Filtration: How does it work?
Improving water quality almost always involves some sort of filtration process. Filtration barriers remove larger particles from water, including those formed by coagulation. This section is about the filtration process which can be used to improve the quality of rural water. An animated version of the filtration process is available below.
Filtration removes particles from water. All filtration systems are designed for specific water uses (for example household, drinking, agri-food sector, industrial, etc.). Two common types of filter media are: Sand / Gravel and Carbon. These filter media can be used in both conventional and biological filters.Conventional rapid filters
- the filter media traps particles
- the flow of water is relatively fast
- the filter media traps particles
- the flow of water is much slower
- in addition, natural living organisms purify the water
Many other types of filter media are available (for example anthracite, birm) depending on the type of water treatment desired. Water processed by any filter requires further purification or disinfection before it can be considered to be safe drinking water.
Conventional rapid filters
Conventional sand filter
Layers of sand and gravel, arranged according to density and sand particle size, trap and strain particles in the water. Water flows relatively fast (5 to 30 metres per hour) through the layers. Filtered water is then collected in a pipe, and passed on to the next treatment process.
The sand and gravel traps
- small bugs or organisms
- suspended dirt
- particles of "floc" formed by coagulation pre-treatment
- any other large particles in the water
Some organisms and dissolved matter pass through (for example bacteria, arsenic). Trapped particles build up in the filter media, and could eventually clog the filter or pass through as a dirty water mixture. Filters must always be cleaned by "backwashing" before this happens. Water (sometimes with air) is forced backwards through the filter to remove the particles. The dirty water is drained to waste. Once the filter is cleaned, the filter can operate normally again. After sand filtration, the water must be treated with additional processes including disinfection before it is safe to use as drinking or household water.
Conventional carbon filter
Granular activated carbon traps some particles, but works primarily through adsorption - a process in which the organic matter present in water "sticks" to the carbon granules. This helps to eliminate colour, taste, odour, and other organic matter problems. The irregular, creviced surface of one gram of carbon has a surface area of about 600 to 1000 m2 ! Microorganisms flourish inside these crevices, feeding on the nutrients in the water and the particles that stick to the carbon.
Critical design features in carbon filters include:
- type of granular activated carbon
- the contact time (that is the length of time the water is in contact with the carbon)
"Contact times" may range from 10 to 20 minutes. If the organic matter does not have enough contact time with the carbon granule, it will not be adsorbed and will pass through the filter. Some organisms and dissolved matter pass through (for example bacteria, calcium).
Carbon filters require regular backwashing to clean out any build-up of trapped particles. Backwashing will not remove any matter that is adsorbed to the carbon. Once the carbon has adsorbed all the organic matter it can, it is said to be "exhausted". Organic matter will then pass through the filter, causing water quality problems. Carbon exhaustion in household filters can occur as often as every one to two months. The carbon should be replaced or regenerated, as backwashing cannot fix carbon exhaustion. Regeneration is only possible in large municipal water treatment plants. After effective carbon filtration, the filtered water is then ready for disinfection and purification before drinking or household use.
Biological sand filter
Biological slow sand filters work primarily by biological activity as well as straining and trapping particles in a sand filter.
Critical design features are:
- fine sand
- slow continuous filtration rate
As the filter works, a layer of dirt, debris, and living and decaying microorganisms builds up on top of the sand. This is called the "schmutzdecke", which is German for "dirty skin!" Dirt, particles and naturally-occurring microorganisms gather in the layer, where they assist in the filtration but slow down the filter flow rate. These filters must therefore be much larger in size than rapid filters. The slow flow rate (0.1 to 0.3 m/hour) and the schmutzdecke make the filter very effective in straining out even very small particles from the water.
Microorganisms living in the schmutzdecke and the top 300 mm of the sand also help to consume and trap other microorganisms, minerals, or nutrients such as iron, arsenic, etc. Some organisms and dissolved matter pass through (for example bacteria, calcium). As the filter operates, it will eventually plug up. The flow may stop when the schmutzdecke becomes too thick and dense. The filter flow is recovered by scraping away the schmutzdecke and the top several centimeters of sand. Slow sand filters may not work well if the raw water is too dirty or turbid. After biological slow sand filtration, the water must be treated with additional processes including disinfection before it is safe to use as drinking or household water.
Biological carbon filter
Biological granular activated carbon filters are similar to conventional carbon filters:
- Water flows slowly through granular activated carbon filter media.
- Dissolved organic matter sticks to the carbon.
Biological carbon filters differ from conventional carbon filters in that they are designed to benefit from biological activity. Important factors in biological carbon filters include:
- the type of granular activated carbon
- contact time (how long the water is organic particles are in contact with the carbon)
- continuous biological activity
Air or oxygen is added to the water before or inside the filter to promote the growth of harmless naturally-occurring microorganisms. The microorganisms flourish in the numerous crevices of the carbon granules and feed on dissolved organic matter, minerals, and other microorganisms passing through the filter. Some organisms and dissolved matter pass through (for example bacteria, calcium).
Biodegradation requires contact times of 30 to 60 minutes (even longer in household filters), and therefore biological granular activated carbon filters are much larger than conventional carbon filters. Well-designed biological carbon filters may operate seven years or more before replacing the carbon, whereas exhaustion in conventional carbon filters can occur in one or two months. By consuming some of the organic build-up, micro-organisms "biologically regenerate" the ability of the carbon granules to adsorb more organic matter. Regular backwashing with air scour and water removes the build-up of particles and decaying microorganisms from biological carbon filters. Backwashing will not remove any matter that is adsorbed to the carbon. After biological granular activated carbon filtration, the filtered water must be disinfected and purified before drinking or household use.
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