Analytical Instrumentation & Environmental Monitoring


Water purification methods - and the South African position

April 2004 Analytical Instrumentation & Environmental Monitoring

Few of us who are privileged to have a supply of running water appreciate what actually goes into the production of this – and the fact that the tap water available in South Africa is amongst the safest in the world. While India is high on every traveller’s list of countries to visit, you do not even use that tap water to brush your teeth!

Few would appreciate that First-World countries, including the USA, also have water purity problems. In New Mexico for example stomach problems encountered after partaking of the local water is referred to as `Mantazuma's Curse' after an old Indian legend. In recent years there have been several incidents in North America where the potentially deadly Cryptosporidium was present in the domestic water supply.

The water industry in South Africa is large by any standards and the Department of Water Affairs and Forestry (DWAF) estimates the total assets under its control together with Water Boards and municipalities is some R102 billion. The industry employs some 56 000 people countrywide and invests R5 billion per year to provide 4600 million kilolitres of pure water per year.

Drinking water

The water treatment process used in South Africa is based on the use of chlorine as the disinfectant. The water intake to the treatment plant is from dams and rivers and it first passes through wire screens that remove any solid objects. Typically the water would then be pre-treated with chlorine to inactivate disease-causing pathogens. This is then followed by a mixing, coagulation and flocculation process, where a chemical coagulant is used to enable the microscopic dirt particles to coagulate into larger flocs, which then sink to the bottom of the sedimentation tank. The clear water is decanted from the top of these sedimentation plants and is passed through large filters made of sand and gravel that remove all the suspended matter. Finally, chlorine is added to kill any remaining germs and the treated water is tested to make sure it is safe for drinking before being pumped to reservoir tanks from which it is transferred as drinking water through a network of pipes to the consumer. Chlorine remains active in the water for some six to eight hours and Rand Water adds another chemical, namely monochloramine (a mixture of chlorine and ammonia) to the water at its booster pumping stations. Although less active than chlorine, the monochloramine protects the water against bacteria for up to eight days.

Wastewater

The treatment of wastewater is critical to the whole water supply process and this is also an expensive procedure. The input is of course raw sewage, which is screened to remove large objects and then passed on to primary settling tanks, where the sludge particles sink to the bottom. The overflow from primary settling tanks then gravitates into the balancing tanks and from there into an activated sludge reactor. Following this the by-now clear effluent goes through an anaerobic zone where it is deprived of oxygen and is then transferred to the secondary settling tanks. Following this, chlorine is added to the treated effluent to kill harmful human bacteria and viruses, after which it would then normally remain in a maturation river for about 12 hours before being discharged into a natural river.

Choice of disinfectant

Despite other disinfection technologies the use of chlorine is still the most widely used, the chlorine destroying target organisms by oxidation of the cellular material. Chlorine technology is well established and it remains more cost-effective than the other technologies except possibly where de-chlorination of wastewater is required. This is necessary where the purified wastewater is delivered back into a river system - as even at low concentrations the chemical is toxic to aquatic life. Chlorine concentration does however decrease with time (local municipalities often recharge the chlorine content of domestic water before distribution) and Umgeni Water (as an example) avoids the chlorine problem in its treated wastewater by holding it in an artificial river before the final discharge back into a natural river.

There are real concerns about the use of chlorine when the raw water contains certain organic compounds (brown water for example). In this case, chlorine oxidises these compounds to form trihalomethanes (THMs) that are known to be carcinogenic. Despite this the South African water industry prefers chlorine because of the presence of residual disinfectant, and none of our large sources of water supply suffer from the presence of organic compounds. With our high local temperatures however the decision to have a residual anti-bacterial agent in the water supply is a sound one, and with the value of the local chlorine-based infrastructure it is unlikely that we will see major use of other technologies. While the taste of chlorine in tap water may be unpalatable for some consumers a handy tip is to first chill the water in a sealed glass or plastic container. The chlorine taste will then disappear.

Ozone (O3) has been used for the disinfection of drinking water in Europe for more than 100 years and although ozone must be treated with care there is no evidence that its use to purify water has ever resulted in a death. Ozone can be smelt in the air in the aftermath of a lightning strike but it is an extremely unstable gas and rapidly decomposes. By the way: humans can smell ozone at concentrations of between 0,02 and 0,05 parts per million (ppm), while the occupational exposure limit in Europe is 0,1 ppm - for an eight hour shift. One major difference between the use of ozone and chlorine for disinfection is that chlorine stays in the water and provides disinfection right through the piping system. Ozone on the other hand disinfects at the source (the water plant) and is reduced to oxygen with no further protection against bacteria, etc, present in the reticulation system. Several chlorine-free water supply companies in the USA have recently reverted to use of chlorine during the summer months to combat 'nuisance bacteria'. Despite having been used in water treatment for so long, ozone is used in probably less than 1% of the water treatment plants worldwide, mainly as a result of its higher cost. There is, however, renewed interest in the technology because of the THM problem and the insistence by customers in many First World countries to have tasteless and odourless drinking water. Note that as ozone does not provide any residual disinfection (to kill bacteria in the reticulation system) some water companies that have chosen this as the primary disinfectant then add chloramines as a secondary disinfectant to maintain a residual in the distribution system.

As for ozone purification, South African consumers can experience the real thing courtesy of local bottler Coca Cola. The local subsidiary, according to the media, buys Rand Water's product, dechlorinates it, provides some (special) filtration and then ozone gas is passed through the water to finally sterilise the product. The final offering sold as Bonaqua comes at a healthy premium to the equally safe product offered through your taps by Rand Water.

Disinfection of water can also be achieved using the strong UV radiation emitted by mercury lamps, the radiation penetrating the genetic material of microorganisms and retarding their ability to reproduce. Use of UV is a relatively new technology but its use seems to be catching on especially in countries desiring environmentally friendly ways of treating water. New Zealand, as one example, has now taken the decision to install a large-scale UV plant for the processing of drinking water while it also possesses the world's largest UV plant for the treatment of wastewater. Health authorities are also becoming increasingly aware of the benefits of UV disinfection, particularly because no chemicals are used.

As for other methods of treating water ... galvanic water treatment is another no-chemical approach and the technology has been around for some 15 years although it does not seem to have been widely adopted. Iodine is a much more powerful chemical than chlorine and destroys bacteria much more rapidly but the taste of iodinated water is worse that that treated with chlorine and iodine can have adverse effects on users with thyroid problems. It is, however, used in personal water bottle purification systems where the iodine is removed from the water after playing its purification role.

South Africa

Is South Africa behind in terms of water purification technology?

Well, Minister Kasrils has stated that this is one of only a dozen countries in the world where it is safe to drink water from the tap, so that fact speaks for itself. Note however, that no technology is perfect and the quality of South Africa's water is monitored continuously by, for example, Rand Water and Umgeni Water that both operate testing laboratories accredited to the ISO/IEC 17025 standard.

Although we do suffer from droughts the water availability in South Africa is better than most European countries with the UK having problems if it has no rain for a few weeks. What we need to remember is that thanks to government's initiative, more than 9 million new citizens in this country have obtained access to potable water since 1994. The current backlog of between 5 and 6 million people without access to safe water is expected to be eradicated by 2008. The need for more storage facilities for water is therefore obvious and while this country is known for frequent droughts, these might be expected to occur more frequently as a result of the global climate change.

The government has addressed these issues through the release during 2003 of this country's first National Water Resources Strategy. This obviously will take time to implement but will pull together all the available resources and will identify and promote measures to reconcile demand and supply. Minister Kasrils has, however, stressed that no plan will be able to cope with droughts that extend over several years so water restrictions will still be imposed in these situations.

Thanks to Japie Schoeman, water and wastewater treatment specialist at the CSIR, for his help in the preparation of this article.

Dr Maurice McDowell has many years' experience as a technical journalist, editor, business manager and research scientist. His third party analyses of world-class companies and processes, as well as his insight into industry and technology trends are well respected.





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