Development of a hybrid water defluoridation technology in groundwater supply systems

Research was carried out to develop a hybrid water defluoridation technology with natural bauxite, gypsum and magnesite to ensure that changes in; pH, colour, hardness, residual that occur when water is defluoridated with each of these materials are within WHO recommended standards. Batch and fixed bed experiments were carried out with the raw and calcined materials and their composites. Bed Depth Service Time (BDST) design model and Empty Bed Residence Time (EBRT) optimisation model were employed to determine optimum fixed bed parameters. The materials were calcined at eight temperatures through range 150 oC to 500 oC. XRF showed that the raw bauxite was predominantly (30.33%), gypsum was mainly (28.09%) and magnesite was mainly (34.57%). The 200 oC calcine composite of mass ratio 3:2:1 (bauxite: gypsum: magnesite) obtained optimum qualities. Mixed sorption mechanisms were observed with Langmuir and Temkin being pronounced for different calcines indicative of monolayer sorption and chemisorption, respectively. Critical bed depths were 6.56 and 7.71 cm for raw and the 200 oC calcined composites with operating lines and , respectively where is adsorbent exhaustion rate (g/l) and is empty bed residence time (s). A 200 oC calcine composite filter of bauxite, gypsum and magnesite in the ratio 3:2:1, respectively is viable for defluoridation. BDST and EBRT can be used to upscale its fixed bed.