Performance of a Hybrid ED–NF Membrane System for Water Recovery Improvement via NOM Fouling Control
Soyoon Kum, Matthew R. Landsman, Gregory M. Su, Guillaume Freychet, Desmond F. Lawler, Lynn E. Katz
Abstract
Natural organic matter (NOM) complicates water treatment and causes formation of disinfection byproducts (DBPs). NOM is removed well by nanofiltration (NF) but causes extensive membrane fouling, especially in the presence of divalent cations. This research investigated a hybrid electrodialysis-nanofiltration (ED–NF) system to treat freshwaters containing hardness and NOM. ED removes most ions but little to no NOM, and the ED diluate is, then, treated with NF to remove NOM with reduced fouling. Subsequently, some or all of the ED concentrate can be mixed with the NF permeate to increase water recovery, limit NOM concentration (and thereby reduce DBPs), and control the ion content. Compared to NF alone, ED–NF increases product water yield. NF fouling was reduced by the removal of divalent cations in ED. Spectroscopic and resonant scattering measurements of fouled membranes near the calcium K-edge reveal a significant contribution to NF fouling by calcium bridging between carboxyl groups in NOM and on the membrane surface. This novel ED–NF system allows several control “knobs” to increase water recovery and reduce DBP formation potential by adjusting ED water recovery, ED diluate (and concentrate) ionic composition, and the fraction of concentrate remixed with NF permeate to form the product water.