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| August 2004 |
Feature |
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Tanzania: challenges and successes of water defluoridationBy Aloyce Menda
Ingestion of excess fluoride, most commonly in drinking-water, can cause fluorosis which affects teeth and bones. Moderate amounts lead to dental effects, but long-term ingestion of large amounts can lead to potentially severe skeletal deformity. Paradoxically, low levels of fluoride intake help to prevent dental caries. The control of drinking-water quality is therefore critical in preventing fluorosis. Excessive fluoride concentration in water from fluoristic areas can therefore be observed physically in affected people. Fluoride belts
Seeking a solutionDefluoridation of water is more expensive than other normal water treatment methods, but costs can be kept low if defluoridated waters are used for cooking and drinking only. This can be achieved by use of a 'Point of Use treatment methods'. This treatment method allows fluoride contaminated water to continue to be used for washing and other external use. Dr. Hassani Mjengera, a water engineer with the Ministry of Water and Livestock Development is the first Tanzanian researcher to probe for methods of removing excessive fluoride from drinking water by the 'Point of Use treatment method'. Together with a team comprising a chemist, am environmental engineer and two technicians, they may have found as successful solution using bone char. Their research methods have shown promising results with water samples with fluoride concentration of up to 22 mg/L reaching the acceptable levels of less than 21 mg/L. How it worksIn the experiment, raw fresh bones are charred in an easy to use charcoal fuelled kiln at about 500 to 600°C. The charred bones are then pulverized into grains of sizes ranging between 0.5 to 2 mm. The weight of bone char in each unit is determined by the initial fluoride concentration. Higher initial fluoride concentrations need larger amounts of bone char. During the experiments raw water containing fluoride was put in a bucket, which was connected by a tube to the bottom of the column containing the bar. As the water was allowed to percolate upwards through the bone char media, the fluoride was removed through adsorption to the bone char. The overflow outlet passed into another bucket to collect the defluoridated water. If the fluoride content exceeded 2 mg/L, 12 grams of alum and six grams of lime were added to the rest of the water before passing through the bone char media. The ratio of these two chemicals is important as pH effects the adsorption of the fluoride to the bone char. At higher pH the adsorbed fluoride on the bone char grains is detached and hence increases the fluoride concentration considerably in the bone char media effluent. A pH range of 6.5 to 8.0 has been the most effective. The use of alum and lime is in fact an alternative method for defluoridation known as the Nalgonda Technique. The addition of alum and lime to fluoride containing water brings about flocculation of the fluoride which then forms a sediment which can be filtered. This is however a costly technqiue and appears suitable for fluoride concentrations of 10mg/L and below. Any higher and the sorption method using bone char is preferred. From the lab to the fieldAccording to Dr. Mjengera bone char preparation at village level was one of the biggest bottlenecks. He said the problem has been with the arrival of the first portable kilns of different sizes. The biggest kiln has a capacity of charring about 150 kg by using only six kg of charcoal! Using the kilns, the bones are burnt in a controlled air supply so that grey bone char is obtained. Too little air results in black bone char, which can cause bad tasting and discoloured water; too much air into the kiln results in white bone char, which has poor fluoride removing capacity as the hydroxy-appatite is destroyed at higher temperatures. The kilns are designed such that the air inlets are fixed and need no adjustment for any one to be able to operate them. The bone char method is now being adopted in rural Tanzania with projects up and running in the Arusha Region and even at an institutional at a primary school. Research continues into the use of the bone char method for defluoridation of water in Tanzania and its application in the rural settting. With the support of policy makers, the effective transfer of the technology to rural areas may soon be a reality.
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Tanzanians in urban and rural areas face scarcity of safe drinking water, but
the problems are compounded in several rural areas by the presence of high
concentrations of harmful substances such as fluoride.
Fluoride in water is mostly of geological origin. Waters with high levels of
fluoride content are mostly found at the foot of high mountains and in areas
where the sea has made geological deposits. Known fluoride belts on land
include: one that stretches from Syria through Jordan, Egypt, Libya, Algeria,
Sudan, Kenya and Tanzania, and another that stretches from Turkey through Iraq,
Iran, Afghanistan, India, northern Thailand and China. There are similar belts
in the Americas and Japan. Fluorosis has been reported in these areas. The
prevalence of dental and skeletal fluorosis is not entirely clear. It is
believed that fluorosis affects millions of people around the world, but as
regards dental fluorosis the very mild or mild forms are the most frequent.