Surface characterization and performance evaluation of commercial fouling resistant low-pressure RO membranes

This paper describes the characterization and evaluation of various RO/NF membranes for the treatment of seasonally brackish surface water with high organic contents (TOC ≈ mg/L). Twenty commercially available 21 RO and NF membranes were initially evaluated by performing controlled bench-scale flat-sheet tests and surface characterization. Based on the results, four low pressure RO membranes were selected for use in the pilot study. The surface characterization revealed that each of four selected membranes had unique surface characteristics to minimize membrane fouling. Specifically, the LFC1 membrane featured a neutral or low negative surface to minimize electrostatic interactions with charged foulants. The X20 showed a highly negatively charged surface, and thus, was expected to perform well with feed waters containing negatively charged organics and colloids. The BW30FR1 exhibited a relatively neutral and hydrophilic surface, which could be beneficial for lessening organic and/or biofouling. The SG membrane had a smooth surface that made it quite resistant to fouling, particularly for colloidal deposition. In the large-scale pilot study using single element, all of the four membranes experienced a gradual increase in specific flux over time, indicating no fouling occurred during the pilot study. The deterioration of permeate water quality such as TDS was also observed over time, suggesting that the integrity of the membranes might be compromised by the monochloramine used for biofouling control.

Based on controlled bench-scale performance evaluation and surface characterization of 20 commercially available RO and NF membranes, four low-pressure RO membranes were chosen for use in a pilot study to treat high organic brackish surface water. They included FilmTec BW30FR, Trisep X20, Hydranautics LFC1, and Osmonics SG. The surface analysis of these membranes interestingly showed that each membrane exhibited one or two unique surface features, that are favorable for minimizing membrane fouling. Their characteristics were found to be neutral or highly negatively surface charge, less hydrophobic, and smooth surfaces, with a wide range of variations. This finding suggested that the selection of commercial fouling resistant membranes should be done carefully by considering foulants’ characteristics as well as membrane surface properties. The evaluation of these membranes at pilot-scale was performed with two advanced pretreatment processes for the duration of four months. The pretreated water did not cause any significant fouling on all of four membranes. Instead, the enhanced specific flux and reduced TDS removal were observed in the pilot study. This observation could be attributable to membrane oxidation by monochloramine that was used to prevent biofouling, indicating that their chemical compatibility should be carefully considered in the process of developing fouling resistant membranes.