water treatment technology (tas 3010) lecture notes 9d - filtration

Izan Jaafar, Engineering Science, FST, UMTIzan Jaafar, Engineering Science, FST, UMT

© SHAHRUL ISMAIL, DESc.University College of Science and Technology Malaysia

CHAPTER 3:Environmental Microbiology

CHAPTER 9d : CHAPTER 9d : CHAPTER 9d : CHAPTER 9d :

TAS 3101 : WATER TREATMENT TECHNOLOGY

Water Water

Treatment Treatment

Process :Process :

FiltrationFiltration

Water Water

Treatment Treatment

Process :Process :

FiltrationFiltration


Menu

1) Introduction

2) Filtration Methods

3) Filtration Types:

- Gravity Filter

- Pressure Filter

4) Percentage of filtered water

5) Filter Sand

6) Filter Gravel


Introduction

- Water management begins with knowing the quantity of water

available

- Basic information on water measurement units and convenient

conversion factors.

- Conversion factors simplify changing from one unit of measurement to

another.

- There are two conditions under which water is measured—water at

rest and water in motion. Water at rest is measured in units of volume.

Water in motion is measured in units of flow— unit of volume for a

convenient time unit. It is important that the difference between a unit of

volume and a unit of flow be kept in mind.


- Passage of water through a porous medium to remove remaining flocs or suspended solids.

- Needed for most surface waters in order to provide a second barrier against the transmission of water borne disease

1) Suspended matter and colloidal matter are removed

2) Chemical characteristic of water are changed

3) Number of bacteria materially reduced.

- Assist significantly by reducing the load on the disinfections process, increasing disinfection efficiency.

Filtration


METHODSMETHODS

MECHANICAL MECHANICAL STRAININGSTRAINING

SEDIMENTATION & SEDIMENTATION & ADSORPTIONADSORPTION

BIOLOGICAL BIOLOGICAL METABOLISMMETABOLISM

ELECTROLYTIC ELECTROLYTIC ACTIONACTION

Filtration - Mechanism


- Size of suspended solid is bigger than size of porous medium

- Removing such particles of suspended matter as are too large to pass through the interstices between size grains

-

Mechanical Straining


1. The interstices between the sand grains act as sedimentation

basins in which the suspended particles smaller than the

voids in the filter-bed settle upon the sides of the sand grains

2. The particles stick on the grains because the physical

attraction between the two particles of matter and because of

the presence of the gelatinous coating formed on the sand

grains by the previously deposited bacteria and colloidal

matter.

Sedimentation & Absorption


1. “ The growth and life process of the living cells”

2. The surface layer gets coated with a zoogleal film in which the

bacterial activities are the highest and which feed on the organic

impurities.

3. The coated converts organic impurities by a complex biochemical

action into simple, harmless compounds – purification of water.

Biological Metabolism


1. Due to the friction between medium and suspended solids

2. A certain amount of dissolved and suspended matter is

ionized.

3. Suspended matter in water is ionized, carries charge of one polarity

and the particles of sand in filter which are also ionized, possess

electrical charges of opposite polarity.

4. These neutralize each other; change the chemical character of water

Electrolytic Action


1) PRESSURE FILTER SYSTEM

2) GRAVITY FILTER SYSTEM

- Commonly used

- Two (2) general categories

1. Slow Sand Filter (SSF)

- 0.1 – 0.5 m/h ~ 0.2 m/h

2. Rapid Sand Filter (RSF)

- 3 – 12.5 m/h ~ 5 - 7.5m/h

Filtration - Types


1) Action at the surface of the sand bed

2) Surface gets coated with a skin or layer formed due to the

bacterial action of finely suspended matter, plankton and other

organic matter present in raw water, with algae, bacteria etc.

previously coated on the surface of the sand-bed by the water

itself – “ schmutzdecke”.

3) Below this layer and up to 30 cm, present other bacterial zones-

completely oxidize the organic matter, destroy most of the

bacteria present and let only simple and unobjectionable

inorganic salts to pass through the filter bed into the effluent.

Slow Sand Filter


- Straining action – Throughout the depth of the filter sand rather than on

the surface or to a small distance below this.

- Other action – complex biological and chemical changes as water passes

through the sand-bed as SSF.

- Previously coated material is important - Necessary for waters to be

coagulated before they are allowed to pass through the rapid sand filter.

- Other factors : Proper thickness of sand-bed, size of the sand grains.

Rapid Sand Filter (RSF)


Gravity Flow


Rapid Sand Filter (RSF)


Percentage of filtered water :

i) Quantity of filtered water every 24 hours operation, f = Qfto

ii) Filter area, As = Qf / Vof

iii) Quantity of wash water, i) QBW = As x VOBW (ii) BW = QBW x t

iv) Percentage of filtered water = BW / f x 100%

Where;

f = Quantity, m3 QBW = Flow of wash water, m3/min

to = 24 VOBW = Rate of washing

Qf = flow rate As = filter area, m2

Vof = Rate of filtration BW = Quantity of wash water, m3

Filtered Water Percentage


ITEMS SSF RSF

Filtration rate (m3/m2/hr) 0.1 – 0.2 3 – 12.5

Area (m2) 50 – 2000 (more area) < 400 (less area)

Size of sand (mm) 0.3 – 0.35 > 0.55

Distribution of medium size

Not stratum Stratum

Washing method Replace the upper layer of sand

Backwash

Operation Cost Low High

Efficiency of disinfections

99% 60%

Supervision Simple Skilled

Gravity Filter System


ITEMS SSF RSF

Loss of head Very small – 0.6 – 0.9 m

More – 2.5 – 3m

Coagulation Not required Required – satisfactory filtration

Period of cleaning Once every 2 – 3 months

24 – 48 hours

Amount of wash water 0.2 – 0.6 percent of filtered water

2 – 4 percent of filtered

Maintenance cost Small More because of maintenance of

pumps for pumping up wash-water and a compressor unit for

obtaining compressed air.

Gravity Filter System


Pressure Filter

1) Water is allowed to pass under a pressure greater than

atmospheric pressure through a closed cylinder. The force

gravity has no function.


1) Free from clay, loam, vegetable or organic matter

2) Uniform and proper size

i. If the sand is too fine, it tends to quickly clog causing a greater loss

of head in the filter

ii. If the sand is too coarse, it will permit suspended solids and bacteria

passes through the voids between the sand grains.

3) Filter sand classification :

- Effective size, Uniformity coefficient, Percent size

Filter Sand


1) No function in the actual purification of water

2) Slow sand filter :

Simply supports the sand-bed

3) Rapid sand filter :

Serve to distribute the wash water evenly throughout the sand

4) Usually placed in five or six layers; finest layer on top

5) Hard, rounded and durable.

Filter Gravel


1) PRESSURE FILTER

2) DOUBLE FILTRATION

3) MICROSTRAINING

4) DIATOMACEOUS EARTH FILTERS

5) IN-DEPTH FILTRATION

6) DUAL MEDIA FILTERS

i) Dual media filters ii) Mixed Media Filters

Filtration – Other types


Determine the percentage of filtered water

required for washwater based on the following

criteria :

Flow, Qf = 300 L/s

Rate of filtration, Vof = 170m3/m2.day

Time of washing = 10 min

Rate of washing, VoBW = 15 mm/s

Problems!!!!


1) Effective size :

i. The seize size in mm, permits 10 percent of the sand,

which permits 10 percent of the sand, by weight to pass - size

of the grain that is larger than 10 percent by weight of all the

particles comprising the sand.

ii. Indicate the minimum size of 90 percent by weight of

the sand.

iii. Consideration of variable in individual grain size.

[NEXT]

Filter Sand – Effective Size


2) Uniformity coefficient :

i. Expression of the degree of variation

ii. The ratio between the size, that will pass 60 percent by

weight to the effective size – the ratio of the particle

size which is coarser than 60 percent by weight to the sand

to the effective size of sand.

iii. E.g. if sand has an effective size of 0.50 mm and 60

percent of sand passes a 0.80 mm, the uniformity

coefficient =

0.80 0.50

= 1.60

[NEXT]

Filter Sand – Uniformity Coefficient


3) Percent Size

i. The size of the grain that has the given percent, by

weight, of materials finer in size.

ii. Percent size specified = 1, 10, 60 and 90 percent

iii. E.g. A percent size of 10 means that 10 percent of the

sand is smaller than the grain size given.

[BACK]

Filter Sand – Percent Size


THANKS FOR THANKS FOR

YOUR ATTENTION…..YOUR ATTENTION…..

THANKS FOR THANKS FOR

YOUR ATTENTION…..YOUR ATTENTION…..

water treatment technology (tas 3010) lecture notes 9d - filtration

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