Water purification is a procedure, whereby contaminants are removed or destroyed from water, to make it suitable for drinking by humans. These contaminants may be solid waste, chemicals or microorganisms, such as bacteria, viruses or fungi. The aim of water purification is to remove both solid suspended particles and dissolved or particulate contaminants. In this article we will discuss how to purify water using scientifically proven water purification methods.
How to Purify Water
There are many different ways to achieve purified water, and the most suitable method to use depends on what contaminants are present and the type of water that is being purified. It may also depend on the time and equipment that you have available.
Method #1: Boiling
Boiling water is the simplest and cheapest way to effectively purify water of bacteria, viruses and parasites. This is purely because these microorganisms will not survive these high temperatures. It is recommended to bring water to a rolling boil for around 5 minutes, to ensure maximum purification.
- Boiling is often a preferred way to purify water as it is a safe process
- No chemicals are added to the water
- Boiling water will not remove solid suspended particles
- Boiling water will also not remove any chemicals
- Requires an energy source
- How to Purify Water
- Reverse osmosis filtration system
- Ceramic Water filtration
- Fibre filtration
- Nano filtration
- Safe Water Storage
- Choosing the best purification method
- References & Sources
Method #2: Chemical purification
There are several chemicals that may be used to effectively disinfect water. It should be noted, that chemical treatment can eliminate microorganisms, but similarly to boiling, is unable to remove other factors, such as chemical contaminants and solid particles. You can read our buyer’s guide on the best water purification tablets here.
Chlorine is an oxidising agent and works to oxidise microorganisms, rendering them harmless. Chlorine is available in liquid form or as purification tablets which are easy to transport. The guidelines for purifying water with chlorine should be closely followed as adding too much chlorine can be detrimental to your health. The Centres for Disease Control and Prevention, recommends that chlorine levels up to 4 parts per million are safe in drinking water. Chlorine addition can affect the taste and smell of your water, so this is something to bear in mind.
Iodine is also available in liquid and tablet forms and works to eliminate harmful microorganisms. Please note, that some people can be allergic to iodine and that pregnant women and those suffering from a thyroid condition, should consult their doctor before use. Research has shown that iodine treatment is ineffective in eliminating the eggs of the parasite, Cryptosporidium in water. As Cryptosporidium is found commonly in surface water, it is important to use an additional source of purification before consuming the water. Iodine also has a bad taste, which will affect the drinking quality of the purified water.
Ozone is a very powerful oxidiser, which can oxidise bacteria, viruses, moulds and yeast spores to effectively kill them. It is much more powerful then chlorine and iodine and is faster at eliminating microorganisms. In comparison to chlorine and iodine, ozone does not produce any taste or odour and does not leave any harmful by-products behind in the water. As ozone consists of oxygen molecules, it simply converts back to pure oxygen and is released harmlessly back into the atmosphere afterwards. Ozone is still unable to remove any solid particulate matter and using ozone requires specialist equipment and higher operational costs.
Method #3: Ultraviolet (UV) light
UV light is one of the most effective treatments for successfully purifying water. In fact, it can eliminate up to 99.99% of harmful microorganisms. It works by causing direct damage to the DNA (Deoxyribonucleic acid) of harmful pathogens present in the water supply. This damage then prevents the pathogens from reproducing and multiplying.
UV light is a safe purification method, whereby no harmful chemicals are added to the water. It also causes no changes to the taste or smell of the water.
This method of purification requires specialised equipment and running costs. Also, similarly to chemical methods, UV light is only able to eliminate microorganisms, further purification would be needed to get rid of chemicals and solid matter.
The benefits of UV as a natural purifier of water may also be utilised in an emergency situation. If you have no other means of purifying water, you may leave it in a bottle, under the sun for 1-2 days. Here the UV rays from the sun, will naturally eliminate harmful pathogens.
Solar disinfection (SODIS) was developed in the 1980s by the Swiss Federal Institute, as a solution to effectively disinfect water. Its aim was to create a household water treatment technology that could help to reduce the incidence of diarrhoea, due to dirty water sources, in the developing world.
Purifying water using UV rays from the sun:
- Place water in a plastic container and vigorously shake it to oxygenate the water
- The containers are then placed in direct sunlight for 6 hours.
The UV light from the sun cleverly breaks down the contaminants by inducing “DNA damage, thermal inactivation, and photo-oxidative destruction to inactivate disease-causing organisms”. The process does work in less sunny climates, but it is advised to leave the containers out for at least 2 days.
This technique has managed to reduce the incidence of diarrhoeal disease, associated with dirty water sources, in developing countries. UV filters are often added as part of a whole house water filtration system.
- Very easy method
- No major costs, you may just use storage bottles that you already have
- Low risk of recontamination, as water stays in the same bottle throughout the procedure
- Can only treat small volumes of water at once
- Can takes up to several days to purify the water completely
- Need an available supply of suitable storage bottles
Method #4: Plants
Some plants have a natural ability to filter water. An area where this is often exploited is in ponds, where natural filtration methods are preferred. Plants, such as water lilies and iris’s absorb water contaminants through their roots and leaves, and are able to metabolise them and use them as food. Plants are able to absorb carbon dioxide from water, lowering its acidity, and also moderate the water’s nutrient levels.
Plants are able to absorb certain toxins and metals, some harmful bacteria and parasites as well as additional chemicals.
Benefits of this natural filtration method include:
- No use of chemicals which can harm humans and wildlife
- No maintenance required
- No electricity required
- Low cost
Water Filtration Methods
The goal of filtration methods of purification is to separate solid and particulate matter from the water by passing it through a filter. This can be used before or after chemical methods in order to further purify the water. Whilst filters effectively capture most microorganisms, very small viruses may escape through. Also, additional contaminants may enter the water in storage, following filtration. Therefore, it is recommended to combine a filtration purification method with a chemical one.
There are many different types of filtration that vary in their effectiveness and ease of use.
Slow sand filtration
Slow sand filtration is a very effective, yet simple method of purifying water. This method does not need any specialist equipment or electricity running costs. It is also a safe method, as no chemicals are added to the water supply.
Slow sand filtration involves the formation of a biofilm of beneficial microorganisms which forms on top of the sand. As water flows through this layer, contaminants are metabolised by the helpful microorganisms and eliminated from the water supply.
The diagram below shows the setup of a typical slow sand filtration system:
- Reasonably simple design to the system
- Cheap to operate and maintain
- Chemical-free method of purification
- Water purified in this way has more health benefits than that purified using other methods, as the mineral content of the water remains unchanged
- The major drawback of this method is that it does take a long time to achieve purification of the water
- This system becomes ineffective over time, as the biofilm grows thicker, decreasing the rate of flow of the water through the filter
- Slow sand filtration systems are large and therefore take up large areas of land
- A lot of labour is required to operate and maintain the system
- Final chlorination will be required to ensure completely safe water
Rapid sand filtration
Rapid sand filtration is usually used as one stage of a large multiple treatment system, for creating municipal drinking water. In contrast to slow sand filtration, this process uses only physical and not biological filtration. Rapid sand filters can also be cleaned automatically, through backwashing and as its name suggests, is much quicker than slow sand filtration. Large capacity rapid sand filtration systems may be open or closed tanks.
Water is filtered through layers of course sand and gravel, under gravity. The setup of a typical rapid sand filtration system is shown below:
- Not surprisingly, a quicker process than slow sand filtration
- Cleaned automatically
- Strong filters
- More expensive than slow sand filtration, to operate and maintain
- Must be cleaned frequently
- Post-purification disinfection is required, as with slow sand filtration
Carbon filtration is an effective and cost-efficient method of filtering water. The Egyptians discovered that storing water in charcoal ensured that the water stayed fresh for longer and also gave it a better taste. Not surprisingly therefore, this is a filtration method that has been used for centuries since.
The filter most frequently used to purify the water source, is a carbon charcoal. There are many variations of this “charcoal water filter” which is commonly used in remote destinations or can be used in an emergency situation. Carbon water filters are also commonly used for general household use, for example, in filtered water pitchers, countertop water filters and in coffee makers. They are also used in domestic fish aquariums to keep the water supply fresh.
Carbon charcoal filters are capable of removing particulate matter, toxins, some drugs, chlorine and small amounts of metals. They can also remove poor taste and odour. However, they are not able to filter fluoride, sodium, ammonia or any microorganisms from the water. Therefore, further treatment would be required to completely purify the water ready for ingestion.
It is testament to the effectiveness of activated charcoal as an adsorber of toxins, that it is used in the medical setting to remove poisons from the body after ingestion.
With carbon filtration, water is passed through the highly porous activated charcoal (carbon treated with oxygen). The activated charcoal adsorbs contaminants onto its surface, where they remain, enabling the purified water to filter through. This chemical adsorption occurs to a much greater degree when the water flow through the filter is reasonably slow.
Carbon filters are available in a wide range of different pore sizes and flow rates, which have differing levels of effectiveness. The choice of filter will depend on what you will be using it for. Pore size determines the size of particles that can be removed. The different filters vary greatly in price too.
Carbon water filters will also need to be replaced regularly. Due to the nature of how they work, when the entire surface of the filter has contaminants bound to it, it becomes saturated and can no longer bind any more. As it is hard to visually see when this happens, manufacturers usually provide a recommended time frame for their replacement. Obviously, if the water source is very heavily contaminated, this time frame will reduce.
- The ability of a carbon filter to effectively remove chlorine is one of its main advantages. This is because very few water filtration methods are able to successfully eliminate chlorine and, in high enough levels, chlorine can be detrimental to our health
- Easy maintenance
- Carbon filters enhance the odour and taste of water
- Can soften hard water sources
- A carbon filter is unable to remove microorganisms, therefore a final disinfection stage will be required
- Will need to be replaced regularly
DIY carbon filtration system
The method of carbon filtration can be adapted to be used by individuals in emergency situations, where obtaining a clean water supply is essential for survival. Here is a brief summary of how a sand filter can be made using materials you may have to hand:
Materials you will need include:
- Dirty water source
- A bottle or water vestibule
- Washed Charcoal (preferably 2cm thick)
- Sand / fine gravel
- Tissue / or cloth (clothing)
- If possible, boil the water first
- Cut the bottom off the water bottle
- Pack the tissue inside the bottle to act as a filter
- Add a small amount of sand/gravel into the tissue – covering about half the tissue
- Ensure that the charcoal is washed, place enough charcoal to fill about 1/3 of the bottle
- Add a small amount of sand/gravel on top of the charcoal
- Put remaining tissue in the top of the bottle
Once the setup is complete, you pass the dirty water through the bottle, the first layer of tissue helps to gather any solid material in the water. The sand/gravel compacts when wet to provide an additional filter to remove solid material from the waste water.
The actual purification takes place when the water passes through the charcoal. The charcoal adsorbs the dirty water and acts as a natural filter as the water passes through it. This results in the contaminants being adsorbed by the charcoal and allowing the cleaner water to pass through. The last layer of tissue/cloth gets rid of the remaining debris and the purified water can then be dispensed to the user. The homemade charcoal filter can be used up to 20 times before it needs to be replaced.
Here is an example of a homemade charcoal filtration system. This YouTube video talks you through the process of making a DIY filtration system with household items:
Reverse osmosis filtration system
Reverse osmosis water filtration involves the use of a semipermeable filter membrane, which enables dissolved and suspended particles to be removed from a water source. Reverse osmosis is also capable of removing microorganisms effectively.
Reverse osmosis is used worldwide in residential, commercial and industrial settings. It is most commonly used to produce purified drinking water from seawater, as it is able to desalinate the water. Additionally, reverse osmosis filtration systems are usually installed in scientific laboratories, where water needs to be purified to a molecular level for the purposes of experiments.
This filtration method takes advantage of the natural process of osmosis. Osmosis occurs when molecules of a less concentrated solution move through a semipermeable membrane to a higher concentrated solution. This occurs to restore chemical equilibrium. Reverse osmosis uses applied pressure to force molecules from a highly concentrated solution into a lower concentrated solution, i.e. the reverse of osmosis. Therefore, contaminants remain on the pressurised side of the membrane, with purified water on the other side.
Concerns regarding reverse osmosis
The use of reverse osmosis as a means to purify water for drinking, raises some concerns. The World Health Organisation has highlighted that purification of water via reverse osmosis, ‘depletes the source water of its mineral contents’. Whilst removing contaminants from our water supply is highly important, water contains other molecules which are actually beneficial to our health. Some of the minerals that are present in water, that reverse osmosis removes include calcium and magnesium. A lack of these minerals can lead to mild symptoms such as tiredness and muscle cramps, but also to more serious problems, such as cardiovascular disorders.
That’s why if you purchase a reverse osmosis system, you’ll want a remineralizer to ensure you are getting sufficient calcium and magnesium.
- Able to remove microorganisms
- Able to remove salt from seawater, to create drinking water
- Reverse osmosis filtration systems are reasonably small
- Removes chlorine, enabling an improved taste and odour
- The membranes used for reverse osmosis filtration vary greatly as with carbon water filters and will need regularly replacing too
- The cost of a reverse osmosis filtration system can be quite high, as can the cost to maintain it
- The high pressure pumps required to drive the system use a lot of electricity and are not very environmentally friendly
- Concerns discussed above regarding the demineralisation of the water
- Reverse osmosis filtration is quite a slow process compared to other methods
Ceramic Water filtration
A ceramic water filter is another method of water filtration which is popular as it is relatively simple to use. The ceramic filter was discovered in the UK by Henry Dalton during the industrial revolution. Ceramic is a natural product with a complex pore structure which makes it an ideal filter for water. As water passes through the ceramic material, it gathers the impurities present, only allowing the purified water through and dissolved minerals that it contains.
Often these filters will be coated in silver, silver will kill any bacteria and also assist in keeping the ceramic clean. Over time, the ceramic can gather mould and algae, which can contaminate the water if the filter is not cleaned or replaced.
Ceramic filters are often used in developing countries as a cheap method of cleaning water, commonly found types are the pot ceramic filter, or candle ceramic filter. The most popular design is the Potters for Peace ceramic filter. It resembles a large flower pot which can be stacked for bulk purification. The pot is filled with water, which flows into a storage vestibule. They are coated with silver which aids in filtration.
The effectiveness of the ceramic filters depends a lot on the quality of the ceramic manufacturing. Most ceramic filters will remove bacteria and protozoans, however it must be advised that this method will not remove all viruses and pathogens. Studies have shown a 60-70% reduction in diarrheal disease by using these filters.
- A proven effectiveness at removing bacteria and protozoa from the water source
- Simple to use
- A proven decrease in the incidence of diarrhoeal disease when using water filtered in this way
- Ceramic filters have a long life. They only require replacing if they become broken, unlike carbon and reverse osmosis filters
- Low cost to purchase as the ceramic filters are cheap to manufacture
- They can be used in a large filtration setup, to supply entire villages
- Low effectiveness at removing viruses
- No residual protection after filtration, therefore this can lead to recontamination
- Locally produced filters have variable levels of quality control
- Ceramic filters may break over time, thus requiring costs for spare parts or replacements
- They have a slow water flow rate of 1-3 litres per hour for non-turbid water sources
- Regular cleaning of filters is required, particularly if filtering turbid water sources
A fibre filter (or hollow fibre membrane) is a filtration system that combines many hollow fibre membranes in a densely packed unit. It often resembles a stack of straws grasped together. Water, usually pressurised, passes through the dense fibre to allow only water and dissolved minerals to be dispensed. The fibres can be made from many types of material, but are most commonly glass fibre with a carbon or quartz coating. The fibres are placed under tension and water is passed through them. The variations in tension and the materials used to manufacture, combine as a filter to the water.
This filter can be used in emergency situations, and some manufacturers have created portable units which can be used for outdoor recreation activities. However, most commonly this method is used on already clean drinking water, the fibre filtration gives another layer of filtration to improve taste and purity of the water.
Here is a YouTube video describing fibre filtration in a bit more detail:
- Also able to purify turbid waters, removing sediment
- May improve bad taste and odour of water
- Chemicals that are dissolved in the water, such as chlorine, are not removed
- Fibre filters need to be replaced regularly
- Fibre filters also need to be regularly cleaned to remove any build-up of bacteria etc.
Nano filtration is another membrane fibre filtration technique, not too dissimilar to fibre filtration. The main difference is that Nano filtration membranes have pores ranging from 1 to 10 nanometres (nm). One nanometre is one billionth of a meter. These are microscopic sized holes which allow the water to pass through and filter out vast amounts of contaminants. These Nano membranes are made primarily from a polymer thin film usually made from polyethylene terephthalate. (Roy et al., 2017).
Nano filtration started being used as a method of water filtration, and has been developed more recently in the food processing industry.
There are 4 classifications of this membrane process, Microfiltration and Ultrafiltration, often used in processing waste water, as well as Nano filtration and reverse osmosis. Nano filtration has more water permeability than reverse osmosis, and is great at desalination. It requires less energy and provides higher flux rates. It is commonly used in the textile industry, pharmaceuticals and demineralisation of the dairy industry.
Nano filtration is great at treating and purifying surface water, which has found much use in disaster relief efforts worldwide. This process was widely used after the Tsunami in Japan 2011. Contaminating molecules are relatively large compared to the Nano filtration pores allowing vast amounts of the contaminant to be filtered.
In the developed world, Nano filtration is commonly used to soften hard water, as no organic matter or salt are able to pass through the membrane. Calcium and magnesium, both salts, are often found in hard water areas, neither of these elements will pass through Nano filtration.
- No chemicals added to the water source
- Able to soften hard water
- Able to remove microorganisms
- Also removes heavy metals
- Able to improve the taste and odour of the water
- Inability to remove soluble elements
- The membranes for Nano filtration systems are more expensive than those for reverse osmosis
Methods of distilling water
Distillation involves boiling a water source in order to produce vapour, which is collected and condensed to form pure distilled water. This enables the majority of contaminants to be removed from the water.
You can distill water in your home using household items, or you can purchase a home water distiller.
The process of distillation for purification has many advantages and disadvantages:
- It effectively removes microorganisms
- It is able to desalinate water, therefore in countries where water is in short supply, sea water can be distilled to produce drinking water
- It is believed to improve the quality of the water, both in taste and odour
- It can demineralise the water, taking away the beneficial minerals too
- Lots of energy is used to heat the water – this has cost implications and implications for the environment
- The energy required is often provided by non-renewable fossil fuels – which has heavy environmental issues
- The cost of running the system, is very high – this would not be viable for poorer countries
- The systems are often highly inefficient, with a lot of water wastage
- The process is time consuming – it can take over 4 hours to produce just a gallon of purified water
- Many chemicals are also capable of vaporising and then re-condensing in the purified water
Distilled water is used in areas where very high purity is required, for example, in scientific laboratories. It is also used in battery units and automobile cooling systems, where ions present in non-distilled water may lead to problems with corrosion.
Solar water distillation
Solar water distillation was described as far back as the 4th Century by Aristotle. In the late 19th century, the first solar distillation plant was built in Chile.
Solar water distillation uses solar energy to vaporise the water source, after which the purified water condenses and is collected. This process, effectively mimics the natural water cycle, whereby water evaporates off the ground and condenses into clouds.
Even though this form of distillation uses the power of the sun, research has shown that water can still be effectively purified in winter and in countries where solar energy is limited.
This technique can be used to purify dirty water sources or even to create ingestible water from seawater, by effectively removing the salt.
- No energy costs
- Less environmental impact than standard distillation methods. Non-replaceable fossils fuels are not used, which are known to increase carbon emissions
- Can be used to desalinate sea water to make it suitable for drinking
- Easy to maintain the equipment, as there are essentially no moving parts
- Can be inefficient, with a lot of water wastage, similar to standard distillation methods
- Slow process
This interesting video, shows how you can produce a simple solar water distillation unit with household items:
Safe Water Storage
All these methods of water filtration would be worthless if the resulting purified water is not stored correctly. Failure to store it safely will often result in recontamination, for example with microorganisms present in the environment. Water should be stored properly in appropriate plastic, ceramic or metal containers.
It is advised not to dispense the water with hands, cups or other drinking apparatus to avoid contamination. Instead, water should be poured into a drinking mechanism from, for example, a Jerry can. If possible, a tap or dispenser should be attached to the water collection container, thus improving cleanliness and preventing further contamination. This view is supported by the WHO as they advocate the use of narrow water dispensers (http://www.who.int/household_water/research/safe_storage/en/).
As well as operating strict hygiene practices, there are other methods that can be used to prevent recontamination. Chlorine can be added after purification to maintain the purity of water during storage and distribution. As discussed previously, chlorine works to kill any microorganisms that make their way into the water supply. Chlorine treatment also helps to prevent the formation of microbial biofilms, which can form in water distribution systems. The amount of chlorine added should be closely monitored, as discussed previously.
Choosing the best purification method
There are many methods which enable us to effectively purify water for human consumption. These have been discussed in detail above. Each of these methods has its advantages and disadvantages and vary in their level of effectiveness at removing contaminants.
In order to choose the most appropriate purification technique, it is important to first decide what your requirements are and what the resulting water will be used for.
Water that is intended for human consumption, most importantly, needs to be safe for use. This means that all contaminating chemicals, microorganisms and suspended solid matter need to be effectively removed. This water should also be appealing and palatable to the drinker. We must also consider maintaining the natural benefits of drinking water, which come in the form of their constituent minerals. Some methods of purification can remove the minerals, thus removing the beneficial properties of the water.
Purified water is also essential in industry. As an example, purified water is regularly used in the manufacture of medications and also in research laboratories. Water used in these settings, needs to be purified to the molecular level. For this reason, more specialised methods of purification such as reverse osmosis and double distillation are more appropriate.
Many of these techniques can also be adapted to be used at home or even in emergency situations.
It’s important to know both how to purify water and how much water you should drink each day.
There are several options available to anyone who needs to purify water. However, it is also vital to remember the importance of the safe storage of water, after it has been cleaned, to prevent it from being re-contaminated. This is of particular interest in the developing world, where producing clean, safe drinking water is of greater significance than in the developed world. In the developed world, many of the filtration methods discussed above are implemented by simply turning on your taps.
Why is it important to purify water?
It is very rare to find water that is safe to drink without being purified. Even water that looks clean and fresh can contain harmful microorganisms, which if ingested could lead to a variety of conditions, including some which are quite serious.
Examples of diseases which can be transmitted via unpurified water include:
This most commonly causes diarrhoea, vomiting, nausea and other gut stress symptoms. In extreme cases, this can lead to death.
Also known as parasitic worms, are often caused by water bound parasites like flatworms, such as flukes and tapeworms, and roundworms such as a hookworm.
This is a rare, but often life-threating condition caused by the Clostridium botulinum bacteria. These bacteria release toxins which attack the nervous system and in extreme cases can cause paralysis.
This is a potentially fatal infection which causes severe diarrhoea, sickness and stomach spasms. It is widespread in the developing world, but can be easily treated using oral rehydration fluids.
Malaria carrying mosquitoes breed in water sources in many developing countries. Infected people are bitten by these mosquitoes and the worse cases can be fatal. The World Health Organisation (WHO) puts malaria in the top 5 causes of death of children under 5.
This is a fever resulting from a bacterial infection caused by a bacterium called Salmonella typhi. It is easily passed on by contaminated water from faeces and occasionally urine.
This is caused by the Shigella and Entamoeba bacterium found in contaminated water. It can cause painful stomach cramps, vomiting and a fever.
This virus can be passed on through the ingestion of water and is caused by an unsafe water supply and poor sanitation. It is not usually serious, unlike its sister viruses Hepatitis B and C.
As briefly discussed before, some of these disorders can be very serious, even fatal. In fact, the WHO, states that diarrhoeal diseases resulting from contaminated water lead to 2 million deaths per year, with children under the age of 5 being most affected.
Hence it is of paramount importance to ensure that any water for human consumption is appropriately purified first.
Pie chart showing the causes of deaths among the under fives
In the under-fives, cases of diarrhea have been shown to cause more deaths than malaria, measles and meningitis.
*Includes neonatal deaths
Figure reproduced using data from Liu et al., Lancet, 2012
References & Sources
How Activated Carbon adsorption works – http://www.lenntech.com/library/adsorption/adsorption.htm
What does Activated Carbon remove – https://iaspub.epa.gov/tdb/pages/treatment/treatmentOverview.do?treatmentProcessId=2074826383
How does Granular Activated Carbon Work –
What does Activated Carbon remove http://www.purewaterproducts.com/articles/carbon
Microbiological contaminants –
Lui et al., (2012). Global, regional, and national causes of child mortality: an updated systematic analysis for 2010 with time trends since 2000. Lancet. Retrieved from http://www.thelancet.com/action/showFullTextImages?pii=S0140-6736%2812%2960560-1
Roy, Yagnaseni; Warsinger, David M.; Lienhard, John H. (2017). “Effect of temperature on ion transport in Nano filtration membranes: Diffusion, convection and electromigration”. Desalination. Elsevier BV. 420: 241–257. doi:10.1016/j.desal.2017.07.020. ISSN 0011-9164