title: Factors affecting fluoride and natural organic matter (NOM) removal from natural waters in Tanzania by nanofiltration/reverse osmosis.
abstract: This study examined the feasibility of nanofiltration (NF) and reverse osmosis (RO) in treating challenging natural tropical waters containing high fluoride and natural organic matter (NOM). A total of 166 water samples were collected from 120 sources within northern Tanzania over a period of 16 months. Chemical analysis showed that 81% of the samples have fluoride levels exceeding the WHO drinking guideline of 1.5mg/L. The highest fluoride levels were detected in waters characterized by high ionic strength, high inorganic carbon and on some occasions high total organic carbon (TOC) concentrations. Bench-scale experiments with 22 representative waters (selected based on fluoride concentration, salinity, origin and in some instances organic matter) and 6 NF/RO membranes revealed that ionic strength and recovery affected fluoride retention and permeate flux. This is predominantly due to osmotic pressure and hence the variation of diffusion/convection contributes to fluoride transport. Different membranes had distinct fluoride removal capacities, showing different raw water concentration treatability limits regarding the WHO guideline compliance. BW30, BW30-LE and NF90 membranes had a feed concentration limit of 30-40 mg/L at 50% recovery. NOM retention was independent of water matrices but is governed predominantly by size exclusion. NOM was observed to have a positive impact on fluoride removal. Several mechanisms could contribute but further studies are required before a conclusion could be drawn. In summary, NF/RO membranes were proved to remove both fluoride and NOM reliably even from the most challenging Tanzanian waters, increasing the available drinking water sources.
authors.affiliation: School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom; The Nelson Mandela African Institute of Science and Technology, P.O. Box 447, Tengeru, Arusha, Tanzania.
authors.forename: Junjie
authors.initials: J
authors.lastname: Shen
authors.affiliation: Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Electronic address: andrea.iris.schaefer@kit.edu.
authors.forename: Andrea I
authors.initials: AI
authors.lastname: Schäfer
mesh_terms: D005374:Filtration / Q000379:methods*; D005459:Fluorides / Q000032:analysis*; D006812:Humic Substances / Q000032:analysis*; D009995:Osmosis; D010710:Phosphates / Q000032:analysis*; D013636:Tanzania; D014874:Water Pollutants, Chemical / Q000032:analysis*; D018508:Water Purification / Q000379:methods*
chemical_list: D006812:Humic Substances; D010710:Phosphates; D014874:Water Pollutants, Chemical; C012980:fluorophosphate; D005459:Fluorides
keywords: Fluoride; NOM; Nanofiltration; Natural water; Reverse osmosis