Factors affecting fluoride and natural organic matter (NOM) removal from natural waters in Tanzania by nanofiltration/reverse osmosis

Junjie Shen; Andrea I. Schäfer

doi:10.1016/j.scitotenv.2015.04.037

Factors affecting fluoride and natural organic matter (NOM) removal from natural waters in Tanzania by nanofiltration/reverse osmosis

Junjie Shen, Andrea I. Schäfer

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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 1.5. mg/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.

Original language	English
Pages (from-to)	520-529
Number of pages	10
Journal	Science of the Total Environment
Volume	527-528
DOIs	https://doi.org/10.1016/j.scitotenv.2015.04.037
Publication status	Published - 5 Sept 2015

Funding

The authors would like to thank Leverhulme Royal Society Africa Award SADWAT-Tanzania for project funding. PhD studentship for Junjie Shen was provided by Energy Technology Partnership (ETP) Scholarship with the Drinking Water Quality Regulator for Scotland (DWQR, UK) being the industrial sponsor. A very special thank you goes to Prof Bryce Richards (KIT, Germany) for his financial, technical and material support to this project. We also thank Prof Anthony Szymczyk (Institut des Sciences Chimiques de Rennes, France) for streaming potential measurement of the BW30-LE membrane; Mr Matthew Bower (DWQR, UK) for providing valuable comments on the water classification scheme; Mr John Tobin (Heriot-Watt University, UK) for careful proofreading of the manuscript. Membrane samples were kindly supplied by Dow Chemicals and Koch Membrane Systems. Appendix A

Keywords

Fluoride
Nanofiltration
Natural water
NOM
Reverse osmosis

ASJC Scopus subject areas

Environmental Engineering
Environmental Chemistry
Waste Management and Disposal
Pollution

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10.1016/j.scitotenv.2015.04.037

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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 1.5. mg/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.",

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AB - 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 1.5. mg/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.

KW - Fluoride

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Factors affecting fluoride and natural organic matter (NOM) removal from natural waters in Tanzania by nanofiltration/reverse osmosis

Abstract

Funding

Keywords

ASJC Scopus subject areas

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