The removal of selected personal care products from a municipal membrane bioreactor secondary effluent with reverse osmosis membranes

Abstract

Recent studies have observed an abundance of chemicals of emerging concern (CEC) in various water sources. As a result of their presence, there is a significant concern regarding their negative impact on humans and the environment. Personal care products (PCPs) form part of CECs and are commonly used for health and hygiene. The trace amounts of PCPs found in the environment, and surrounding water sources are evidence of the non-uniform removal efficiency of conventional wastewater treatment systems. Reverse Osmosis (RO) systems are recommended for PCP removal as they have been reported to be successful in removing trace organic material, and they produce quality effluent In this study, the focus was on the removal of Triclosan (TCS), methylparaben (MeP), and Ethylhexyl methoxycinnamate (EHMC) from a synthetic municipal secondary membrane bioreactor effluent. The research investigated the influence of membrane characteristics, physicochemical properties of the products, and water quality on the removal of selected PCPs. It was achieved by determining the effect of feed pH (3, 6, 10) and temperature (15°C, 25°C, 35°C) on the removal of chemical oxygen demand (COD), total dissolved solids (TDS) measured as turbidity and selected Inorganics ammonia and phosphate ions. A bench-scale reverse osmosis system was used to evaluate the efficiency of reverse osmosis (XLE) efficiency and nanofiltration (NF270) aromatic polyamide thin film composite membranes' removal of the selected PCPs. The synthetic feed consisted of organic and inorganic substances spiked with the commercial model PCPs with concentrations ranging from 450-480 μg/L blended with deionized water. Experimental runs were operated over approximately 8 hours with 45-minute intervals. Grab samples were analyzed for Electric Conductivity (EC), Total Dissolved solids (TDS), and feed, brine and permeate temperature to monitor the system operation. After treatment, the composite permeate samples were prepared for quantitative analysis through solid phase extraction (SPE) before being analyzed using gas chromatography-mass spectrometry (GC-MS). Chemical analysis of various inorganics was conducted to calculate the percentage removal. Results uncovered considerable effects of pH control on eliminating the inorganics of interest and the carbon-oxygen demand (COD). Upon thorough analysis, results showed that both membranes had high overall efficiency at feed pH 6 and temperature at 35°C. The recorded percentage removal for COD, TDS, salt rejection, ammonia, and phosphorus was 92,59%, 87,53%, 91,68%, 96,46%, and 99,6%, respectively, for XLE. At the same time, XLE outperformed the NF270 under the same conditions.

Adjustment of feed pH for the XLE membrane was shown to be a factor of consideration for improving inorganic removal in the advanced treatment of domestic secondary MBR effluent. It was shown that water quality obtained with reverse osmosis and nanofiltration membranes could meet quality requirements for reuse applications in cooling systems and irrigation. The membranes were characterized before and after treatment using Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX), and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The results showed that pH and temperature had more effect on membrane structure for NF270 than XLE. Even though XLE outperformed NF270, NF could still produce quality effluent that met the criteria for reuse application. The SEM-EDX results show that the virgin XLE and NF270 membranes contained 78,08% and 76,14% of C, indicative of the aromatic functional group and 5,51% and 6,3% of S, respectively, representing polysulfone, the mechanical layer supporting filtration. After treatment, the analysis showed that the change in pH resulted in compromised membrane layers. The S content was relatively low for both membranes after each treatment, especially at pH 3 and 10. The lack of C for all pH conditions showed that the aromatic functional group was removed due to changing pH. The effect of feed pH and temperature after treatment was nicely demonstrated by the ATR-FTIR spectra before and after treatment. The spectra for virgin membranes showed pronounced peaks, while the fouled membranes demonstrated deformed peaks at the same wave numbers due to temperature change. XLE spectra included a C-C and C-O stretch, C-O antisymmetric stretch, C-O-C asymmetric stretch vibration of the polysulfone layer, and aliphatic C-H deformation. The functional groups observed for NF270 were the carbonyl functional group (C=O), C-O and C-C stretching and an indication of the polysulfone support layer. When comparing NF270 to XLE, the spectrum of NF270 suggested a most significant effect of pH on the membrane than it did for XLE. At pH 6 and 35°C, the overall PCP removal favoured NF270, with EHMC, MeP, and TCS reporting rejection of 99,92%, 99,67%, and 99,9%, respectively, making it a viable option for PCP removal even though considered a loose membrane. The target compounds were removed due to size exclusion, electrostatic repulsion, and hydrophobic interactions. The rejection for XLE resulted from size exclusion, while NF270 rejections were dominated by electrostatic repulsion due to the change in pH and varying ionization constants. The temperature had no significant difference in PCP rejection, and that rejection increased with pH for all membranes.

Consequently, municipal MBR secondary wastewater effluent treated by a bench-scale RO unit with RO and NF membranes is acceptable for effectively removing selected personal care products (EHMC, MeP and TCS). The temperature does not affect the removal of target analytes by both RO or NF membranes. However, increasing the feed pH has proven to be more effective in its removal. Ultimately, using a hybrid system could assist in further abatement for reuse applications.