Download pptx - Filtration unit in water treatment plant

Semester- 6, Division-C

Guided by,Prof. H.H. Jariwala

L.D. College of Engineering, Ahmedabad-15

Filtration [Water and Wastewater Engineering]

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Prepared by,

Sr. No. Name of Student Roll No. Enrollment No.

1. Baloliya Payal R. 6062001 130280106007

2. Khandor Bhavya M. 6062005 130280106046

3. Merin Issac 6062007 130280106057

4. Patni Mo.Aamir Z. 6062016 130280106091

5. Shah Vrushti K. 6062023 130280106110

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ContentSr. No. Topic Slide No.

1. Introduction [04-05]2. Theory of filter [06-10]3. Filter materials [11-15]4. Classification of Filters [16-16]5. Slow Sand Filter [17-28]6. Rapid Sand Filter [29-40]7. Backwashing [41-45]8. Comparison of SSF & RSF [46-47]9. Design of Rapid Sand Filter [48-53]

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Sedimentation removes a large percentages of settable solids,

suspended solids, organic matter and small percentage of bacteria.

But water still contains fine suspended solids, microorganisms and

color(if present).

To remove these impurities, still further and to produce potable and

palatable water, the water is filtered through the beds of granular

materials like sand and gravel.

Introduction

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The process of passing the water

through the beds of granular

material(sand and gravel) is known

as Filtration.

By doing filtration, we can remove

bacteria, colour, taste, odours and

produce clear and sparkling water.

Introduction

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When water is filtered through the bed of filter media, usually

consisting of clean sand, the following factors take place:

◦ (1) Mechanical straining

◦ (2) Sedimentation

◦ (3) Biological action &

◦ (4) Electrolytic action

Theory of filter

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Mechanical strainingSand contains small pores. The suspended particles which are bigger

than the size of the voids in the sand layer, cannot pass through these

voids and get arrested.

These arrested particles forms a mat on the top of the bed which

further helps in straining out

the impurities.

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In mechanical straining only those particles which are coarser than

void size are arrested. Finer particles are removed by sedimentation.

The voids between sand grains of filter acts like small sedimentation

tanks.

The colloidal matter arrested in the voids is the gelatinous mass and

therefore attract other finer particles.

Sedimentation

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Suspended impurities contain some portion of organic impurities such

as algae, plankton, etc., which are food of various types of micro-

organisms.

These organic impurities form a layer on the top of sand bed which is

known as ‘Schmutzdecke’ or ‘dirty skin’.

This layer further helps in absorbing and

straining out the impurities.

Biological action

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Another function of the filter is to remove the particulate matter by

electrostatic exchange.

The charge of the filter medium neutralize the charge of floc, thereby

permitting the floc to be removed.

During the process of ‘back washing’ of filter, the electrostatically

neutral material is removed and the charge of the filter

media is replaced.

Electrolytic action

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Sand(fine/coarse) is generally used as filter media.

The layer of sand may be supported on gravel, which permits the

filtered water to move freely to the under drains and allows the wash

water to move uniformly upward.

Filter materials:

◦ 1. Sand

◦ 2. Gravel

◦ 3. Anthracite

Filter materials

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It should have following properties:

Obtained from hard rock such as – Quartzite, Trap, Basalt, etc.

Free from – Clay, Loam and Organic matter

Uniform size and nature

Hard and resistant

If placed in HCl for 24 hr, it should not

loose more than 5% of its weight.

Sand

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Uniformity Coefficient(Cu):

◦ It is a measure of particle range and is

given by,

Sasasasaaaaaaaaaaaaaaaaaaaadsd

ddfsssssssssssssssds

Uniformity coefficient shall be,

◦ 3.0-5.0 for Slow Sand Filter

◦ 1.3-1.7 for Rapid Sand Filter

SandEffective size of sand(D10):

◦ Very fine sand: Clogged quickly &

reduce rate of filteration

◦ Very coarse sand: Suspended particles

and bacteria pass through the sand bed

Effective size shall be,

0.20-0.30 mm for Slow Sand Filter

0.45-0.70 mm for Rapid Sand Filter

Depth of sand bed should be between 60 cm to 90 cm

𝐶𝑢=𝐷60𝐷10

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It should be – hard, durable, free from impurities, properly rounded

and have a density of about 1600 kg/cubic meter.

It supports the sand and allows the filtered water to move freely

towards the underdrains.

Gravel

It allows the wash water to move upward

uniformly on sand.

The gravel is placed in 5 to 6 layers having

finest size on top.

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Substitute for sand

Can be used in conjunction with sand

Cost is more as compared to sand

Anthracite

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Filters may be classified as:

Classification of FiltersFilters

Gravity filters:Slow sand filter

(Biological Filter)

Rapid sand filter(Mechanical Filter)

Pressure filters:Horizontal pressure filter

Vertical pressure filter

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Used in rural areas in place of a rapid gravity filter

Filtration rate is 50 to 100 times slower than that of a rapid gravity

filter (0.1 to 0.3 m/hour - 0.2 m/hr is the typical rate)

Used for the removal of turbidity (colloidal particles), suspended

solids and pathogens

Replaces the coagulation-flocculation-settling, the filtration and the

disinfection by chlorination treatments in rural areas

Slow Sand Filter

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Filtered water has < 0.3 NTU turbidity (the goal is < 0.1 NTU)

Output water may require chlorination (for quality improvement)

A pre-treatment in roughing filters may be needed specially when the

turbidity is high (greater than 20-50 NTU)

Oxfam filters (use of geo-textile fabric on the top of the sand layer for

straining out the suspended matter (pre-treatment!)

Slow Sand Filter

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Slow Sand FilterEssential features:

1) Enclosure tank 2) Filter media 3) Base material

4) Under drainage system 5) Appurtenances

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SSF is open basin, rectangular shape and built below finished ground

level.

Floor has Bed slope of 1:100 to 1:200 towards central drain

Surface area (As) of tank varies from

50 to 1000 sqm

Filtration rate – 100 to 200 lit/sqm/hr.

Depth – 2.5 to 4 m

1. Enclosure tank

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Thickness of sand layer - 90 to 110 cm

Effective size – 0.20 to 0.35 (Common value -0.3)

Coefficient of uniformity – 2.0 to 3.0 (Common value - 2.5)

2. Filter media: Sand

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Thickness of gravel bed - 30 to 75 cm

3. Base material: Gravel

Layer Depth Size(mm)

Top most 15 cm 3 to 6

Intermediate 15 cm 6 to 20


Bottom 15 cm 40 to 65

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Base material and filter media are supported by under drainage

system.

Under drainage system collects filtered water and delivers it to the

reservoir

Laterals – earthenware pipes of 7.5 to

10 cm dia.

Spacing of laterals- 2 to 3 m c/c

4. Under drainage system

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Devices are required for

Gauge – to measure loss of head

Vertical air vent pipe – for proper functioning of filtering layers

Telescopic tube – to maintain constant discharge

A meter – to measure flow

5. Appurtenances

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In a slow sand filter impurities in the water are removed by a

combination of processes: sedimentation, straining, adsorption, and

chemical and bacteriological action.

During the first few days, water is purified mainly by mechanical and

physical-chemical processes. The resulting accumulation of sediment

and organic matter forms a thin layer on the sand surface, which

remains permeable and retains particles even smaller than the spaces

between the sand grains.

Working of Slow Sand Filter

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As this layer (referred to as “Schmutzdecke”) develops, it becomes

living quarters of vast numbers of micro-organisms which break down

organic material retained from the water, converting it into water,

carbon dioxide and other oxides.

Most impurities, including bacteria and viruses, are removed from the

raw water as it passes through the filter skin and the layer of filter bed

sand just below.

Working of Slow Sand Filter(contd..)

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The purification mechanisms extend from the filter skin to approx.

0.3-0.4 m below surface of filter bed, gradually decreasing in activity

at lower-levels as water becomes pure & contains less organic matter.

When the micro-organisms become well established, the filter will

work efficiently and produce high quality effluent which is virtually

free of disease carrying organisms and biodegradable organic matter.

They are suitable for treating waters with low colors, low turbidities

and low bacterial contents.

Working of Slow Sand Filter(contd..)

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Disadvantages:

Old fashioned and outdated

method of water purification

(but still in use)

Initial cost is low but

maintenance cost is much more

than rapid sand filter

These filters need a lot of space

Advantages:

Simple to construct and operate

Cheaper

Physical, Chemical and

Bacteriological quality of water

is very high

Reduces bacterial count by

99.9% & E. coli by 99%

Advantages & Disadvantages of SSF

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Rapid Sand FilterEssential features:

1) Enclosure tank 2) Filter media 3) Base material

4) Under drainage system 5) Appurtenances

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Smaller in size, therefore can be placed under roof

Rectangular in shape and constructed of concrete or masonry

Depth – 2.5 to 3.5

Surface area – 20 to 50 m2.

L/B ratio – 1.25 to 1.35

Designed filtration rate are 3000 to

6000 lit/sqm/hr

1. Enclosure tank

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Should be free from dirt, organic matter and other suspended solids

It should be hard and resistant

Depth of sand media – 0.6 to 0.9 m

Effective size – 0.35 to 0.6 mm (Common value 0.45)

Uniformity coefficient – 1.2 to 1.7 (Common value -1.5)

2. Filter media: Sand

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The depth of sand bed should be such that flocs should not break through the sand

bed.

Depth varies from 60 to 90 cm

Min depth required is given by Hudson’s formula

where,q = Filtration rate in cum/sqm/hr [Assumed filtration rate x Factor of safety (2)]D = sand size in mmH = terminal head loss in ml = depth of sand bed in mBi = Break through index = 4 x 10^(-4) to 6 x 10^(-3)

Estimation of depth

𝑞∗𝐷3∗𝐻𝑙 =𝐵𝑖∗29323

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Thickness of gravel bed – 45 to 60 cm

3. Base material: Gravel

Layer Depth Size(mm)

Top most 15 cm 3 to 6



Bottom 15 cm 20 to 50

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To start with, a size gradation of 2 mm at top and 50 mm at bottom is

assumed.

The required depth (l) in cm of a component of gravel layer of size d

(mm) can be computed by following equation

where,

K can be taken as 12

d = gravel size in mm

Estimation of depth

𝑙=2.54∗𝐾∗ log (𝑑 )

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Objectives of under drainage system

1. To collect filtered water uniformly

over the area of gravel bed

2. It provides uniform distribution of

back wash water without disturbing

or upsetting gravel layer and filter

media

4. Under drainage system

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Typical devices required are,

1. Wash water troughs

2. Air compressors

3. Rate control device

5. Appurtenances

[Wash water trough]

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Working of Rapid Sand Filter

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All valves are kept closed except valves A and B

Valve A is opened to permit water from clarifier

Valve B is opened to carry filtered water to clear water sump

Head of 2m over sand bed is maintained

Designed filtration rate are 3000 to 6000 lit/sqm/hr

Filter run depends on quality of feed water


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Filter run may range between less than a day to several days

Objective of backwash is to remove accumulated particles on the

surface and within the filter medium

Backwash is performed using wash water or air scouring.


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Disadvantages:

Relatively high skill operation

Costlier

Removes bacteria by 98-99%

Advantages:

Deal with raw water directly

Occupies less space

Filtration is rapid

Washing of filter is easy

More flexibility in operation

Advantages & Disadvantages of RSF

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Rapid Sand Filter

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Filter is back washed when head loss through it has reached the

maximum permissible.

RSF are washed by sending air and water upwards through the bed by

reverse flow through the collector system.

2% - 4% filtered water is used for backwashing

Backwashing

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1. Close influent valve A

2. Close effluent valve B

3. Open air valve F, so that air blows at rate of 1 to 1.5 m3 free air

/min/m2 of bed area for @ 2 to 3 min. this will break up the scum

and loosen the dirt.

4. Close the air valve F and open the wash water valve E gradually to

prevent the dislodgement of finer gravel.

Steps in Back Washing

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5. Open the wastewater valve D to carry wash water to drain. Continue

backwashing till wash water appears fairly clear.

6. Close the wash water valve E. Close the wastewater valve D. wait for some

time till all matter in bed settles down.

7. Open valve A slightly, open valve C for carrying filtered water to drains for

few minutes.

8. Close the valve C and open valve B. Open valve A completely to resume

normal filtration

Steps in Back Washing(contd…)

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Back washing at Treatment Plant

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Comparison of SSF & RSF

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Comparison of SSF & RSF(contd…)

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Problem: Design a rapid sand filter to treat 10 million litres of raw water per day allowing 0.5% of filtered water for backwashing. Half hour per day is used for backwashing. Assume necessary data.Solution: Total filtered water = 10.05 x 24 x 106 = 0.42766 Ml/h 24 x 23.5Let the rate of filtration be 5000 l / h / m2 of bed.Area of filter = 10.05 x 106 x 1 = 85.5 m2 23.5 5000Provide 2 units. Each bed area 85.5/2 = 42.77. L/B = 1.3; 1.3B2 = 42.77B = 5.75 m ; L = 5.75 x 1.3 = 7.5 mAssume depth of sand = 50 to 75 cm.

Rapid Sand Filter Design

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Underdrainage system:Total area of holes = 0.2 to 0.5% of bed area.Assume 0.2% of bed area = 0.2 x 42.77 = 0.086 m2

100Area of lateral = 2 (Area of holes of lateral)Area of manifold = 2 (Area of laterals)So, area of manifold = 4 x area of holes = 4 x 0.086 = 0.344 = 0.35 m2 .So, Diameter of manifold = (4 x 0.35 /p)1/2 = 66 cmAssume c/c of lateral = 30 cm. Total numbers = 7.5/ 0.3 = 25 on either side.

Rapid Sand Filter Design(contd…)

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Length of lateral = 5.75/2 - 0.66/2 = 2.545 m.C.S. area of lateral = 2 x area of perforations per lateral. Take dia of holes = 13 mmNumber of holes: n p (1.3)2 = 0.086 x 104 = 860 cm2

4 So, n = 4 x 860 = 648, say 650 p (1.3)2

Number of holes per lateral = 650/50 = 13Area of perforations per lateral = 13 x p (1.3)2 /4 = 17.24 cm2

Spacing of holes = 2.545/13 = 19.5 cm.


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C.S. area of lateral = 2 x area of perforations per lateral = 2 x 17.24 = 34.5 cm2.

So, Diameter of lateral = (4 x 34.5/p)1/2 = 6.63 cmCheck: Length of lateral < 60 d = 60 x 6.63 = 3.98 m. l = 2.545 m (Hence acceptable).Rising washwater velocity in bed = 50 cm/min.Washwater discharge per bed = (0.5/60) x 5.75 x 7.5 = 0.36 m3/s.Velocity of flow in lateral = 0.36 = 0.36 x 10 4 =2.08 m/s (ok) Total lateral area 50 x 34.5Manifold velocity = 0.36 = 1.04 m/s < 2.25 m/s (ok) 0.345


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Washwater gutterDischarge of washwater per bed = 0.36 m3/s. Size of bed = 7.5 x 5.75 m.Assume 3 troughs running lengthwise at 5.75/3 = 1.9 m c/c.Discharge of each trough = Q/3 = 0.36/3 = 0.12 m3/s. Q =1.71 x b x h3/2

Assume b =0.3 m h3/2 = 0.12 = 0.234 1.71 x 0.3So, h = 0.378 m = 37.8 cm = 40 cm = 40 + (free board) 5 cm = 45 cm; slope 1 in 40


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Clear water reservoir for backwashingFor 4 h filter capacity, Capacity of tank = 4 x 5000 x 7.5 x 5.75 x 2 1000

= 1725 m3

Assume depth d = 5 m. Surface area = 1725/5 = 345 m2

L/B = 2; 2B2 = 345; B = 13 m & L = 26 m.Dia of inlet pipe coming from two filter = 50 cm.Velocity < 0.6 m/s. Dia. of wash water pipe to overhead tank = 67.5 cm.Air compressor unit = 1000 l of air/ min/ m2 bed area.For 5 min, air required = 1000 x 5 x 7.5 x 5.77 x 2 = 4.32 m3 of air.


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Water supply engineering by S.K.Garg,1977Picture Courtesy: http://www.google.comwww.historyofwaterfilterrs.com/filteration

References

http://www.google.com/

http://www.historyofwaterfilterrs.com/filteration

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