Performance of a Hybrid ED–NF Membrane System for Water Recovery Improvement via NOM Fouling Control

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.

X-ray Absorption Spectroscopy Reveals Mechanisms of Calcium and Silicon Fouling on Reverse Osmosis Membranes Used in Wastewater Reclamation

“…Based on our conclusion that organic-bound Ca is abundant, we expect that Ca plays an important role in adhering OM to the membrane surface and in enhancing OM aggregation, leading to further deposition. This is consistent with previous studies, which observed that Ca 2+ enhances organic fouling in laboratory-based RO and nanofiltration reactors. ,,,− To our knowledge, however, this is the first time that Ca-bound organics have been observed directly on membranes harvested from a pilot or full-scale system. We observed that Ca-tannic acid represents the majority of the Ca species present in the fouling layer.…”

“…Current pretreatment for Ca removal includes ion-exchange columns, 76 seeded precipitation, 77 and electrodialysis. 23,77 Although these methods lead to higher capital costs, they could reduce operating costs by avoiding caustic and expensive Si cleaning and minimizing the use of an antiscalant. 78−81 Moreover, we expect that pretreatment may be a more desirable fouling mitigation strategy due to the combination of foulants (Ca minerals, silica and organics) that could be difficult to remove simultaneously by cleaning.…”

“…Based on the results of this study, Ca removal would be easiest to achieve and have the greatest overall benefit, as this could minimize Si and organic fouling because Ca would no longer be present to serve as a bridging ion. Indeed, recent work by Kum and colleagues 23 demonstrates that removal of divalent ions, including Ca 2+ , via electrodialysis could improve water recovery by nanofiltration due to a decrease in OM fouling in the absence of bridging cations. Future research should examine to what extent silica and organic fouling of RO membranes can be minimized by Ca removal.…”

“…Future research should examine to what extent silica and organic fouling of RO membranes can be minimized by Ca removal. Current pretreatment for Ca removal includes ion-exchange columns, seeded precipitation, and electrodialysis. , Although these methods lead to higher capital costs, they could reduce operating costs by avoiding caustic and expensive Si cleaning and minimizing the use of an antiscalant. − Moreover, we expect that pretreatment may be a more desirable fouling mitigation strategy due to the combination of foulants (Ca minerals, silica and organics) that could be difficult to remove simultaneously by cleaning. Potential cleaning strategies could include EDTA at pH 11, which can be partly effective for removing organics in the presence of Ca and silica .…”

“…X-ray characterization can provide key insights into fouling layer compositions under real-world conditions, but it has not been widely employed for this and other purposes in the water treatment community. , Specifically, synchrotron-based X-ray spectromicroscopy is a promising technique to determine elemental composition and molecular speciation of foulants and their spatial distribution on membranes, which has long been utilized to characterize speciation in heterogeneous natural systems comprising mixtures of minerals and natural organic matter. , Indeed, X-ray absorption spectroscopy (XAS) is uniquely suited to the detection of chemical speciation regardless as to whether a species is a molecular complex, sorbate, or amorphous or crystalline solid. It is one of the few tools that can be used to identify mixtures of these types of species within complex mixtures. , Laboratory-based spectroscopic methods, namely, X-ray photoemission spectroscopy (XPS), have, in contrast, been utilized widely to study membrane fouling.…”