What specific water purification methods are there?

Water that is distributed in cities or communities is treated extensively. Specific water purification steps are taken, in order to make the water meet current water standards.
Purification methods can be divided up into settling of suspended matter, physical/ chemical treatment of colloids and biological treatment. All these treatment methods have several different applications.

How do specific water purification methods work?

1 Physical water purification

Physical water purification is primarily concerned with filtration techniques. Filtration is a purification instrument to remove solids from liquids. There are several different filtration techniques. A typical filter consists of a tank, the filter media and a controller to enable backflow.

Screens

Filtration through screens is usually done at the beginning of the water purification process. The shape of the screens depends on the particles that have to be removed.

Sand filtration

Sand filtration is a frequently used, very robust method to remove suspended solids from water. The filter medium consists of a multiple layer of sand with a variety in size and specific gravity. When water flows through the filter, the suspended solids precipitate in the sand layers as residue and the water, which is reduced in suspended solids, flows out of the filter. When the filters are loaded with particles the flow-direction is reversed, in order to regenerate it. Smaller suspended solids have the ability to pass through a sand filter, so that secondary filtration is often required.

Cross flow filtration

Cross flow membrane filtration removes both salts and dissolved organic matter, using a permeable membrane that only permeates the contaminants. The remaining concentrate flows along across the membrane and out of the system and the permeate is removed as it flows along the other side of the membrane.
There are several different membrane filtration techniques, these are: micro filtration, ultra filtration, nano filtration and Reversed Osmosis (RO). Which one of these techniques is implemented depends upon the kind of compounds that needs to be removed and their particle size. Below, the techniques of membrane filtration are clarified.

1) Microfiltration
Microfiltration is a membrane separation technique in which very fine particles or other suspended matters, with a particle size in the range of 0.1 to 1.5 microns, are separated from a liquid. It is capable of removing suspended solids, bacteria or other impurities. Microfiltration membranes have a nominal pore size of 0.2 microns.

2) Ultrafiltration
Ultrafiltration is a membrane separation technique in which very fine particles or other suspended matters, with a particle size in the range of 0.005 to 0.1 microns, are separated from a liquid. It is capable of removing salts, proteins and other impurities within its range. Ultrafiltration membranes have a nominal pore size of 0.0025 to 0.1 microns.

3) Nanofiltration
Nanofiltration is a membrane separation technique in which very fine particles or other suspended matters, with a particle size in the range of approximately
0.0001 to 0.005 microns, are separated from a liquid. It is capable of removing viruses, pesticides and herbicides.

4) Reversed Osmosis (RO)
Reversed Osmosis, or RO, is the finest available membrane separation technique. RO separates very fine particles or other suspended matters, with a particle size up to
0.001 microns, from a liquid. It is capable of removing metal ions and fully removing aqueous salts.

More on RO

Cartridge filtration

Cartridge filtration units consist of fibres. They generally operate most effectively and economically on applications having contamination levels of less than 100 ppm. For heavier contamination applications, cartridges are normally used as final polishing filters.

2 Chemical water purification

Chemical water purification is concerned with a lot of different methods. Which methods are applied depends on the kind of contamination in the (waste)water. Below, many of these chemical purification techniques are summed up.

Chemical addition

There are various situations in which chemicals are added, for instance to prevent the formation of certain reaction products. Below, a few of these additions are summed up:
- Chelating agents are often added to water, in order to prevent negative effects of hardness, caused by the deposition of calcium and magnesium.
- Oxidizing agents are added to act as a biocide, or to neutralize reducing agents.
- Reducing agents are added to neutralize oxidizing agents, such as ozone and chlorine. They also help prevent the degradation of purification membranes.

Clarification

Clarification is a multi-step process to remove suspended solids. First, coagulants are added. Coagulants reduce the charges of ions, so that they will accumulate into larger particles called flocs. The flocs then settle by gravity in settling tanks or are removed as the water flows through a gravity filter. Particles larger than 25 microns are effectively removed by clarification. Water that is treated through clarification may still contain some suspended solids and therefore needs further treatment.

Deionisation and softening

Deionisation is commonly processed through ion exchange. Ion exchange systems consist of a tank with small beds of synthetic resin, which is treated to selectively absorb certain cations or anions and replace them by counter-ions. The process of ion exchange lasts, until all available spaces are filled up with ions. The ion-exchanging device than has to be regenerated by suitable chemicals.
One of the most commonly used ion exchangers is a water softener. This device removes calcium and magnesium ions from hard water, by replacing them with other positively charged ions, such as sodium.

For specific information on water softening move to the water softener FAQ

Disinfection

Disinfection is one of the most important steps in the purification of water from cities and communities. It serves the purpose of killing the present undesired microrganisms in the water; therefore disinfectants are often referred to as biocides. There are a variety of techniques available to disinfect fluids and surfaces, such as: ozone disinfection, chlorine disinfection and UV disinfection.

Chlorine has a downside: it can react to chloramines and chlorinated hydrocarbons, which are dangerous carcinogens. To prevent this problem chlorine dioxide can be applied. Chlorine dioxide is an effective biocide at concentrations as low as
0.1 ppm and over a wide pH range. ClO₂ penetrates the bacteria cell wall and reacts with vital amino acids in the cytoplasm of the cell to kill the organism. The by-product of this reaction is chlorite. Toxicological studies have shown that the chlorine dioxide disinfection by-product, chlorite, poses no significant adverse risk to human health.

Ozone has been used for disinfection of drinking water in the municipal water industry in Europe for over a hundred years and is used by a large number of water companies, where ozone generator capacities up to the range of a hundred kilograms per hour are common. When ozone faces odours, bacteria or viruses, the extra atom of oxygen destroys them completely by oxidation. During this process the extra atom of oxygen is destroyed and there are no odours, bacteria or extra atoms left. Ozone is not only an effective disinfectant, it is also particularly safe to use.

UV-radiation is also used for disinfection nowadays. When exposed to sunlight, germs are killed and bacteria and fungi are prevented from spreading. This natural disinfection process can be utilised most effectively by applying UV radiation in a controlled way.

Distillation

Distillation is the collection of water vapour, after boiling the wastewater. With a properly designed system removal of organic and inorganic contaminants and biological impurities can be obtained, because most contaminants do not vaporize. Water will than pass to the condensate and the contaminants will remain in the evaporation unit.

Electro dialysis

Electro dialysis is a technique that employs an electrical current and special membranes, which are semi permeable to ions, based on their charge. Membranes that permeate cations and membranes that permeate anions are placed alternately, with flow channels between them, and electrodes are placed on each side of the membranes. The electrodes draw their counter ions through the membranes, so that these are removed from the water.

pH-adjustment

Municipal water is often pH-adjusted, in order to prevent corrosion from pipes and to prevent dissolution of lead into water supplies. The pH is brought up or down through addition of hydrogen chloride, in case of a basic liquid, or natrium hydroxide, in case of an acidic liquid. The pH will be converted to approximately 7 to 7.5, after addition of certain concentrations of these substances.

Scavenging

Most naturally occurring organics have a slightly negative charge. Organic scavenging is done by addition of strong-base anion resin. The organics will fill up the resin and when it is loaded it is regenerated with high concentrations of sodium chloride.

3 Biological water purification

Biological water purification is performed to lower the organic load of dissolved organic compounds. Microrganisms, mainly bacteria, do the decomposition of these compounds. There are two main categories of biological treatment: aerobic treatment and anaerobic treatment.
The Biological Oxygen Demand (BOD) defines the organic load. In aerobic systems the water is aerated with compressed air (in some cases merely oxygen), whereas anaerobic systems run under oxygen free conditions.

For water terminology check out our Water Glossary or ga back to water FAQ overview

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