Water purification methods

What specific water purification methods are there?

Water that is distributed in cities or communities goes through extensive treatment. Purification methods can be divided into settling of suspended matter, physical/chemical treatment of colloids and biological treatment.

How do water purification methods work

Physical water purification

Physical water purification is mostly concerned with filtration techniques. Filtration is a purification method to remove solids from liquids. A typical filter consists of a tank, the filter media and a controller to enable backflow.

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

Sand filtration
Sand filtration removes suspended solids from water. The filter medium comprises multiple sand layers with different densities. When water flows through the filter, the suspended solids precipitate in the sand layers as residue and the water flows out of the filter. When the filters are loaded with particles the flow-direction is reversed, in order to regenerate it.

Cross flow filtration
Cross flow filtration removes both salts and dissolved organic matter, using a permeable membrane. The remaining concentrate flows along across the membrane and out of the system.

There are many different membrane filtration techniques:

  • Microfiltration
  • Ultrafiltration
  • Nanofiltration
  • Reverse Osmosis (RO).

Deciding which techniques should be implemented depends on the kind of compounds that need to be removed and their particle size.

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 proficient in 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 proficient in 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 proficient of removing viruses, pesticides and herbicides.

4) Reversed Osmosis (RO)
Reversed Osmosis (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.

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 can be used as final polishing filters.

Chemical water purification

Chemical addition

There are different situations in which chemicals are added, for example, to prevent the formation of certain reaction products. Below, a few of these additions are explained:

  • Chelating agents are often added to water to prevent negative effects of hardness caused by the deposition of calcium and magnesium.
  • Oxidising agents are added to act as a biocide, or to neutralise reducing agents.
  • Reducing agents are added to neutralise oxidising agents, such as ozone and chlorine. They also help prevent the degradation of purification membranes.

Clarification is a multi-step process to eliminate suspended solids. As a first step, 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 normally 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 renewed by suitable chemicals.

Water softeners are one of the more commonly used systems. Softeners remove calcium and magnesium ions from hard water, by replacing them with other positively charged ions, such as sodium.

Disinfection is a vital step in the purification of water from cities and communities. It has the important job of killing the present undesired micro-organisms in the water; therefore disinfectants are often referred to as biocides. There are a few techniques available to disinfect fluids and surfaces, such as:

  • Ozone disinfection
  • Chlorine disinfection
  • UV disinfection.

Chlorine has a con:

It can react to chloramines and chlorinated hydrocarbons, which are harmful carcinogens. To prevent this issue chlorine dioxide can be administrated. Chlorine dioxide is an effective biocide at concentrations as low as0.1 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 municipalities across Europe for over a hundred years and is used by a 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. Ozone is an effective disinfectant as well as having the advantage of being very safe to use.

UV-radiation is also used for disinfection. 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 (UV lights and systems).

Distillation is the collection of water vapour after boiling the wastewater. With a correctly 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 stay in the evaporation unit.

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

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 nitrogen 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.

If water is sourced from a borehole or underground water source there may be low pH levels which could affect the copper or steel pipes. Sometimes a blue/ green residue is found in sinks, baths and toilets which is from the oxidation of the copper or metal pipes. The pH levels need to be adjusted to get back to the 7 – 7.5 range.

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.

Biological water purification

Biological water purification is operated to lower the organic load of dissolved organic compounds. Micro-organisms, mainly bacteria, do the decomposition of these compounds. There are two main categories of biological treatment:

  • Aerobic treatment
  • Anaerobic treatment.

The Biological Oxygen Demand (BOD) defines the organic load.

Aerobic systems:

  • the water is aerated with compressed air

Anaerobic systems:

  • Run under oxygen free conditions.