Class 6

IPRC MUSANZE
Integrated Polytechnic
Regional College
WATER SUPPLY SYSTEM DESIGN

WASDW801
Lecturer
Dr Janvier MUNYANEZA
Email: karindwi777@gmail.com
Mobile: 0792997916/ 0735398681
Chap1. Design and operation of waste water treatment plant
(Cont'd)
2. Adsorption
Adsorption is the accumulation or
concentration of of substances at the
surface or interface.
or
Adsorption is capability of all solid
substances to attract to their surfaces
molecules of gases or solutions with
which they are in contact.
or
Adsorption is the process in which a
substance, usually a gas, forms a very
thin layer on the surface
2. Adsorption
The adsorbing phase is termed "adsorbent"
and the material being adsorbed the
"adsorbate".
The process for adsorbates to release from
adsorbent surfaces into water is termed as "
desorption".
Adsorption can occur between two phases,
namely liquid-liquid, gas-liquid, gas-solid or
liquid-solid interfaces.
When activated carbon is used, the
adsorbing phase is a solid.
2. Adsorption
Mechanism of Adsorption
• When the particle on the surface and the particle in the bulk of the adsorbent are not
in the same environment, adsorption occurs. In other words, the net force acting on
them is not the same.
• The particle on the surface or adsorbate has unbalanced forces acting on it which are
also called residual attractive forces.
• During the process of adsorption, charge transfer occurs between the adsorbate and
the adsorbent.
• This results in a dipole moment.
• Hence, there is the adhesion of ions, atoms and molecules from liquid, gas and
dissolved solids to a particular surface.
• As a result, an adsorbate film is formed on the adsorbent surface.
• During adsorption, there is always a decrease in the residual attractive forces of the
surface.
• It is an exothermic process which means that energy is liberated during this process.
• The energy of the surface decreases and this appears as heat. This is called the heat
of adsorption
• At constant temperature and pressure, adsorption occurs spontaneously.
2. Adsorption
Types of Adsorption
Chemisorption
When the gas molecules or atoms are held to
the solid surface by chemical bonds, the
adsorption is termed chemical adsorption or
chemisorption.
Adsorption
Physisorption
If the accumulation of gas on the surface of a
solid occurs on account of weak van der
Waals’ forces, the adsorption is termed
physical adsorption or physisorption.
2. Adsorption
Characteristics
Physisorption Chemisorption
•It is reversible in nature and is

dependent on the pressure as well as • It is accompanied by an increase in
temperature. temperature.
•It is a multi-layered process. • The adsorbate and adsorbent get
•It can take place all over the attached by chemical bonds which
adsorbent. are either covalent or ionic in nature.
•Activation energy is less in • High activation energy is required.
physisorption. • High temperature is also favourable.
•It is favourable at lesser temperatures. • Chemisorption is an irreversible
•Physisorption depends on the surface process.
area of the adsorbent.
Classification
2. Adsorption
Factors Affecting Extent and Rates of
Adsorption
i) Pressure
• In the case of Physisorption, the extent of adsorption increases with an
increase in pressure
as the volume of the gases decreases during adsorption.
• In the case of Chemisorption, The chemisorption process occurs slowly at low
temperatures and occurs at a higher rate with an increase in pressure.
ii) Temperature
• At lower temperatures, Physisorption has a high rate as it does not require much
energy to form the weak bonds using van der Waals’ forces.
• At higher temperatures, Chemisorption has a high rate as more energy from the
covalent or ionic bonds by exchange of charges.
• For H2 on finely divided nickel, at lower temperatures, hydrogen molecules get
adsorped by van der Waals’ forces. However, as the temperature increases, hydrogen
molecules dissociate into hydrogen atoms which are held on the surface by
chemisorption.
2. Adsorption
Factors Affecting Extent and Rates of
Adsorption
iii) Nature of the Adsorbate
• The gases which are more easily liquefiable or are more soluble in water are
more readily adsorbed than others because the intermolecular forces are stronger
in more easily liquefiable gases.
• Hence, they are adsorbed readily.
iv) Nature of the Adsorbent

• Different adsorbents would adsorb different amounts of the same adsorbates even
under similar conditions.
• The substances that are porous in nature and have rough surfaces are better
adsorbents.
• Hence, Substances like charcoal and silica gel are excellent adsorbents.
v) Pressure
• The extent of adsorption also depends upon the surface area of the solid.
• The greater the surface area the more is the surface available for adsorption and the
greater is the adsorption.
2. Adsorption
Physisorption Vs Chemisorption
Physisorption Chemisorption
It is also known as physical adsorption. It is also known as chemical adsorption.
It arises because of van der Waals’ It is caused by chemical bond formation.
forces.
It is not specific in nature. It is highly specific in nature
It is reversible in nature. It is irreversible in nature.
It depends on the nature of the gas. It also depends on the nature of the gas.
More easily liquefiable gases are Gases that can react with the adsorbent
adsorbed readily. show chemisorption
Low temperature is favourable for High temperature is favourable for
adsorption. It decreases with an adsorption. It increases with the
increase in temperature. increase in temperature.
Physisorption has a very low adsorption Chemisorption has higher adsorption
enthalpy of nearly about 20 to 40 enthalpy, nearly about 80 to 240 kJ/mol,
kJ/mol. than that of physisorption.
Physical adsorption has less activation Chemical adsorption has high activation
energy. energy.
2. Adsorption
Adsorption Vs absorption
Properties Absorption Adsorption
Definition Assimilation of molecular Accumulation of the
species throughout the molecular species at the
bulk of the solid or liquid surface rather than in the
is termed absorption. bulk of the solid or liquid
is termed adsorption.
Phenomenon It is a bulk phenomenon It is a surface

phenomenon.
Heat exchange Endothermic process Exothermic process
Temperature It is not affected by It is favoured by low
temperature temperature
Rate of reaction It occurs at a uniform It steadily increases and
rate. reaches equilibrium
Concentration It is the same throughout Concentration on the
the material. surface of the adsorbent
is different from that in
the bulk
2. Adsorption
2. Adsorption
2. Adsorption
Note: Activated carbon is the most common adsorbent for industrial

wastewater treatment.
Classes of organic compounds adsorbed
on activated carbon
2. Adsorption
To achieve an economically effective treatment of metal-bearing wastewater, several
low cost adsorbents have been studied
2. Adsorption
To achieve an economically effective treatment of metal-bearing wastewater, several
low cost adsorbents have been studied
Application in water treatment
o Water treatment
Application in wastewater treatment
o Wastewater treatment (Secondary & tertiary)
At the current state of development, the integrated systems are
basically divided into two based on the configuration of the
Biomass.
i) Suspended growth biological systems receiving activated

carbon dosage:
Powdered Activated Carbon Treatment (PACT) process and
the PAC added membrane bioreactor (PAC-MBR) process.
ii) Attached growth (biofilm) biological systems containing GAC

media, namely the Biological Activated Carbon (BAC) process
Use of PAC in secondary treatment of WW
Use of PAC in secondary treatment of WW
Use of GAC in secondary treatment of WW
2. Adsorption
Recent development of adsorption
2. Adsorption
Recent development of adsorption
Biomass-derived carbon for enhanced adsorption
Biomass-derived carbon for enhanced adsorption
2. Adsorption
2. Adsorption
2. Adsorption
2. Adsorption
Use of Zeolitic waste as adsorbent

2. Adsorption
Use of polymers
2. Adsorption
Advantages & disadvantages
i) Advantages
o High removal efficiency
o Flexibility in design and operation
o Simple installation and maintenance
o A large variety of adsorbents are available
i) Disadvantages
o Adsorption performance depending on the type
of adsorbent and the wastewater composition.
o Relatively high adsorbents costs.
o In some cases adsorption is an irreversible
reaction, not making possible the adsorbent
regeneration and metal recovery, which causes
serious waste disposal problems.

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IPRC MUSANZE

WATER SUPPLY SYSTEM DESIGN

•It is reversible in nature and is

iv) Nature of the Adsorbent

Phenomenon It is a bulk phenomenon It is a surface

Note: Activated carbon is the most common adsorbent for industrial

i) Suspended growth biological systems receiving activated

ii) Attached growth (biofilm) biological systems containing GAC

Use of Zeolitic waste as adsorbent

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