Removal of nitrate and fluoride by nanofiltration – a comparative study

Abstract

High levels of nitrate and fluoride are causing a major threat to drinking water production in many countries worldwide. Besides reverse osmosis, recent publications indicate that nanofiltration (NF) is a promising technology to remove nitrate and fluoride from surface and groundwater. However, the tightness of the NF membranes plays a major role in the removal efficiency. Within this work, therefore, two commercial NF membranes supplied by Dow Water Solutions were selected for testing nitrate and fluoride rejection within a laboratory study. The NF 90 can be regarded as a “dense” membrane and the NF 270 as a “loose” NF membrane. Both membranes are negatively charged (MWCO of NF 90: 200 Da, NF 270: 300 Da). The experiments were conducted with nitrate-spiked and fluoride-spiked model and tap water. With regard to nitrate removal, only the NF 90 has the ability to comply with the recommended Maximum Contaminant Level (MCL) of 50 mg/L in the studied range of feed concentration (100–400 mg/L). For fluoride, the NF 270 can keep the fluoride level below the MCL (1.5 mg/L) up to 10 mg/L feed concentration whereas the NF 90 has the ability to keep fluoride permeate concentration below MCL up to 20 mg/L feed concentration. The effect of pH on the rejection rate is low: fluoride concentration in permeate only exceeded the MCL at pH 5. Eventually two parameters representing water and solute permeability for a solution-diffusion model were used to compare the experimental data and which could also be used for designing a technical device.