Evidence of solute-solute interactions and cake enhanced concentration polarization during removal of pharmaceuticals from urban wastewater by nanofiltration

- Membrane-solute-solute interactions govern the PhACs rejection by loose NF membranes.
- PhACs rejection by tight NF membranes is governed by fouling and flux variations.
- CECP impacts PhACs rejection of tight NF membranes at the start of filtration.
- After a dense cake is formed on tight NF membranes, PhACs rejection becomes enhanced.

The objective of this paper is to help understanding the distinctive influence of the matrix and of the flux decline (e.g. through the cake enhanced concentration polarization (CECP) phenomenon) on the removal mechanisms of four pharmaceutically active compounds (PhACs) from wastewater treatment plant (WWTP) effluent by nanofiltration (NF). PhACs which are commonly encountered in secondary treated effluent were spiked in various matrix (real and synthetic) to investigate the separate and synergetic effects of the organic and ionic environment on PhACs rejection by two commercial membranes (NF-90 and NF-270). With pure water, rejection of NF membranes is dependent on the type of PhACs and of the permeate flux variations. Then, it appeared that the rejection of PhACs by NF-90 was poorly influenced by the type of compounds, because of the prevalence of steric mechanisms, but rather influenced by permeate flux variations and thus to fouling. For this tight NF membrane, CECP impacts PhACs rejection at the start of filtration while after a dense cake is formed, it became enhanced. On the contrary, rejections of PhACs by the NF-270 were enhanced during the filtration of the real wastewater in comparison with spiked pure water. It appeared that for loose-NF membranes, PhACs rejection is mainly governed by solute-solute interactions (EfOM-compound association) or electrostatic membrane-solute interactions. Finally, it seems that calcium concentration of the effluent is a critical parameter for the rejection of PhACs as it alters both the availability of sites for PhACs association and the fouling layer density. Rejections of the NF-270 were negatively impacted in the presence of Ca2+. Such a study has practical implications for further understanding of the fate of trace organic compounds during nanofiltration of wastewater for reuse applications.

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