.

– Wavelength region 2000 to 3000 Ao is effective.

– Water should be free from turbidity and other light absorbing substances.

– 99.99% E-coli killing can be achieved.

– Advantage : No foreign matter is added.

– Disadvantage: No residual effect and expensive.

Other Disinfectant

• Acids and bases– pH below 3 or above 11 is toxic for pathogens and they do not

survive long in such waters.

• Metal ions– Silver, copper, mercury, cobalt, nickel posses significant

bactericidal properties.

– Except silver others are not suitable for drinking water.

– Require long contact time, but very low concentration of 15 x 10-12

g/L is sufficient to destroy most organisms.

– It can be introduced in water as silver salts or silver coated electrodes with electric potential (100V).

Other Disinfectant

water treatment 25-01-2012

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