Development, optimization and promotion of simple and low-cost fluoride removal methods
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Fluoride is the 13th most abundant element in the earth crust (625 mg/kg) and often has a natural rock-derived origin in drinking water (Koritnig, 1951). Different minerals and their corresponding rocks, such as granites, basalts, syenites, shales etc., can contain high fluoride concentrations. Groundwater, interacting with these rocks, can dissolve fluoride and pose a health risk if used for drinking water supply.
Often drinking water is the main source of total fluoride intake, especially in areas where fluoride concentrations in groundwater and/or surface water are high. However, in some areas foodstuff or indoor air pollution, due to burning of coal, may make significant contributions to the daily intake of fluoride (e.g. Nielsen and Dahi, 2002; Ando et al, 2001). The WHO international guideline value for fluoride in drinking water is 1.5 mg/l. In hot climates this value is more stringent and a range between 0.7Ė1 mg F/L is recommended (WHO, 2004), due to higher water consumption. Different fluoride experts suggest the fluoride threshold concentration should be lowered to 0.5 mg/l (e.g. participants comments on WHO draft publication ďfluoride in drinking waterĒ, 2000; Heikens et al, 2005).
Excess fluoride intake causes different types of fluorosis, primarily dental and skeletal fluorosis. White line striations followed by brown patches and, in severe cases, brittling of the enamel are common symptoms of dental fluorosis. Skeletal fluorosis first causes pain in the different joints, then limits joint movement and finally causes skeletal deformities, which become particularly acute if fluoride uptake occurs during growth. Since these ailments are incurable, fluorosis can only be mitigated by preventing intake of excess fluoride. According to estimates of UNESCO (internet source, status January 2007) more than 200 million people worldwide rely on drinking water with fluoride concentrations higher than the present WHO guideline value.
For several decades, different fluoride removal techniques have been studied in various different countries. Nevertheless, implementation especially in developing countries is sparse and most often restricted to limited areas.
Bone char defluoridation
In 1928, Wayne suggested the use of bone char to reduce the fluoride concentration in water. Hence bone char defluoridation was one of the first defluoridation methods at all. The chemical composition of bone char is dominated by hydroxyapatite (Ca5(PO4)3OH) with a significant carbon content (3-5 wt%).
Bone char is an effective filter material for the removal of fluoride, provided the bone char is of high quality. ďUnless carried out properly, the bone charring process may result in a product of low defluoridation capacity and/or deteriorated water qualityĒ (Dahi, 2000). Production of high-quality bone char requires much practical experience..
The Catholic Diocese of Nakuru, Water Quality Section, Kenya (CDN WQ) has successfully developed and is currently producing, implementing and disseminating simple and low-cost defluoridation units, using bone char as a sorption medium. The following production steps are carried out to obtain high quality bone char and to guarantee successful treatment:
∑ Charring: During the charring process organic impurities are partially mineralized and the specific surface area of the bones is increased. Oxygen content, temperature (particularly temperature distribution in the furnace) and charring time are the most important parameters that determine the quality of the bone char.
∑ Crushing and sieving: After charring the bones are brittle and easy to crush. CDN WQ developed a crushing machine that replaced the laborious hand crush. Sieves are attached to the machine that enables the separation of the crushed bone char into three different fractions.
∑ Washing: The bone char may still contain organic impurities and dust from the crushing process that is removed during the washing steps. NaOH and CO2-solutions are used to wash the bones.
∑ Drying and packing: For safe storage and facilitation of transport, the bone char is dried outdoors, packed and stored, ready for use.
Two different designs of household units are available: A defluoridation bucket (20 L) containing bone char and a combined household unit that removes microbiological contamination, heavy metals and fluoride (Picture 1). A monitoring survey, conducted in October 2006, showed that 32% of the household units had a fluoride removal efficiency of above 99%. In 80% of the household units the fluoride concentration in the treated water was < 1.5 mg/L. According to interviews carried out with the users, the bone char of the household units that exceeded 1.5 mg F/L have never been regenerated, hence the filter medium is saturated with fluoride and further uptake is reduced.