Philadelphia University
Faculty of Engineering
Department of Civil Engineering
First Semester, 2013/2014
ENVIRONMENTAL
ENGINEERING
343
Lecture 8:
Water Treatment (3)
Filtration & Disinfection
FILTRATION
Filtration is a process for separating suspended or
colloidal impurities from water by passing
through a porous medium, usually a bed of sand
or other medium.
Settled water (Sedimentation effluent) turbidity
range 1-10 TU – due to residue of flocs particles.
So turbidity need to be reduce to less than 0.3
Common materials for granular bed filters:
Sand (slow, rapid or high)
 Anthracite coal
 Dual media (Coal plus sand)
 Mixed media (coal, sand & garnet)

FILTRATION- FILTER MEDIA

Loading rate
Where,
Vo = water velocity (m/s)
Q = water flow (m3/s)
Ac= cross surface area (m2)
EXAMPLE
Installing a Sand filter after the sedimentation tank
. The design loading rate to the filter is
200m3/d.m2 How much filter surface area should
be provided for the design flow rate of 0.5 m3/s
FILTRATION
Purpose to reduce the turbidity from 1- 10 TU to
less than 0.3 TU
Slow sand filters
 Low filtration rate with the use of smaller sand
 Filter sand is less uniform
 Particles are removed on the surface of the filter
(forming a mat of materials , called schmultzdecke)
 Schmultzdecke forms a complex of biological
community that degrade some organic compounds.
 Pretreatment is not important
 Require large area of land and are operator intensive
 Loading rate 2.9-7.6 m3/d.m2
FILTRATION
Rapid sand filters ( most common)
 Graded (layered) within the bed to optimize the
passage of water while minimizing the passage of
particulate mater
 Cleaned in place by backwashing process
 Pretreatment to destabilize particles is essential
 Loading rate 120 up to 235 m3/d.m2

For larger loading rate, a minimum of 4 filters is
suggested
FILTRATION
FILTRATION
FILTRATION
Dual-media Filters
• Constructed of silica sand and anthracite coal.
• Depth of sand is about 0.3m and coal 0.45 m. Size and
uniformity is selected to produce a distinct separation
after backwashing
• Disadvantage is that filtered materials are held loosely
in the anthracite layer and can dislodge with sudden
changes in hydraulic loading. The material can then
bind to the sand layer
• Loading rate up to 250 m3/d.m2 or more
FILTRATION
Mixed Media Filters
• The perfect filter is composed of a grading of large
media at the top to small at the base. This is best
achieved by the use of 3 or more media with ranging
size, density and uniformity coefficient
• Typical installation – Overall bed depth 0.75m; 60%
anthracite; 30% silica sand; 10% garnet sand
• Size range from 1.0mm anthracite to 0.15 garnet sand
• Filtration rates range from 10 to 20m/hr
FILTRATION
Terminal head loss: As the filter clogs, it become
very hard to force water through the bed and the
filter bed must be cleaned for backwashing
process
 Head loss: (pressure drops) that occurs when
clean water flows through a bed of clean filter
media. It is important in design value to
determine the overall head require to operate the
filter
 Backwashing process; allowing large flow of
water (clean water) to be pumped to enter the
filter bed. This forces the sand bed to release the
colloidal particles that trapped in the pores and
released and escaped with the washwater.

FILTRATION DESIGN

Key Elements


Hydraulics
Parameters to be measure during operation
The head loss across the filter
 The turbidity of the effluent

DISINFECTION

Disinfection is used in water treatment to reduce
pathogens (diseases-producing microorganisms)
to an acceptable level
DISINFECTION- INTRODUCTION
Treatment of water with chemicals to kill
bacteria”
 Two objectives:

Primary disinfection : Kill any pathogen in water
 Secondary (residual) disinfection : Prevent pathogen
re growth in the water.


Method use :
Should be harmless and unobjectionable to the
consumer
 Should be able to retain a residual disinfecting effect
for a long period

DISINFECTION- PROPERTIES
1.
2.
3.
4.
5.
Destroy bacteria / pathogens within a
practicable period of time, over an expected
range of water temperature
Effective at variable compositions,
concentration and conditions of water treated.
Neither toxic to humans and domestic animals
nor unpalatable
Not change water properties
Have residual in a sufficient concentration to
provide protection against recontamination
DISINFECTION- PROPERTIES-CON’D
6) Can be determined easily, quickly, and
preferably automatically.
7) Dispensable at reasonable cost
8) Safe and easy to store, transport, handle and
supply
10) Not form toxic by-products due to their
reactions with any naturally occurring materials
in water.
DISINFECTION- METHODS
1.
2.
3.
Chlorination- chlorine
Ultra violet Radiation
Ozonation
DISINFECTION METHODSCHLORINATION

Free chlorine disinfection

Effective and the most common application

Available in granular, powdered, liquid or gases form

Developed by using chlorine gas (Cl2), sodium hypochlorite
(NaOCl) or calcium hypochlorite (Ca(OCl)2).

Reacts in water to produce dissolved chlorine gas(Cl2(aq)),
hypochlorous acid (HOCl) and hypochlorite (OCl-).
DISINFECTION –WATER REACTION
H2O+Cl2 (g)
 HOCl
 H++ OCl
H+ + HOCl + ClH++ OCl- (pH > 8)
HOCl (pH < 7)
HOCl= hypochlorous acid
OCl- = hypochlorite ion
HOCl is more effective than OCl
Chlorine gas will be injected for water pH less
than 3
CHLORINATION- AMMONIA REACTION
Ammonia chlorine
 HOCl+NH3
H2O+NH2Cl (Monochloramine)
HOCl+NH2Cl
H2O+NHCl2 (Dichloramine*)
 HOCl+NHCl2
H2O+NCl3 (Trichloramine)
*These reactions depend on :





Low pH
Low Temp
Time
high Cl2 : NH3 initial concentration
In favor of dicloramine
AMMONIA REACTION CON’D
The proportion of chloramines depends on
1. pH.
2. Temperature
3. Time
4. Initial chlorine to ammonia ratio
*Combined chlorine: Combination of chlorine with
ammonia nitrogen or organic nitrogen
compounds. Combined chlorine is less reactive.
CHLORINATION- ADVANTAGE


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Provide chlorine residual for secondary disinfection.
Chlorine residue must be maintained in the treated
water to the end user. This secondary disinfection
functioned to control pathogen distribution during
water distribution.
Increase lifetime of chlorine residual by adding
ammonia to treated water.
Ammonia reacts with free chlorine to form
chloramines (NH2Cl, NHCl2 and NCl3) which are
termed combined chlorine.Chloramines less effective as oxidants than HOCl,
seldom used as primary disinfectant. However, more
persistent than free chlorine and maintain secondary
disinfectant.
CHLORINATION- DISADVANTAGE
Effectiveness is less with protozoan cysts,
Giardia lamblia and Crytosporidium and virus.
2) Formation of halogenated disinfectant byproducts (DBPs). Includes trihalomethanes
(THMs) such as chloroform (CHCl3) and
haloacetic acids (HAAs).
- THMs and HAAs created when free chlorine
combines with natural organic substances that
may still be present in the water.
1)
CHLORINE EFFECTIVENESS -FACTORS
1.Dosage – Sufficient high concentration to inactive
pathogen
2.Contact time – Physical contact with pathogen
for sufficient time to achieve inactivation.
3.Turbidity – Present particles (turbidity) hides
pathogen from the disinfectant.
4.Other reactive species – Consume disinfectant,
reduce concentration available for inactivation.
5. pH – Most effective at pH values less than 7.5.
6. Water temperature – Disinfection increases,
however chlorine become less stable.
DISINFECTION - OZONE

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


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


Sweet smelling, unstable gas-Form of oxygen in
which three atoms of oxygen combined to form the
molecule O3
O+O2
O3
Air in generating equipment contain up to 13% ozone.
Most powerful disinfectant and widely used in water
More effective against cysts and viruses than free
chlorine.
Leaving no taste or odor problems.
Faster contact time
No residual remain
Not forming THM
Unaffected by the pH or the ammonia content of the
water
High cost – Construction, Operation and
Maintenance
DISINFECTION - UV









Employed by submerged UV lamps into the water to
be treated
Energy is absorbed by genetic material in the
microorganism, interfering with their ability to
reproduce and survive.
Multiple lamps used to provide greater coverage
Ability of UV lights to pass through the water to get
the target organism.
Much higher energy level than visible light
Potential to inactivate pathogens.
Performs well against both bacteria and viruses.
Leaves no residual protection for distribution system.
High cost