High-flux and fouling-resistant membranes for brackish water desalination

Interfacial Polymerization of Zwitterionic Building Blocks for High-Flux Nanofiltration Membranes

A simple scalable strategy is proposed to fabricate highly permeable antifouling nanofiltration membranes. Membranes with a selective thin polyamide layer were prepared via interfacial polymerization incorporating building blocks of zwitterionic copolymers. The zwitterionic copolymer, poly(aminopropyldimethylaminoethyl methacrylate)- co-poly(sulfobetaine methacrylate) with an average molecular weight of 6.1 kg mol^-1, was synthesized in three steps: (i) polymerization of dimethylaminoethyl methacrylate to yield the base polymer by atom transfer radical polymerization (ATRP), (ii) fractional sulfobetainization via quaternization, and (iii) amination via quaternization. The effect of the zwitterionic polymer content on the polyamide surface characteristics, fouling resistance, and permeance is demonstrated. The zwitterion-modified membrane becomes more hydrophilic with lower surface roughness, as the zwitterionic polymer fraction increases. The excellent fouling resistance of the zwitterion-modified membrane was confirmed by the negligible protein adsorption and low bacteria fouling compared to a pristine membrane without zwitterionic segments. In addition, the zwitterion-modified membranes achieve a water permeation around 135 L m^-2 h^-1bar^-1, which is 27-fold higher than that of the pristine membrane, along with good selectivity in the nanofiltration range, confirmed by the rejection of organic dyes. This permeance is about 10 times higher than that of other reported loose nanofiltration membranes with comparable dye rejection. The newly designed membrane is promising as a highly permeable fouling resistant cross-linked polyamide network for various water treatment applications.

Long-Term Stability of Low-Pressure Reverse Osmosis (RO) Membrane Operation—A Pilot Scale Study

Reverse osmosis (RO) elements operating at a low pressure (LP) or a low energy (LE) are generally called “LPRO” or “LERO”, and the nomenclature “LP” and “LE” are convertible due to the interrelated features of the pressure and the energy in the RO process. Not only can LPRO be operated at lower pressures, which enables energy saving, but also at the standard operating pressure with an enhanced permeate flux. In this study, the feasibility of the LPRO element was evaluated in the face of high fouling potential feed water. The commercially available standard RO and LPRO were chosen, and the membrane properties including the fouling susceptibility and the surface characteristics were thoroughly evaluated. The variations of various performance parameters were monitored during an 872 h operation in a pilot system, which was operated in a constant flux mode. Then, the used membranes were analyzed to further verify the fouling load localization and the fouling intensities. The average flux variation of the individual RO elements in a vessel and the economic feasibility of LPRO were also evaluated through a simulation study using an RO system design software. This study showed that the localization of fouling load within a pressure vessel of an LPRO system caused about 20% higher flux decline and almost 2-times higher salt passage than those of a standard RO membrane system. Furthermore, the simulation study predicted that average operating pressure difference ratio (%) between two RO membranes decreased from 24.4% to 17.8% and a substantial quantity of LPRO elements (83.3%) must be replaced to meet the designated water criteria only after 2 years’ operation.

Chlorine-Resistant Polyamide Reverse Osmosis Membrane with Monitorable and Regenerative Sacrificial Layers

Improving chlorine stability is a high priority for aromatic polyamide (PA) reverse osmosis (RO) membranes especially in long-term desalination. In this Research Article, PA RO membranes of sustainable chlorine resistance was synthesized. Glycylglycine (Gly) was grafted onto the membrane surface as a regenerative chlorine sacrificial layer, and the zeta-potential was used to monitor the membrane performance and to conduct timely regeneration operations for chlorinated Gly. The Gly-grafted PA membrane exhibited ameliorative chlorine resistance in which the N-H moiety of glycylglycine served as sacrificial pendants against chlorine attacks. Cyclic chlorination experiments, combined with FT-IR and XPS analysis, were carried out to characterize the membrane. Results indicated that the resulting N-halamines could be fast regenerated by a simple alkaline reduction step (pH 10). A synchronous relationship between the zeta-potential and the chlorination extent of the sacrificial layer was observed. This indicated that the zeta-potential can be used as an on-site sensor to conduct a timely regeneration operation. The intrinsic mechanism of the surface sacrificial process was also studied.