Membrane bioreactor followed by solar photo-Fenton oxidation: Bacterial community structure changes and bacterial reduction

Abstract

The removal of antibiotic resistance genes (ARGs) and taxon-specific markers, the bacterial community structure changes and the permanent inactivation of total bacteria including their antibiotic-resistant counterparts (ARB) in actual wastewater during a Membrane BioReactor (MBR) application followed by solar photo-Fenton oxidation at bench- and then pilot-scale under solar irradiation, were investigated. The presence of enterococci- and pseudomonad-specific taxon markers and of sul1 and ampC ARGs in the MBR effluent was confirmed, indicating the challenge of such processes, for the removal of biological molecules. On the other hand, >99 % reduction of all types of cultivable bacteria examined was observed after MBR treatment, with a 5-log reduction of E. coli and 6-log reduction of P. aeruginosa and Klebsiella spp. There was a shift in the bacterial community structure in the MBR effluent after the bench- and pilot-scale solar photo-Fenton oxidation. Notably, thermotolerant bacterial genera like Ignavibacterium and Thermomonas were prevalent during the pilot-scale process operated at a high ambient temperature, while the most prevalent genera were Mycobacterium, Nocardioides and Mesorhizobium, which are primarily not pathogenic and plant-related. In agreement, a different bacterial community structure according to the G-C content after DGGE analysis was noted between the MBR and solar photo-Fenton oxidation-treated effluents, but interestingly also between the bench- and pilot-scale oxidation-treated effluents. There was complete absence of ARGs after the bench-scale solar photo-Fenton oxidation application but not after the pilot-scale treatment (1.56 and 1.53 log₁₀ CE 100 ng^-1 DNA, of sul and ermB, respectively). Taxon-specific markers were found in both oxidation setups. Inactivation of cultivable Escherichia coli, Pseudomonas aeruginosa and Klebsiella spp. (including ARB) was achieved during both oxidation setups, with no further re-activation observed.

Keywords: Advanced oxidation processes; Antibiotic resistance genes; Bacterial community structure; Bacterial inactivation; Wastewater treatment.