A gravity sand filter, also known as a rapid gravity filter, is a widely used type of water treatment filter that relies on gravity to pass water through a bed of filtering material. These filters are most commonly used in Municipal water purification plants worldwide.
How Gravity Sand Filters Work
Gravity sand filters function by allowing water to flow downwards through a layer (or layers) of filtering media. The name "gravity" comes from the fact that no external pressure is applied; the natural force of gravity pulls the water through the filter bed.
The filter media typically consists of relatively coarse sand and other granular materials. As the water passes through this bed, particles and impurities are physically strained out or adsorbed onto the media surfaces. A key aspect, as highlighted in the reference, is their effectiveness in removing particles and impurities that have been trapped in a floc through the use of flocculation chemicals in a prior treatment step. Flocculation causes small suspended particles to clump together into larger, more easily filterable masses called floc.
Key Features and Components
While designs can vary, common elements of a gravity sand filter include:
- Filter Bed: The layer(s) of granular media (sand, gravel, anthracite, etc.). The size and type of media are crucial for effective filtration.
- Underdrain System: Located at the bottom of the filter, this system collects the filtered water and also distributes water and air during the cleaning process (backwashing).
- Filter Box/Basin: The structure that contains the filter bed and underdrain.
- Inlet and Outlet Controls: Systems to manage the flow of raw water into the filter and filtered water out.
Applications and Importance
Given their efficiency and cost-effectiveness, gravity sand filters are a cornerstone of large-scale water treatment. As mentioned, they are most commonly used in Municipal water purification plants worldwide to treat raw water from sources like rivers, lakes, and reservoirs before it is distributed to consumers. They are effective in removing suspended solids, turbidity, and some microorganisms, making the water clearer and safer.
- Primary Use: Municipal drinking water treatment.
- Role in Treatment Process: Usually follows flocculation and sedimentation steps.
- Benefits: Relatively simple operation, high flow rates (compared to slow sand filters), effective turbidity removal.
Filter Cleaning (Backwashing)
Over time, the filter bed accumulates removed particles, leading to increased resistance to flow and reduced efficiency. To restore filter performance, gravity sand filters undergo a cleaning process called backwashing. This involves reversing the flow of water (sometimes with air) up through the filter bed. This action lifts and expands the media, releasing the trapped impurities, which are then flushed away as waste.
