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Arsenic Contamination:  Searching for Solutions - 1997

by Sylvia Mortoza


In Bangladesh today, more than 90 per cent of the population get their drinking water from the million plus tube-wells that were installed for the anti-diarrhoeal campaign. This was one of the few success stories in the public health sector for it helped reduce water-borne diseases. But tubewells are now dispensing arsenic-laced water, with the result that many people are showing arsenical skin lesions in the late stages of manifestation of arsenic toxicity. This came as a shock for no one was prepared for what could be a disaster - arsenic contamination. The source of arsenic is believed to be geological and most water samples show a mixture of arsenic and arsenate. None showed any methylarsonic or dimethylarsenic acid.

Water is one of the major means of transport of arsenic in the environment. Arsenic in the aquatic environment is predominant in places with high geo-thermal activity. Soil erosion and agricultural run-offs are also large contributors to the arsenic concentration in sediments. High arsenic levels have been reported to be associated with sediments and a potential exists that it may be released in hazardous amounts to the overlying waters. Industrial effluents are also a major source of arsenic to the environment. Arsenic and arsenical compounds are found in effluent from metallurgical industry; glassware and ceramic industry; dye and pesticide manufacturing industry; petroleum refining; rare earth industry and other organic and inorganic chemical industries. It finds an application in the manufacture of herbicides and pesticides. Other industries using arsenic include wood and hide preservatives; lead shot manufacture; phosphate detergent builder; and pre-soaks used in many fertilisers. A chemical factory manufacturing several chemicals including the insecticide Paris-Green (acetocopper arsenic) was responsible for the contamination of wells in the southern part of Calcutta, India. Over 7000 people were consuming the arsenic contaminated water for several years but this fact remained unnoticed until September 1989. A few died and some of the victims were hospitalised while symptoms of arsenic poisoning were evident in many families living in the area.

As arsenic is a cause for skin, liver, lung and kidney or bladder cancer, it is a big headache for the nation. Due to this carcinogenicity of some arsenic compounds, the objective should now be to reduce exposure to arsenic contaminated water to a level as close to zero as possible, taking into consideration its health effects and toxicology, occurrence and human exposure, availability and cost of treatment technology, the practical quantitation limit of arsenic normally found in drinking water.

Addressing the problem

Various treatment methods have been adopted to remove arsenic from drinking water under both laboratory and field conditions. The major mode of removing arsenic from water by physical-chemical treatment methods. Various treatment methods include:

  • adsorption-co-precipitation using iron and aluminium salts
  • adsorption on activated alumina/activated carbon/activated bauxite
  • reverse osmosis
  • ion exchange
  • oxidation followed by filtration

Scientists and health and water experts have intensified their search for treatment methodologies and reliable alternative sources of water supply. The United States Environmental Protection Agency (USEPA) have summarised coagulation with iron and aluminium salts and lime softening as the most effective treatment process for removing arsenic from water to meet the interim primary drinking water regulations standard of 0.05 mg/L [30]. That regulatory agencies are reviewing maximum allowable concentration levels in drinking water is comforting but, as arsenic contamination of ground water and the possibility of poisoning is too alarming to wait for solutions, it is essential to locate alternative sources.

The most logical answer would be to go back to using surface water sources. But as most of these sources are heavily polluted with bacteria, some simple method of disinfection is required. Boiling is an effective method of disinfection but, in Bangladesh, poverty, the difficulty with which fuel can be obtained as well as the current rate of illiteracy, prevents people from boiling water.

Solar decontamination

One alternative may possibly be solar decontamination. During the eighties, Professor Acra and his colleagues at the University of Beirut demonstrated how a wide range of microbes, including pathogenic bacteria and viruses, can be inactivated by exposing the contaminated water to sunlight. The process that was adopted was simple. All that was needed to make the contaminated water drinkable was to place the water inside a transparent glass container or a transparent plastic container and place it in the direct sunlight for two to three hours before drinking.

Although some doubt has been cast on the effectiveness of solar disinfection, the anti-microbial properties of sunlight have been known for a long time but it is only recently that solar radiation has been seriously proposed as a means for decontaminating water. All that is really needed is a reasonably constant and reliable source of sunlight. Several researchers have successfully demonstrated that sunlight will destroy much of the faecal bacteria present in contaminated drinking water but this process is especially effective if the water contains a sufficient amount of oxygen. An easy way to ensure this is to oxygenate the water before hand by mixing it with air.

That sufficient oxygen is needed to be effective has also been confirmed by other researchers in the UK who have shown that the effectiveness of solar decontamination of water is strongly dependent upon its oxygen status. Under controlled conditions, tests using water contaminated with either pure cultures of faecal bacteria, freshly voided faeces or raw sewage, were exposed to full strength, natural sunlight for several hours causing a substantial decrease in the bacterial count when the water was fully oxygenated. De-oxygenated water gave a far slower rate of decrease.

This means that over a period of several hours, sunlight and oxygen can act together to inactivate faecal bacteria. But the most significant practical aspect of this kind of solar "photo-oxidative" disinfection is that the oxygen level of the water must be kept close to maximum value during the exposure of water to full strength sunlight. Simple experiments undertaken by the team showed that a clear plastic or glass bottle, three-quarters filled with de-oxygenated water and capped, shaken vigorously for a couple of minutes will create air bubbles which will restore the oxygen level of the water to near saturation point. As microbes present in the water may consume the dissolved oxygen, this will reduce the effectiveness of this disinfection process. Therefore it is essential to shake each bottle a few times during the period of exposure to sunlight to ensure the oxygen level is kept close to maximum.

If the above routine can be followed to the letter, solar photo-oxidative disinfection will give consistently effective results provided the bottles are illuminated by sunlight of a sufficient intensity to give clearly defined shadows. Professor Acra's pioneering work has shown that the most favourable solar conditions are obtained between the latitudes of 15" and 35" North and South of the equator where sunlight is both consistent and predictable. With slightly less favourable conditions such as in equatorial and tropical regions (between latitudes of 0" and 15") it may not be as effective, due to the higher cloud cover, for this process is slowed down under cloudy conditions.

Nevertheless this technology has important implications for us for small-scale water treatment. Clear glass bottles however, restrict the penetration of short-wave ultra-violet component of sunlight which may lead to a slower rate of microbial inactivation, in which case clear plastic containers may be a better option. Glass containers may. however, be more durable for as the temperature rises during illumination, some plastic bottles may leach small amounts of plasticisers. Also they have a tendency to becoming increasing opaque to short-wave-length light from any prolonged exposure to the sun.

Water turbidity and colour may reduce the rate of bacterial inactivation. Such negative effects are insignificant except when the water is highly turbid or coloured when light transmission is reduced to less than half the surface value. Solar photo-oxidative disinfection works better if the water source is relatively clear or when turbidity or colour does not substantially restrict the penetration of sunlight. This could be a problem in time of flood when water turbidity and colour normally increase, in which case the water will need some initial processing before being placed in sunlight. This could have an advantage for a proportion of the contaminating microbes would be removed prior to solar photo-oxidation.

Solar photo-disinfection can be carried out in the home by individuals, families or small communities without any need for a significant financial investment or external agency support - (the only requirement being a sufficient number of bottles to provide enough drinking water to meet each person's daily needs). Therefore, solar water treatment might be the answer to the problem of arsenic contamination for any low-income community where the cost of engineering solutions may prove too high.

In places where there is already a water supply system, it may make greater sense to install large solar powered water purification systems which can treat large quantities of water. Ultraviolet disinfection uses an ultraviolet (UV) light source enclosed in a transparent protective sleeve, mounted in such a manner that the water that passes through the flow chamber admit the UV rays and absorb them into the stream. These rays are able to destroy bacteria and inactivate many viruses. This kind of system disinfects the water without the need for adding chemicals and as a result, possesses some of the benefits of distillation. It neither creates new chemical complexes nor changes the taste or odor of the water - and - it does not remove any beneficial minerals that may be in the water. However if the water is partially treated by the sediment process and carbon filter prior to passing through the UV flow chamber, it is more effective. Some of the bacteria that can be rendered ineffective through this process include - Leptospira interrogans (Infective Jaundice); Salmonella paratyphi (enteric fever); Salmonella typhosa (typhoid fever) Shigella dysenterai (dysentery); Shigella Flexneri (dysentery) Vibrio cholerae; Streptococcus; Staphylococcus; Escherichia coil; Hepatitis virus; Influenza virus; Poliovirus (poliomyelitis); Rotavirus and Bacteriophage (E.col) and yeast and fungi, all common in Bangladesh.

No. of words: 1857

AIT, Bangkok, Thailand
Disaster Forum, Bangladesh


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