Purification of water so that it is safe and healthy to drink is a primary requirement for sustaining human life on the earth.
The history of water purification dates back to ancient times, but in recent years several new and innovative water purification methods have been developed.
One of the earliest forms of water purification was the use of clean sand to filter organic contaminants such as algae and slime, as well as small organisms, out of water.
Although this form of water filtration is believed to date back to the ancient Egyptians, the earliest documented use of sand to filter a public water supply is from 1829 in London, with the first water quality legislation being passed in 1855. Drinking water standards were first introduced in the USA in 1914.
Another simple type of filter, which is recorded even earlier than the sand filter, is the cloth filter, where water is passed through one or more pieces of finely woven cloth to remove any non-dissolved solids. Hippocrates designed a simple cloth filter, the ‘Hippocratic sleeve’ for filtering sediment out of boiled water, in the 5th century BC.
Use of chemicals such as chlorine as a disinfectant to render drinking water ‘germ-free’ dates back more than a hundred years, to 1905, when chlorination of water was used to stop a cholera epidemic in Lincoln, England.
Since then, water purification has become much more comprehensive in its scope and complex in its operation. In some countries water treatment plants are mainly government-run and in other countries the function is largely undertaken by the private sector utility companies.
Yet there are still concerns about the quality of domestic water supplies in many areas, and point-of-entry household water filtration systems, as well as point-of-use single faucet water filters are widely used.
Modern Water Purification Processes
Modern large-scale water utilities and wastewater treatment works use a combination of these categories to produce potable water for distribution or recycling. Some of these methods have been successfully adapted for use at household level and even at personal level for hikers and emergency workers. Household water filtration systems also often use a combination of methods.
A water purification plant at a large water utility will usually pre-filter raw water through a screen or mesh filter to remove large solids such as leaves, bark, sand and sediment, after which a coagulant is added to the water to bind or ‘flocculate’ the fine sediments and muds into lumps which can be more easily removed by letting the water stand in settlement ponds or by aerating it to make the flocculation float to the surface.
After this the main filtration process takes place, to remove dissolved solids, chemicals, bacteria and viruses. This process will consist of a variety of filtering methods and pore sizes, including filters composed of sand, gravel and charcoal.
Cities in water scarce areas, such as the Persian Gulf and North African states, Namibia, and parts of Australia, amongst others, use large-scale Reverse Osmosis (RO) plants to desalinate seawater for domestic use. These systems, however, use a lot of energy and are relatively expensive to operate compared with standard water purification systems.
The final stage is one of disinfection, which is either achieved through the addition of chemicals such as chlorine or chloramine, by ultraviolet (UV) radiation of the water, by ozonation, or by a combination of these processes.
4 Water Purification Methods
Methods of water purification fall generally into four broad categories; mechanical/physical, chemical, electromechanical/radiation, and biological. Below is a summary of some of the more common technologies found within these categories.
Rapid sand filters are still used in the treatment plants of water utilities, but, in the modern context, sand filters are probably best known in the form of swimming pool water filters. Sand filtration is not suited to small-scale use such as in household water filters.
Cloth and Paper Filters
Although cloth filters may still be used in emergencies, the modern equivalent of the cloth filter is available in the form of the hollow fiber membrane (HFM) filter. HFM filters were developed in the 1970’s as a spin-off of research into reverse osmosis desalination technology.
They are available in many forms and sizes, with pore sizes less than 25 microns or less, that is, less than one-third of the thickness of a human hair.
For domestic use, they are generally produced in the form of a woven cartridge of hollow synthetic fibers with an internal diameter of as small as 0.2 microns, and can be installed in a whole-house water filtration system, or in a single shower head system, to remove pathogens including Legionella bacteria.
Simple pleated-paper sediment filters which are relatively cheap and easy to replace, are widely used as pre-filters in household water filtration systems.
Carbon adsorption is the process that Granular Activated Carbon (GAC) filters, commonly known as charcoal filters, use to purify water.
Carbon in the form of charcoal has a high porosity based on the tubular structure of the wood from which it is made, and is able to take in and ‘trap’ microscopic particles and micro-organisms in the matrix of its structure. Because it is light and has a high capacity, GAC is widely used in household water treatment.
It has the ability to remove unpleasant odors and tastes, including that of chlorine, as well as many gases, toxic compounds, chemicals such as chloramine and some microorganisms.
Depending on the design of the filter and the period of contact with the water, certain types of GAC are also able to remove organic chemicals such as pesticides and pharmaceutical products.
Bringing water to the boil for several minutes is recommended by the US Centre for Disease Control and Prevention (CDC) as the most effective means of ensuring the destruction of pathogens, including bacteria and viruses, in drinking water.
Whilst this method will neutralize microbial contaminants, it will not remove chemicals and inorganic compounds which may be harmful. For this, one must resort to distilling the water by condensing the steam produced by the boiling water and only drinking the resulting distilled water.
Even then, however, there is a danger that organic compounds such as some herbicides and pesticides, which have boiling points lower than that of water, will not be totally removed. While distilled water is the purest form of water available, it lacks nutritious elements and is therefore not suitable for long term use as a drinking water supply.
In addition, owing to the relatively high amount of energy required to distill a small amount of water, this is not a commonly used form of water filtration, but may be applied for specialist purposes.
Reverse Osmosis (RO) uses pressure to force the water molecules through a porous membrane filter system which is fine enough to prevent the passage of most bacteria, minerals and dissolved solids from the water.
RO systems are usually equipped with an automatic rinsing system clean the filter membrane by backwashing clean water through the membrane in the reverse direction. The resulting waste water is flushed away, reducing the production efficiency of the system.
Nevertheless, with a porous membrane filter much finer than even HFM filters, RO is considered one of the most economical processes for removing up to 99% of all contaminants for water.
RO systems require pre-filtering to reduce the frequency of rinsing, and are used at both utility and domestic point-of-entry and (more commonly) point-of-use scale.
Chlorine has been known to be a potent biocide that destroys pathogens for over a century, and has been used as a water disinfectant since 1905.
In more recent times, chloramines, which generate less by-products than chlorine, have been increasingly used as secondary disinfectants for municipal water supplies, applied before distribution of the drinking water.
Chloramines are compounds of amine nitrogen which bind readily to chlorine making them less chemically active than chlorine, while having a longer lasting disinfectant effect.
Ion exchange systems are commonly used at household level as water softeners, but can also be used by water utilities to chemically remove toxic ions including mercury, nitrite, arsenic, lead and others.
It has little effect on bacterial contamination, however, and is almost always used in conjunction with other water filtering methods.
Ozone, an unstable gas with three oxygen atoms, is made by passing oxygen through UV light or an electrical charge. It is a powerful broad spectrum disinfectant used widely in water treatment plants in Europe for over a century.
Because it has a short lifespan, it must be produced and used on site to treat water by bubble contact. It generates fewer by-products than chlorine and does not affect the odor or taste of the water, but its strong oxidation potential has a greater ability than chlorine to degrade most organic micro-pollutants including viruses and cyst-forming protozoan parasites such as Giardia and Cryptosporidium.
Ozone also oxidizes iron, sulfur and manganese in water to produce insoluble metal oxides, which are then removed by post-filtration of the water.
Although the USFDA considers ozone as safe, it is capable of producing a suspected carcinogen in the form of bromate when reacting with bromide ions in water. It is therefore recommended that ozonation should be accompanied by the removal of minerals from the water activated carbon filtration or reverse osmosis.
Domestic ozonation systems are less common and generally more expensive than other types of water purification system, but they can be found in point-of-use faucet format as well as point-of-entry whole house systems.
3. Electrochemical / Radiation
Ultraviolet Radiation (UV)
As bacteria develop increased tolerance to chemical treatments, ultraviolet (UV) disinfection is becoming more widely used. In clear water, UV is extremely good at deactivating microbial contaminants, including cysts, by disrupting their DNA. UV water treatment does not add anything to the water and produces no by-products so is a very safe form of disinfection.
Like ozone treatment, however, it has no residual disinfectant effect and so, in large water treatment works, a secondary disinfectant such as chloramine is added after treatment, to prevent re-growth of bacteria.
Ultraviolet light is produced by special lamps that do not use excessive energy, and hand-held UV water disinfectant devices are available for hikers. Point-of-entry UV devices can be used with other water filtration systems such as domestic reverse osmosis or HFM filter systems to provide final disinfection to household water supplies.
Similar to ion-exchange treatment, electro-deionization (DI) uses two electrodes separated by ion-exchange membranes to only allow positive ions in the water to move in one direction and negative ions in the other.
The result is highly purified deionized water with heavy metals such as iron, lead, mercury and cadmium removed by being concentrated on the electrodes.DI does not, however, effectively remove bacteria or micro-organisms, and this process is often used in combination with CAG or RO filtration systems.
Since the system is relatively energy intensive, it is not used in domestic-scale water treatment systems.
The use of naturally occurring bacteria as a means of removing contaminants in water is a process that is receiving increasing attention as a means of pre-filtering raw water containing a wide range of chemical and organic pollutants.
Much research has been done in Canada and several water purification plants are now using biological pre-filtration in conjunction with their RO systems.
This method of water treatment is also widely used in waste water (sewage) treatment plants and for addressing oil pollution, but is unlikely to be used in domestic-scale water purification in the foreseeable future.
With such a wide range of water purification methods, you will always be able to find a system that suits your household water treatment needs as well as your budget.
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