Filtration/slow sand filter

filtration of water treatment. types of filters and filtration. slow sand filter. environmental engineering-1. water treatment methods. filtration of raw water. design and construction of slow sand filters. cleaning of slow sand filters. slow sand filters were explained. about rapid sand filters and pressure filters will be explained in next publication.

1. FILTRATION A WATER PURIFICATION METHOD GARRE RAVI KUMAR

2. FILTRATION Screening and sedimentation removes a large percentage of the suspended solids and organic matter present in raw supplies. And the resultant water will not be free from impurities, and may contain some very fine suspended particles and bacteria present in it. To remove or to reduce the remaining impurities still further, and to produce safe water, the water is filtered through the beds of fine granular material, such as sands, etc. The process of passing the water through the beds of such granular materials is known as filtration. Filtration may help in removing colour, odour, turbidity and some pathogenic bacteria from water.

3. FILTER MATERIALS SAND: The filter sand should generally be obtained from rocks like quartzite and should contain following properties: It should be free from dirt and other impurities It should be uniform in nature and size It should be hard and resistant It should be such as not loose more than 5% of its weight after being placed in hydrochloric acid for 24 hours The selection of the correct effective size is very important, because too smaller size will lead to clogging and too large will lead to escaping of suspended particles and bacteria.

4. FILTER MATERIALS GRAVEL: The gravel which may be used below the sand should be hard, durable, free from impurities, properly rounded and should have a density of about 1600 kg/m3. Gravel of different sizes are usually placed in 3 to 4 layers each of 15 to 20cm depth, with coarsest size – 20 to 60cm placed at bottom-most layer and finest size – 3 to 6cm as top most layer. ANTHRO FILT: Anthro filt is also can be used as filter media, instead of sand. It is cheaper and has been able to give a high rate of filtration. Anthro filt is made from Anthracite, which is a type of coal-stone, that burns without smoke or flames. The use of anthrafilt as a filter media in two or multi layers have proved very successful at abroad. However, this type of coal is not available in India.

5. Theory of filtration The filters purify water under four different processes, as summarized below: Mechanical straining Flocculation and sedimentation Biological metabolism Electrolytic changes

6. MECHANICAL STRAINING The suspended particles present in water, and which are of bigger size than the size of voids in the sand layers of the filter, cannot pass through these voids and get arrested in them. Therefore, the resultant water will be free from these impurities. Most of the particles are removed in the upper sand layers. The arrested particles including the coagulated flocs form a mat on the top of the sand bed, which further helps in straining out the impurities.

7. Flocculation and sedimentation The filters whose voids size is more than the size of particles, also able to remove such particles. This fact is possible by assuming the voids spaces acting as tiny coagulation-sedimentation tanks. The colloidal matter arrested in these voids is a gelatinous mass and , therefore, attract other finer particles. These finer particles settle down in the voids and get removed.

8. Biological metabolism Generally micro-organisms and bacteria are reside in voids as coatings over sand grains during the initial process of filtration. And, these organisms use organic impurities as their food and convert them into harmless compounds. Such harmless compounds form a layer on the top, which is called schutzdecke or dirty skin. This layer further, helps in absorbing and straining out the impurities.

9. ELECTROLYTIC CHANGES The purifying action of filter can also be explained by the theory of ionisation. This may be explained by that, the sand grains of filter media and the impurities in water, carry electrical charges of opposite nature. When these oppositely charged particles and the impurities come in contact with each other, they neutralise each other, there by changing the character of water and making it purer. After a certain interval, the electrical charges of sand grains get exhausted, and have to be restored by cleaning the filter.

10. Types of Filters The various types of filters are commonly used for treating municipal water supplies are: Slow sand filters Rapid sand filters Pressure filters

11. SLOW SAND FILTERS Slow sand filters were first invented in England by James Simpson in 1829. Slow sand filters normally utilise effluents from the plain sedimentation tanks, and are used for relatively clearer waters.

12. Construction of Slow Sand Filters The various parts of the slow sand filter are: Enclosure tank Filter media Base material Under drainage system Inlet and outlet arrangements

13. Construction of slow sand filter Enclosure tanks: An open water tight rectangular tank, made of masonry or concrete The bed slope is kept at about 1 in 100 towards the central drain The depth of tank is 2.5m to 3.5m The plan area of tank – 100 t0 2000 sq.km. Filter media: The filter media consists of sand layer about 90 to 110 cm in depth and placed over a gravel support. The effective size of sand varies from 0.2 to 0.4mm and uniformity coe . Varies from 1.8 to 2.5

14. Construction of slow sand filter Filter media: The filter media consists of sand layer about 90 to 110 cm in depth and placed over a gravel support. The effective size of sand varies from 0.2 to 0.4mm and uniformity coefficient Varies from 1.8 to 2.5 The coarser layer should be at bottom and finer layer should be at top. Top layer should contain uniform in grain size

15. Construction of slow sand filter Base material: The base material is gravel, and it supports the sand. It consists of 30 to 75mm thick, with gravels of different sizes, which placed in 3 to 4 layers. Thickness of each layer around 15 to 20cm are used. The coarsest gravel is used in the bottom most layer and the finest layer is used in the top most layer. The size of gravel in each layer should be as : bottom most layer – 40-65 mm Intermediate layers – 20-40mm & 6-20mm Top most layer – 3-6mm

16. Construction of slow sand filter Under drainage system: The gravel support is laid on the top of an under drainage system. The under drainage system consists of a central drain and lateral drains. The lateral are open jointed pipe drains or some other kind of porous drains placed 3 to 5m apart on the bottom floor and sloping towards a main central covered drain. The laterals collect the filtered water and discharge it into the main drain, which leads the water to the filtered water well.

17. Construction of slow sand filter Inlet and outlet arrangements: An inlet chamber is constructed for admitting the clarifier’s effluent without disturbing the sand layers of the filter and to distribute it uniformly over the filter bed. A ‘filtered water well’ is also constructed on the outlet side in order to collect the filtered water coming out from the main-under drain. Inlets and outlets are generally governed by automatic valves.

18. Operation of Slow Sand Filter The effluent from the sedimentation tank is allowed to enter the inlet chamber of the filter unit and get distributed uniformly over the filter bed. The water percolates through the filter media and gets purified. Through gravels, the water comes out as filtered water. It gets collected in the layers through the open joints, which into the main drain. The main drain finally discharges into the filtered water well.

19. Limitations to Operation The water entering the slow sand filter should not be treated by coagulants. The depth of water on the filter should also be decided and should not be allowed to under go large variations. The filter head is generally limited to 0.7 to 1.2m.

20. Cleaning of slow sand filter For cleaning slow sand filters, lot of manual labour is required and also small quantities of wash waters are needed. The cleaning is done by scrapping and removing the 1.5 to 3cm of top sand layer and The amount of wash water required is very small, for filtering Cleaning is repeated until the sand depth is reduced to about 40cm or so. The interval between two successive cleanings, depends upon Nature of impurities and Size of filter media This interval normal ranges between one to three months.

21. Slow Sand Filters Rate of Filtration: The rate of filtration for slow sand filters ranges between 100 to 200 litres/ hour/ sq. m. of filter area. Efficiency and Performance: Highly efficient in removing bacteria and suspended matter, and also removes colours and turbidity up to 50 mg/l. The extent of bacteria removal is up to 98 to 99% Disadvantages: Because of their smaller rate of filtration, they require huge surface areas, and large volumes of filtering materials. This makes them costly and uneconomical.