Process Safety and Environmental Protection 124 (2019) 172–180 Contents lists available at ScienceDirect Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep Enhancing biodegradation of toxic industrial wastewaters in a continuous two-phase partitioning bioreactor operated with effluent recycle M. Concetta Tomei ∗ , Domenica Mosca Angelucci Water Research Institute, C.N.R., Via Salaria km 29.300, CP 10, 00015, Monterotondo Stazione (Rome), Italy a r t i c l e i n f o Article history: Received 29 November 2018 Received in revised form 13 February 2019 Accepted 14 February 2019 Available online 20 February 2019 Keywords: Industrial wastewater treatment Continuous two-phase partitioning bioreactor (C-TPPB) Toxic organic compounds Process optimization Biological treatment a b s t r a c t In this study, we propose the application of a Continuous Two-Phase Partitioning Bioreactor (C-TPPB) operated with the tubing effluent recycle to enhance the biodegradation of toxic substrates in industrial wastewaters under severe loading conditions. Stepwise increasing influent concentrations (from 200 to 900 mg L −1 ) of 2,4-dichlorophenol (2,4-DCP) were fed to a C-TPPB operated with Hytrel G3548 tubing to simulate phenolic wastewater. Practically complete 2,4-DCP removal has been achieved during the entire experimental period, but the increased load reduced biodegradation efficiency. At influent concentration of 700 mg L −1 , the first effluent recycle (recycle /influent flow rate ratio = 0.3) was applied: biodegradation efficiency doubled from 40 to 80% and was maintained until influent concentrations of 800 mg L −1 . Higher influent concentrations caused a decrease in 2,4-DCP biodegradation, so the effluent recycle ratio was increased to 0.5 at 900 mg L −1 and, also in this case, the bioreactor showed a fast recovery (∼ 24 h) of the biodegradation efficiency at 80%. Mass transfer data analysis showed that the effluent recycle resulted in an increase of the mass transfer coefficient. This positive effect, joined with the reduction of the influent concentrations, demonstrated the feasibility of recycle application in enhancing the C-TPPB performance. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 1. Introduction Critical aspects in biological treatment of industrial wastew- ater relate to the presence of self-inhibitory organic pollutants, generally xenobiotics and “hostile” reaction environments caused by extreme pH values, high salinity and inorganic toxic com- pounds (such metals) often characterizing these streams. To avoid serious reduction of the process performance, safe concentration levels ≤ EC 50 of organics in contact with the biomass are required, and the impact of operating conditions potentially detrimental for the biomass activity, has to be minimized. Livingston et al. (1998) proposed and extensively investigated extractive membrane biore- actors (EMBs), a powerful technology able to avoid the direct contact wastewater-biomass through single tubing (Ruben and Livingston, 2000; Zhang et al., 1998) or tubing modules (Splendiani et al., 2003; Mehdizadeh et al., 2011) fed with the wastewater and immersed in the bioreactor containing the microbial culture. Main tubing feature is the capacity of selectively transferring the toxic organics through the walls to the bulk liquid phase containing the ∗ Corresponding author. E-mail address: tomei@irsa.cnr.it (M.C. Tomei). biomass, and retaining the toxic inorganics present in ionic form. In the original EMB configuration, tubing was made of silicone rubber, an organophilic polymer permeable to many volatile chlorinated hydrocarbons (VOCs), and effectiveness of EMBs for their removal has been extensively tested and demonstrated. With the aim of extending the applicability of tubing systems to compounds other than VOCs, tubing made of specifically selected polymers with high affinity for the target substrates have been tested for the removal of phenolic compounds in continuous two-phase partitioning biore- actors (C-TPPBs) operated with the same EMB configuration. The idea behind this approach is to extend the advantages of absorp- tive polymers demonstrated in solid-liquid TPPBs, operated with polymer beads and mainly advantageous in discontinuous biore- actors (Tomei et al., 2011, 2016a) to continuous systems in which the partitioning phase is the tubing. Recent studies on pheno- lic compounds (Tomei et al., 2016b, 2017a), on synthetic tannery (Mosca Angelucci et al., 2017) and hypersaline wastewater (Tomei et al., 2017b, 2018) demonstrated the validity of the application of absorptive polymer tubing in such systems showing not limiting mass transfer rates of organic substrates and complete retention of the ionic species in tubing. The next step in the applicability evaluation of C-TPPB is the optimization of the operating parameters to control the process https://doi.org/10.1016/j.psep.2019.02.011 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.