A hybrid system comprising an aerobic biological process and electrochemical oxidation for the treatment of black table olive processing wastewaters Triantafyllos I. Tatoulis a , Stamoulis Zapantiotis b , Zacharias Frontistis b , Christos S. Akratos a , Athanasia G. Tekerlekopoulou a, * , Stavros Pavlou b, c , Dionissios Mantzavinos b , Dimitrios V. Vayenas b, c a Department of Environmental and Natural Resources Management, University of Patras, 2 Seferi St., GR-30100 Agrinio, Greece b Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece c Institute of Chemical Engineering Sciences (ICE-HT), Stadiou St., Platani, GR-26504 Patras, Greece article info Article history: Received 24 December 2015 Received in revised form 20 January 2016 Accepted 20 January 2016 Available online 1 February 2016 Keywords: Agro-industrial effluents Attached-growth reactors Boron-doped diamond Decolorization Integrated treatment Salinity abstract In this work table olive processing wastewaters (TOPW) were treated by aerobic biological processes using indigenous microorganisms originating from TOPW, as well as the combination of two successive steps, i.e. aerobic biological treatment followed by electrochemical oxidation over a boron-doped dia- mond anode. In the single aerobic biological processes, experiments in suspended and attached growth reactors (trickling filters) were carried out using different TOPW feed concentrations of 5500 ± 350, 7500 ± 650 and 15,000 ± 1050 mg dissolved COD L 1 . Two different operating modes were used to investigate the optimum performance of the filter, i.e. batch and SBR with recirculation. The latter mode with recircu- lation of 0.5 L min 1 led to high removal rates of dissolved COD and total phenolic compounds, up to 96.5% and 64.5%, respectively, for the initial COD concentration of 7500 mg dissolved COD L 1 . Depending on the type and operating conditions of the bioreactors, residual COD ranged between a few hundred and a few thousand mg L 1 , while decolorization could not be achieved even under the most favorable conditions. A biologically treated effluent with residual dissolved COD of 5100 mg L 1 was completely mineralized and decolorized at 187.5 mA cm 2 applied current density; complete removal of COD, color and total phenolic compounds was achieved in 180e240 min, 30e60 min and 30 min of electrochemical oxidation, respectively. Lower treatment times and current densities were needed to polish effluents with lower organic loads. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Wastewater generated in green and black table olive processing industries (TOPW) is characterized by seasonal peaks, high organic matter and phenolic content, as well as high salt concentration. Unfortunately, this type of wastewater is sometimes discharged untreated into small streams or directly into the sea. In the best case, it is transported either to evaporating ponds for water evap- oration (where anaerobic conditions are quickly established lead- ing to malodors and the risk of polluting surface or ground waters) or to large municipal wastewater treatment plants where it is mixed with municipal wastewater (Rivas et al., 2000a, 2000b; Aggelis et al., 2001; Kotsou et al., 2004; Ferrer-Polonio et al., 2015). However, as environmental regulations and their enforce- ment have become more stringent, a growing interest in the development of new treatment methods for TOPW has emerged. Several technologies have been proposed for treating TOPW such as aerobic and anaerobic biological processes (Borja et al., 1994; Brenes et al., 2000; Aggelis et al., 2001; Beltran et al., 2008; Lasaridi et al., 2010; Zarkadas and Pilidis, 2011; Ayed et al., 2013; Ferrer-Polonio et al., 2015), oxidation processes (Rivas et al., 2000a, 2003a, 2003b; Beltr an et al., 2001; Deligiorgis et al., 2008; Katsoni et al., 2008; García-García et al., 2011), ultrafiltration (Garcia-Ivars et al., 2015), membrane distillation (Kiai et al., 2014) * Corresponding author. E-mail address: atekerle@upatras.gr (A.G. Tekerlekopoulou). Contents lists available at ScienceDirect International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod http://dx.doi.org/10.1016/j.ibiod.2016.01.013 0964-8305/© 2016 Elsevier Ltd. All rights reserved. International Biodeterioration & Biodegradation 109 (2016) 104e112