Water purification generally means freeing water from any kind of impurity it contains, such as contaminants or micro organisms.
Water purification is not a very one-sided process; the purification process contains many steps. The steps that need to be progressed depend on the kind of impurities that are found in the water. This can differ very much for different types of water.
Before the purification process begins some contaminants, such as oil, can be settled in a settling tank. They can then be removed easily, after they have reached the bottom of the tank.
Removal of dangerous microorganisms
Often polluted water has to be freed from microorganisms. The water is than disinfected, usually by means of chlorination.
Removal of dissolved solids
Microrganisms are not only a threat to water; they can also be an advantage when it comes to water purification processes. They can convert harmful contaminants to harmless substances. This biological purification process usually takes a long time and it is only used for water that is polluted with contaminants that the microrganisms, usually bacteria, can convert.
Physical/ chemical techniques
When treatment by microrganisms is not an option we often use different treatment techniques, called physical/ chemical treatment techniques. Chemical treatment often deals with the addition of certain chemicals, in order to make sure that the contaminants change structure and can then be removed more easily. Fertilizers such as nitrates are removed this way. Removal of contaminants can also be done through more difficult specific chemical processes. It takes a lot of education to fully understand these purification steps. Physical treatment usually deals with purification steps such as filtration.
More information on water treatment chemicals
Water pollution treatment process
More detailed descriptions of water purification steps are available here
Bacteria and other microorganisms are removed from water through disinfection. This means that certain substances are added to kill the bacteria, these are called biocides. Sometimes disinfection can also be done with UV-light.
More information on disinfection
When bacteria are used for water purification there are two sorts of transfer; one of these is aerobic transfer. This means, that bacteria that are oxygen dependent are converting the contaminants in the water. Aerobic bacteria can only convert compounds when plenty of oxygen is present, because they need it to perform any kind of chemical conversion. Usually the products they convert the contaminants to are carbon dioxide and water.
When bacteria are used for water purification there are two sorts of conversion; one of these is anaerobic transfer. This means, that bacteria that are NOT oxygen dependent are converting the contaminants in the water. Anaerobic bacteria can only convert when oxygen levels are low, because they use other sorts of substances to perform chemical conversion. Anaerobic bacteria do not just develop carbon dioxide and water during conversion, but also methane gas. This can be used to keep the machinery that supports the purification going. The anaerobic conversion of a substance requires more steps than aerobic conversion, but the final result is often less satisfactory. After anaerobic conversion usually aerobic bacteria (bacteria that do use oxygen) still need to finish the process, because the water is not clean enough yet.
Fertilizers such as phosphate are removed through addition of another chemical, usually iron. The substances than become solid precipitates, that can be filtered from the water.
The removal of ammonium and nitrates is a little bit more complicated; it is a purification process that takes both aerobic and anaerobic conversion to remove them.
In the aerobic conversion stage there are two bacterial species involved. Nitrosomonas bacteria that convert ammonia to nitrite and Nitrobacter bacteria that convert nitrite to nitrate after that.
Although nitrate does not represent a direct health threat to most fish, high levels are still undesirable. Apart from encouraging abnormal extensive algal growth, it is now believed that high nitrate levels are implicated in some fish diseases. This means that the process cannot be stopped here.
The anaerobic bacteria take over; they convert nitrate to atmospheric nitrogen gas. This process only occurs in the absence of oxygen. The first stage is the reverse of the nitrification process, it converts nitrate back to nitrite. The second stage of denitrification converts nitrite to nitrogen gas (N2). This gas can be freely released into the atmosphere without causing environmental damage.
For water terminology check out our Water Glossary or go back to water FAQ overview
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