Combined aluminium sulfate/hydroxide process for fluoride removal from drinking water

Abstract

In this study, aluminium hydro(oxide), AO was prepared from locally available aluminium sulfate, and used for fluoride removal from water by combining with Nalgonda Technique. The fluoride removal performance of the combined process was investigated as a function of contact time, proportion of alum-lime/AO dose, initial fluoride concentration and solution pH in series of batch experiments. The rate of fluoride removal was investigated using conventional kinetic models. The fluoride removal efficiency of the combined process was also tested for real water sample from selected community water supply system in the Rift Valley Region of Ethiopia. The removal of fluoride was rapid in the first 15 min. The combined process efficiency was about 93% with an optimum combined alum/AO dose (80 mg alum/mg F, 5 mg AO/mg F and lime = 35% of alum) using initial fluoride concentration of 15 mg/L. The optimum solution pH for fluoride removal was in the range of 5-9, which is suitable for practical application in the Ethiopia Rift Valley Region. Kinetic studies showed that the rate of fluoride removal by alum/AO can be well described by a pseudo-second-order rate equation with an average rate constant of 0.096 g/min^.mg. The optimum dose required to attain 85-93% fluoride removal efficiency from real water sample was 90 mg alum/mg F, 5 mg AO/mg F, and 15% lime. Hence, the combined defluoridation process has higher fluoride removal efficiency than the Nalgonda Technique with an additional advantage of minimizing chemical dose. The reported removal efficiency of Nalgonda Technique is 70% at alum dose of 150-170 mg alum/mg F. Besides, sludge production is also minimized. Therefore, this process is highly efficient and could be applied in areas where the fluoride concentration is higher than 10 mg/L, both at household and community level.

KEY WORDS: Nalgonda Technique, Aluminium sulfate, Aluminium hydroxide, Fluoride, Drinking water

Bull. Chem. Soc. Ethiop. 2016, 30(3), 391-401

DOI: http://dx.doi.org/10.4314/bcse.v30i3.7