SAB 4973:
HAZARDOUS WASTE
TREATMENT TECHNOLOGIES
Technologies
• Chemical methods
Coagulation, flocculation, combined with flotation
and filtration, precipitation, ion exchange,
electroflotation, electrokinetic coagulation.
• Physical methods
Membrane-filtration processes (nanofiltration,
reverse osmosis, electrodialysis, . . .) and adsorption
techniques.
• Biological treatments
Biodegradation methods such as fungal
decolorization, microbial degradation, adsorption
by (living or dead) microbial biomass and
bioremediation systems
Advantages and disadvantages
Chemical methods
Advantages :
• Rapid and efficient process
• Removes all pollutants types, produce a highquality treated effluent
• No loss of sorbent on regeneration and effective
Disadvantages :
• Expensive, and although the pollutants are
removed, accumulation of concentrated sludge
creates a disposal problem
• High energy cost, chemicals required.
Advantages and disadvantages
Physical methods
Advantages :
• The most effective adsorbent, great, capacity,
produce a high-quality treated effluent
• No sludge production, little or no consumption
of chemicals.
Disadvantages :
• Economically unfeasible, formation of byproducts, technical constraints
Advantages and disadvantages
Biological treatments
Advantages :
• Economically attractive, publicly acceptable
treatment
Disadvantages :
• Slow process, necessary to create an optimal
favorable environment, maintenance and
nutrition requirements
COAGULATION
• Definition
Destabilisation of colloid particles by the
addition of chemicals (coagulant)
• Applications
Industrial waste containing colloidal and
suspended solids (e.g. pulp and paper,
textile)
Coagulant type
• Metal coagulants :aluminium-based
coagulants, Fero-based coagulants
magnesium chloride (MgCl2)
• Organic polymer coagulants :
Polyacrylamide, Chitosan, Moringa olifeira
Alginates (brown seaweed extracts)
Coagulant agent
Alum
Magnesium chloride
Polyacrylamide
Chitosan
Moringa oleifera
Coagulant - Reaction
• Some of the coagulants used include:
 Aluminium sulphate
 Ferric chloride
 Ferric sulphate
 Lime (not true coagulant)
 Polymer as coagulant aid eg cationic, anionic, nonionic.
 PAC – new types
Al2(SO4)3.18H20+ 3Ca(HCO3)
2AI(OH)3+
3CaSO4+ 6C02 + 18H20
AI(OH)3 or Al2O3 ( form as floc is the key element
causing destabilisation of charge).
Raw waste
Floc Formation
Settle floc
Flocculation
• is a process of forming aggregate of flocs to
form larger settleable particle. The process
can be described as follows:
 Mutual collision of small floc resulting in
bigger size.
 Usually slow speed or gentle mixing is
used so as not to break the large flocs due
to shear.
 Polymer or large molecular wt compound
is added to enhance floc build up. Most of
them are proprietary chemicals.
Flocculation mechanism
Flocculation mechanism
Flocculation mechanism
Flocculation
• The benefits of flocculation are:
 To improve settling of particles in
sedimentaion tank
 To increase removal of
suspended solids and BOD
 To improve performance of
settling tanks
Differences
• Coagulation: is a
chemical technique
which is directed towards
the destabilisation of the
charged colloidal
particals.
• Flocculation: is the slow
mixing technique which
promotes the
agglomeration of the
stabilised particles.
CHEMICAL PRECIPITATION
• Definition:
Removal of metal ions
from solution by changing
the solution composition,
thus causing the metal ions
to form insoluble metal
complexes.
solution with
soluble ions
chemical
reaction
insoluble
complexes
+
“clean
Water”
Natural methods of precipitation include settling or
sedimentation, where a solid forms over a period of time due to
ambient forces like gravity or centrifugation
CHEMICAL PRECIPITATION
(Applications)
• Removal of metals from waste stream
– e.g. plating and polishing operations, mining, steel
manufacturing, electronics manufacturing
– include arsenic, barium, chromium, cadmium, lead,
mercury, silver
• Treatment of “hard” water – removal of
Mg2+ and Ca2+
• Phosphorus removal
• Making pigments
• Removing salts from water in water
treatment
CHEMICAL PRECIPITATION
(Theoretical Background)
• Solubility equilibria
A chemical reaction is said to have reached
equilibrium when the rate of forward reaction is
equal to the rate of the reverse reaction
ABs  A+ + Bwhere ABs : solid; A+, B- - ionic species
CHEMICAL PRECIPITATION
(Theoretical Background)

K eq
-
(A )(B )

(ABs )
Due to dilute concentration,
Ksp = [A+] [B-]
= solubility product constant
where [ ] refer to molar concentration
Eg.
A+ + B-
ABs
Compound
Solubility
(mg/L)
Ksp
CaCO3
18
5 x 10-9
CaCl
745000
159 x 106
CHEMICAL PRECIPITATION
(Basic Principles)
A. Add chemical
precipitants to
waste stream
B. Mix thoroughly
C. Allow solid
precipitates to
form floc by
slow mixing
D. Allow floc to
settle in clarifier
CHEMICAL PRECIPITATION
(Types of Precipitation)
Heavy metals removal
• Hydroxide precipitation (OH-)
• Sulphide precipitation (S2-)
• Carbonate precipitation (CO32-)
Phosphorus removal
• Phosphate precipitation (PO42-)
CHEMICAL PRECIPITATION
(Hydroxide Precipitation)
• Add lime (CaO) or sodium hydroxide (NaOH) to
waste stream to precipitate heavy metals in the
form of metal hydroxides.
Cd2+ + Ca(OH)2  Cd (OH)2  + Ca2+
• CaO in the form of slurry (Ca(OH)2) while NaOH
in the form of solution.
• NaOH is easier to handle but is very corrosive.
• Will form floc and settle in clarifier
CHEMICAL PRECIPITATION
(Sulphide Precipitation)
• Use of sulphide in the form of FeS, Na2S or NaHS
• Better metal removal as sulphide salt has low
solubility limit
Cu2+ + FeS  CuS  + Fe2+
• Limitation: can produce H2S (g) at low pH
2H+ + FeS  H2S + Fe2+
• At low pH, reaction will proceed to the right.
Thus, require pH > 8 for safe sulphide
precipitation.
CHEMICAL PRECIPITATION
Reaction rate
• Reaction rate is a measure of how fast a reaction
occurs, or how something changes during a given
time period.
• Consider the oxidation of glucose, C6H12O6 :
C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(g)
• One of the things that happens during this reaction
is simply that glucose gets used up as it reacts with
oxygen in the air, and carbon dioxide and water
start to form.
• A common measure of reaction rate is to express how
the concentration of a reaction participant changes
over time. It could be how the concentration of a
reactant decreases, or how the concentration of a
product increases. This is the standard method we will
be using.
• Now that we have something that changes to measure,
we must consider the second key aspect of
determining rate - time. Rate is a measure of how
something changes over time.
Change in concentration
Change in time
Chemistry Notation
• In chemistry, we typically represent concentration by using
square brackets around the chemical formula of the
substance. For example to indicate the concentration of
SO2(g) in the following reaction we would write it as [SO2].
• Also, the delta symbol, Δ is used to indicate a change. ΔT, for
example, means "the change in temperature."
• Therefore, if we wanted to express the rate of the following
reaction:
SO2(g) + NO2(g) → SO3(g) + NO(g)
• Let's try an example of calculating a reaction rate.
Consider the following reaction:
A→B
• The following data were obtained for how the
concentration of these substances changed during the
experiment.
Time
(min)
0.0
3.0
6.0
A
mol/L
1.000
0.400
0.250
B
mol/L
0.000
0.600
0.750
We could measure the rate of the reaction either by measuring how
the concentration of reactant A changes or how the concentration of
product B changes. Let's measure A's average rate of change first:
Compare this rate to the rate of just the first three minutes of the
reaction:
If we calculate the average rate based on the production of
product B:
Factors that Affect the Chemical Reaction
Rate
• Concentration of Reactants
A higher concentration of reactants leads to more effective
collisions per unit time, which leads to an increasing
reaction rate (except for zero order reactions).
• Temperature
Usually, an increase in temperature is accompanied by an
increase in the reaction rate. Temperature is a measure of
the kinetic energy of a system, so higher temperature
implies higher average kinetic energy of molecules and
more collisions per unit time.
Factors that Affect the Chemical Reaction
Rate
• Medium
The rate of a chemical reaction depends on the medium in
which the reaction occurs. It may make a difference
whether a medium is aqueous or organic; polar or
nonpolar; or liquid, solid, or gaseous.
• Presence of Catalysts and Competitors
Catalysts (e.g., enzymes) lower the activation energy of a
chemical reaction and increase the rate of a chemical
reaction without being consumed in the process. Catalysts
work by increasing the frequency of collisions between
reactants, altering the orientation of reactants so that more
collisions are effective, reducing intramolecular bonding
within reactant molecules, or donating electron density to
the reactants.
OXIDATION
a method by which wastewater is treated by using
oxidizing agents.
Generally, two forms viz.
• Chemical oxidation and
• UV assisted oxidation using chlorine, hydrogen
peroxide, fenton’s reagent, ozone, or potassium
permanganate are used for treating the effluents,
especially those obtained from primary treatment
(sedimentation)
CHEMICAL OXIDATION
(Oxidants)
• Rapid and efficient process
• High energy cost, chemicals required
REDOX
Oxidation and reduction in terms of oxygen transfer
Definitions
Oxidation is gain of oxygen.
Reduction is loss of oxygen.
• Fe2O3 + 3CO  2Fe + 3CO2
Another definition
Oxidation and reduction in terms of hydrogen transfer
– These are old definitions which aren't used very
much nowadays. The most likely place you will
come across them is in organic chemistry.
Definitions
•Oxidation is loss of hydrogen.
•Reduction is gain of hydrogen.
CH3CH2OH CH3CHO
Oxidation by loses of hydrogen
Another definition
Oxidation and reduction in terms of electron
transfer
• This is easily the most important use of the
terms oxidation and reduction at A' level.
Definitions
• Oxidation is loss of electrons.
• Reduction is gain of electrons.
OIL RIG  oxidation is loss, reduction is gain
CuO + Mg  Cu + MgO
Cu2+ + Mg  Cu + Mg2+
OXIDATION STATES (OXIDATION
NUMBERS)
• Oxidation state shows the total number of
electrons which have been removed from an
element (a positive oxidation state) or added
to an element (a negative oxidation state) to
get to its present state.
– Oxidation involves an increase in
oxidation state
– Reduction involves a decrease in
oxidation state
Some elements almost always have the
same oxidation states in their compounds:
• Group 1 metals : always +1
• Group 2 metals : always +2
• Oxygen : usually -2 except in peroxides and
F2O
• Hydrogen : usually +1 except in metal
hydrides where it is -1
• Fluorine : always -1
• Chlorine : usually -1 except in compounds
with O or F
Example 1:
• This is the reaction between magnesium and
hydrochloric acid or hydrogen chloride gas:
Mg + 2HCl MgCl2 + H2
0
+1 -1 +2 -1 0
• The magnesium's oxidation state has increased - it
has been oxidised. The hydrogen's oxidation state has
fallen - it has been reduced. The chlorine is in the
same oxidation state on both sides of the equation - it
hasn't been oxidised or reduced.
Example 2:
• The reaction between sodium hydroxide and
hydrochloric acid is:
NaOH + HCl  NaCl + H2O
+1 -2 +1 +1 -1
+1 -1 +1 -2
• Nothing has changed. This isn't a redox
reaction.
Example 3:
• The reaction between chlorine and cold dilute
sodium hydroxide solution is:
2NaOH + Cl2  NaCl + NaClO + H2O
+1 -2 +1 0
+1 -1 +1 +1 -2 +1 -2
• One atom has been reduced because its
oxidation state has fallen. The other has been
oxidised.
Symbols
European Union chemical hazard symbol for
oxidizing agents
Dangerous goods label for oxidizing agents
Common oxidizing agents
• Hydrogen peroxide and other inorganic peroxides
• Nitric acid and Nitrates
• Chlorites, chlorate, perchlorate, and other analogous
halogen compounds
• Hypochlorite and other hypohalite compounds such
as bleach
• Fluorine and other halogens
• Ozone
• Nitrous oxide(N2O)
• Silver oxide
• Permanganate salts
Hydrogen peroxide
• In acidic solutions H2O2 is one of the most powerful
oxidizers known—stronger than chlorine, chlorine
dioxide, and potassium permanganate.
• Also, through catalysis, H2O2 can be converted into
hydroxyl radicals (.OH), which are highly reactive.
• H2 + O2 → H2O2
• It is used as a disinfectant, antiseptic, oxidizer,
propellant in rocket. Hydrogen peroxide is naturally
produced in organisms as a by-product of oxidative
metabolism. Nearly all living things (specifically, all
obligate and facultative aerobes) possess enzymes
known as peroxidase.
Nitric acid
• Nitric acid is made by reacting nitrogen dioxide
(NO2) with water.
– 3 NO2 + H2O → 2 HNO3 + NO
• Nitric acid reacts with most metals.
3 Cu + 8 HNO2 → 3 Cu2+ + 2 NO + 4 H2O + 6 NO3Cu + 4 H+ + 2 NO3-→ Cu2+ + 2 NO2 + 2 H2O
ION EXCHANGE
• Definition
Ion exchange is basically a reversible chemical process
wherein an ion from solution is exchanged for a
similarly charged ion attached to an immobile solid
particle.
Removal of undesirable anions and cations from
solution through the use of ion exchange resin
• Applications
– Water softening
– Removal of non-metal inorganic
– Removal or recovery of metal
ION EXCHANGE
(Medium - resin)
• Consists of an organic or
inorganic network structure
with attached functional group
• Synthetic resin made by the
polymerisation of organic
compounds into a porous three
dimensional structure
• Exchange capacity is
determined by the number of
functional groups per unit mass
of resin
ION EXCHANGE
(Type of Resin)
a. Cationic resin - exchange positive ions
b. Anionic resin – exchange negative ions
(a)
(b)
ION EXCHANGE
(Exchange Reactions)
• Cation exchange on the sodium cycle:
Na2 · R + Ca2+  Ca · R + 2Na+
where R represents the exchange resin. When all
exchange sites are substantially replaced with
calcium, resin is regenerated by passing a
concentrated solution of sodium ions (5-10%)
through the bed:
2Na+ + Ca · R  Na2 · R + Ca2+
ION EXCHANGE
(Exchange Reactions)
• Anion exchange replaces anions with hydroxyl ions:
SO42- + R · (OH)2  R · SO4 + 2OHwhere R represents the exchange resin. When all
exchange sites are substantially replaced with
sulphate, resin is regenerated by passing a
concentrated solution of hydroxide ions (5-10%)
through the bed:
R · SO4 + 2OH-  SO42- + R · (OH)2
ION EXCHANGE
(Basic Principles)
H+, CN-
Cation
Resin
Cr3+, CN-
H+, OH-
Anion
Resin
Clean
water
ION EXCHANGE
(Selectivity)
• Cations:
Ra2+ > Ba2+ > Sr2+ > Ca2+ > Ni2+ > Cu2+ > Co2+ > Zn2+ > Mn2+ > Ag+
>Cs+ > K+ > NH4+ > Na+ > Li+
• Anions:
HCRO4- > CrO42- > ClO4- > SeO42- > SO42- > NO3- > Br- > HPO4- >
HAsO4- > SeO32- > CO32- > CN- > NO2- > Cl- > H2PO4-, H2AsO4-,
HCO3- > OH- > CH3COO- > F-
Note: The least preferred has the shortest retention time, and
appears first in the effluent and vice versa for the most
preferred.
Ion exchange-electrochemistry
• During redox reactions, electrons pass from one substance to
another. Electrochemistry is the branch of chemistry that deals
with the conversion between chemical and electrical energy.
• The fact that different substances are oxidized more readily
than others is the driving force behind electrochemical cells,
and it is this force that forces electrons through the external
circuit from the anode (site of oxidation) to the cathode (site of
reduction). This force is known as the potential difference or
electromotive force (emf or E). Potential difference is
measured in volts (V), and thus is also referred to as the
voltage of the cell. Voltage is a measure of the tendency of
electrons to flow. The higher the voltage, the greater the
tendency for electrons to flow from the anode to the cathode.
• For example, if copper and hydrogen half-cells are joined
together we find that the copper half-cell will gain electrons
from the hydrogen half-cell. Thus the copper half-cell is
given a positive voltage and given a relative value of +0.34
V:
Cu2+(aq) + 2e- → Cu(s) E° = 0.34 V
• Since both half-reactions cannot undergo reduction, we
must reverse the equation of the reaction that will undergo
oxidation. This will give us an electrochemical cell voltage
of 0.34 V:
E°
Cu2+(aq) + 2e- → Cu(s)
0.34 V
H2 (g) → 2H+(aq) + 2e0.00 V
Cu2+(aq) + H2 (g) → 2H+(aq) + Cu(s) 0.34 V
• We see in the Table of Standard Reduction Potentials that
zinc has a negative E° indicating that it is not as good at
competing for electrons as hydrogen.
Zn2+(aq) + 2e- → Zn(s) E° = -0.76 V
• Therefore if zinc and hydrogen are paired together in an
electrochemical cell, the hydrogen would be reduced (gain
the electrons) and zinc would be oxidized (losing electrons).
To determine the net redox reaction as well as the voltage of
the electrochemical cell we reverse the zinc equation, and
also reverse it's sign before adding the equations and E°
together:
E°
Zn(s) → Znu2+(aq) + 2e0.76 V
2H+(aq) + 2e- → H2 (g)
0.00 V
Zn(s)+ 2H+(aq) → Zn2+(aq) + H2 (g)
0.76 V