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Colloids and stable suspensionsColloids and stable suspensionsColloids and stable suspensionsColloids and stable suspensionsColloids and stable suspensionsColloids and stable suspensionsColloids and stable suspensionsColloids and stable suspensions ColloidsColloidsColloidsColloids: particles that do not settle under the influence of
gravity but remain stable in suspension in a fluid medium
Size range: 1 nm to 1 micron (or even 10 microns) Inorganic particles like asbestos fibers, silt, clay particles
Organic particles: NOM (humic and non-humic substances),viruses bacteria lankton microbes
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Particles in natural watersParticles in natural watersParticles in natural watersParticles in natural waters (generally in pH range of 6 to 8) arenegatively charged
Stable particlesStable particlesStable particlesStable particles: Like charges repel each other and remainsuspended in solution indefinitely
No aggregation is possible due to net repulsive energybetween them
Examples: Silt in rivers, turbidity of lake waters, andground waters
Coagulation and flocculationCoagulation and flocculationCoagulation and flocculationCoagulation and flocculationCoagulation and flocculationCoagulation and flocculationCoagulation and flocculationCoagulation and flocculation Process for combining small particles (colloids) into larger,Process for combining small particles (colloids) into larger,Process for combining small particles (colloids) into larger,Process for combining small particles (colloids) into larger,
settleable aggregates (flocs)settleable aggregates (flocs)settleable aggregates (flocs)settleable aggregates (flocs) Addition of coagulants like Al or Fe salts, organic polymers to
‘destabilize’ particles so that they will aggregate, form floc and settle Primary objective in water treatmentPrimary objective in water treatmentPrimary objective in water treatmentPrimary objective in water treatment: Removal of turbidity
(particles) and suspended solids (SS) Light is scattered most easily by colloidal suspensions
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oagu a on o ows se men a on an screen ng w ere arger eav er particles or floc are removed by discrete settling
Design objective is removal of colloidal particles (1 nm to 1Design objective is removal of colloidal particles (1 nm to 1Design objective is removal of colloidal particles (1 nm to 1Design objective is removal of colloidal particles (1 nm to 1micron)micron)micron)micron)
Can remove bacteria, soil, sand and clay particles
Concomitant removal of associated compounds or smaller particles like NOM, heavy metals, pesticides, etc. Enhanced coagulation: when an elevated coagulant dose (higher
than the optimum required for turbidity removal) is used to remove
TOC (or NOM) to ensure that DBP conc are not exceeded
Suspended versus dissolved solidsSuspended versus dissolved solidsSuspended versus dissolved solidsSuspended versus dissolved solidsSuspended versus dissolved solidsSuspended versus dissolved solidsSuspended versus dissolved solidsSuspended versus dissolved solids Distinction between dissolved and suspended solids varies greatly in the
literature.
There are no theoretical cut-offs for these particles
Can only have a cutoff defined based on operational convenience
Choice of filter pore size (anywhere from 1.5 micron to 0.2 micron) isbased on operational requirement or objective.
Colloids fall into either category ‘operationally’ speaking.
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Standard methods Dissolved solids are measured after filtration through glass fiber
filters (no ash residue on burning)
Recommended filters: Whatman 934AH has nominal pore size of 1.5 micron; Millipore AP40 -?
Bacteriological requirements Separation of bacteria from water (especially drinking water
Coagulation, Flocculation and PrecipitationCoagulation, Flocculation and PrecipitationCoagulation, Flocculation and PrecipitationCoagulation, Flocculation and PrecipitationCoagulation, Flocculation and PrecipitationCoagulation, Flocculation and PrecipitationCoagulation, Flocculation and PrecipitationCoagulation, Flocculation and Precipitation
Coagulation:Coagulation:Coagulation:Coagulation: chemical conditioning of particleschemical conditioning of particleschemical conditioning of particleschemical conditioning of particles
Destabilization and change in physicoDestabilization and change in physicoDestabilization and change in physicoDestabilization and change in physico----chemicalchemicalchemicalchemicalproperties of colloidal particlesproperties of colloidal particlesproperties of colloidal particlesproperties of colloidal particles
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Gentle mixing of destabilized suspensions to accelerateGentle mixing of destabilized suspensions to accelerateGentle mixing of destabilized suspensions to accelerateGentle mixing of destabilized suspensions to accelerateinterparticle contact, promoting aggregation and settlinginterparticle contact, promoting aggregation and settlinginterparticle contact, promoting aggregation and settlinginterparticle contact, promoting aggregation and settling
PrecipitationPrecipitationPrecipitationPrecipitation:::: Dissolved minerals (TDS) like Ca, Mg in water can beDissolved minerals (TDS) like Ca, Mg in water can beDissolved minerals (TDS) like Ca, Mg in water can beDissolved minerals (TDS) like Ca, Mg in water can be
precipitated as SS by addition of chemicals like lime andprecipitated as SS by addition of chemicals like lime andprecipitated as SS by addition of chemicals like lime andprecipitated as SS by addition of chemicals like lime andsoda ashsoda ashsoda ashsoda ash
Zeta potential or electrokinetic potentialZeta potential or electrokinetic potentialZeta potential or electrokinetic potentialZeta potential or electrokinetic potentialZeta potential or electrokinetic potentialZeta potential or electrokinetic potentialZeta potential or electrokinetic potentialZeta potential or electrokinetic potential: Potential that
causes charged particles to move towards an oppositely
charged electrode and is associated with the plane of
Double layer modelDouble layer modelDouble layer modelDouble layer modelDouble layer modelDouble layer modelDouble layer modelDouble layer model Accumulation of counter ions on and around a charged particle results in Accumulation of counter ions on and around a charged particle results in Accumulation of counter ions on and around a charged particle results in Accumulation of counter ions on and around a charged particle results in
Increase in solution I results in higher conc of counterions in the diffuseIncrease in solution I results in higher conc of counterions in the diffuseIncrease in solution I results in higher conc of counterions in the diffuseIncrease in solution I results in higher conc of counterions in the diffuse
layer layer layer layer Volume of diffuse layer required to maintain electroneutrality
reduces
Destabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloids
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Example: when freshwater (low I) meets seawater (high I) – particlesin freshwater (silt) get destabilized and settle forming deltas
As double layer gets compressed As double layer gets compressed As double layer gets compressed As double layer gets compressed
Particles can come closer to each other (electrostatic repulsion is
reduced)
VDW forces dominate and net interaction energy becomes attractive
Destabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloidsDestabilization of colloids
Adsorption and charge neutralizationAdsorption and charge neutralizationAdsorption and charge neutralizationAdsorption and charge neutralizationAdsorption and charge neutralizationAdsorption and charge neutralizationAdsorption and charge neutralizationAdsorption and charge neutralization Chargeneutralization(QMZ):Chargeneutralization(QMZ):Chargeneutralization(QMZ):Chargeneutralization(QMZ): AdditionofAlorFesaltsandorganicpolymersprovideshighconcentrationsofcounterionsthatneutralizenegativesurfacechargesonparticles
Al, Fe and Si polymeric species and pH reduction on addition of these salts Al, Fe and Si polymeric species and pH reduction on addition of these salts Al, Fe and Si polymeric species and pH reduction on addition of these salts Al, Fe and Si polymeric species and pH reduction on addition of these salts
Coagulation practiceCoagulation practiceCoagulation practiceCoagulation practiceCoagulation practiceCoagulation practiceCoagulation practiceCoagulation practice Alum doses are generally in the range of 5 to 50 mg/L Effective from pH of 5 to 7.5 (see solubility diagram)
Ferric chloride Effective from pH 4.5 to 9
Surface waters can be grouped into 4 categories Group 1: high turbidity – low alkalinity
Lowering of pH makes coagulation more effective, but neutralization maybecome necessary
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Group 2: high turbidity – high alkalinity Buffering by alkalinity is adequate for maintaining pH
Adsorption and charge neutralization will be less effective than in lowalkalinity waters
Higher coagulant dose for sweep floc formation
Group 3: low turbidity – high alkalinity Colloid conc is low, therefore adding turbidity causing particles like clay will
Particle Transport ProcessesParticle Transport ProcessesParticle Transport ProcessesParticle Transport ProcessesParticle Transport ProcessesParticle Transport ProcessesParticle Transport ProcessesParticle Transport Processes Aggregation of destabilized colloids requires flocculation
Flocculation is based on enhancing interparticle collisions
Greater the number of collisions, greater the probability that the particleswill aggregate and floc will be formed
Three physical processes for interparticle collisions to occur 1. Brownian diffusion (perikinetic flocculation)
Random motion of particles due to collision with water molecules
Driving force is velocity gradient (G) in either laminar or turbulent fluidfields
Gt = design parameter = measure of the relative velocity of twoGt = design parameter = measure of the relative velocity of twoGt = design parameter = measure of the relative velocity of twoGt = design parameter = measure of the relative velocity of two
particles of fluid and the distance between themparticles of fluid and the distance between themparticles of fluid and the distance between themparticles of fluid and the distance between them Gt values range from 104 to 105 for t ranging from 10 to 30 min
3. Differential settling
Vertical transport of particles results in collisions
Driving force is gravity, controlling parameter is settling velocity of particle
Flocculation or mixingFlocculation or mixingFlocculation or mixingFlocculation or mixing
Rapid mixing: for mixing the coagulant
Detention time is approx. 0.5 min, ideally should be 2 min
Impellers or in line blenders
Conventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatment
Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)
Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)Plate and tube settlers (M&E)
LAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIERLAYOUT OF TUBE SETTLERS CLARIFIER
(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)(Gangtok Water Treatment Plant)
COLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIERCOLLECTING CHANNEL OF CLARIFIER
to prevent biological growth on filter to prevent biological growth on filter to prevent biological growth on filter to prevent biological growth on filter
mediamediamediamedia
47Sincero and Sincero, 1996Sincero and Sincero, 1996Sincero and Sincero, 1996Sincero and Sincero, 1996