Investigation of Household Defluoridation of Water Using Locally Available Materials as Sorbent Media: A Case of Keren Community in Eritrea

Abstract

Fluoride in water in some parts of Eritrea is above the WHO guideline of 1.5 mg/l. The Keren community suffers dental and skeletal fluorosis as a result of being exposed to water with high fluoride content. A survey of 16 water sources in 13 villages around Keren town in Eritrea was made and 87% of the samples exceeded the guideline, having fluoride levels ranging between 1.40-3.98 mg/l. Therefore, defluoridation is required to make the water safe for drinking. Fluoride sorbent materials namely crushed burnt clay pot, Keren soil, Adigerghish soil and household ash were studied in a batch study for the removal of fluoride from water. At pH of 7, crushed burnt clay pot, Adigerghish and Keren soils had higher removal of fluoride but household ash had optimum fluoride removal at pH of 4. However, the contact time was similar and the equilibrium time was 120 minutes (2 hours). Particle size had no significant effect in the case of crushed burnt clay pot, however, in other three adsorbents, fine particles performed better than coarse ones. In all adsorbents studied, similar increasing trend in adsorption were observed when the initial fluoride concentration was increased. Comparison of the adsorbent materials was made based on the fluoride removal efficiency. The study revealed that crushed burnt clay pot has superior fluoride removal capacity than the other three adsorbents. Its average fluoride removal capacity was about 0.26 mg F - /g of the medium compared to the capacity of the other adsorbents which ranged from 0.08-0.1 mg/g for the same mass of 7g. Crushed burnt clay pot is a suitable low cost adsorbent to remove fluoride from water and was thus selected for further defluoridation tests carried out in a mini column unit. The result from the mini column indicated that optimum removal of fluoride was obtained at a bed depth of 25 cm and a flow rate of 2.5 ml/min, treating 7.3 litres of water meeting the WHO drinking water guidelines. The set-up of the mini column was scaled up to a pilot scale unit. The pilot scale managed to treat 324 litres of water while satisfying the WHO standards of fluoride concentration. The performance of the pilot column agreed with that obtained from the mini column and therefore, can be upgraded for larger application. Moreover, the fixed bed data of the mini column were investigated for simplified fixed bed models (BDST and EBRT) application. The results show that the data could be successfully applied to analyze the column performance and evaluate the model parameters. The BDST model gave a good prediction for the service time when compared to the observed result from the pilot scale experiment and thus could be used in designing and optimizing of fluoride adsorption system onto crushed burnt clay pot.