Takahiro Fujioka

N-nitrosamine rejection by reverse osmosis membranes: A full-scale study

This study aims to provide longitudinal and spatial insights to the rejection of N-nitrosamines by reverse osmosis (RO) membranes during sampling campaigns at three full-scale water recycling plants. Samples were collected at all individual filtration stages as well as at a cool and a warm weather period to elucidate the impact of recovery and feed temperature on the rejection of N-nitrosamines. N-nitrosodimethylamine (NDMA) was detected in all RO feed samples varying between 7 and 32 ng/L. Concentrations of most other N-nitrosamines in the feed solutions were determined to be lower than their detection limits (3-5 ng/L) but higher concentrations were detected in the feed after each filtration stage. As a notable exception, in one plant, N-nitrosomorpholine (NMOR) was observed at high concentrations in RO feed (177-475 ng/L) and permeate (34-76 ng/L). Overall rejection of NDMA among the three RO systems varied widely from 4 to 47%. Data presented here suggest that the feed temperature can influence rejection of NDMA. A considerable variation in NDMA rejection across the three RO stages (14-78%) was also observed. Overall NMOR rejections were consistently high ranging from 81 to 84%. On the other hand, overall rejection of N-nitrosodiethylamine (NDEA) varied from negligible to 53%, which was considerably lower than values reported in previous laboratory-scale studies. A comparison between results reported here and the literature indicates that there can be some discrepancy in N-nitrosamine rejection data between laboratory- and full-scale studies probably due to differences in water recoveries and operating conditions (e.g. temperature, membrane fouling, and hydraulic conditions).

Removal of N-nitrosamines by an aerobic membrane bioreactor

This study investigated the fate of eight N-nitrosamines during membrane bioreactor (MBR) treatment. The results suggest that biodegradation is mainly responsible for the removal of N-nitrosamines during MBR treatment. Other removal mechanisms were insignificant (e.g. adsorption to sludge) or not expected (e.g. photolysis and volatilization) given the experimental conditions and physicochemical properties of the N-nitrosamines studied here. N-nitrosamine removal efficiencies were from 24% to 94%, depending on their molecular properties. High removal of N-nitrosamines such as N-nitrosodimethylamine and N-nitrosodiethylamine could be explained by the presence of strong electron donating functional groups (EDG) in their structure. In contrast, N-nitrosomorpholine possessing the weak EDG morpholine was persistent to biodegradation. The removal efficiency of N-nitrosomorpholine was 24% and was the lowest amongst all N-nitrosamines investigated in this study.

Rejection of trace organic chemicals by a nanofiltration membrane: the role of molecular properties and effects of caustic cleaning

This study aims to provide further insights to the rejection mechanisms of trace organic chemicals (TrOCs) by nanofiltration (NF). The separation mechanisms of TrOCs by an NF membrane were elucidated by assessing the role of molecular properties and the impact of caustic cleaning on their rejection. All charged TrOCs were rejected by the NF270 membrane by more than 80%. However, the rejection of positively charged TrOCs was lower than that of negatively charged TrOCs with similar molecular sizes and was similar to the rejection of neutral TrOCs. The results suggest that size exclusion, rather than electrostatic repulsion, was a major factor attributing to the rejection of these positively charged TrOCs. The results also showed that the minimum projection area was a better surrogate parameter for molecular dimensions than molecular weight. Our study highlights the need to monitor the rejection of neutral and positively charged TrOCs (particularly those that are normally moderately rejected by the membrane) following caustic cleaning.

Boron as a surrogate for N‑nitrosodimethylamine rejection by reverse osmosis membranes in potable water reuse applications

The results of this study reveal a strong linear correlation (R(2) = 0.95) between the rejections of boron and N-nitrosodimethylamine (NDMA) by six different reverse osmosis (RO) membranes, suggesting that boron can be used as a surrogate for NDMA rejection. This proposal is based on the premise that the rejection of both boric acid and NDMA is governed by steric hindrance and that they have similar molecular dimensions. The concept proposed here is shown to be valid at pH 8 or below where boron exists as the neutral boric acid species and NDMA is also a neutral solute. Observed changes in the rejections of these two species, as a function of permeate fluxes and feed solution temperatures, were also almost identical. Boron rejection increased from 21 to 79%, and the correlation coefficient of the linear regression between boron and NDMA rejections was 0.99 as the permeate flux increased from 5 to 60 L m(-2)h(-1). Similarly, a linear correlation between boron and NDMA rejections was observed as the feed solution temperature increased from 10 to 40 °C. This linear correlation was also validated in a tertiary treated effluent matrix.

Ozonation of N-Nitrosamines in the Reverse Osmosis Concentrate from Water Recycling Applications

This study aims to evaluate the performance of ozone treatment for removing N-nitrosamines from reverse osmosis (RO) concentrate in water recycling applications. In the absence of any N-nitrosamine precursors, the destruction efficiency of N-nitrosamines was dependent on their molecular weight or the length of the alkyl chain in their molecular structure. Experiments conducted with RO concentrate showed that ozonation could lead to the formation of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA), resulting in an increase in concentrations of these N-nitrosamines. Nevertheless, ozonation was effective for destruction of N-nitrosamines with molecular weight greater than that of NDEA (102 g/mol).

High rejection reverse osmosis membrane for removal of N-nitrosamines and their precursors

Direct potable reuse is becoming a feasible option to cope with water shortages. It requires more stringent water quality assurance than indirect potable reuse. Thus, the development of a high-rejection reverse osmosis (RO) membrane for the removal of one of the most challenging chemicals in potable reuse - N-nitrosodimethylamine (NDMA) - ensures further system confidence in reclaimed water quality. This study aimed to achieve over 90% removal of NDMA by modifying three commercial and one prototype RO membrane using heat treatment. Application of heat treatment to a prototype membrane resulted in a record high removal of 92% (1.1-log) of NDMA. Heat treatment reduced conductivity rejection and permeability, while secondary amines, selected as N-nitrosamine precursors, were still well rejected (>98%) regardless of RO membrane type. This study also demonstrated the highly stable separation performance of the heat-treated prototype membrane under conditions of varying feed temperature and permeate flux. Fouling propensity of the prototype membrane was lower than a commercial RO membrane. This study identified a need to develop highly selective RO membranes with high permeability to ensure the feasibility of using these membranes at full scale.

Removal of Emerging Contaminants for Water Reuse by Membrane Technology

High-pressure membrane separation processes including reverse osmosis (RO) and nanofiltration (NF) have been widely used for drinking water purification and water recycling as well as for the removal of trace organic contaminants (TrOCs). Rejection of TrOCs by NF/RO membranes is governed mainly by size exclusion, electrostatic repulsion and hydrophobic interaction. A detailed characterisation of the membrane and physicochemical properties of the TrOC is key to recognising the dominant mechanism. TrOC rejection by NF/RO membranes can also be influenced by fouling and subsequent chemical cleaning. It is, therefore, essential to understand the transport of TrOCs through NF/RO membranes in order to ensure adequate treatment and at the same time avoid over-engineering. In addition to NF/RO, TrOCs removal by several emerging membrane technologies, including forward osmosis, membrane distillation and membrane electrodialysis, is reviewed briefly.

Role of membrane fouling substances on the rejection of N-nitrosamines by reverse osmosis

The impact of fouling substances on the rejection of four N-nitrosamines by a reverse osmosis (RO) membrane was evaluated by characterizing individual organic fractions in a secondary wastewater effluent and deploying a novel high-performance liquid chromatography-photochemical reaction-chemiluminescence (HPLC-PR-CL) analytical technique. The HPLC-PR-CL analytical technique allowed for a systematic examination of the correlation between the fouling level and the permeation of N-nitrosamines in the secondary wastewater effluent and synthetic wastewaters through an RO membrane. Membrane fouling caused by the secondary wastewater effluent led to a notable decrease in the permeation of N-nitrosodimethylamine (NDMA) while a smaller but nevertheless discernible decrease in the permeation of N-nitrosomethylethylamine (NMEA), N-nitrosopyrrolidine (NPYR) and N-nitrosomorpholine (NMOR) was also observed. Fluorescence spectrometry analysis revealed that major foulants in the secondary wastewater effluent were humic and fulvic acid-like substances. Analysis using the size exclusion chromatography technique also identified polysaccharides and proteins as additional fouling substances. Thus, further examination was conducted using solutions containing model foulants (i.e., sodium alginate, bovine serum albumin, humic acid and two fulvic acids). Similar to the secondary wastewater effluent, membrane fouling with fulvic acid solutions resulted in a decrease in N-nitrosamine permeation. In contrast, membrane fouling with the other model foulants resulted in a negligible impact on N-nitrosamine permeation. Overall, these results suggest that the impact of fouling on the permeation of N-nitrosamines by RO is governed by specific small organic fractions (e.g. fulvic acid-like organics) in the secondary wastewater effluent.

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.

Emerging investigators series: a steric pore-flow model to predict the transport of small and uncharged solutes through a reverse osmosis membrane

This study proposed a new approach to apply the steric pore-flow model to predict the rejection of eight N-nitrosamines and seven VOCs that are of great concern in potable water reuse through an RO membrane. In this approach, solute rejection is predicted by estimating the free-volume hole-size. The free-volume hole-radius was determined with pure water permeability of a membrane and a single reference compound – N-nitrosodimethylamine (NDMA) – by minimizing the variance between the experimentally obtained and calculated NDMA rejection values at the permeate flux of 20 L m−2 h−1. The obtained free-volume hole-radius of the ESPA2 RO membrane was 0.348 nm, which was larger than the value previously determined by positron annihilation lifetime spectroscopy (PALS) analysis (0.289 nm). The model incorporated with the estimated free-volume hole-radius could accurately predict the rejection of eight N-nitrosamines under a range of permeate flux (2.6–20 L m−2 h−1). The model was also validated using experimentally obtained VOC rejection values. The predicted VOC rejections at the permeate flux of 20 L m−2 h−1 were almost identical to their experimentally obtained rejections. However, VOC rejection prediction at a lower permeate flux was less accurate. Further improvement and validation of the model with a variety of trace organic chemicals is required to allow for a more accurate prediction. The model was also validated using the membrane free-volume hole-radius value previously obtained from PALS analysis. Using PALS data resulted in some over-prediction. The results suggest that additional adjustment is necessary when using data from PALS analysis for predicting the rejection of small and uncharged solutes.