Supercritical water oxidation of recalcitrant organic pollutants present in retentates from RO desalination of secondary effluents

This work focuses on applying supercritical water oxidation (SCWO) for eliminating residues of pharmaceutical, endocrine-disruptor and pesticide species from the reject of reverse-osmosis-treated secondary municipal wastewater. SCWO, a well-known treatment method for organic sludge and highly concentrated wastewaters, has not yet been investigated with reverse osmosis (RO) retentates of municipal secondary effluents. The results shown here, focusing on batch SCWO of municipal effluent RO retentate spiked with emerging pollutants, demonstrate the potential of utilizing this single advanced-oxidation generic treatment to fully-mineralize the vast variety of organic species encountered in municipal wastewater effluents, while at the same time lowering its total organic carbon concentration by up to 99%. The chosen model pollutants (carbamazepine and 17β-Estradiol both in synthetic solution and within a real effluent RO retentate matrix) and the effluent’s background organic matter, started to degrade already at subcritical conditions, however near-complete mineralization of the full matrix (including transformation products) was invariably attained only at fully-established supercritical conditions (450°C–540°C). The extreme hydrothermal oxidation conditions applied in this work allowed for simultaneous oxidation of target molecules and background organic species throughout the applied temperature range, while the phenomenon known as “radicals scavenging” by the background organic matter, was not observed. A new technique is introduced in this work for controlling the pressure within semi-batch SCWO experiments.