Filtration unit in water treatment plant

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Semester- 6, Division-C

Guided by,Prof. H.H. Jariwala

L.D. College of Engineering, Ahmedabad-15 Filtration [Water and Wastewater Engineering]


Prepared by,Sr. No.Name of StudentRoll No.Enrollment No.1.Baloliya Payal R.60620011302801060072.Khandor Bhavya M.60620051302801060463.Merin Issac60620071302801060574.Patni Mo.Aamir Z.60620161302801060915.Shah Vrushti K.6062023130280106110


Content3Sr. No.TopicSlide 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]


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.Introduction4

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


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 actionTheory of filter6

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.7

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.Sedimentation8

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


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


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. Sand2. Gravel3. AnthraciteFilter materials


It should have following properties:Obtained from hard rock such as Quartzite, Trap, Basalt, etc.Free from Clay, Loam and Organic matterUniform size and natureHard and resistantIf placed in HCl for 24 hr, it should not loose more than 5% of its weight.Sand


Uniformity Coefficient(Cu):It is a measure of particle range and is given by,Sasasasaaaaaaaaaaaaaaaaaaaadsd ddfsssssssssssssssdsUniformity coefficient shall be,3.0-5.0 for Slow Sand Filter1.3-1.7 for Rapid Sand Filter

SandEffective size of sand(D10):Very fine sand: Clogged quickly & reduce rate of filterationVery coarse sand: Suspended particles and bacteria pass through the sand bedEffective size shall be,0.20-0.30 mm for Slow Sand Filter0.45-0.70 mm for Rapid Sand FilterDepth of sand bed should be between 60 cm to 90 cm13

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.14

Substitute for sandCan be used in conjunction with sandCost is more as compared to sandAnthracite


Filters may be classified as:Classification of Filters16

Used in rural areas in place of a rapid gravity filterFiltration 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 pathogensReplaces the coagulation-flocculation-settling, the filtration and the disinfection by chlorination treatments in rural areas Slow Sand Filter17

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 Filter18

Slow Sand Filter

Essential features:1) Enclosure tank2) Filter media3) Base material4) Under drainage system5) Appurtenances19

SSF is open basin, rectangular shape and built below finished ground level.Floor has Bed slope of 1:100 to 1:200 towards central drainSurface area (As) of tank varies from 50 to 1000 sqmFiltration rate 100 to 200 lit/sqm/hr.Depth 2.5 to 4 m1. Enclosure tank


Thickness of sand layer - 90 to 110 cmEffective 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: Sand21

Thickness of gravel bed - 30 to 75 cm3. Base material: GravelLayerDepthSize(mm)Top most15 cm3 to 6Intermediate15 cm6 to 20Intermediate15 cm20 to 40 Bottom15 cm40 to 65


Base material and filter media are supported by under drainage system.Under drainage system collects filtered water and delivers it to the reservoirLaterals earthenware pipes of 7.5 to 10 cm dia.Spacing of laterals- 2 to 3 m c/c4. Under drainage system


Devices are required forGauge to measure loss of headVertical air vent pipe for proper functioning of filtering layersTelescopic tube to maintain constant dischargeA meter to measure flow5. Appurtenances24

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 Filter25

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..)26

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..)27

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 filterThese filters need a lot of space Advantages:Simple to construct and operateCheaperPhysical, Chemical and Bacteriological quality of water is very highReduces bacterial count by 99.9% & E. coli by 99%Advantages & Disadvantages of SSF28

Rapid Sand Filter

Essential features:1) Enclosure tank2) Filter media3) Base material4) Under drainage system5) Appurtenances29

Smaller in size, therefore can be placed under roofRectangular in shape and constructed of concrete or masonryDepth 2.5 to 3.5Surface area 20 to 50 m2.L/B ratio 1.25 to 1.35Designed filtration rate are 3000 to 6000 lit/sqm/hr1. Enclosure tank


Should be free from dirt, organic matter and other suspended solidsIt should be hard and resistantDepth of sand media 0.6 to 0.9 mEffective 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: Sand31


The depth of sand bed should be such that flocs should not break through the sand bed.Depth varies from 60 to 90 cmMin depth required is given by Hudsons 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 32

Thickness of gravel bed 45 to 60 cm3. Base material: GravelLayerDepthSize(mm)Top most15 cm3 to 6Intermediate15 cm6 to 12Intermediate15


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