Water disinfection explained

Saturday June 01, 2019 Written by Published in Weekend
Water disinfection explained

Do we need to disinfect our drinking water?

Several reports have highlighted the need to improve

the quality of the drinking water throughout the Cook Islands – Rarotonga and the Pa Enua. In Rarotonga work has started on the provision of a more robust supply of water with better distribution and increased water storage for times of drought. This should result in cleaner water being available, but not necessarily safer water. By Dr Ian Calhaem

 Water for drinking is normally obtained from one of three places, deep underground from subterranean aquafers; ground water such as streams or shallow bores; and from the rain via roofs and water tanks. In the Cook Islands only the last two are available.

The problem with water collected from ground water or roofs is that it almost always contains micro organisms that can affect our health. These are bacteria such as E.Coli, Viruses such as Dengue, and protozoa such as Cryptosporidium and Giardia. Collectively these micro-organisms are referred to as pathogens.

The removal of these potentially harmful micro organisms is called disinfection.

The question of disinfection is a vexed one, with authorities knowing that disinfection is necessary to prevent waterborne pathogens from affecting public health, but the public is aware that they want to preserve the quality of the lagoons and fisheries.

Water borne pathogens can cause health problems in three ways. Most people are aware of the need to treat drinking water to avoid stomach upsets, but are not aware of the other ways in which pathogens can affect them. People can be affected by breathing in fine droplets of water or by skin contact.

Pathogens are found worldwide and Cook Islands is no exception. Some pathogens live in the soils, such as legionella, others in the guts of animals such as E.coli. When conditions are right, such as in a hot, humid climate these pathogens can multiply and cause health problems.

The United Nations considers that access to water and sanitation is a basic human right. There must be Sufficient water for each person to have a continuous supply available for personal and domestic use – drinking, cooking, washing and personal hygiene. The water must be Safe, that is free from microorganisms, chemical and radiological hazards; Acceptable meaning that it has acceptable colour, taste and odour; Accessible which means that it should be available in the immediate vicinity of where it is needed; and Affordable.

Providing water that meets the requirements of the United Nations and the World Health Organisation standards requires a multi-pronged approach.

•             The source of the water must be managed to the highest standard available

•             The water should be treated to remove soil, colour etc to make it acceptable

•             Storage should be provided to ensure that it is always available

•             The water should be treated to kill any pathogens

In the Cook Islands the water comes from streams and shallow bores, or from roofs.

The Te Mato Vai project was designed to manage the water collected from the ten intakes around Rarotonga. The water will be fed into settling tanks and a chemical added to assist the small particles to drop to the bottom of the tank. This process is called flocculation.

The water from the top of the tank is then passed through a sand filter to remove the fine particles and the colour.

At this point the water should look clean and the larger pathogens – the protozoa, should have been reduced in number, but the bacteria and viruses will not have been removed.

Before being fed into a storage tank this water needs to be disinfected.

The big question is how should this be done?

There are many possible ways to disinfect a water supply system, but some are more effective than others. In particular, not all methods leave a Residual in the water to ensure that the water does not get re-contaminated as it travels through the distribution system.

Most people are familiar with the “Boil Water Notice”. This is issued by the Authority after pathogens are found in the drinking water. Boiling the water before drinking will kill most common micro-organisms.

If this is done by heating the water over an open fire, the water can be kept boiling for several minutes – the recommendation is 5 minutes at a rolling boil to kill the hard to kill pathogens such as protozoa. In todays world with electric kettles this is not possible as the kettle is designed to turn off as soon as it reaches boiling point. The electric kettle cannot be guaranteed to kill all pathogens, just some of them.

Boiling does not prevent the water from being contaminated again and does not address the dangers associated with showering or bathing.

There are four distinct methods of disinfecting water – Ozone, UV Radiation, Chemical and Electro-Chemical using different technologies. Each one has its advantages and disadvantages.

Ozone Plant

Ozone is produced from Oxygen and high voltage electricity. It is effective on all pathogens but does not leave a residual to protect the water after initial treatment.

An electrical supply would be needed at each of the intakes and continuous monitoring is required as inhalation of Ozone can be dangerous.

This is not a practical option for installation at the 10 intakes in Rarotonga, and is impractical for the Pa Enua. It does not provide the Residual required to maintain disinfection to the consumer.

UV Plant

Ultra Violet Radiation is a good means of killing or stunning pathogens but is not easy to use. It works by absorbing the UV radiation into the water and thus is totally ineffective if the water is not very clear. The lamps producing the radiation have a limited life and need to be monitored continuously.

Many pathogens are not actually killed by UV treatment, only stunned. They undergo a process called dark recovery, whereby the damage caused by the radiation is repaired. This can happen as quickly as 60 minutes after treatment.

Like ozone, UV treatment does not provide a Residual to maintain the disinfection of the water as it travels through the distribution system.

Chemical Plant

Chemical disinfection of water is the best known and most widely used. Until recently is was by far the easiest to install but does come with some major disadvantages.

It does not kill all pathogens and special handling is required as it is a dangerous good in all its forms.

The most widely used chemicals to sanitise water are chlorine-based compounds. These can be powder, liquid or gas.

Gas is pure chlorine and the most concentrated form of chlorine, but the most dangerous.

Chlorine powder and liquid are not pure chlorine but are mixed with other substances such as calcium, sodium and water. The liquid form of chlorine, usually sodium hypochlorite loses it strength over time, particularly in hot climates, and is therefore unsuited to use in the Cook Islands.

Powdered chlorine, usually calcium hypochlorite is mixed with water prior to use and then the resulting liquid used to dose the water after filtering out the slurry of calcium hydroxide that forms.

Like liquid sodium hypochlorite, calcium hypochlorite also loses its strength so cannot be stored for a long time, either in powder form or dissolved in water. World Health Organisation quote the loss of effectiveness as 5-18% after 40 days for powder.

Special precautions are required when handling calcium hypochlorite as, even in powder form, it reacts violently with a number of common substances. The manufacturers safety data sheet (MSDS) warns against contact with anything containing carbon, or any acids, as chlorine gas is released. If heated it will also explode even in sealed containers. Powdered Calcium Hypochlorite must be stored in a specially built Dangerous Goods store, and transported in suitably marked vehicles.

Nevertheless, disinfection of water using calcium hypochlorite is widely used throughout the world as it is effective for most pathogens and leaves a Residual in the water to maintain disinfection throughout the distribution system. The dangers of using Calcium Hypochlorite have to be balanced against the health danger of not treating the water.

When using Calcium Hypochlorite to treat the water other considerations also have to be taken into account.

Most people can taste and smell chlorine concentrations or 0.6 mg/L (also expressed as 0.6ppm). The World Health Organisation recommends that the target residual chlorine concentration at the consumer should be 0.2 to 0.5 mg/L, with 0.2 mg/L being the absolute minimum at all points in the distribution network.

The initial dosage at the treatment plant has to be higher than the recommended 0.5mg/L because chlorine will be used up as it travels through the pipes of the distribution network.

To avoid excessive, and unacceptably high levels in the initial part of the network secondary chlorination plants are often used.

Another consideration when us Another consideration when using Calcium Hypochlorite to sanitise water are the by-products that the hypochlorite is converted into.

The formation of a slurry of calcium hydroxide was mentioned earlier as it has to be filtered out and not allowed to enter the distribution system as it will cause blockages of pipes and valves.

From a health point of view, more important are the by-products such as chlorates, chlorites and trichloramine as these are toxic in high concentration. They are formed when chlorine reacts with organic material.

Sufficient hypochlorite has to be added to the water to leave a residual to kill the pathogens but adding too much will leave high levels of toxic by-products.

When hypochlorite is used to sanitise water, it is also critical to measure and control the pH.

pH is a number that represents whether the water is acidic or alkaline.

Chlorine is most effective at killing pathogens when the pH is in the range 6-7, and gets progressively weaker as the pH increases, becoming a very poor sanitizer at pH 9.

Because acids can be corrosive, it is usual to keep the pH of the water to the range 6.5- 7.5.

If the pH is higher than 7.5 more chlorine must be added to achieve the same killing power. The maximum concentration of chlorine must always be below 5mg/L but at these high concentrations taste and smell will be obvious.

Chemical disinfection using Calcium Hypochlorite is used throughout the world and is effective at killing most pathogens, but not protozoa. It is usual to use filters remove these remaining micro-organisms.

Advantages of using Hypochlorites

Calcium Hypochlorite is readily available and if stored in suitable dry containers is relatively stable.

It can be dosed at non-toxic levels to leave a residual in the reticulation system to prevent downstream contamination.

It is relatively cheap to purchase, but does require the necessary infrastructure to store and manage it safely, as if mistakes do occur the consequences can be serious.

Disadvantages of using Hypochlorites

Although the dangers associated with Hypochlorite have been known for a long time, it has continued to be used extensively throughout the world because there was no viable alternative.

It is effective in deactivating most bacteria but has limited effect on viruses as these are moderately tolerant to Chlorination, and is ineffective for Protozoa which are highly tolerant to chlorine.

When in solution Hypochlorite has a short shelf life.

It is highly toxic to aquatic life so provision must be made to ensure that any accidental spills cannot enter the lagoon.

The use of Hypochlorite requires the provision of a fully equipped and trained response team to deal quickly and efficiently with any spillage. Hypochlorite must the treated in the same way as Chlorine Gas since a number of common substances react with Hypochlorite to produce Chlorine Gas.

Electro-chemical Plant

The application of Electro-Chemical disinfection is relatively new for the treatment of water supplies but the technology has been studied for many years. Studies began in early 1800’s when it was discovered that the human white blood cells make a substance call Hypochlorous Acid that was extremely effective at killing pathogens but leaves the good bacteria alone. The problem was that Hypochlorous acid formed outside the human body was short lived so it was not practical to use it for disinfection.

Eventually it was discovered that when a weak brine solution was electrolysed using a high current, the resulting hypochlorous acid was stabilised by the electrical charge stored in the water. The resulting hypochlorous acid had a shelf life of over nine months.

How does Electro-Chemical disinfection relate to Chemical disinfection?

When Calcium Hypochlorite is mixed with water both Hypochlorous Acid and Hypochlorite ions are formed in solution. The balance between Hypochlorous Acid and Hypochlorite ions is controlled by the pH. If the water is acidic then Hypochlorous Acid predominates, whilst if the water is alkaline Hypochlorite ions predominate. Hypochlorous Acid is far more effective at killing pathogens so the disinfection is much better in slightly acid or neutral solutions.

Scientists were puzzled when they tested Hypochlorous Acid produced by electrolysis because although the Hypochlorous Acid was chemically identical to the Hypochlorous Acid produced by Calcium Hypochlorite, it was 80-100 more effective at killing pathogens, including protozoa that the chemical based Hypochlorous Acid did not kill.

A number of companies experimented with different designs of electrodes and electrical currents and modern technology has resulted in pure Hypochlorous Acid at pH 7 –Neutral Anolyte. Because of the purity, the resulting liquid is 100% safe. It is an extremely broad-spectrum disinfectant able to be used to treat water supplies, but also with numerous other uses in the food, agriculture and medical industries.

Since it is made from common salt, water and electricity the Anolyte is ideally suited for developing countries with little infrastructure and a lack of knowledge in handling dangerous goods. It also produces very few by-products as it reverts back to salt and water.

Whilst the exact mechanism by which Electro-Chemically produced Hypochlorous Acid was killing pathogens remained a mystery, a lot of work was done to verify how effective it was.

It was the NASA space shuttle program that eventually solved the scientists’ puzzle. Early space shuttles returned to earth heavily oxidised on the outside. The Glen Science found that the oxygen in the upper atmosphere was being split into Atomic Oxygen by the UV radiation. It was then realised that the oxygen that is produced when the Hypochlorous Acid kills the pathogens was also being split into atomic oxygen, this time using the energy stored in the electrolysed water.

Advantages of Electro-Chemical Disinfection

ANK Neutral Anolyte has very little chemical content as its efficacy comes from the ability to generate Atomic Oxygen from the Hypochlorous Acid and Hypochlorite ions in electrolysed water.

It is 100% non-toxic even at full strength. When used to disinfect water it is added at 1% by volume to the water.

ANK Neutral Anolyte is produced using only common salt, water and electricity. No dangerous goods are involved, and no special training is required for its on-site manufacture, storage or use.

The process has been used overseas to dose water in small mountain villages and also in large cities. It is therefore a viable option for the Pa Enua as well as in Rarotonga.

Disadvantages of Electro-Chemical Sanitation

Electricity is required to electrolyse the water.

Since it is not planned to provide power at the intakes, Anolyte would need to be manufactured at a central site and transported to the intakes by water tanker.

Since it is essentially transporting electrolysed water, no special requirements are needed, nor special handling. A central factory also has additional advantages, as the disinfectant could be made available to households to dose their rainwater tanks and for treatment of sewerage and for horticulture.


WHO Chlorination Guide, 2016

A modern Approach to Disinfection, as Old as the Evolution of Vertebrates, F Migliarina et al; Health Care 2014.

Photoreactivation and Dark Repair in UV-Treated Microorganisms: Effect of Temperature; I Salcedo et at, Journal Applied Microbiology, 2007

Efficacy and Safety of Liquid Chemical

THIS PUBLIC INFORMATION article was written for Cook Islands News by Dr Ian Calhaem of Forensic Imaging Ltd • Sponsored by AWS Group • Design and printed June 2019 by CINews.PRINT in Rarotonga


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