Simultaneous removal of dissolved organic matter and nitrate from sewage treatment plant effluents using photocatalytic membranes

Abstract

The residual dissolved organic matter (DOM) and nitrate in sewage treatment plant (STP) effluent have potential negative impacts on the aqueous environment. To that end, we used formic acid (FA) to enhance the photochemical behavior of the photocatalytic membrane for the simultaneous removal of DOM and nitrate from secondary STP effluent. Effluent samples were collected from two different biological treatment processes, Anaerobic-Oxic and Anaerobic-Anoxic-Oxic-membrane bioreactor, respectively. Through Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis, we found that the addition of FA resulted in a similar molecular transformation in different STP effluent samples. Besides, the radical signal of the carboxyl anion could be observed during the photocatalytic process. Based on the results, we proposed the mechanism of the process that carboxyl anion radicals generated by FA could attack DOM and result in further oxidation of the DOM transition state to CO2 or small molecule by nitrate. Meanwhile, CHON and CHOS compounds in DOM were attacked by the carboxyl anion radicals more easily than CHO compounds. Moreover, long-term use of the membrane confirmed its durability and reusability in practical applications. At a moderate FA concentration and lower hydraulic retention time, the nitrate and DOM removal efficiencies for the sample from JX STP were 68% and 70%, respectively, whereas those of the CD STP sample were 85% and 60%. The removal of DOM and nitrate from different STP effluents using photocatalytic membranes is an advanced approach for the treatment of secondary effluent, and may be applicable to other membranes or systems.