Fouling substances causing variable rejection of a small and uncharged trace organic chemical by reverse osmosis membranes

The safety of recycled water for potable water reuse can be enhanced by improving the reliability of reverse osmosis (RO) treatment for the removal of trace organic chemicals. This study assessed the mechanisms underlying the variable rejection of a carcinogenic N-nitrosamine, namely N-nitrosodimethylamine (NDMA), caused by RO membrane fouling. Foulants that cause the variable rejection were evaluated through rejection tests and foulant characterization. The RO treatment of wastewaters with and without pre-treatment using an ultrafiltration or nanofiltration membrane showed that NDMA rejection commonly increased with increasing membrane fouling. The characterization of organics in the treated wastewater samples revealed that increased NDMA rejection can be caused by foulants composed of low-molecular-weight organics (<300 Da), including tryptophan (or tryptophan-like substances). It is speculated that small organics such as tryptophan form a densely packed cake layer on the membrane surface, which may function as an additional barrier for the membrane transport of NDMA. The results of this study indicate that RO membrane fouling that occurs during long-term wastewater treatment can increase NDMA rejection. The enhanced separation performance can yield positive consequences for the credibility of RO treatment in potable water reuse

Assessment of 265-nm UV-LED for direct photolysis and advanced oxidation of N-nitrosamines and 1,4-dioxane

The advanced oxidation process (AOP), using conventional low-pressure mercury (LP Hg)-vapor UV lamps, forms a critical barrier for pathogens and contaminants of emerging concern (CECs) in potable water reuse. In recent years, UV-light-emitting diode (UV-LED) has attracted considerable attention as an alternative UV source because it has a long lifetime and is mercury-free. This study assesses the effectiveness of 265-nm UV-LED in removal of characteristic CECs, 1,4-dioxane (100 μg/L) and N-nitrosamines (500 ng/L), including N-nitrosodimethylamine (NDMA), in recycled potable water. Direct photolysis using a UV-LED lamp, at the maximum UV dose of 900 mJ/cm2, achieved a maximum of 31% NDMA reduction, as opposed to a 93% reduction by a conventional LP Hg UV lamp. We attributed this to the longer emission wavelength of the UV-LED (265 nm) than that for the LP Hg UV lamp (254 nm). Both UV lamps failed to photolyze 1,4-dioxane. Using hydrogen peroxide or monochloramine remarkably enhanced the effectiveness of the LP Hg UV-based AOP in removing 1,4-dioxane, and the percentage of NDMA removed also increased slightly; however, it did not improve the effectiveness of the UV-LED-based AOP. We conclude from this study that despite the advantages of the UV-LED over the conventional LP Hg UV lamp, the former performs ineffectively at attenuating major CECs in recycled water

Near real-time N-nitrosodimethylamine monitoring in potable water reuse via online high-performance liquid chromatography-photochemical reaction-chemiluminescence

Direct potable reuse requires stringent water quality assurance to protect public health. This study developed an online analytical technique?high-performance liquid chromatography followed by photochemical reaction and chemiluminescence detection (HPLC-PR-CL)?for determination of the concentration of N-nitrosodimethylamine (NDMA) and three other N-nitrosamines. Its feasibility for near real-time analysis was evaluated by analyzing an ultrafiltration (UF)-treated wastewater before and after a pilot-scale reverse osmosis (RO) treatment system. The online instrument with a method detection limit of 0.3?2.7 ng L?1 requires a direct injection (i.e., no sample pre-concentration) of only 20?200 μL sample volume for the determination of N-nitrosamine concentrations every 20 min. NDMA concentrations in UF-treated wastewater were successfully monitored in a range of 50?200 ng L?1 over the course of 24 h. Likewise, NDMA concentrations in RO permeate ranged from 26?81 ng L?1 over the course of 48 h. The online monitor was capable of recording variations in N-nitrosamine concentration in RO permeate that occurred following changes in feedwater concentration and temperature. This study demonstrates the potential for online water quality assurance via direct measurement of trace levels of organic contaminants, which is highly relevant to the implementation of potable reuse

Membrane fouling, chemical cleaning and separation performance assessment of a chlorine-resistant nanofiltration membrane for water recycling applications

The effectiveness of hypochlorite cleaning for fouling mitigation of a prototype chlorine-resistant nanofiltration (NF) membrane was assessed for direct filtration of a secondary treated effluent. The chlorine resistance and separation performance of the prototype NF membrane were also compared to commercial NF and reverse osmosis membranes. The prototype chlorine resistant NF membrane did not show any changes in permeability and conductivity rejection after exposing a NaOCl solution for up to 5 × 104 ppm-h. By contrast, a considerable deterioration in rejection was observed for the other two commercial membranes. Direct filtration of a secondary treated effluent by the prototype NF membrane resulted in a progressive permeability reduction by up to 25% after 10 h of filtration. The membrane permeability was fully restored by hypochlorite cleaning with a 2000 ppm NaOCl solution for 1 h. Effective permeability recovery by hypochlorite cleaning was demonstrated with multiple hypochlorite cleaning cycles. Membrane fouling and hypochlorite cleaning were also simulated using solutions containing a model foulant (sodium alginate, humic acids or bovine serum albumin). Among them, an insufficient permeability recovery was observed for membrane fouling caused by humic acids. Further research is recommended to develop an improved hypochlorite cleaning protocol to control various membrane fouling

Online monitoring of N-nitrosodimethylamine rejection as a performance indicator of trace organic chemical removal by reverse osmosis

The security of recycled water quality in potable reuse can be enhanced by improving the credibility of reverse osmosis (RO) treatment for the removal of trace organic chemicals (TOrCs). This study evaluated the potential of online monitoring of N-nitrosodimethylamine (NDMA) before and after RO treatment as a surrogate indicator for TOrC removal by RO. This pilot-scale study monitored NDMA concentrations in RO feedwater (ultrafiltration-treated wastewater) and RO permeate every 22 min using novel online NDMA analyzers?high-performance liquid chromatography followed by photochemical reaction and chemiluminescence detection. NDMA rejection by RO varied considerably in response to changes in operating conditions (permeate flux and feedwater temperature). A high linear correlation between NDMA rejection and the rejection of six other TOrCs was observed. The linear correlation was also identified for an RO membrane damaged with chlorine. The correlation between another potential surrogate indicator (conductivity rejection) and TOrC rejection was relatively low. NDMA, which is the smallest compound among regulated TOrCs, revealed rejections lower than the other TOrCs, indicating that NDMA rejection can be a conservative surrogate indicator capable of predicting changes in TOrC removal

A rapid and reliable technique for N-nitrosodimethylamine analysis in reclaimed water by HPLC-photochemical reaction-chemiluminescence

A fast and reliable analytical technique was evaluated and validated for determination of N-nitrosodimethylamine (NDMA) formation and rejection by reverse osmosis (RO) membranes in potable water reuse applications. The analytical instrument used in this study is high-performance liquid chromatography (HPLC), photochemical reaction (PR) and chemiluminescence (CL) e namely HPLC-PR-CL. Results reported here show that HPLC-PR-CL can be used to measure NDMA with a similar level of accuracy compared to conventional and more time-consuming techniques using gas chromatography and tandem mass spectrometry detection in combination with solid phase extraction. Among key residual chemicals (i.e. monochloramine, hydrogen peroxide and hypochlorite) in reclaimed wastewater, hypochlorite was the only constituent that interfered with the determination of NDMA by HPLC-PR-CL. However, hypochlorite interference was eliminated by adding ascorbic acid as a reducing agent. Direct injection of ultrafiltration (UF)-treated wastewater samples into HPLC-PR-CL also resulted in an underestimation of the NDMA concentration possibly due to interference by organic substances in the UF-treated wastewater. Accurate determination of NDMA concentrations in UF-treated wastewater was achieved by reducing the sample injection volume from 200 to 20 mL, though this increased the method detection limit from 0.2 to 2 ng/L. In contrast, no interference was observed with RO permeate. These results suggest that RO membranes could remove part of substances that interfere with the NDMA analysis b