Chemical Treatment
Processes of Industrial
Waste
“Compare and choose the chemical
treatment methods for waste treatment in
industries. Calculate and design the basic
structure of waste treatment unit
operations”.
Chemical treatment / Unit operation
Introduction
 Chemical treatment usually are used in
combination with the Physical Unit Operations;
- Screening, coarse solids reduction, mixing
and flocculation, gravity separation, grit
removal, sedimentation, flotation, aeration,
etc.
and also with Biological Unit Operations.
Role of Chemical Unit Processes in
Wastewater Treatment
Chemical coagulation
Chemical precipitation
Chemical disinfection
Chemical oxidation
Advanced oxidation process
Ion exchange
Chemical neutralization, scale control, and
stabilization
Application of chemical Unit Processes in wastewater treatment
Considerations & Issues of Chemical
Treatment….
“Chemical treatment – additive processes”
Net increase
in dissolved
constituents
Handling,
treatment and
disposal of the large
volumes of sludge
produced
Increase in
cost of energy
& chemical
costs
CHEMICAL
COAGULATION
Chemical Coagulation
• Colloidal particles found in wastewater :
- net negative surface charge,
- 0.01 to 1 µm in size
- attractive body forces between particles < repelling forces
- this stable conditions, Brownian motion (i.e., random
movement) keeps the particles in suspension.
- Brownian motion is brought about by constant thermal
bombardment of colloidal particles by small water molecules
that surround them.
• Coagulation is the process of destabilizing colloidal
particles so that particle growth can occur as a
result of particle collisions.
Brownian motion (random movement)
Basic definitions
- Chemical coagulation – all reactions and mechanisms
involved in the chemical destabilization of particles and in the
formation of larger particles through perikinetic flocculation
(aggregation of particles in the size range from 0.01 to 1 µm )
- Coagulant – chemical that is added to destabilize the colloidal
particles in wastewater so that floc formation can occur.
- Flocculent – chemical, typically organic, added to enhance
the flocculation process.
- Coagulant & Flocculant : natural and synthetic organic
polymers, metal salts, and prehydrolized metal salts (ex:alum,
ferric sulfate, polyaluminum chloride (PACl) and polyiron
chloride (PICl), etc )
• Flocculants are also used to enhance the performance of
granular medium filters and dewatering of digested
biosolids
Filter aids
Flocculation: the process of increasing the size of particles
as a result of particle collisions.
Microflocculation
Macroflocculation
(perikinetic flocculation)
- Particle aggregation is brought
about by the random thermal
motion of fluid molecules known
as Brownian motion
(orthokinetic flocculation)
- Particle aggregation is brought about
by inducing velocity gradients and
mixing in the fluid containing the
particles to be flocculated
Reaching 1-10 µm size, then separated by
gravity sedimentation and filtration
Nature of particles in wastewater
• Suspended particles > 1.0 µm, can be removed by gravity sedimentation
• Colloidal particles cannot be removed by sedimentation (need
coagulants & flocculant aids)
Important factors that contribute to the Characteristics of
Colloidal Particles:
a) Particle size and number
 0.01 to 1.0 µm
 number in untreated wastewater and after primary sedimentation
= 106 to 1012 /mL
b) Particle shape and flexibility
 spherical,
ellipsoids, disklike, various length, D, and random coils .
 shapes affect electrical properties, particle-particle interaction,
and particle-solvent interaction
c) Particle-solvent interaction
 Hydrophobic – have relatively little attraction for water
 Hydrophilic – much greater attraction for water
 Association colloids – made up of surface-active agents, ex: soaps,
synthetic detergents, and dyestuff which form organized
aggregates known as micelles.
d) Surface properties including electrical characteristics (surface
charge)
e) Particle-particle interaction
We will discuss more on (d) and (e) in the following slides…
Development of Surface Charge
**surface charge is an important factor in the stability of colloids!
**Develop through:
a) Isomorphous replacement
- occurs in clays and other soil particles, ions in lattice structure
replaced with ions from solution, ex: Si4+ replaced with Al3+
b) Structural imperfections
- occurs in clay or similar particles, due to broken bonds on crystal
edge (imperfections in crystal formation)
c) Preferential Adsorption
- when oil droplets, gas bubbles, or other inert substances are
dispersed in water, they will acquire –ve charge through adsorption
of ions (hydroxyl ions)
d) Ionization
- ionization of carboxyl and amino groups (at different level of pH)
What is Electrical Double Layer?
• When the colloid or particle surface become charged, some ions of
the opposite charge (known as counterions) become attached to
the surface.
• They are held there through electrostatic and van der Waals forces
of attraction strongly enough to overcome thermal agitation,
forming a layer called Stern layer.
• Surrounding this fixed Stern layer is a diffuse layer of ions.
• The electrical double layer consists of compact layer (Stern) in
which the potential drops from ψ0 to ψ1 and a diffuse layer in
which the potential drops from ψs to 0 in bulk solution.
Surface potential- depends on distance from particle surface
The Electrical Double Layer
Electrical double layer or electrostatic interaction force
Particle-particle interactions
Involve 2 principal forces: repulsion force and van der Waals force of attraction
*Refer page 483 in textbook for explaination on the graph
Particle Destabilization
 Required to reduce particle charge or to overcome effect of this charge.
Therefore aggregation of particles (microflocculation) can be achieved.
1) Particle Destabilization and Aggregation with
Polyelectrolytes
Actions of polyelectrolytes:
a) Charge neutralization
b) Polimer bridge formation
c) Charge neutralization and polimer bridge formation
a) Charge neutralization
- act as coagulants that neutralize or lower the charge of the
wastewater particles (why?relates to zeta potential)
- normally the wastewater particles are –ve charge, so,
cationic (+ve charge) polyelectrolytes are used.
- polyelectrolytes must be adsorbed to the particles
used
sufficient and high intensity of mixing (prevent folding back
of polyelectrolytes)
b) Polymer bridge formation
Anionic or nonionic
polyelectrolytes
Removed by sedimentation
c) Charge neutralization and Polymer bridge
formation
- use cationic polyelectrolytes having extremely
high molecular weight
- can form both charge neutralization and
polymer bridge
2) Particle Destabilization with Potential-determining
Ions and electrolytes
a) Addition of potential determining ions
- add strong acids or bases to reduce charge of metal oxides
or hydroxides to near 0 so that coagulation can occur
- not feasible due to massive concentrations of ions to be added
b) Use of Electrolytes
- added to coagulate colloidal suspension
- cause decrease in zeta potential and corresponding
decrease in repulsive forces.
- also not feasible in waste treatment.
3) Particle destabilization and removal with
hydrolyzed metal ions
- Addition of alum or ferric sulfate (Fe3+ & Al3+)
- Complex formation of metal ion hydrolysis products
From Eq 6-7 above, one or more of hydrolisis products and/or polymers
may be responsible for observed actions of Al or Fe.
Action of hydrolyzed metal ions:
i) Adsorption and charge neutralization
- mononuclear and polynuclear metal hydrolysis species adsorb on the
colloidal particles.
ii) Adsorption and interparticle bridging
- involve the adsorption of polynuclear metal hydrolysis species and
polymer species which in turn will form particle-polymer bridges
- if enough coagulant requirement & charge neutralization, metal
hydroxides precipitates and soluble metal hydrolysis products form
- if sufficient metal salts added, large amount of metal hydroxide floc will
form
settle
iii) Enmeshment (trapped) in sweep floc
- floc particles settle and sweep through water containing colloidal
particles
- colloidal particles enmesh in the floc – removed by sedimetation.
CHEMICAL
PRECIPITATION
FOR IMPROVED
PLANT PERFORMANCE
Chemical Precipitations
-involves the addition of chemicals to alter the physical state of dissolved
and SS , and facilitate removal by sedimentation
1) Alum
Alkalinity
(or Magnesium
bicarbonate)
Precipitate
The quantity of alkalinity (as CaCO3 having Mw =
100) required to react with 10 mg/L of alum is;
!! Note: If less than this amount of alkalinity is available,
it must be added, ex: Lime
If lime alone is added as precipitant, much more lime is required than
when sulfate of iron is used.
2) Lime (or calcium hydroxide)
- Reactions for carbonic acid – clarification;
- Alkalinity;
3) Ferrous sulfate and lime
Ferrous sulfate alone added to wastewater;
FeSO4 ·7H2O + Ca(HCO3)2
Fe(HCO3)2 + CaSO4 + 7H2O
Ferrous sulfate Calcium bicarbonate Ferrous bicarbonate
(soluble)
(soluble)
(soluble)
278
100
Calcium
sulfate
(soluble)
- Addition of Ferrous sulfate & lime
Fe(HCO3)2 + 2Ca(OH)2
Fe(OH)2 + 2CaCO3 + 2H2O
Ferrous
bicarbonate
(soluble)
178
Calcium
hydroxide
(slightly soluble)
2 x 56
Ferrous
hydroxide
(very slightly
soluble)
Calcium
carbonate
(somewhat
soluble)
Further oxidation,
Fe(OH)2 + 1/4O2 + 1/2H2O
Fe(OH)3
Ferrous hyroxide
Ferric hydroxide (insoluble)
89.9
Oxygen
¼ x 32
Water
½ x 18
The alkalinity required for 10 mg/L dosage of ferrous sulfate,
10 mg/L x (100/278) = 3.6 mg/L
The lime required,
10 mg/L x 2(56)/278 = 4 mg/L
The oxygen required,
10 mg/L x (32/4)/278 = 0.29 mg/L
Because the formation of ferric hyroxide is dependent on the presence
of O2, ferrous sulfate is not used commonly.
replace with ferric chloride (equations in page 496).
Example 6.1 – Estimation of sludge volume from
chemical precipitation of untreated wastewater
a) Estimate the mass and volume of sludge produced from
untreated wastewater without and with the use of ferric
chloride for the enhanced removal of TSS.
b) Also estimate the amount of lime required for the specified
ferric chloride dose.
- Assume that 60% of the TSS is removed in the primary
settling tank without the addition of chemicals, and that the
addition of ferric chloride results in an increased removal of
TSS to 85%.
- Also, assume that the following data apply to this situation:
1.
2.
3.
4.
5.
Wastewater flow rate
= 1000 m3 /d
Wastewater TSS
= 220 mg/L
Wastewater alkalinity as CaCO3 = 136 mg/L
Ferric chloride (FeCl3) added
= 40 kg/1000m3
Raw sludge properties:
Specific gravity
Moisture content
= 1.03
= 94 %
6. Chemical sludge properties:
Specific gravity
Moisture content
= 1.05
= 92.5 %
Solutions:
1)
2)
3)
4)
5)
6)
Compute the mass of TSS removed without chemicals
Compute the mass of TSS removed with chemicals
Using Equation (6-16), determine the mass of ferric hyroxide
produced from addition of ferric chloride
Determine the mass of lime required using Eq (6-17)
Determine total amount of sludge (TSS + Fe (OH)3)
Determine the total volume of sludge (use specific gravity and
moisture content info) for
i) from chemical precipitation
ii) without chemical precipitation
Answer : refer page 499-500, textbook
Recommended design for
Primary Sedimentation
Types of Precipitation
Chemical Precipitation
Precipitation without
chemical additives
Percentage removal %
TSS
BOD
Bacteria
80 – 90 %
50 – 80 %
80 – 90 %
50 – 70 %
25 – 40 %
25 – 75 %
Surface Loading Rate (SLR) or “surface settling rate” or “surface overflow rate”
: is a hydraulic loading factor expressed in terms of flow per surface area.
CHEMICAL
PRECIPITATION
FOR PHOSPHORUS
REMOVAL
Introduction
The removal of phosphorus from
wastewater involves the incorporation of
phosphate into TSS and the subsequent
removal of these solids.
Incorporation into
biological solids (during
biological treatment)
Incorporation into
Chemical precipitates
Phosphate Precipitation
- Addition of salts of multivalent metal ions, ex:
Ca(II), Al(III), and Fe(III).
1) Phosphate precipitation with Calcium
- Calcium is added in the form of lime Ca(OH)2.
- usually, when lime is added, it reacts with natural
bicarbonate alkalinity to precipitate CaCO3.
- As pH > 10, excess calcium ions will react with phosphate,
to precipitate hydroxylapatite [Ca10(PO4)6(OH)2].
- Quantity of lime required to precipitate P - independent of
phosphate amount present, but dependent of wastewater
alkalinity (about 1.4 – 1.5 times total alkalinity as CaCO3)
- because need high pH- not feasible.
2) Phosphate precipitation with Aluminum and Iron
-
-
Because many competing reactions and effects of pH, alkalinity, trace
elements, etc., Eqs 6-20 & 6-21 cannot be used to estimate the
required chemical dosages.
So, achieved by bench-scale tests
Figure 6-12.
Shaded area: pure metal phosphates are precipitated
Solid lines: Conc. of residual soluble phosphates after precipitation
Example 6.2 – Determination of Alum
Dosage for Phosphorus Removal
Determine the amount of liquid alum required to precipitate
phosphorus in a wastewater that contains 8 mg P/L.
Also determine the required alum storage capacity if a 30-d
supply is to be stored at the treatment facility. Based on
laboratory testing, 1.5 mole of Al will be required per
mole of P. The flow rate is 12 000 m3/d. the following data
are for the liquid alum supply.
1. Formula for liquid alum Al2(SO4)3 ·18H2O
2. Alum strength = 48 %
3. Density of liquid alum solution = 1.2 kg/L
Solution (page 503):
1) Determine the weight of Al/L
2) Determine the weight of Al required per
unit weight of P
3) Determine the amaunt of alum solution
required per kg P
4) Determine the amount of alum solution
required per day
5) Determine the required alum solution
storage capacity.
CHEMICAL
OXIDATION
Chemical Oxidation
- Use of Ozone (O3), Hydrogen peroxide
(H2O2), Permanganate (MnO4), Chloride
dioxide (ClO2), Chlorine (Cl2) or (HOCl),
and Oxygen (O2)
- To reduce /degrade BOD, COD, ammonia,
nonbiodegradable organic compounds.
Fundamentals of chemical oxidation
1) Oxidation-reduction reactions (redox)
Cu2+ + Zn
Cu + Zn2+
2) Half-reaction potentials
+ve(tendency to proceed
to the right
-ve (tendency to the left)
Reaction potential
• To predict whether a reaction comprised of 2 half
reactions will proceed as written.
• Equation:
• Example:
E°reaction = E°reduction – E°oxidation
Cu2+ + Zn
Cu + Zn2+
Potential of
overall reaction, E°reaction = E°reduction – E°oxidation
= 0.34 – (-0.763)
= +1.103 volts
Therefore, the reaction will proceed as written.
CHEMICAL NEUTRALIZATION
Is the removal of excess acidity or alkalinity by
treatment with a chemical of opposite composition
Chemical selection depends on suitability for
particular application and economic consideration.
Thank you