Filtration

Water treatment

Theory of filtration

• Mechanical straining• Sedimentation• Biological Action: schmutzdecke or dirty skin

• Electrolytic action

Classification of filter

• Slow sand filter• Rapid sand filter- Gravity type• Pressure type• Head required for filtration

• Rate of filtration• Composition of filter media• Method and frequency of cleaning the filter media.

Filter Media

• Sand• Antharacite• Garnet sand and ilmente• Other locally available materials

sand

• Cheapest filter medium• It should be free from clay, silt, loam,

suspended matter and other organic matter,

Sand

• It should be free from clay, silt, loam, suspended matter and other organic matter.

• It should be fairly uniform and of the proper size.

• The uniformity characteristics of sand are expressed I terms of

• (i) effective size• (ii) uniformity coefficient

Effective size or diameter(D10)

• It represents a size, in mm, such that 10% of the particles are finer than this size. Similarly, the sand particles finer than D60 are 60% of by the total weight of the sample.

• The uniformity coefficient (Cu) is a measure of particle range, and is given by the ratio of D60andD10 sizes.

• Cu= D60/D10• Cu= 1

Properties of uniformly graded sand Cu

• It shall be of hard and resistant quartz or quartzite and free from clay, fine particles , soft grains and dirt.

• Ignition loss should not exceed 0.7%• Soluble fraction in HCl shall not exceed 5% by weight.• Specific gravity shall be in the range between 2.55 t 2.65• Wearing loss shall not exceed 3%• Effective size shall be

– 0.2 to 0.3 mm for slow sand filters– 0.45 to 0.7 mm for rapid sand filters.The uniformity coefficient shall be

3.0 to 5.0 for slow sand filtersnot less than 1.3 more than 1.7 in rapid sand filter

Anthracite

• Crushed anthracite has been used as filter medium as a substitute for sand in some filters

• It is more costly per tonne than sand but its greater volume per unit weight and lower transportation cost per unit volume may make it more economical.

Garnet Sand

• It has high specific gravity 4.2 and is a dense material. Because of high cost, it cannot be used as a sole filler material.

• Other materials such as coconut husks, burned rise husks, crushed glass, slag, metallic ores have been used as filler material.

Essential features of Slow Sand Filter

• Pretreatment of primary settling• Enclosure tanker • Filter media• Base material• Under drainage tank• Appurtenanaces.

Enclosure tank

• Open basin rectangular in size, built below finished ground level.

• The water tight basin or tank is constructed either in stone or brick masonry with a coating of water proof material.

• The floor has a bed slope of 1 in 100 to 1 to 200 towards the central drains .

• The surface area of the tank varies between 50 m2 to 1000 m2, and is found on the basis of filtration rate which varies between 100 to 200 litres of water per sq.mt.

• The depth of tank varies from 2.5 to 4 m.

Filter Media

• The filter media consists of sand layer 90 to 110 cm thick. • The effective size of sand varies from 0.2 to 0.35. with a

common value of 0.3.• The coefficient of uniformity varies from 2 to 3. the

common value being 2.5• The finer the sand, better will be bacterial efficiency, but

slower will be filtration rate.• If pretreatment is good and water to be treated is not

highly polluted, coarse sand is suitable.• The sand should not contain more than 2% of calcium and

magnesium computed as carbonates.

Base Material

• The filter media is supported on the base material consisting of 30 to 75 cm thick gravel bed.

• The gravel bed is graded and laid in layers of 15 cm with top most layer of finer size and bottom most layer of coarse size.

Base materialDepth Size

Top most layer 15 cm 3 mm to 6 mm

Intermediate layer 15 cm 6 mmto20mm

15 cm 20 mm to 40 mm

Bottom layer 15 cm 40 mm to 65 mm

Under drainage system

• The filter media and the base material are supported over the under drainage system which eventually collects the filtered water and delivers it to the clean water reservoir.

• The lateral drains are either earthenware pipe perforated pipes of 7.5 to 10 cm dia or may consist of patented drain devices, spaced 2 to 3 m centre to centre. They may be of open jointed tiles.

appurtenances

• Measuring loss of head through filter media• Controlling depth of water above the filter media• Maintaining constant rate of flow through the filter.• When a newly constructed or newly washed filter unit is put

in operation, the filter head ( the difference in water level between the filter basin and outlet chamber) is only 10 to 15 cm.

• But as the upper portion if filter sand gets clogged due to impurities of the water, the filter head increases.

• In order to maintain a constant discharge, the telescope tube is adjusted manually.

Working of the filter

• Water depth over the filter media should be neither be too of the sand medium.

• It is collected by the under drainage system. – The surface coating, called schmutzdecke– The autotrophic zone consiting a few milimetres

below the schmutzdecke– The heterotrophic zone which may extend some

30 cm in the bed.

smutzdeckeche• It is formed by reddish brown sticky deposit of partly decomposed

organic matter together with iron, mangenese, aluminum and silica.• This coating further absorbs organic matter in the colloidal state.g• After 2-3 weeks of commissioning the filter, there exists in the

uppermost layer of sand a film of algae, bacteria, protozoa, suspended particles and organic matter.

• In the autotrophic zone, the growing plant life breaks down organic matter, decomposes the plankton, and uses up available nitrogen, phosphates and carbon dioxide providing oxygen in their place thus oxidizing the filtrate.

• In the heterotrophic zone, extending a depth to a depth of 30 cm, the bacteria multiply in large number, to break down competely the organic matter, resulting in the presence of only simple inorganic substances and unobjectionable salts.

Working of filter

• The difference of water level in the filter basin and the outlet chamber is known as the filer head.

• Slow sand filters are worked upto a maximum filter head of either 75 cm or equal to 65 to 85% of the thickness of filter medium.

• In the beginning, the filtration head is only 10 to 15 cm.

guideline for design of slow sand filters

Description Recommended design value

Design period 10 yrs

Number of filter

mimimum 2

Area upto 20 m2 2

A= 20 to 249 m2 3

A= 250 to 649 m2 4

A= 650 to 1200 m2 5

A= 1201 to 2000 m2 6

Depth of supernatant water 1.0m

Filtration rate

Normal oepration 0,1m/hr

Max. over load rate 0.2 m/hr

Guidelines for design of slow sand filterDescription Recommended design value

Free board 0.2m

Depth of filter sand

Initial 1.0

final 0.4

Sand specifications

Effective size 0.2 to 0.3 mm

Uniformity coefficient 5

Gravell ( l3- 4 layers0 depth 0.3 m

Under drains 0.2m

Comparison between slow and rapid gravity filter

Item Slow sand filter Rapid Gravity Filter

Area Require very large area Require small area

Quantity of sand Requires considerable quantity of sand

Require less quantity of sand