SAB 4973:HAZARDOUS WASTE

TREATMENT TECHNOLOGIES

Technologies

• Chemical methodsCoagulation, flocculation, combined with flotation and filtration, precipitation, ion exchange, electroflotation, electrokinetic coagulation.

• Physical methodsMembrane-filtration processes (nanofiltration, reverse osmosis, electrodialysis, . . .) and adsorption techniques.

• Biological treatmentsBiodegradation methods such as fungal decolorization, microbial degradation, adsorption by (living or dead) microbial biomass and bioremediation systems

Advantages and disadvantages

Chemical methodsAdvantages :• Rapid and efficient process• Removes all pollutants types, produce a

high-quality treated effluent• No loss of sorbent on regeneration and

effectiveDisadvantages :• Expensive, and although the pollutants are

removed, accumulation of concentrated sludge creates a disposal problem

• High energy cost, chemicals required.

Advantages and disadvantages

Physical methodsAdvantages :• 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 by-

products, technical constraints

Advantages and disadvantages

Biological treatmentsAdvantages :• Economically attractive, publicly

acceptable treatmentDisadvantages :• Slow process, necessary to create an

optimal favorable environment, maintenance and nutrition requirements

COAGULATION

• DefinitionDestabilisation of colloid particles by the addition of chemicals (coagulant)

• ApplicationsIndustrial 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

AlumMagnesium chloride

Polyacrylamide

ChitosanMoringa 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,

non-ionic. 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 withsoluble ions

insolublecomplexes

“cleanWater”

+chemicalreaction

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 equilibriaA 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+ + B-

where ABs : solid; A+, B- - ionic species

CHEMICAL PRECIPITATION(Theoretical Background)

Due to dilute concentration,

Ksp = [A+] [B-] = solubility product

constant

where [ ] refer to molar concentration

Eg.

ABs

A+ + B-

)(AB))(B(A

Ks

-

eq

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 thoroughlyC. 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 concentrationChange 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 A B (min) mol/L mol/L 0.0 1.000 0.000 3.0 0.400 0.600 6.0 0.250 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).

• TemperatureUsually, 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

REDOXOxidation and reduction in terms of oxygen transfer

DefinitionsOxidation 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 CH3CHOOxidation 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 gainCuO + Mg Cu + MgOCu2+ + 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

NO3-

Cu + 4 H+ + 2 NO3-→ Cu2+ + 2 NO2 + 2 H2O

ION EXCHANGE

• DefinitionIon 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 ionsb. 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 + 2OH-

where 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)

CationResin

Cr3+, CN-

H+, CN-

AnionResin

H+, OH- Cleanwater

ION EXCHANGE(Selectivity)

• Cations:

Ra2+ > Ba2+ > Sr2+ > Ca2+ > Ni2+ > Cu2+ > Co2+ > Zn2+ > Mn2+ > Ag+ >Cs+ > K+ > NH4

+ > Na+ > Li+

• Anions:

HCRO4- > CrO4

2- > ClO4- > SeO4

2- > SO42- > NO3

- > Br- > HPO4

- > HAsO4- > SeO3

2- > 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.