The water treatment is the purposeful change in water quality. It is a branch of process engineering and essentially comprises two groups of treatment:
- Removal of substances from the water (e.g. cleaning, disinfection / sterilization, iron removal and manganese removal , softening , desalination );
- Adding substances and setting parameters for the water (e.g. dosage, setting the pH value, dissolved ions and conductivity).
Water treatment is an essential process step in the production of drinking water for human consumption (drinking water supply).
The water treatment is also used for the purification of waste water , especially for the recovery (recycling) of z. B. Metals (including precious metals) from wastewater.
The first experiments in the field of water filtration go back to the 17th century. Sir Francis Bacon tried to desalinate seawater by passing it through a sand filter . Although this experiment was unsuccessful, the experiment marked the beginning of a new area of interest. The fathers of microscopy, Antonie van Leeuwenhoek and Robert Hooke , used the newly invented microscope to observe suspended particles in water for the first time, laying the foundation for an understanding of waterborne diseases.
The first documented use of sand filters to purify water can be traced back to 1804. John Gibb, the owner of a bleaching factory in Scotland, used the experimental filter and sold the unwanted excess. The method was further developed by private water companies in the decades that followed. The first public installation for water supply was finally realized in 1829 by the engineer James Simpson for the Chelsea Waterworks Company in London. The installation provided the households in the area with filtered water and the network was copied many times in the ensuing period, with the new technology reaching large parts of the British population.
John Snow was the first to successfully disinfect water with chlorine. Even William Soper used 1879 chlorine to treat the sewage of typhoid patients. In a paper from 1894, Moritz Traube formally recommended adding calcium hypochlorite to water in order to disinfect it. Traube's findings could later be replicated. The earliest attempts to implement the chlorination of water in a water treatment plant were made in Hamburg in 1893. The permanent chlorination of water was first used in 1905 when a defective filter in Lincoln led to a typhoid outbreak . The chlorination of the water was able to stop the spread of the disease and the precautionary treatment was continued until 1911 when a new water supply system was introduced. This form of "protective chlorination" has not been permitted in Germany since 1991, which is why, depending on the area of application, alternatives such as filtration, ozonation and UV radiation are used.
Process of water treatment
The processing procedures can be broken down as follows:
- mechanical processing (e.g. rakes, sieves , filters )
- physical processes (e.g. aeration, atomization , sedimentation , flotation , adsorption , vacuum processes, thermal effects)
- chemical processes (e.g. oxidation , disinfection , flocculation , decarbonisation , ion exchange , electrodeionisation )
- Membrane processes (e.g. nanofiltration , osmosis )
- biological processes (biochemical oxidation, sludge digestion, activated sludge process , anaerobic wastewater treatment )
The following table gives an overview of the various preparation processes and their purposes.
|adsorption||Activated carbon filter||Accumulation of z. B. adsorbable halogenated hydrocarbon compounds (AOX) or dyes|
|Biochemical process||Example: denitrification||Exploitation of biochemical processes. In denitrification , the nitrate content of polluted raw water is reduced either by adding carbon in the subsoil or in a reactor .|
|disinfection||Special basin, addition to the pipe network||Addition of chlorine , chlorine dioxide or ozone (previously partly also iodine ), UV radiation or ultrafiltration|
|dosage||Dosing systems for solids, liquids and gases||Addition of chemicals to specifically influence the water quality, e.g. B. the tendency to deposit or corrosion|
|Decarbonization||Decarbonization plant||Reduction of the temporary hardness to reduce the deposition of calcium carbonate on the surfaces of the pipelines and heat exchangers|
|softening||Water softener, nanofiltration, low pressure reverse osmosis||Removal of calcium and magnesium ions (Ca 2+ and Mg 2+ ; limited with nanofiltration)|
|Desalination ( Desalination )||Desalination plant, reverse osmosis||Removal of salts e.g. B. for the treatment of sea water for drinking water and for irrigation|
|Deacidification||Deacidification system||Removal of aggressive carbon dioxide. Used to prevent corrosion in the pipe network.|
|precipitation||Precipitation basin or precipitation filtration||Conversion of dissolved substances into undissolved substances and subsequent sedimentation or flocculation .|
|Filtration||Filters , sand filters , candle filters, precoat filters, microfiltration , ultrafiltration , nanofiltration||Removal of suspended matter ( particles ) etc. a. Iron or manganese|
|Flocculation||Flocculation basin||Removal of colloids and fine dirt particles by adding flocculants (discharge of the particles) and adjusting the pH value . Can also be combined with a filtration (flocculation filtration).|
|Flotation||Flotation tank||Removal of fine dirt particles by blowing in air|
|oxidation||Ventilation systems and gravel filters||Removal of dissolved iron and manganese ions ( iron and manganese ). The oxidized iron and manganese are deposited on the gravel and are removed by backwashing the filter.|
|sedimentation||Sand trap, sedimentation basin||Particulate matter, flakes from flocculation|
|Selective exchange||Ion exchange systems with special resins||z. B. Removal of uranium and other heavy metals from wastewater, recycling of metals|
|Sieving||Rake , drum screen, microscreen||Removal of larger solids and floating matter|
|Special treatment||Special basin for contaminated sewage||Special treatment of wastewater that is contaminated with initially unknown substances or living beings. Examples: Untreated wastewater from electroplating plants or roundworms.|
|Strip||Stripping pool||Removal by blowing in air / gases. In this way, dissolved water constituents are converted into the gaseous phase in accordance with the vapor pressure and thus removed from the water.|
|Full demineralization||VE systems, regenerable mixed-bed exchangers , cartridge exchangers, reverse osmosis , electrodeionization||Removal of salts e.g. B. for the production of pure and ultrapure water|
Areas of application
The most important processes are listed in the table above. However, a system for the treatment of water rarely consists of just one single process, but very often of several. The required number and type of procedures are determined by:
- Type and quality of the raw water and its dissolved and undissolved ingredients
- Type of use and requirements for the treated water (pure water)
Raw water : The most important raw water suppliers in Central Europe are:
- Groundwater from shallow wells or deep wells
- Bank filtrate from shallow wells near the surface
- Surface water from rivers or lakes and reservoirs
Surface water , and particularly flowing water, contains at least temporarily (flood periods) high levels of undissolved particles. It can also contain organic contaminants, although the legal regulations for direct discharges of wastewater and for treated wastewater mean that they only occur in small amounts. In the case of lake and dam water, on the other hand, the content of undissolved and organic substances is significantly lower.
Depending on the type of soil layer from which it is extracted, groundwater is more or less polluted by dissolved iron and manganese compounds and higher levels of free carbon dioxide (CO 2 ). Dissolved or colloidal organic compounds are only present in small amounts, if at all. Well water near the surface, especially in areas with swampy soil layers and from landfills, can also contain higher concentrations of organic contaminants.
Use of the pure water
With the exception of wastewater treatment, pure water is mainly used as drinking water or as service water for industry and commerce. Another area of application is irrigation in agriculture, where the water rarely needs to be treated. Depending on the respective requirements, the water can go through various treatment stages and combinations in order to achieve the required pure water quality. In individual cases, the quality of the raw water may already be sufficient for use, so that further treatment does not appear necessary. Legal and normative specifications (e.g. German Drinking Water Ordinance , DIN 2000 ) and the requirements of the distribution network are decisive, especially when producing drinking water .
An example of a typical treatment plant for surface water as raw water consists of a pre-cleaning (e.g. rake, sieve belt, sieve drum) or filter stage (possibly two-layer filter) to separate the undissolved substances, including additional flocculation to improve the cleaning effect of the filter stage and disinfection of the pure water. If the raw water has an increased carbonate hardness , an additional decarbonization system can be arranged upstream of the filter stage. In the use of surface water of the filter stage nor a mechanical pre-treatment is with rake and drum filters or Siebbandanlagen upstream. The reverse osmosis process is used to produce drinking water from salty raw water.
In the case of groundwater from shallow or deep wells, an aeration stage is often required for the oxidation and precipitation of iron and manganese oxide hydrates before filtering. At the same time, this increases the oxygen content of the water and, with open ventilation, removes excess carbon dioxide. This is important for iron-made pipe networks in order to reduce corrosion by forming a protective layer of lime rust .
A special form that is partially comparable to the requirements for drinking water is the treatment of bathing water . The bathing water in public outdoor and indoor swimming pools is treated according to the German DIN 19643 or the Swiss SIA 385/1. Typical process steps for this are the standard processes for flocculation, filtration and disinfection.
Industrial water is required in large quantities for power plants ( cooling and feed water ), industrial plants, chemical processes, pharmaceuticals, the food industry and laundries. Very extensive changes to the water properties are often required. Furthermore, different qualities of pure water are often required in a system. For example, the make-up water for cooling systems almost always has different requirements than the process water.
Treatment systems for cooling water often consist of mechanical pre-cleaning using screens, drum filters or belt filter systems and subsequent flocculation with filtration. If the carbonate hardness is too high , a decarbonization stage follows. In addition, the circulating water is mixed with hardness stabilizing and anti-corrosion chemicals via dosing stations. When using well water as raw water, a filter stage can usually be dispensed with if no decarbonisation is necessary.
Since the requirements for the quality of process water are almost always higher, part of the treated water for the cooling systems is separated and further cleaned. Additional treatment processes such as adsorption filters for the removal of organic impurities, reverse osmosis or ion exchangers for partial or full desalination and degassers for the removal of dissolved gases are required.
- Hormones in the environment
- Sewage treatment plant
- Mobile water treatment
- Herbal sewage treatment plant (for cleaning the wastewater)
- Water treatment plant
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