The water filtration process involves letting water run through some type of filter material, or, in some cases, using pressure to force the water through it. Depending on the filtering medium used, filtration can remove particulate matter, metals, and microbial pathogens. Filters come in all sizes, from small models that attach to household faucets, all the way up to those used in municipal water treatment systems, but they all operate under the same basic principles.
Filter media, the materials that capture contaminants from the water, are the main components in any water filtration process. Sand is a commonly used in large water treatment systems and is sometimes mixed with anthracite coal. Granular activated carbon — carbon material that has been processed to have a higher surface area, so that it can absorb more contaminants — is a popular choice for small household filters. Manganese greensand is a filter material specially treated to remove iron and manganese from water. Swimming pools frequently use diatomaceous earth filters.
Small filters generally contain only one material, but in larger ones, the filter media are layered. For example, sand filters have a layer a gravel with a layer of sand over it. If the filter also includes anthracite coal or some other fine-grained filter material, it is layered on the top. The water flows slowly through the small particles in the upper layers of the filter, then moves more quickly through the gravel layer and finally collects in the underdrain until it moves on to its next destination.
The only visible action in the water filtration process is that of the water flowing through, or being forced through, the filter, but in reality several processes are taking place to remove contaminants from the water. Larger contaminants are sieved out because they cannot move through the filter material. Particles of filter material adsorb contaminants, such as gases and particulates like metals and minerals. Adsorption means that the contaminants become attached to the surfaces of the filter material.
For a certain type of filter called a slow sand filter, biological breakdown is another part of the water filtration process that removes contaminants. Slow sand filters have a layer of microorganisms, called the schmutzdecke, that forms on top of the sand in the filter. The microorganisms in the schmutzdecke break down organic contaminants, and can also consume harmful bacteria that may be present in the water.