Sand gravity filtration, a common water treatment method, effectively removes turbidity and suspended solids by using gravity to draw water through a bed of sand and gravel. It's highly effective for removing turbidity, with well-operated filters reducing it to less than 1 NTU (often < 0.1 NTU).
Here's a more detailed look at sand gravity filtration:
Mechanism:
Relatively large suspended particles are mechanically strained as they pass through the sand and gravel bed.
Smaller particles adhere to the surface of the sand grains due to the van der Waals forces (physical adsorption).
Chemical filter aids (coagulants or flocculants) can be added to promote additional adhesion.
Types:
Rapid Gravity Filters: These are commonly used in municipal drinking water facilities as part of a multi-stage treatment system.
Slow Sand Filters: These are another type of sand filter that can produce high-quality water with pathogen removal, but require a larger footprint and slower filtration rates.
Performance:
Turbidity Removal: Rapid sand filters are highly effective at removing turbidity, with well-operated filters reducing it to less than 1 NTU (often < 0.1 NTU).
Other Contaminants: While effective for turbidity, rapid sand filters are less effective at removing other contaminants.
Filtration Rates: Filtration rates can vary in the range of 5–10 m3/h.m2.
Operation:
Pre-treatment: Rapid sand filters often require pre-treatment (coagulation-flocculation) to prepare the water for filtration.
Post-treatment: Disinfection (e.g., with chlorine) is usually required after filtration to ensure safe drinking water.
Backwashing: Regular backwashing is necessary to remove accumulated solids and maintain filter performance.
Advantages:
Cost-effective and space-saving.
Simple operation.
Effective for removing turbidity.
Disadvantages:
Require pre-treatment and post-treatment.
Can be complex and expensive to operate and maintain.
Not suitable for small communities or developing nations due to complexity and cost.
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