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
In entrapment in precipitate, how much coagulant is required for low colloidal concentrations and why?
Large excess of coagulant required to produce enough precipitate to enmesh the few colloidal particles as the precipitates settle.
(Advantageous to add turbidity)
In sweep-floc coagulation, how much coagulant is required for high colloidal concentrations and why?
Requires less coagulate dosage
Colloids serve as condensation nuclei for precipitate formation.
What does entrapment in precipitate depend on?
pH - optimum pH depends on the solubility-pH relationship of the coagulant, influencing precipitate formation.
NOT surface charge neutralisation / minimum zeta potential
Rank importance of methods in water treatment from most to least important.
- Entrapment in precipitate
- Surface charge neutralisation
3. - Double layer compression
Give two examples of coagulants used in sweep-floc coagulation.
Metal salts to form precipitates of metal hydroxides.
E.g. Aluminum sulfate (Al₂(SO₄)₃) to form aluminium hydroxide Al(OH)₃
Ferric chloride (FeCl₃) to form iron (III) hydroxide Fe(OH)₃
What is the Schulze-Hardy rule?
As valence of ion increases, coagulating power of ion increases.
Monovalent (1+) ions → 1
Divalent (2+) ions → 10
Trivalent (3+) ions → 1000
1:2:3 to 1:10:1000
E.g., Fe³⁺ is more effective than Fe²⁺.
How does particle bridging occur?
One end of a polymer is adsorbed by a particle and the polymer extends into the solution to attach to the surface of another particle through electrostatic attraction, forming a bridge.
What is particle bridging?
Using high molecular weight polymers (natural or synthetic) to attach to multiple particles, forming bridges between them to create larger aggregates.
What types of polymers are used in particle bridging?
Polymers can be anionic, cationic, or non-ionic and may be natural (e.g., starch, cellulose) or synthetic.
What type of electrostatic attraction occurs between the polymer and particle under different charge conditions?
Opposite charges = coulumbic attraction
Similar charges = ion exchange, hydrogen bonding and van der Waals forces
What happens if too much polymer is added in particle bridging?
Overdosing can saturate surfaces of colloidal particles, reducing sites for bridge formation.
What effect does mixing have on particle bridging?
Prolonged or intense mixing can break existing bridges, disrupting the coagulation process.
What are flocs?
Aggregates of particles that form when suspended particles in water bind together during the coagulation and flocculation process.
They typically have favorable settling characteristics due to their larger size and weight compared to individual particles.
Why are polymers often used with metal salts?
Using both coagulants and polymers produce flocs with favourable settling characteristics, thus improving particle removal efficiency.
What are the two steps in aggregation?
- Destabilisation- Allows attachment of particles when contact occurs.
- Transport- Affects the inter-particle collisions to promote contact between particles and form aggregates.
What is the role of chemical coagulants?
To hydrolyse and neutralise electrical charges on particles,
enabling them to aggregate, enmesh the aggregates and form flocs for settling.
What is coagulation?
The destabilisation and initial coalescing of colloidal particles.
Coalescing = merging of particles to form larger aggregates
List the three main features of the coagulation process.
- Flash or rapid mixing with high turbulence.
- Short contact time (secs or mins).
- Occurs in influent channels , tank basin with mixing devices or pipeline to flocculation basin tank.
Influent channels = pipelines directing water into treatment plant.
What factors are critical when considering coagulation performance?
Mixing through:
- Hydraulic mixing – flow energy using baffles for turbulence.
- Mechanical mixers – paddles or propellers.
- Pumped blenders – coagulant added through a diffuser in pipes.
What is flocculation?
A process that promotes agglomeration of destabilised small particles into larger, fast-settling flocs / particles.
What is the size range of flocs formed during flocculation?
0.1 – 3 mm, depending on the removal process.
Compare the mixing between coagulation and flocculation.
Purpose
C: destablise and initiate particle aggregation.
F: promote growth or larger flocs for settling.
Mixing intensity
C: Flash or rapid mixing with high turbulance.
F: slow and gentle mixing to avoid breaking flocs.
List the three main features of the flocculation process.
- Slow and gentle mixing.
- Contact time between 10 –30 minutes depending on flocculators (mechanical / gravitational) and
tank basin design to promote contact. - Mixing velocity and energy input may be decreased as size of floc
increases.
Compare the contact time between coagulation and flocculation.
Significance
C: not a critical factor.
F: contact time is important to promote contact.
Time measured
C: secs to mins.
F: 10 - 30 mins depending on flocculators (mechanical / gravitational) and
tank basin design.
What factors are critical when considering flocculation performance?
- Correct detention time.
- Correct mixing intensity.
- Suitable tank basin design.
to promote contact between microflocs.
What happens if mixing in flocculation is inefficient?
There would be insufficient collisions, leading to poor floc formation.
What happens if mixing in flocculation is too vigorous?
Flocs break apart into smaller particles, making them more difficult to remove downstream.
What are primary coagulants and why are they commonly used?
Primary coagulants are salts of aluminum or iron.
Commonly used because:
- effective
- low cost
- readily available
- easy to handle, store, and apply.
What is a potential adverse effect of aluminum sulfate as a coagulant?
Aluminum sulfate can cause the dissolution of aluminum, which may have adverse health effects.
What are the issues with ferric sulfate and chloride as coagulants?
Ferric sulfate and chloride are corrosive acidic liquids, therefore hazardous to handle and apply.
Why was ferrous sulfate widely used in potable water treatment but now has concerns?
Cost effective
Excess chlorine used in disinfection can cause the formation of trihalomethanes (THMs) which is carcinogenic.
List the challenges of using iron coagulants.
Requires close process control to avoid excessive residual iron
Excess residual iron stains clothes in washing which leads to customer complaints.
List three primary coagulants that are hydrolysing metallic salts.
Alum (aluminium sulphate)
Ferric chloride
Ferric sulphate
List the advantages of alum as a primary coagulant.
Used as an industry standard in coagulation/flocculation.
Attracts inorganic suspended solids effectively.
List the disadvantages of alum as a primary coagulant.
Requires fast mixing.
pH of 5.5 and 7.5 required to avoid excessive dosage requirements, using alkaline additives to achieve optimum pH.
Lower temperatures = worse performance
Bad at removing organic suspended solids.
Requires larger dosages when only alum is used as primary coagulant.
List the advantages of ferric chloride and ferric sulphate as primary coagulants.
Good for attracting inorganic SS.
Produces more compact sludge.
Less pH sensitivity than alum.
Suitable for lime-softening processes (at pH 9).
List the disadvantages of ferric chloride and ferric sulphate as primary coagulants.
Less effective for removing organic SS than alum.
Requires fast mixing.
Needs pH between 5.5–8.5, often requiring alkaline additives.
Requires larger dosages than alum.
List three primary coagulants that are pre-hydrolysed metallic salts.
PACl / PAC
Polyaluminium sulphate
Polyiron chloride
List the advantages of pre-hydrolysed metal salts as primary coagulants.
Does not need alkali addition for pH adjustment.
Less pH sensitive and works in a wider pH range (4.5–9).
Mixing time is less critical.
Forms tougher flocs.
May reduce dosage and avoid use of coagulant aids.
Suitable for high-colour applications.
List the disadvantages of PACl / PAC / polyaluminium sulphate as primary coagulants.
Requires an onsite process to prepare pre-hydrolysed metallic salts from alum.
List the disadvantages of polyiron chloride as primary coagulants.
Requires an onsite process to prepare pre-hydrolysed metallic salts from iron chloride.
Which hydrolysed metallic salt is best for attracting inorganic SS?
Ferric chloride
Which hydrolysed metallic salt is best for attracting organic SS?
Alum
What do coagulant aids do?
Assist coagulation and flocculation processes by:
Accelerating flocculation
Strengthening flocs, making them easier to separate.
How are coagulant aids classified into two groups?
Coagulant aids are classified into two types based on their mechanism:
- Bind to particles (like coagulant chemicals).
- Act as sites of nucleation to speed floc formation by increasing floc density.
What are the chemical classes of coagulant aids?
Synthetic polymers (cationic, neutral, anionic).
Natural polymers.
Inorganic coagulant aids.
Why are synthetic polymers widely used in water treatment?
Synthetic polymers can be used as both coagulant aids or primary coagulants.
Are synthetic polymers better as coagulant aids or primary coagulants?
Coagulant aids
Not as efficient when they are used as primary coagulants alone.
What do synthetic polymers do as coagulant aids?
Speeds up flocculation by improve sludge properties (density, strength, and floc size) to make separation easier.
List the treatment stages for a potable water treatment works.
Coagulation
Flocculation
Sedimentation
Filtration
Disinfection
What characteristics of raw water are different between potable and wastewater treatment?
Concentration of the suspension
Characteristics of the substances
List the advantages of synthetic cationic / neutral polymers as coagulant aids.
Used with metallic coagulants to produce denser, sheer-resistant SLUDGE.
Improves process performance and economy when used correctly.
List the advantages of synthetic anionic polymers as coagulant aids.
Promotes bridging to produces larger, sheer-resistant FLOCS.
Improves process performance and economy when used correctly.
List the disadvantage of synthetic polymers as coagulant aids.
Increases complexity of coagulation and flocculation processes.
What is sodium alginate particularly suited for, and how can it be effective?
Used with ferric salts and effective with alum if used properly.
What is a disadvantage of natural polymers like sodium alginate?
Less efficient than synthetic polymers.
What are the advantages of natural polymers such as chitosan and starch as a coagulant aid?
Inexpensive
Increases settling velocity while reducing coagulant dosage.
What is the main use of the inorganic coagulant- aluminum chloride- in coagulation?
Used with organic polymer coagulants.
What are the advantages of inorganic coagulant aids such as activated silica, bentonite, kaolinite and calcium carbonate?
Inexpensive additives that increase settling velocity and reduce dosage.
Improves process performance and economy when used properly.
List the main disadvantage of inorganic coagulant aids.
Increases complexity of coagulation and flocculation processes.
What is polyacrylamide (PAC) commonly used for in the water industry?
Coagulant aid to assist in flocculation processes.
What is acrylamide (AA) and how does it relate to PAC?
AA is the monomer impurity in PAC.
0.005% to 0.02% of monomers may remain unconverted in PAC
What are the issues with acrylamide?
AA is carcinogenic at high levels.
Potable water has AA limit of 0.1 µg/L set by the Drinking Water Inspectorate (DWI).
What are potential solutions for water companies to address acrylamide (AA) concerns?
- Use alternatives – e.g., Wispafloc, which has no acrylamide.
- Optimise PAC dosing to reduce acrylamide, but the challenge is maintaining effective coagulation and flocculation.
- Add additional process steps to remove AA from head of works (HOW) returns.
- Keep sampling to analyse water quality.
Using the solids contact unit diagram, describe the function of the chemicals feed.
Introduces coagulants or flocculants to initiate particle destabilization and floc formation.
Using the solids contact unit diagram, describe the function of the gear drive.
Powers the mixing mechanism for proper blending and flocculation.
Using the solids contact unit diagram, describe the function of the radial launder.
Collects clarified water (effluent) from the top of the unit.
Using the solids contact unit diagram, describe the function of the sludge scraper.
Moves settled sludge towards the sludge sump for removal.
Using the solids contact unit diagram (SCU diagram), describe the function of the sludge sump.
Collects settled sludge at the bottom, allowing periodic removal through the sludge drain.
What is the purpose of the rapid mix zone? (SCU diagram)
Ensures fast blending of chemicals with raw water to destabilize particles for coagulation.
What is the purpose of the flocculation and solids contact zone? (SCU diagram)
Promotes particle collisions to form larger, settleable flocs through gentle mixing.
What is the purpose of the upflow and sludge blanket zone? (SCU diagram)
Provides a settling area for flocs as water flows upwards, creating a sludge blanket that traps additional particles.
How can the SCU system be optimized for varying influent water quality conditions?
Adjust chemical dosage and mixing speeds based on influent turbidity and pH to maintain effective coagulation and flocculation.
What adjustments can be made to improve performance during periods of high turbidity in SCU?
Increase coagulant dosage, enhance mixing intensity in Zone A, and monitor sludge blanket levels to prevent carryover.
Identify three methods of mixing for coagulation and analyse the performance for each.
Hydraulic (flow energy of system using baffles)
- Dependent on rate of flow and so there is the potential for step 1 to be incomplete if rate of flow through the system is reduced or slow.
Mechanical (paddles, turbines, propellers)
- Versatile and generally reliable but is the largest consumer of energy (electrical) for mixing the coagulant with the water.
Pumped blenders (direct addition through diffuser in a pipe)
- Provides rapid dispersion of the coagulant and no significant head loss created
- Consumes energy but less than mechanical.
