Influence of organic loading rate on ciprofloxacin and sulfamethoxazole biodegradation in anaerobic fixed bed biofilm reactors

Antibiotic compounds, notably sulfamethoxazole (SMX) and ciprofloxacin (CIP), are ubiquitous emerging contaminants (ECs), which are often found in domestic sewage. They are associated with the development of antimicrobial resistance. Operational parameters, e.g. organic loading rate (OLR), hydraulic retention time (HRT) and sludge retention time, may influence EC biodegradation in wastewater treatment plants. This study assessed the impact of the OLR variation on the biodegradation of CIP and SMX, applying two configurations of anaerobic fixed bed reactors: anaerobic packed bed biofilm reactor (APBBR) and anaerobic structured bed biofilm reactor (ASBBR). A significant reduction in the biodegradation of SMX (APBBR: 93–69%; ASBBR: 94–81%) and CIP (APBBR: 85–66%; ASBBR: 85–64%) was observed increasing OLR from 0.6 to 2.0 kgCOD m−3 d−1. The decrease in the HRT from 12 to 4 h resulted in higher liquid-phase mass transfer coefficient (APBBR: ks from 0.01 to 0.05 cm h−1; ASBBR: ks from 0.07 to 0.24 cm h−1), but this was not enough to overcome the decrease in the antibiotic-biomass contact time on biofilm, thus reducing the bioreactors’ performance. The ASBBR favored biomethane production (from 7 to 17 mLCH4 g−1VSS L−1 d−1) and biodegradation kinetics (kbio from 1.7 to 4.2 and for SMX and from 2.1 to 4.8 L g−1VSS d−1 for CIP) due to the higher relative abundance of the archaea community in the biofilm and the lower liquid-phase mass transfer resistance in the structured bed. CIP and SMX cometabolic biodegradation was associated to the hydrogenotrophic methanogenesis (mainly Methanobacterium genus) in co-culture with fermentative bacteria (notably the genera Clostridium, Bacillus, Lactivibrio, Syntrophobacter and Syntrophorhabdus). The anaerobic fixed bed biofilm reactors proved to be highly efficient in biodegrading the antibiotics, preventing them from spreading to the environment.