22 Questions
What is the main purpose of a jar test?
To determine the proper coagulant and coagulant aid, and chemical dosage required
What does the velocity gradient (G) measure?
The relative velocity of two fluid particles per unit distance
What is the formula to calculate the power input (P) for mixing by impellers?
P = KT n^2 Di^3 μ
What is the condition for laminar flow in a mixing system?
NRe < 20
What is the formula to calculate the power input (P) for pneumatic mixing?
P = C1 Ga log (h+C2)/C2
What is the purpose of a velocity gradient in a baffle basin?
To measure the degree of mixing
What happens to colloids when they undergo destabilization?
They aggregate in size and start to settle
What is the purpose of adding Ca(OH)2 or NaHCO3 during the coagulation process?
To overcome the problem of reduced pH
What is the reaction between aluminum sulfate and bicarbonate ions?
Al2(SO4)3.14 H2O + 6HCO3- → 2Al(OH)3 + 6CO2 + 14H2O + 3SO4-2
What is the reaction between ferrous sulfate and calcium hydroxide?
2 FeSO4.7H2O + 2 Ca(OH)2 + 0.5 O2 → 2Fe(OH)3 + 2 CaSO4 + 13 H2O
What is the range of pH required for ferric sulfate coagulation?
pH 4 to 12
What type of mixing is required for coagulation?
Rapid mixing
What is the purpose of coagulation?
To aggregate colloids and form larger flocs
What is the type of agitator commonly used in coagulation and flocculation?
Mechanical agitators
What is the primary goal of the coagulation/flocculation process in water treatment?
To reduce turbidity in water
What is turbidity a measure of in a water sample?
The amount of suspended solids
What is the role of a coagulant in the coagulation process?
To cause small particles to come together to form large particles
What is the purpose of adding alum (Aluminum sulfate) to water during the coagulation process?
To cause small particles to come together to form large particles
What are non-settleable solids?
Solids that can not settle down by virtue of gravity
What is the process by which colloids can be destabilized?
Charge neutralization
What is the role of positively charged ions in the coagulation process?
They destabilize the colloids
What is the result of removing turbidity from water?
All of the above
Study Notes
Coagulation and Flocculation
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The main goal of coagulation/flocculation process is to reduce turbidity in water, which is a measure of the amount of suspended solids in water sample that cause cloudiness.
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Turbidity affects taste, odor, and disinfection rates of water, as lower turbidity allows for higher rates of disinfection.
Coagulants
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A coagulant is a chemical that is added to water to cause small particles to come together and form large particles that can settle due to gravity.
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Typical coagulants include Alum (Aluminum sulfate) and Ferric sulfate (Fe2(SO4)3).
Coagulation
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Coagulation is a chemical process in which a coagulant is added to water and suspended by rapid mixing to aggregate dispersed non-settleable solids by destabilization.
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The process involves destabilization of colloids, which are particles that stay in suspension due to repulsive forces.
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Positively charged ions (e.g., Na+, Mg2+, Al3+, Fe3+) can neutralize the colloidal negative charges and thus destabilize them, allowing them to aggregate and settle.
Theory of Coagulation
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Destabilization of colloids can be achieved through charge neutralization or bridging with organic polymers.
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Colloids can be destabilized by adding positively charged ions or polymers that react with the colloidal surface, allowing them to aggregate and settle.
Coagulants Properties
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Aluminum sulfate (Alum):
- Consumes 6 moles of bicarbonate (HCO3-) per 1 mole of alum.
- Can lower pH if alkalinity is not enough.
- Can be overcome by adding Ca(OH)2 or NaHCO3 (Soda Ash).
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Ferrous sulfate:
- Forms a dense, quick-settling floc (Fe(OH)3).
- Requires pH to be raised to 9.5.
- Can be oxidized to form Ferric sulfate (Fe2(SO4)3) at pH 4.
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Ferric sulfate:
- Forms a floc that can settle quickly.
- Requires pH to be in the range from 4 to 12.
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Ferric chloride:
- Forms a floc that can settle quickly.
- Requires pH to be in the range from 4 to 12.
Coagulation and Flocculation Process
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Coagulation involves rapid mixing to form microflocs, while flocculation involves gentle mixing to form larger flocs.
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Agitation devices include mechanical agitators, pneumatic agitators, and baffle basins.
Jar Test
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A laboratory technique used to determine the proper coagulant and coagulant aid, as well as the chemical dosage required.
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Variables include speed of rotation, time of settling, dosage, size of flocs, turbidity removal, color removal, and pH.
Mixing and Agitation
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Velocity gradient (G) measures the degree of mixing, with higher G values indicating more intense mixing.
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Velocity gradient can be calculated using power input, tank volume, and dynamic viscosity.
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Mixing can be achieved through impellers, pneumatic agitation, or baffle basins, each with its own unique characteristics and design equations.
Learn about the importance of coagulation and flocculation in water treatment, including reducing turbidity, improving taste and odor, and enhancing disinfection.
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