Media filter

Media filtration is a type of granular depth filtration process in which water passes through one or more layers of filter media. It is primarily used for the removal of suspended solids, turbidity, and some microorganisms from water.^[1] Depending on the number and arrangement of filter media, media filtration can be classified into single-media, dual-media, and multi-media filtration systems. The material is arranged in such a way that the density and particle size decrease with increasing depth.^[2]

The single-media filtration typically uses sand as the sole filter material. It is commonly applied for the removal of relatively small particles and turbidity.^[3]

The dual-media filtration combines sand with a coarser and less dense material such as anthracite. This configuration improves the removal of suspended solids and reduces the accumulation of particles at the surface of the sand layer, allowing for longer filtration cycles and more efficient operation.^[4]

Multi-media consists out of three or more layers of different filter media, such as anthracite, sand, and gravel. The varying physical properties of these materials, including particle size and density, enhance the filtration performance by enabling more effective removal of particles throughout the depth of the filter bed.^[5]

One design brings the water in the top of a container through a "header" which distributes the water evenly. The filter "media" start with fine sand on the top and then becomes gradually coarser sand in a number of layers followed by gravel on the bottom, in gradually larger sizes. The top sand physically removes particles from the water. The job of the subsequent layers is to support the finer layer above and provide efficient drainage.

As particles become trapped in the media, the differential pressure across the bed increases. Periodically, a backwash may be initiated to remove the solids trapped in the bed. During backwash, flow is directed in the opposite direction from normal flow. In multi-media filters, the layers in the media re-stratify due to density differences prior to resuming normal filtration. Multimedia filter can remove particles down to 10-25 microns.

1. Multimedia filters use multiple layers of different filter media to achieve more effective and efficient filtration than single-media filters like sand filters.    
2. They can remove a wider range of particle sizes and types than single-media filters, resulting in more efficient filtration and longer filter life.  
3. They are effective at removing suspended solids, turbidity, and other contaminants from water.    
4. They can be used for a wide range of flow rates and particle sizes. They can be easily backwashed to clean the filter media and restore filtration efficiency.    
5. They require little to no electricity to operate.  

1. Multimedia filters have a higher capital cost compared to single-media filters like sand filters.
2. They have a larger footprint and require more space than single-media filters.
3. They may not be effective at removing some types of contaminants, such as dissolved organic compounds and bacteria.
4. They may require pre-treatment to remove large particles or debris that could clog the filter media.
5. They can create waste material (backwash water) that needs to be treated or disposed of properly.