Coagulation and Flocculation Flashcards
Name the three classifications of substances in water.
- Suspended solids removed via filtration.
- Dissolved solids removed with phase change (e.g distillation).
- Colloidal (1nm to 1µm) particles are microscopic dispersed insoluble particles that have a
clear phase boundary.
Define colloidal particle.
Colloidal particles are very small microscopically dispersed insoluble particles that have a clear phase boundary.
Properties:
-
List four properties of colloidal particles.
- Do not aggregate (clump together).
- Large surface area to mass (Big SA:V).
- Insufficient mass to overcome the fluid forces (e.g surface tension, drag) so particles do not settle.
- Colloidal particles are negatively charged, preventing aggregation and settling in water.
Why are colloidal particles difficult to separate from water?
- Does not settle by gravity.
- Small enough to pass through sand filtration units.
How do you remove colloidal particles from water?
Aggregate particles to form larger particles that can be separated in downstream clarification using sedimentation or filtration.
Do colloidal dispersions have a net charge? Why / why not?
Colloidal dispersions (group of colloids) have no net charge
This is due to the presence of counter-ions (electrical double layer) in the diffuse layer to balance the negative charges.
Define zeta potential.
The magnitude of charge at the shear surface of a single colloidal particle in a colloidal dispersion.
Zeta potential indicates the stability of colloidal dispersions by representing the repulsive or attractive forces between particles based on their individual surface charges.
Give the equation for zeta potential.
z = 4 π q d / D
q = particle charge
d = thickness of the effective charge layer surrounding shear plane.
D = Dielectric constant of the medium (e.g., water = 78.35 at 25°C).
What does a high or low zeta potential represent?
High zeta potential = Stable dispersion (particles repel each other).
Low zeta potential = Unstable dispersion (particles aggregate, leading to settling).
Describe the principle of destabilising colloidal systems?
Overcome net repulsion force (energy barrier) by adding coagulates to the water to create net attractive force, allowing particles to aggregate.
Coagulates neutralises the electrical charge and overcome London forces to reduce repulsion.
Name the four mechanisms that can cause the destablisation of colloidal dispersions.
- Double layer compression
- Surface charge neutralisation
- Entrapment in precipitate
- Particle bridging
What is double layer compression?
Destablising colloidal systems by adding an electrolyte containing ions of the opposite charge to the colloid particles.
In double layer compression, how does adding electrolyte destabilise the system?
Oppositely charged electrolyte ions enter the diffuse layer surrounding the particles.
When sufficient amount of electrolyte is added, the counter ions compress the diffuse layer.
Thus, reducing the energy required to move particles with like charge closer together.
Does the amount of electrolyte required for double layer compression dependent on colloid concentration?
No, the amount of electrolyte needed to achieve coagulation is independent of the colloid concentration in the water.
Give an example of double layer compression in natural environments.
When river water (low ionic strength) mixes with sea water (high ionic strength), particles in the river water are destabilised by double layer compression, leading to coagulation and settling.
Give two limitations of double layer compression method in water treatment.
Slow particle aggregation.
High electrolyte concentrations required e.g NaCl concentrations near seawater levels.
What does the double compression layer technique change to cause coagulation?
Changes the characteristics of the medium by adding electrolyte to reduce the thickness of the diffuse layer.
What does the surface charge neutralisation technique change to cause coagulation?
Changes the characteristics of the colloid particles so that the colloidal particles have a neutral charge.
How does surface charge neutralisation cause coagulation?
Coagulants (oppositely charged ions) adsorb to the particle surface, reducing the net surface charge and repulsion.
Therefore, the thickness of the diffuse layer and energy required to move particles with like charge closer together are reduced.
Why is surface charge neutralisation more preferable than double layer compression?
Surface charge neutralisation is more effective because the sorbable species requires a lower dosage to destablise colloidal systems non-sorbable ions.
How is the dosage of coagulant determined in surface charge neutralisation?
Stoichimetry
As colloidal concentration increases, coagulant dosage increases.
What happens if too much coagulant is added?
Overdosing of adsorable species causes re-stablisation of system due to the reversal of charge on colloidal particles.
What is entrapment in precipitate / sweep-floc coagulation?
A coagulation process where metal salts are added to water, forming precipitates that enmesh or trap colloids as they settle.
What is the relationship between optimum coagulant dosage and colloidal concentration in sweep-floc coagulation?
Inverse
Low colloidal concentration = high coagulant dosage
High colloidal concentration = low coagulant dosage