Water defluoridation: nanofiltration vs membrane distillation

MORAN, LUCIA; PAQUET, MARIE; JANOWSKA, KATARZYNA; JAMARD, PAUL; QUIST-JENSEN, CEJNA ANNA; BOSIO, GABRIELA NATALIA; MÁRTIRE, DANIEL; FABBRI, DEBORA; BOFFA, VITTORIO

INDUSTRIAL & ENGINEERING CHEMICAL RESEARCH

AMER CHEMICAL SOC

Lugar: Washington; Año: 2018

0888-5885

Nowadays fluoride contamination of drinking water is a major problem for various countries, becausehigh concentrations of fluoride pose a risk of dental and skeletal fluorosis. Over the past years,membrane nanofiltration (NF) has been proposed as convenient defluoridation technology. However,NF cannot be applied to water systems with high fluoride concentration and the disposal of themembrane concentrate remains an issue. In this work, we compared a commercial polyester NFmembrane and a polypropylene hollow-fiber membrane distillation (MD) module for their ability toremove fluoride ions from water in the presence of hardness ions and organic fouling agents. The NFmembrane can offer more than 10 times higher water productivity than MD, under realistic gradientsof temperature and pressure, respectively. Despite that, after reaching a concentration factor of about3, fouling and scaling caused the flux to drop to about 80 % respect to its initial value. Moreover, Fretentiondecreased from 90% to below 80%, thus providing a permeate of scarce quality. MD wasoperated in the direct-contact mode on a polypropylene hollow-fiber membrane, which was chargedwith a hot feed flow (average T = 58 ºC) on one side and a cooled (20 ºC) permeate flow of distilledwater on the other side. The concentration of fluoride ion in the permeate was always below thedetection limit of our electrode (0.2 ppm), regardless of the fluoride concentration in the feed.Moreover, the MD module showed higher resistance to fouling and scaling than NF and CaF2 crystalswere recovered from the MD concentrate after cooling. These results suggest that the synergiccombination of the two techniques might be beneficial for the purification of fluoride-contaminatedwater systems: MD can be used to further concentrate the NF retentate, thus producing high-puritywater and recovering CaF2 crystals.