1Module 5

Module 5 Water TreatmentOn completion of this module you should be able to:

• Be aware of the objectives of water treatment

• Have an appreciation of the location, layout of a plant

• Describe the processes involved in water treatment

• Discuss the types of separation processes

• Design a simple sedimentation tank

2Module 5

Basic Methods for Correcting Water Quality Deficiencies

• The processes and extent of required treatment are dependent on the nature and degree of quality deficiencies to be corrected.

• There is virtually no water that cannot be treated to potable standards. Cost effectiveness is one of the guiding principles

• The basic methods are physical and chemical processes and to a lesser extent, biological

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WaterTreatmentmatrix

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5Module 5

Plant Layout and Headloss Through the Plant

• Planning and environmental constraints

• Selected source

• Plant design factors

• Site factors

• Environmental factors

• Unit processes should lie on the system gravity hydraulic grade line

6Module 5

Preliminary TreatmentDepending on the source, the following unit processes are likely

• Intake screens

• Aeration

• Preliminary settling tanks

• Pre-chlorination and algal control

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Intake screen

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Aeration

• Increase dissolved oxygen in ‘stale’ water

• Remove or reduce dissolved CO2 and other gases

• Precipitate out dissolved ferrous and manganese compounds

• Reduce volatile impurities and odour

• Various methods of aeration e.g. spray, cascade, tray and diffused air

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Preliminary settling tank

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Chemical treatment through coagulation

• Coagulants are chemicals that react with colloidal matter to form absorbent bulky precipitates (flocs)

• Destabilisation of colloidal particles (10-3 - 1 m), hydrophilic or hydrophobic in nature

• Salts of aluminium and iron form insoluble hydroxides

• Reaction is pH dependent (6 - 7 optimum range)

11Module 5

Aluminium salts are commonly used

• Aluminium sulfate; sodium aluminate

• Natural or added alkalinity is required

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

• Reaction is sensitive to pH

• May revert to soluble for if pH increases/decreases

• Some recent concerns relating to health issues

12Module 5

Aluminium sulfate

• Al2(SO4)3 + 6H2O 2Al(OH)3 + 3H2SO4

• H2SO4 + Ca(OH)2 CaSO4 + 2H2O

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

In the absence of alkalinity

Natural alkalinity

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Iron salts are more difficult to control

• Ferric chloride/iron(III) chloride; ferric sulfate

• 2FeCl3 + 3Ca(HCO3)2 2Fe(OH)3 + 3CaCl2 + 6CO2

• Natural or added alkalinity is required

• Wider operating pH range

• Cheaper material and forms heavier floc

• Iron salts cake and are dirty to handle, difficult sludge to dispose

14Module 5

Coagulant aids

• They assist difficult coagulant processes and result in dramatic improvement with increased floc formation and faster settling

• Polyelectrolytes of organic synthetic high molecular weight material with electrical charges

• Clays, lime, soda ash and activated silica are other examples of coagulant aids

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Optimum coagulant dosage

• Use of laboratory jar test

• Determine least cost of chemicals that remove turbidity, colour in an shortest possible time

• Comparison of first floc appearance, floc size, dosage and settling time

• Optimum dosage also tested against pH

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Optimumcoagulantdosageusing the jar test

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Flash/Rapid MixingTo cause rapid dispersion at minimum power input

• Use of various devices e.g. bends, baffles, can result in energy losses

• Energy for good mixing requires 3 - 15 kW.s/m3

• 30 - 60 sec detention time at maximum flow

• Rate of chemical diffusion is quantified by the shear velocity gradient, G = [P/(V)]0.5

18Module 5

Flash/Rapid Mixing (cont)

• G = 500 - 600 s-1 at 30 - 60 s residence time

• Mechanical power for head loss, P = Q g h watt

• Head loss from hydraulic mixing varies 0.15 - 0.5 m

• Excessive G values can be harmful

• Increased contact time of 120 s or more achieve little

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Flash/Rapid Mixer

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FlocculationGentle stirring following rapid mixing so that floc particles can coalesce and agglomerate

• Two phases are involved; initial perikinetic, orthokinetic > 1 m

• Shear velocity gradient, G = 20 - 75 s-1

• Detention time, t = 20 - 60 minutes

• Camp No, G t of (12 to 270) x 103

• Mechanical flocculation power input

• Tapered flocculation using high G values and progressively lower as floc size increase

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Relationship between Shear Gradient and time, t

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A Mechanical Flocculator

23Module 5

SedimentationRemoval of suspended particles in an aqueous medium through gravity settling

• Class I Unhindered settling of discrete particles

• Class II Settling of dilute suspension of flocculent particles

• Class III Hindered settling and zone settling

• Class IV Compressive settling (compaction)

24Module 5

Class I settlingFor discrete particles settling freely, the terminal velocity is reached when gravitational force is balanced by frictional drag force

• vs = g d2 (1 - )/(18 )

• As particle size increases, vs increases

• As CD increases vs decreases

• CD varies inversely as NR

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26Module 5

Class I settling (cont)

• Detention time, t = Volume/Q

• Depth of tank is not relevant, vs = Q/surface area

• Performance is influenced by overflow rate and detention time

• High water temperature decreases CD and thus increases vs

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Drag coefficient

28Module 5

Shallow depth sedimentation

• Proposed as early as 1904 with initial failure

• Obvious inherent advantages

• Tube clarifiers with high surface loading rates achieve 9 m/h

• Plated tanks in zig-zag pattern, vh = 44 m/h, HRT of 22 min

• Lamella separator with vs = 20 m/h

Module 5 29Tube clarifier at Mt Kynoch settling tank

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Shallow depth sedimentation

Plate settler tank

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Shallow depth sedimentation

Lamella separator tank

32Module 5

Difficult settling operation conditions

• Excessive suspended solids

• High colloidal content

• Coincidence of peak demand and high turbidity

• Low coefficient of fineness < 1

• Low temperature, overturn

• Persistent wind condition

• Streaming caused by density currents, temperature gradients

33Module 5

Settlement in horizontal flow tanks

• Overflow rates varies from 18 to 54 m/d

• Typically 28 m/d for a 3.5 m depth and 3 h HRT

• In tropical countries with more turbid water, 18 m/d with 4 h HRT is appropriate with depths of 3 - 3.5 m

• In practice, particles are not wholly discrete and there is merit in depth

• As a preliminary guide use HRT x (TSS/900)0.5 h to adjust for varying TSS in water

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A typical horizontal flow sedimentation tank

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Settlement in upward flow tanks

• Area of tank to ensure vs > v = Q/A

• In practice, vs 2 v• vs = 3 m/h for well formed floc

• = 6 - 10 m/h with coagulant aids

• = 8 m/h in water softening plants

• Types: hopper bottomed sludge blanket square tanks, solid contact clarifiers, pulsator

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Vertical flow tank

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Pulsator

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Solids contact clarifier

39Module 5

FlotationAn effective means of removal of particles of density less than the liquid medium

• Use of air bubbles to separate solids/particulates from a liquid phase

• Air bubbles (20 - 100 m) generated by dissolved air flotation, diffused air flotation and vacuum filtration

• Attachment of solids to bubbles in a 3 phase system; size of flocs less important

• Solids separation through a floating scum and removed by a skimmer

40Module 5

Flotation (cont)

• Advantage of high surface loading rates 5 - 12 m/h, and the ability to remove oils, grease and algae

• Short HRT of 40 - 80 minutes; bubbles rise at 1 - 1.5 mm/s

• Flotation units are smaller in size than normal clarifiers

• Saving in chemical costs

• Optimum amount of air is determined from pilot studies

• Disadvantage of additional equipment cost, high operating cost and energy use

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Dissolved Air Flotation (DAF)

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Dissolved Air Flotation (DAF)

Dissolved air flotation

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FiltrationA process of passing water through a sand bed or other suitable medium at low speed to remove suspended solids

• Removal of non-settleable flocs after coagulation and sedimentation

• Properties of the medium (effective size, hardness etc)

• More than a mechanism of straining

44Module 5

Mechanisms of filtration

• Straining

• Sedimentation

• Interception

• Adhesion

• Flocculation

• Adsorption

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Rapid sand filterA process of depth filtration as solids are removed within the granular medium

• Sand bed 0.6 - 0.75 m deep of 0.4 - 0.7 mm effective size and a uniformity coefficient 1.7

• Supporting gravel layer 0.3 - 0.5 m (graded 2 - 60 mm)

• Underdrain system to collect filtered water and to discharge air scour and backwash water uniformly

• Filtration rate varies from 4 - 15 m/h

46Module 5

Rapid sand filter (cont)

• Backwash when head loss 2 m

• Application of backwash water assumes practical importance in the design of filters

• Some problems associated with rapid sand filters are mud balls, air-binding, surface cracks and shrinkage

• Other forms are direct filtration, and up-flow filtration

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Arrangement of filter media

clogs up readily ideal but unattainable

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Typical rapid sand filter

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Rapid sand filter isometric view (Droste 1997, p. 418)

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Types of filter underdrain system (McGhee, 1991, p.212)

51Module 5

Flow control for rapid sand filtersMost systems include some means of automatic flow control valves operated by signals from level-sensing or flow-sensing elements

• Flow control systems are usually operated hydraulically or pneumatically

• Avoid control conditions that lead to controller instability e.g. 'hunting' caused by continual over correction

• Downstream flow control

• Upstream flow control

• Control system with common head loss

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Rapid sand filter flow control systems

53Module 5

Problems associated with rapid sand filters

• Negative head

• Dirty filter media (mud ball formation)

• Mineral deposits

• Gravel movement during backwashing

• Underdrain failure

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Negative head

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Slow sand filtersThese are the oldest and effective method for removing pathogenic microorganisms in water. Cake filtration when solids are removed on entering the face of the granular material

• No pre-treatment or chemicals are required

• Filter media 0.7 - 1.2 m layer of 0.2 - 0.4 mm effective size with a uniformity coefficient 3

• Supported on gravel layer 0.1 m (graded 5 - 25 mm)

• Relies on surface straining and microbial action (schmutzdecke)

• Slow filtration rates of 350 - 700 L/s.ha (3 - 6 m/d)

56Module 5

Slow sand filters (cont)

• 1 - 3 months filter run or when head loss 1 m

• Surface renewal by removing 12 - 25 mm of surface layer each time until 600 mm of sand layer is left

• Requires large land area

• Labour intensive to remove and clean the sand

• Suitable for reservoir-fed supply and small communities requiring no technical supervision

• Does not remove colour but is able to deliver bacteriologically superior water

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Slow sand filter

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Pressure filtration

• No different from rapid sand filters

• Filter lies on the HGL

59Module 5

Chlorine disinfectionIt is presently the most cost-effective disinfection method but it has some adverse effects

• Properties of chlorine

• Reaction is highly pH dependent

• Cl2 + H2O HOCl + HCL

• As pH increase the hypochlorous acid (HOCl) will further dissociate to H+ and OCl- (hypochlorite ions)

• HOCL and OCl represent the free available chlorine

60Module 5

Chlorine disinfection (cont)

At 20o CpH %HOCL %Ocl

6 97 3

7 79 21

8 21 73

9 4 96

Chlorine:ammonia reaction

Breakpoint chlorination

Superchlorination

61Module 5

Chlorine - ammonia reaction

Formation of monochloramine (NH2Cl)

HOCl + NH3 H2O + NH2Cl Cl2:NH3 < 5:1; pH 7

Formation of dichloramine (NHCl2)

NH2Cl + HOCl H2O + NHCl2 Cl2:NH3 < 10:1

Formation of trichloramine (NCl3)

NHCl + HOCl H O + NCl3 Cl2:NH3 < 20:1; pH < 4

Monochloramine and dichloramine represent the combined available chlorine, with less disinfecting power compared with the free available chlorine

62Module 5

Breakpoint chlorination

• Oxidation of chloramines until appearance of free available chlorine

• At this point the free available chlorine residual is lowest

• Taste, odour are reduced through oxidation

• Some colour may also be removed

• Good control required to ascertain that breakpoint is reached

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Breakpoint chlorination

64Module 5

Superchlorination

• High concentration of Cl2 is used to completely oxidise ammonia, organics, chlorophenols, colour, taste & odour

• Short contact time and effective when contamination is anticipated

• Dechlorination is necessary using SO2, sodium bisulfate or activated carbon to remove the high chlorine residue

65Module 5

Factors affecting chlorine disinfecting efficiency

• Turbidity and organic matter

• Metallic compounds

• Contact time

• Temperature

• pH value

• Type of microorganisms

66Module 5

Other disfecting agents

• Ozone gas, O3

• Chlorine dioxide

• Iodine, bromine (halogens)

• Silver (metal ions)

• Simple retention time

• Heat

• Ultra-violet light

• Ultrasonic radiation

• Ultra-filtration

67Module 5

Water softening

• Chemical precipitation and ion-exchange

• Carbonate hardness

Ca2+ requires lime to raise the pH to 9.5 – 10 when HCO3- is

changed to CO3

Mg2+ requires more lime to pH 10.5 – 11 when HCO3- is changed to

• Non-carbonate hardness

Ca2+ requires soda ash to precipitate CaSO4

Mg2+ requires soda ash and lime to precipitate to Mg(OH)2 and

CaCO3

68Module 5

Ion-exchange

• Ca2+ and Mg2+ are exchanged for sodium ions from zeolites or resin compounds (Na2R)

• Regeneration by washing with brine solution and CaCl2 and MgCl2 are discharged

• Ion-exchange plants are easy to operate, in-line with the hydraulic gradient

69Module 5

Ion-exchange (cont)

• Flow rate 15 m/h and no solid sludge discharge but not suitable for turbid water or iron > 5 mg/L

• For very hard water, precede with lime-soda ash softening. Water should first be treated by coagulation, sedimentation & filtration prior to ion-exchange process

• Does not reduce total dissolved solids (TDS)

70Module 5

Taste and odourThey are often the immediate and main sources of consumer complaints

• Action by microorganisms, decomposition

• Reduction of sulfates to sulfides under anaerobic condition

• Sewage and industrial discharges

• Reaction with phenols & organics by chlorine

• Urban runoff from asphaltic surfaces

• Leachates from landfills

71Module 5

Taste and odourRemedies

• Aeration may precipitate out iron and remove sulfuretted hydrogen odours found in deep bores

• Superchlorination and dechlorination

• Chloramine of 1:2 – 1:4 of NH3:Cl2 to produce combined available chlorine

• Chlorine dioxide has a stronger oxidising property than chlorine

• Ozonation is more powerful than chlorine and leaves no after-taste, but is also more expensive

• Activated carbon in the form of powder, granular or filter beds, which removes taste and odour by adsorption and also removes a wide range of complex organics eg. pesticides

72Module 5

Iron & manganese removalProblem lies with the variety of reactions that can occur with these element

Oxidation• Aeration followed by sedimentation and filtration

• Use of oxidising agents eg. Chlorine, chlorine dioxide, potassium permanganate

Lime softening

• Increasing pH after aeration to precipitate as Fe(OH)3

Catalytic action

• Oxidation of manganese in zeolite or pyrolusite ore with

KMnO4

73Module 5

Iron & manganese removal (cont)

Ion-exchange

Zeolite or ion-exchange resins to remove Fe3+ and Mn2+ if associated with HCO3

Suitable for groundwater devoid of O otherwise Fe3+ or Mn2+ ions will clog the ion-exchange resin bed

Sequestering

Use of complex molecules to encase the ions of Fe3+ and Mn2+ so that they no longer participate in future reactions

Common sequestering agents are polyphospates or organic compounds eg. ‘Calgon’ (sodium hexametaphosphate) but subsequent heating may destroy treatment

74Module 5

Desalination

Distillation

• Simple to multi-stage distillation

• Multi-stage flash distillation

• Vapour compression

• Solar stills

Freezing

• H2O molecules form and attach to ice crystal, while salt molecules remain in solution

• Latent heat of fusion is 333 kJ/kg

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Desalination

Multi-stage flash distillation (Barnes et al 1986, p.348)

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Vapour compression

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Desalination (cont)

Ion-exchange

• Use of cation and anion exchange resins

• Simple recharge using acids for cation resin and alkaline for anion resin

• Na+ + HR H+ + NaR

• R(OH) + Cl- OH- + RCl

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Desalination

Anion and cation exchange (Barnes et al 1986, p.352)

79Module 5

Desalination (cont)

Reverse osmosis

• Membranes permitting only water molecules but not solute to pass

• Under normal osmosis water flows from low solute concentration to high solute concentration

• Osmotic pressure is pressure to stop this flow

• When pressure applied > osmotic, then reverse osmosis occurs

• Turbidity, iron, Mn2+, CaCO3 must first be removed

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Desalination

Reverse osmosis (Barnes et al 1986, p.354)

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Desalination• DALBY commissioned a DESALINATION

PLANT (2004) to supplement water for its 10,000 residents whose regular supply from bores is too brackish

• The $2.8m reverse osmosis plant will supply a quarter of its annual needs at roughly the same price - 32c/kL - as current price for treated summer water drawn from the Condamine River

82Module 5

Desalination (cont)

Electro-dialysis

• Use of electrodes to maintain an electric field in which ions will move as in electrolysis

• 2 kinds of special membrane selective each to cations and anions

• Conditions tend to be acid at anode and alkaline at cathode. To prevent CaCO3 deposits at the cathode it is continually washed with acid rinse

• Operating potential difference 1000 V

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Desalination

Electrodialysis process (Barnes et al 1986, p.357)

84Module 5

End of Module 5 Water Treatment