Ceramic Nanofiltration Of NOM Rich Brine

Abstract

The presence of organic and inorganic contaminants in aquatic ecosystem has been threat to public health and environment. To produce drinking water, removal of natural organic matter has been of importance due to formation of DBPs. Of the many available treatments, ion exchange (IEX) has the potential to remove natural organic matter (NOM). However, IEX resins need to be regenerated upon saturation with NOM, which generates brine. Usually, NaCl is used to regenerate the resins, hence the desorbed NOM ends in the brine with sodium chloride and other ions like sulphate and nitrate which are present in surface or ground water. The disposal of high saline brine is complex due to aftereffects on the environment and the associated costs. Therefore, an interesting physical alternative is to separate the NOM from brine that can be further reprocessed as fertilisers for agriculture. Ceramic nanofiltration (NF) appears to be an alternative to treat the brine. This type of membrane is chemically and mechanically robust, can be operated at extreme pH conditions, withstands backwashing and chemical cleaning. Besides, ceramic nanofiltration membranes have the capability to separate multivalent ions which makes it suitable to treat the brine. However, NOM possess a complex matrix and relies on various factors for its high removal. In this research, it was interesting to recognize the NOM fractions and the behaviour alleviating NOM retention. Various IEX brines from water treatment plants were studied. For this, NOM was characterised using two different methods i.e., LC-OCD and NSM. The LC-OCD characterization done by Het Waterlaboratorium characterized NOM fraction based on the size of fractions into Humic substances, building blocks, Low molecular weight neutrals and low molecular weight acids. Characterisation of NSM was done by Udine University, Italy wherein the characterization of humics was done on the principle of the selective resin adsorption and precipitation. The characterisation of NOM by LC-OCD predicted the NOM rejection on membrane quite accurately. The effect of ionic strength was investigated to reflect NOM removal. NOM rejection for same NOM source and membrane pore size remained unaffected by the ionic strength of the brine. However, when different membrane pore sizes (600Da and 900Da) for different NOM source, NOM removal by ceramic nanofiltration was governed a combination of steric exclusion, electrostatic repulsion and hydrophobic nature of the humic content and. In longer duration, ceramic NF membranes may foul due to filtration process. To reflect the same, fouling test was conducted. Fouling test was conducted with the high ionic strength brine because during the filtration experiments, it showed highest permeability drop. The sudden drop in permeate flux was due to osmotic pressure difference. However, during NOM filtration period, the flux and permeability were quite steady which suggested no major fouling in that phase. Irreversible fouling did not affect the membrane pore size.