WATER WATER PURIFICATION PURIFICATION

ON ON LARGE SCALELARGE SCALE

• First used in 1804 in Scotland & subsequently in London.

• During 19th Century used through out the world.

• consists of concrete rectangular basin containing carefully selected graded sand supported on gravel and stones.

Slow sand filter

Filter box

1-1.5 m

1 m

0.3 m

Slow sand filter

1.1. Supernatant waterSupernatant water above the sand bed Depth 1-1.5 mtr

2. 2. A bed of graded sandA bed of graded sand– Most important part of the filter– Thickness – about 1 mtr1 mtr– Sand : preferably rounded with effective diameter

of 0.2-0.3 mm0.2-0.3 mm– Supported by graded gravel (30-40 cm deep)– Water percolates through the bed and gets

filtered by • Mechanical straining, sedimentation, adsorption,

oxidation and bacterial action

– Rate of filtration: 0.1-0.4 0.1-0.4 m3/hour/m2

Slow sand filter

Vital LayerVital Layer

– The Heart of Slow Sand FilterThe Heart of Slow Sand FilterSurface of the sand bed gets covered with a slimy growth known as

‘SchmutzdeckeSchmutzdecke’, vital layer or zoological layer or biological layer’, vital layer or zoological layer or biological layerIt is slimy geletinuos, consists of algae, plankton, diatoms and

bacteria.The formation of this layer is known as RIPENINGRIPENING of the filter, which

may take several days to fully develop (2-3 cm)Removes organic matter, holds back bacteria, oxidizes ammonical Removes organic matter, holds back bacteria, oxidizes ammonical

nitrogennitrogen to nitrates and yield a bacteria free water to nitrates and yield a bacteria free waterUntil the vital layer is fully formed, filter only worked as a

mechanical strainer & so for the first few days filter water run to the waste.

3. 3. An under-drainage systemAn under-drainage system porous of perforated pipes porous of perforated pipes at the bottom of filter

– Provides an outlet to the filtered water and

– supports the filter medium

Slow sand filter

Slow sand filter

4. 4. A system of filter control valvesA system of filter control valvesVenturi meter -regulation system

– To maintain constant rate of filtration

– measures the resistance of the sand bad or “loss of head”

• Filter cleaningFilter cleaning

– When the bad resistance increases to such an extent that the regulating valve has to be kept fully open, filter bed needs to be cleaned.

– Cleaning is done by• Draining of supernatant water• Scrapping of the top portion of the sand bed to a depth of

1-2 cm– After 20-30 scrapings , new bed (filter) is required to

be constructed

Slow sand filter

• Advantages :Advantages :

Simple to construct and operate Cheaper Physical, Chemical & Bacteriological quality of

water is very high Reduces bacterial count by 99.9% & E. coli by

99%

Slow sand filter

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

Rapid sand filter• In 18851885, the first rapid sand filters were installed in

USA• 2 types

– Open/gravity type— PATERSON FILTERPATERSON FILTER– Closed/presure type— Candy’s FilterCandy’s Filter

• Steps of filtration CoagulationCoagulation Rapid MixingRapid Mixing FlocculationFlocculation

SedimentationSedimentation FiltrationFiltration

• CoagulationCoagulation– Alum is added – 5-40 mg/liter depending upon turbidity, color, pH

& temperature• Rapid mixing of AlumRapid mixing of Alum

– By violent agitation in a mixing chamber for few minutes• FlocculationFlocculation

– Slow & gentle stirring of the treated water for about 30 minutes– Results in formation of thick, copious, white flocculent precipitate

of aluminium hydroxide• SedimentationSedimentation

– For 2-6 hours– Flocculent precipitate together with impurities and bacteria– 95% of the flocculent needs to be removed before the water

enters the rapid sand filter

STEPSSTEPS

Filter beds –Filter beds –

Each unit has a surface of 80-90 m2

Supernatant water – 1 to 1.5 mtr

Depth of sand bed – 1 mtrEffective size of the sand particle is 0.4-0.7 mm

Supported graded gravel – 30-40 cm

Filtration rate – 5-15 m3 / m2/ hr

Filtration Filtration –

– Retained alum- flocs is held back on sand bed

• Forms a slimy layer [comparable to the zoological layer in slow sand filter]

• adsorbs bacteria from water

– Oxidation of ammonia also takes place during the passage of water

Rapid sand filter

• Cleaning & MaintenanceCleaning & Maintenance

–When the loss of head reaches 7-8 ft, it needs to be cleaned

–cleaned by Backwashing.Backwashing.•Backwashing is done by reversing the flow of water through the sand bed.dislodges the impurities and cleans up sand bed.

•Compressed air can be used in back washing

Rapid sand filter

Rapid sand filter

• Advantages –Advantages –

deal with raw water deal with raw water directlydirectly

Occupies less spaceOccupies less spaceFiltration is rapidFiltration is rapidWashing of filter is easyWashing of filter is easyMore flexibility in More flexibility in

operationoperation

• DisadvantagesDisadvantages

×Relatively high Relatively high skill operationskill operation

×CostlierCostlier×Removes bacteria Removes bacteria

by 98-99%by 98-99%

Slow sand filterSlow sand filter Rapid sand filterRapid sand filter

SpaceSpace Occupies large area Small area

Rate filtrationRate filtration 0.1-0.4 m3/m2/ hour 5-15 m3 / m2/ hour

Effective size of sandEffective size of sand 0.2-0.3 mm 0.4-0.7 mm

Preliminary treatmentPreliminary treatment Plain sedimentation Chemical coagulation & sedimentation

WashingWashing By scraping the sand bed

Backwashing

OperationOperation Less skilled Highly skilled

Loss of head allowedLoss of head allowed 4 feet 6-8 feet

Removal of turbidityRemoval of turbidity Good Good

Removal of colourRemoval of colour fair Good

Removal of bacteriaRemoval of bacteria 99.9-99.99 % 98-99%

THANK YOUTHANK YOU