Filtration introduction

CIVL 1101 Introduction to Filtration 1/13

Water Treatment Water Treatment

 Basis water treatment  Coagulation
consists of four processes:  This process helps removes
 Coagulation/Flocculation
g particles suspended in water.
 Sedimentation  Chemicals are added to
water to form tiny sticky
 Filtration
particles called "floc" which
 Disinfection attract the particles.


 Flocculation  Sedimentation
 Flocculation refers to  The heavy particles (floc)
water treatment processes settle to the bottom and the
that combine or coagulate clear water moves to
small particles into larger filtration.
particles, which settle out
of the water as sediment.


 Filtration  Disinfection
 The water passes through  A small amount of chlorine is
filters, some made of layers added or some other
of sand, gravel, and charcoal disinfection method is used
that help remove even to kill any bacteria or
smaller particles. microorganisms that may be
in the water.


1. Coagulation
2. Flocculation
3. Sedimentation
4. Filtration
5. Disinfection
6. Fluoridation
7. Stabilization
8. Collect and test water
samples

1. Coagulation - Aluminum or iron salts plus chemicals
called polymers are mixed with the water to make 5. Disinfection - Chlorine is added to reduce risks from
the particles in the water stick together. remaining bacteria and other disease-causing
2. Flocculation - The coagulated particles are slowly organisms and to maintain water quality through the
mixed so th t th can collide and form l
i d that they llid df larger distribution pipe system
system.
particles, known as "floc." 6. Fluoridation - Fluoride is added to provide dental
3. Sedimentation - Water flows through a large tank benefits.
which allows the "floc" to settle to the bottom of 7. Stabilization - Small amounts of lime (calcium
the tank and be removed. hydroxide) or sodium hydroxide are added to make
4. Filtration - Water is passed through filters made of the water less corrosive to pipes and plumbing.
sand and anthracite coal to filter out remaining 8. Collect and test water samples
particles.

Water Filtration Water Filtration
 Filters may be classified according to the
 Filtration is used to separate nonsettleable solids types of media used as follows:
from water and wastewater by passing it through a
porous medium
 Single–media filters: These have one type of
yp
 The most common system is filtration through a media, usually sand or crushed anthracite coal.
layered bed of granular media, usually a coarse
anthracite coal underlain by a finer sand.  Dual–media filters: These have two types of
media, usually crushed anthracite coal and sand.

 Multi–media filters: These have three types of
media, usually crushed anthracite coal, sand, and
garnet.



 In water treatment all three types are used; however,  Filtration was actually developed prior to the discovery
the dual– and multimedia filters are becoming of the germ theory by Louis Pasteur in France.
increasingly popular.

 Particle removal is accomplished only when the  Louis Pasteur (1822 – 1895) was a
particles make physical contact with the surface of French chemist and microbiologist.
the filter medium.
 He is remembered for his remarkable
breakthroughs in the causes and
preventions of diseases.


 In the 1700s the first water filters for domestic  In 1854 it was discovered that a cholera epidemic
application were applied. These were made of wool, spread through water.
sponge and charcoal.
 The outbreak seemed less severe in areas where sand
 In 1804 the first actual municipal water treatment filters were installed.
plant designed by Robert Thom, was built in Paisley,
Scotland.  British scientist John Snow found that the direct
cause of the outbreak was water pump contamination
 The water treatment was based on slow sand by sewage water.
filtration, and horse and cart distributed the water.
 He applied chlorine to purify the water, and this paved
 Some three years later, the first water pipes were the way for water disinfection.
installed.


 John Snow (1813 – 1858) was an
English physician and a leader in the
adoption of anaesthesia and medical
hyg n .
hygiene.
 He is considered to be one of the
fathers of epidemiology, because of
his work in tracing the source of a
cholera outbreak in Soho, England, in
1854.


Floc Particles
 Larger particles may be removed by straining
Interception
Straining
 Particles may also be removed by
Flocculation
sedimentation
 Others may be intercepted by and adhere to
the surface of the medium due to inertia
 Filtration efficiency is greatly increased by
destabilization or coagulation of the particles
prior to filtration
Sedimentation
Filter Media

Gravity Granular–Media Filtration Gravity Granular–Media Filtration

 Gravity filtration through beds of granular  Because of the reduction in pore area, the
media is the most common method removing velocity of water through the remaining voids
colloidal impurities in water processing
ll id l i i i i i increases, shearing off pieces of capture fl
i h i ff i f floc
and carrying impurities deeper into the filter
 Initially, surface straining and interstitial
bed
removal results in accumulation of deposits in
the upper portion of the filter media  The effective zone of removal passes deeper
and deeper into the filter

Gravity Granular–Media Filtration Turbidity

 Eventually, clean bed depth is no longer  Turbidity is a measurement of the clarity of
available and breakthrough occurs, carrying water run
solids out in the underflow and causing
lid i h d fl d i
termination of the filter run  Clouded water is caused by suspended
particles scattering or absorbing the light

 Turbidity is an indirect measurement of the
amount of suspended matter in the water


Turbidity Slow Sand Filtration

 However, since solids of different sizes, shapes,  The early filtration units developed in Great
and surfaces reflect light differently, turbidity Britain used a process in which the hydraulic
and suspended solids d not correlate well.
d d d lid do l ll loading
l di rate i relatively l
is l i l low.
 Turbidity is normally gauged with an instrument  Typical slow sand filtration velocities are only
that measures the amount of light scattered at about 0.4m/hr.
an angle of 90° from a source beam.
 At these low rates, the filtered contaminants do
 The units of turbidity are usually in not penetrate to an appreciable depth within the
Nephelometric Turbidity Units (NTU). filtration medium.

Slow Sand Filtration Slow Sand Filtration

 The filter builds up a layer of filtered
contaminants on the surface, which becomes the
active fil i medium
i filtering di
 Slow sand filters are cleaned by taking them off
line and draining them. The organic or
contaminant layer is then scraped off.
 The filter can then be restarted. After water
quality reaches an acceptable level, the filter
can then be put back on line.


Rapid Sand Filtration Rapid Sand Filtration
 In rapid sand filtration much higher application velocities  In the United States, filter application rates are often
are used expressed as volumetric flowrate per area, or
 Filtration occurs through the depth of the filter gal/m n ft wh ch s
gal/min–ft2, which is actually a velocity with atypical
veloc ty w th atyp cal
units.
 A comparison of rapid and slow sand filtration is shown in
the table below

Filtration Type Application Rate Filtration Type Application Rate
m/hr gal/ft2–day m/hr gal/ft2–day
Slow Sand 0.04 to 0.4 340 to 3400 Slow Sand 0.04 to 0.4 340 to 3400
Rapid Sand 0.4 to 3.1 3400 to 26,000 Rapid Sand 0.4 to 3.1 3400 to 26,000


Hydraulic Loading Rate Hydraulic Loading Rate
Let’s compute the hydraulic loading rate on our A hydraulic loading rate of 3.954 gpm/ft2 could be
filters in lab: classified as:
Flowrate: 1,000 ml/min 1. A high-end direct filtration (1–6 gpm/ft2)
Area of filter: 3.5 inch diameter filter 2. A mid-range rapid filter (range of 2–10
Flowrate gpm/ft2 with 5 gpm/ft2 normally the maximum
Loading Rate  design rate)
Area
1, 000 ml min 1 gallon 144in 2 gpm
    3.954
2
 (3.5in )
4
3,785 ml ft 2 ft 2

To convert the hydraulic loading rate to the U.S. A hydraulic loading rate of 5,694 gpd/ft2 could be
standard of gpd/ft2, convert minutes to days qualifies as a rapid sand filter
Flowrate
Fl t
Loading Rate 
Area Filtration Type Application Rate
gpm
 3.954  60 min  24 hr m/hr gal/ft2–day
ft 2 hr day Slow Sand 0.04 to 0.4 340 to 3400
gpd Rapid Sand 0.4 to 3.1 3400 to 26,000
 5,694
ft 2

Let’s compute the hydraulic loading rate for Let’s compute the hydraulic loading rate for
flowrates in class: flowrates in class:

Flowrate: 1,250 and 1,500 ml/min Flowrate: 1,250 and 1,500 ml/min

Area of filter: 3.5 inch diameter filter Area of filter: 3.5 inch diameter filter

Flowrate
Loading Rate  Flowrate of 1,250 ml/min  4.943 gpm/ft2
Area



Flowrate ml min   1 gallon 
144in 2
Flowrate of 1,500 ml/min  5.931 gpm/ft2

3,785 ml ft 2
2
 (3.5in )
4


Hydraulic Loading Rate Rapid Sand Filtration
Let’s compute the hydraulic loading rate for  The water above the filter provides the hydraulic
flowrates in class: pressure (head) for the process.

Flowrate: 1,250 and 1,500 ml/min  The filter medium is above a larger gravel, rock, or other
h fl d b l l k h
media for support.
Area of filter: 3.5 inch diameter filter
 Below the rock is usually an underdrain support of some
type.
Flowrate of 1,250 ml/min  7,117 gpd/ft2
 The water flows through the filter and support media,
Flowrate of 1,500 ml/min  8,541 gpd/ft2 exiting from a pipe below.




 Most modern filters employ two separate filter media in  As the filter begins to clog from accumulated solids, less
layers: water will pass through it. At some point cleaning is
 The lower layer is composed of a dense fine media, often sand
dense, media requ red.
required.
 The upper layer is composed of a less dense, coarse media, often
anthracite coal  Usual filter operation before cleaning is from a few
hours to 2 days.
 The coarse upper layer removes larger particles before  Cleaning is accomplished by reversing the flow of water
they reach the fine layer, allowing the filter to operate to the filter, or backwashing.
for a longer period before clogging.



Water supply Backflush water out  The backwash velocity is sufficient to fluidize the bed
– that is, to suspend the bed with the reverse flow.

Backflush Backflush  After backwashing, the filter is again placed in
supply supply operation
Fluidized filter
Filter media media

Filtered water
Underdrain support Underdrain support

Operation during filtration Operation during cleaning


http://www.fbleopold.com/flash/media.swf


Backwash Velocity Backwash Velocity
The backwash velocity may be estimated using the Once the backwash velocity has been estimated, the depth
following equation of the expanded filter bed may be computed

L(1   )
v  v s e4.5 Le 
 
0.22

where v is the backwash velocity (ft/s)
1  vv
s

vs is the settling velocity of the filter media (ft/s) where L is depth of the filter media (ft)

e is the porosity of the expanded filter Le is depth of the expanded filter media (ft)

 is the porosity of the filter media



Backwash Velocity Example Backwash Velocity Example
Determine the required backwash velocity to expand the The backwash velocity may be estimated using the
sand filters in lab to a porosity of 0.70. following equation
Also, determine the depth of the expanded filter bed.
v  v s e4.5
Assume the following data about our lab filters:
1. Depth of sand bed 0.5 ft 
 0.27 ft s   0.70  4.5

2. Sand with a particle diameter of 0.5 mm or 0.02 inches with a settling

0.054 ft s
velocity of 0.27 ft/s

3. Sand porosity is 0.35


Backwash Velocity Example Backwash Velocity Example
Determine the hydraulic loading rate of the backwash Once the backwash velocity has been estimated, the depth
of the expanded filter bed may be computed
V l it  0 054ft s  0 054ft
3
Velocity 0.054 0.054 L(   )
(1
ft 2s
Le 
 
0.22

 0.054ft
3

7.48 gallons

86, 400s 1  vv
ft 2s ft 3 day s

0.5ft (1  0.35)
 34, 900
gpd The backwash loading rate  0.22  1.26 ft
ft 2 is about 7 times larger than  0.054 ft 
1 s 
the filter loading rate  0.27 ft 
 s 


Backwash Velocity Group Problem Traditional Filtration
Determine the required backwash velocity to expand the A typical scheme for water filtration consists of
sand filters in lab to a porosity of 0.75. flocculation with a chemical coagulant and sedimentation
prior to filtration
filtration.
Also, determine the depth of the expanded filter bed.
Alum or other
Assume the following data about our lab filters: coagulant Polymer coagulant

1. Depth of sand bed 0.5 ft
Influent Effluent
Flocculation Sedimentation Filtration
2. Sand with a particle diameter of 0.5 mm or 0.02 inches with a
t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2
settling velocity of 0.27 ft/s
Rapid mixing
3. Sand porosity is 0.30 t = 30 minutes



Traditional Filtration Traditional Filtration
Under the force of gravity water passes downward When the media become filled or solids break through,
through the media that collect the floc and particles. a filter bed is cleaned by backwashing.

Alum or other Alum or other
coagulant Polymer coagulant coagulant Polymer coagulant

Influent Effluent Influent Effluent
Flocculation Sedimentation Filtration Flocculation Sedimentation Filtration
t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2 t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2

Rapid mixing Rapid mixing
t = 30 minutes t = 30 minutes


Traditional Filtration Direct Filtration
Filtration rates following flocculation and sedimentation The process of direct filtration does not include
are in the range of 2–10 gpm/ft2 with 5 gpm/ft2 sedimentation prior to filtration.
normally the maximum desi n rate
design rate.
Alum or other
Alum or other coagulant Polymer coagulant
coagulant Polymer coagulant

Influent Effluent
Influent Effluent Optional mixing Filtration
Flocculation Sedimentation Filtration T > 30 minutes R = 1 – 10 gpm/ft2
t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2
Rapid mixing
Rapid mixing t = 30 minutes
t = 30 minutes


Direct Filtration Direct Filtration
The impurities removed from the water are collected Contact flocculation of the chemically coagulated
and stored in the filter. particles in the water takes place in the granular media.

Alum or other Alum or other
coagulant Polymer coagulant coagulant Polymer coagulant

Influent Effluent Influent Effluent
Optional mixing Filtration Optional mixing Filtration
T > 30 minutes R = 1 – 10 gpm/ft2 T > 30 minutes R = 1 – 10 gpm/ft2
Rapid mixing Rapid mixing
t = 30 minutes t = 30 minutes



Direct Filtration Description of a Typical Gravity Filter System

Successful advances in direct filtration are
attributed to: During filtration, the water enters above the filter
media through an inlet flume.
Development of coarse–to–fine multimedia filters
Improved backwashing systems, and After passing downward through the granular media and
Availability of better polymer coagulants the supporting gravel bed, it is collected in the
underdrain system
Filtration rates in direct filtration are usually
1–6 gpm/ft2

Operating Table
Filter Bed
Description of a Typical Gravity Filter System
Concrete
Floor Wall

Floor During backwashing, wash water passing upward through
the filter carries out the impurities that accumulated in
Hydraulic
Lines
for Values

the media
Drain
Influent Line
Waste

The flow is directed upward, hydraulically expanding the
Effluent Line
Wash Line to Clearwell

filter media

The water is collected in the wash–water troughs that
Wash Trough Concrete Wall

discharge to the outlet flume
Filter Sand

Graded Gravel

Perforated Laterals
Manifold

Description of a Typical Gravity Filter System

The filters are placed on both sides of a pipe gallery
that contains inlet and outlet piping, wash–water inlet
lines, and wash–water drains.

A clear well for storage of filtered water is located
under a portion of the filter bed area



Filter Media – Ideal Filter

Bed Depth
Increasing
Grain Size

Pore Size


Filter Media – Single Medium Filter Filter Media – Dual-Medium Filter

Increasing
Grain Size
Bed Depth

Bed Depth

Increasing
Grain Size

Increasing
Grain Size

Pore Size Pore Size


Filter Media Filter Media
Broadly speaking, filter media should possess the These attributes are not compatible. For example:
following qualities:

1. Fine sand retains floc and tends to shorten the filter run
1. Coarse enough to retain large quantities of floc,
2. Sufficiently fine particles to prevent passage of 2. For a course sand the opposite would be true
suspended solids,
3. Deep enough to allow relatively long filter runs, and
4. Graded to permit backwash cleaning.



Filter Media Filter Media
A filter medium is defined by effective size and Conventional sand medium has an effective size of 0.45–
uniformity coefficient. 0.55 mm, a uniformity coefficient less than 1.65

Effective size is the 10–percentile diameter; that is, 10%
by weight of the filter material is less than this diameter, A sand filter bed with a relatively uniform grain size can
D10 provide effective filtration throughout its depth

Uniformity coefficient is the ratio of the 60–percentile
size to the 10–percentile size (D60 /D10)


Multimedia Filters Multimedia Filters
Dual–media filter beds usually employ anthracite and sand The main advantages of multimedia filters compared to
single–medium filters are:
However,
However other materials have been used, such as
used
activated carbon and sand
1. Longer filtration runs,
Multimedia filter beds generally use anthracite, sand, and
2. Higher filtration rates, and
garnet.
3. The ability to filter a water with higher turbidity
However, other materials have been used, such as
activated carbon, sand, and garnet.


Multimedia Filters
The advantages of the multimedia filters are due to:

Any Questions?
1. The
1 Th media particle size,
di ti l i
2. The different specific gravities of the media, and
3. The media gradation.

Filtration introduction

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