Effects of chemical preservation on flux and solute rejection by reverse osmosis membranes

Impacts of Desalination Brine Discharge on Benthic Ecosystems

Seawater reverse osmosis (SWRO) desalination facilities produce freshwater and, at the same time, discharge hypersaline brine that often includes various chemical additives such as antiscalants and coagulants. This dense brine can sink to the sea bottom and creep over the seabed, reaching up to 5 km from the discharge point. Previous reviews have discussed the effects of SWRO desalination brine on various marine ecosystems, yet little attention has been paid to the impacts on benthic habitats. This review comprehensibly discusses the effects of SWRO brine discharge on marine benthic fauna and flora. We review previous studies that indicated a suite of impacts by SWRO brine on benthic organisms, including bacteria, seagrasses, polychaetes, and corals. The effects within the discharge mixing zones range from impaired activities and morphological deformations to changes in the community composition. Recent modeling work demonstrated that brine could spread over the seabed, beyond the mixing zone, for up to several tens of kilometers and impair nutrient fluxes from the sediment to the water column. We also provide a possible perspective on brine's impact on the biogeochemical process within the mixing zone subsurface. Desalination brine can infiltrate into the sandy bottom around the discharge area due to gravity currents. Accumulation of brine and associated chemical additives, such as polyphosphonate-based antiscalants and ferric-based coagulants in the porewater, may change the redox zones and, hence, impact biogeochemical processes in sediments. With the demand for drinking water escalating worldwide, the volumes of brine discharge are predicted to triple during the current century. Future efforts should focus on the development and operation of viable technologies to minimize the volumes of brine discharged into marine environments, along with a change to environmentally friendly additives. However, the application of these technologies should be partly subsidized by governmental stakeholders to safeguard coastal ecosystems around desalination facilities.

Roles of Sulfites in Reverse Osmosis (RO) Plants and Adverse Effects in RO Operation

More than 60 years have passed since UCLA first announced the development of an innovative asymmetric cellulose acetate reverse osmosis (RO) membrane in 1960. This innovation opened a gate to use RO for commercial use. RO is now ubiquitous in water treatment and has been used for various applications, including seawater desalination, municipal water treatment, wastewater reuse, ultra-pure water (UPW) production, and industrial process waters, etc. RO is a highly integrated system consisting of a series of unit processes: (1) intake system, (2) pretreatment, (3) RO system, (4) post-treatment, and (5) effluent treatment and discharge system. In each step, a variety of chemicals are used. Among those, sulfites (sodium bisulfite and sodium metabisulfite) have played significant roles in RO, such as dechlorination, preservatives, shock treatment, and sanitization, etc. Sulfites especially became necessary as dechlorinating agents because polyamide hollow-fiber and aromatic thin-film composite RO membranes developed in the late 1960s and 1970s were less tolerable with residual chlorine. In this review, key applications of sulfites are explained in detail. Furthermore, as it is reported that sulfites have some adverse effects on RO membranes and processes, such phenomena will be clarified. In particular, the following two are significant concerns using sulfites: RO membrane oxidation catalyzed by heavy metals and a trigger of biofouling. This review sheds light on the mechanism of membrane oxidation and triggering biofouling by sulfites. Some countermeasures are also introduced to alleviate such problems.

Composite nanofiltration membrane with asymmetric selective separation layer for enhanced separation efficiency to anionic dye aqueous solution

The development of high performance membrane is crucial for enhanced separation efficiency of nanofiltration process in treating wastewaters from dye and textile industries. In this work, a novel thin-film composite nanofiltration membrane with an asymmetric separation layer composed of relatively compact skin and loose body was designed and fabricated for improved perm-selectivity and antifouling property. Nascent composite membrane with loose poly(piperazine amide) (PA) separation layer was interfacially synthesized and performed with secondary surface reaction using an aqueous tannic acid (TA) solution. Membrane characterizations verified the formation of compact TA skin on top of the loose PA layer through forming covalent bonds via esterification reaction. Compared with the composite nanofiltration membrane with PA symmetric separation layer having the same glucose rejection of about 82.0%, the permeability to pure water of the composite membrane with PA/TA asymmetric separation layer was higher by 28.0%, the water fluxes to aqueous solutions dissolved with model anionic dyes of alizarin yellow R, sunset yellow and Congo red under steady state were higher by 50.3, 43.9 and 36.1%, and the steady-state flux declines were lowered by 27.8, 43.4 and 23.5%, respectively. Furthermore, the PA/TA-based membrane also exhibited better antifouling property and rejection ability to alizarin yellow R.

Reverse osmosis membrane composition, structure and performance modification by bisulphite, iron(III), bromide and chlorite exposure

Reverse osmosis (RO) membrane exposure to bisulphite, chlorite, bromide and iron(III) was assessed in terms of membrane composition, structure and performance. Membrane composition was determined by Rutherford backscattering spectrometry (RBS) and membrane performance was assessed by water and chloride permeation, using a modified version of the solution-diffusion model. Iron(III) dosage in presence of bisulphite led to an autooxidation of the latter, probably generating free radicals which damaged the membrane. It comprised a significant raise in chloride passage (chloride permeation coefficient increased 5.3-5.1 fold compared to the virgin membrane under the conditions studied) rapidly. No major differences in terms of water permeability and membrane composition were observed. Nevertheless, an increase in the size of the network pores, and a raise in the fraction of aggregate pores of the polyamide (PA) layer were identified, but no amide bond cleavage was observed. These structural changes were therefore, in accordance with the transport properties observed.

Effects of DMSO and glycerol additives on the property of polyamide reverse osmosis membrane

The polyamide reverse osmosis (RO) membranes were prepared through interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC). The use of dimethyl sulfoxide (DMSO) and glycerol as additives for the formation of thin-film composite (TFC) was investigated. We studied the effect of DMSO and glycerol addition on membrane property and RO performance. Microscopic morphology was examined by atomic force microscopy and scanning electron microscopy. The surface hydrophilicity was characterized on the basis of water contact angle and surface solid-liquid interfacial free energy (-ΔG_SL). Water flux and salt rejection ability of the membranes prepared with or without the additives were evaluated by cross-flow RO tests. The results reveal that the addition of DMSO and glycerol strongly influences the property of the TFC RO membrane. Compared to the MPD/TMC membrane fabricated without DMSO and glycerol, the MPD/TMC/DMSO/glycerol membrane has a rougher surface and is more hydrophilic, showing smaller water contact angle and larger -ΔG_SL value. Without decrease in salt rejection ability, the MPD/TMC/DMSO/glycerol membrane shows water flux significantly larger than that of the MPD/TMC membrane. The unique property of the MPD/TMC/DMSO/glycerol membrane is attributed to the cooperative effect of DMSO and glycerol on membrane structure during the interfacial polymerization process.

A critical review on recent polymeric and nano-enhanced membranes for reverse osmosis

Current and recent advances in polymeric and nano-enhanced membrane developments for reverse osmosis are reported in terms of membrane performance and fouling.