The removal of selected pharmaceuticals from a municipal membrane bioreactor secondary effluent with reverse osmosis membranes

Abstract

The complexity of water and its need for sustainability is, without a doubt, a global crisis. Water is the precondition for sustainable development, human and animal survival, and healthy ecosystems are critical for socio-economic development. Recycling and reusing municipal secondary wastewater effluent can strengthen the water supply. Membrane technology such as membrane bioreactors (MBR), nanofiltration (NF), and reverse osmosis (RO) have gained a great deal of popularity due to their effectiveness in removing organics, inorganics, emerging micropollutants (EMPs) and contaminants of emerging concerns (CECs). This research used a bench-scale RO system to investigate the removal of selected inorganics and pharmaceuticals from a municipal membrane bioreactor secondary effluent with reverse osmosis membranes. The study was divided into the RO process, solid-phase extraction (SPE), and gas chromatography-mass spectrometry (GCMS) for quantification analysis. The effects of operating conditions on the elimination of inorganics by NF and RO membranes were assessed. Experimental runs were performed on the bench-scale RO cell in recycle mode, adjusting the feed pressure. Chemical analysis of various inorganics was conducted to calculate the percentage removal. Results uncovered considerable effects of feed pressure control on eliminating the inorganics of interest and the carbon-oxygen demand (COD). Adjustment of flux due to the feed pressure for the RO membrane was shown to be a factor of consideration for the improvement of inorganic removal in the advanced treatment of domestic secondary MBR effluent. It was shown that water quality obtained with the RO and NF membranes could meet quality requirements for reuse application in cooling systems and irrigation, among others. Attenuated total reflection, Fourier-transform infrared spectroscopy (ATR-FTIR) was used to identify functional groups on the membrane’s surface. The relationship between initial concentration and functional group deposition was monitored. It can be confirmed that the increase in feed pressure resulted in a higher flux and higher rejection of organics and inorganics. However, the feed concentration had negligible changes in the removal efficiencies; it was depicted that there was an increase in fouling with the rise in feed concentration. Scanning electron microscopy and energy dispersive X-ray (SEM-EDX) were used to analyse the morphology of the membrane's surface. The results showed that NF fouled more than the RO. The foulant deposition onto the polyamide (PA) layer increased for both membranes following the feed pressure increases. A 100-hour-long experimental run was conducted to compare the performance and sustainability of both membranes under the same conditions. The EDX results indicated that the NF had a 13.6% increase in carbon and an 18.8% decrease in oxygen compared to the RO membrane. The selected pharmaceuticals, namely: aspirin (ASP), carbamazepine (CBZ), ibuprofen (IBU) and diclofenac (DCF), were assessed using solid-phase extraction (SPE) and gas chromatography-mass spectrometry (GCMS). The results indicated that the higher feed pressure resulted in greater removal of target analytes. The lowest rejection of DCF was 87% and >95% for CBZ with the NF membranes at the same feed pressure. The feed concentration did not significantly influence the rejection of CBZ, IBU, DCF and ASP, which was most likely ruled by steric hindrance, electrostatic repulsion (donnon exclusion molecular weight cut-off (MWCO) and hydrophobic/super molecular interactions simultaneously. RO resulted in higher rejections than NF, with average rejections greater than 95% for all CECs. Consequently, municipal MBR secondary wastewater effluent treated by a bench-scale RO unit with RO and NF membranes is acceptable for effectively removing selected pharmaceuticals (CBZ, DCF, IBU and ASP). The feed concentration does not affect the removal of target analytes by both RO and NF membranes. However, increasing the feed pressure has proven to be more effective in its removal. Ultimately, using a hybrid system could assist in further abatement for reuse applications.