Mechanism of Slow Sand Filtration

Drinking Water Microbiology
Types of Water
Outline of Water Treatment
Sources of Drinking Water
Preliminary Treatment Processes
Aeration
Water Fall Aerators
Bubble Aerators
Sedimentaion
Coagulation and Flocculation
Sand Filtration
Rapid Sand Filters
Pressure Filters
Upflow Filters
Multiple MediaFilters
Slow Sand Filters
Mechanism of Slow Sand Filtration
Comparison of Rapid and Slow Sand Filters

Mechanism of Slow Sand Filtration

	Mechanism of Slow Sand Filtration In slow sand filters, as in the case of rapid sand filters, removal of impurities from raw water is brought about by a combination of different processes such as straining, sedimentation, adsorption and biochemical and microbiological processes. As filtration proceeds day after day a slimy, gelatinous film accumulates around each grain of sand and in the interstices. This slimy layer is composed of millions of bacteria, protozoa, filamentous algae and other microorganisms and is called schmutzdecke. [German word meaning dirt layer].
This layer is usually formed in the upper three or four inches of sand. This slowly closes up the pores between sand grains thus making filtration more effective. Actually the microbial and biochemical processes take place in schmutzdecke. Straining removes the suspended particles that are too large to pass through the pores of the filter bed. Straining takes place only at the surface of the fitter. Periodically the accumulated impurities are removed by scraping off the top layer.
Sedimentation removes fine suspended solids when they are deposited onto the surface of sand grains. The larger surface area of the fine particles and low rate of filtration makes sedimentation an effective process. Just like straining sedimentation is also carried out in the upper surface. Charged sand particles adsorb particles of opposite charge by electrostatic attraction. Clean quartz sand has a negative charge and binds positively charged particles such as flocs of carbonates, aluminium hydroxide and cations of iron and manganese.
They can not bind negatively charged bacteria, organic colloidal particles and other anions. However, excessive binding of positively charged particles leads to a reversal of the charge, thus the filter bed becomes positively charged. Now negatively charged particles are adsorbed. The organic matter and other particles adsorbed onto the filter sand grains are subjected to microbial and biochemical activities. A ripening period is required before any biochemical activity takes place. During this period microorganisms from the raw water binds on to the filter bed and establish themselves.
	Organic matter is broken down by microorganisms for their growth and energy requirement. When these microorganisms die out they become the substrate for other microorganisms as they are carried down further into the filter bed. Thus the organic matters are slowly transformed into inorganic compounds such as CQ2, nitrate, sulphates and phosphates which are finally discharged along with effluent. In addition certain chemical reactions also take place in filter beds. This includes conversion of soluble ferrous and manganous compounds into insoluble ferric and manganic oxides I hydrates which are coated around the sand particles. For effective filtration bacteria have to develop and migrate to the deeper layers of the filter bed. This can be achieved by maintaining the depth of filter bed to around 0.7m. Another important objective of slow sand filtration is the removal of bacterial pathogens. This is achieved in a number of ways.

Almost all bacteria are removed from the water and retained at the surface of filter beds by adsorption. Most of the intestinal bacteria experience an unfavourable temperature as the temperature is colder than that in their natural environment. Certain other microbial groups produce antibiotic compounds which either kill bacteria or inactivate them.

Moreover, there are predatory organisms like protozoa which feed on bacteria and thus reducing their number. There are certain limitations in the case of slow sand filters. They are not used for raw waters with more turbidity because of the problems associated with cleaning.

Thus they cannot be used to filter water to which chemical coagulants have been added as there will be some carryover of floc. For the treatment of turbid water slow sand filters are used as secondary filters after the rapid sand filters.

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