Research article Performance improvement of a thermophilic sulfate-reducing bioreactor under acidogenic conditions: Effects of diversified operating strategies Carolina Gil-Garcia * , Leandro Augusto Gouv ^ ea de Godoi, Lucas Tadeu Fuess, Marcia Helena Rissato Zamariolli Damianovic Biological Process Laboratory, S~ ao Carlos School of Engineering, University of S~ ao Paulo (LPB/EESC/USP), Av. Jo~ ao Dagnone 1100, S~ ao Carlos, S~ ao Paulo 13563-120, Brazil article info Article history: Received 11 September 2017 Received in revised form 8 November 2017 Accepted 16 November 2017 Available online 12 December 2017 Keywords: Biological sulfate conversion Phase separation Acidogenic/sulfidogenic reactor Liquid phase recirculation Sulfide removal abstract The establishment of a sulfidogenic environment under thermophilic (55  C) acidogenic conditions was assessed in an innovative structured-bed bioreactor to enhance sulfate removal and acetate production prior to methanogenesis. Diversified operating strategies, i.e., variations in the hydraulic retention time (HRT; 6e12 h), sulfate loading rate (SLR; 8e16 kg SO 4 2 m 3 day 1 ) and liquid phase recirculation ratio (0.0e57.0) were assessed to both enable the establishment of sulfate-reducing conditions and remove H 2 S from the liquid phase. Ethanol was used as the only carbon source. Applying a low HRT (6 h) as the initial operating strategy severely hindered the establishment of sulfate-reducing bacteria (SRB) pop- ulations within the system (sulfate removal < 27.5%). In turn, applying effluent recirculation had a positive impact on the system (sulfate removal ~ 60%) by providing an adequate buffer control along the entire height of the system, as well by displacing over 70% of the H 2 S to the gaseous phase. The main- tenance of pH values above 6.1 proved to be adequate for the sulfidogenic activity, whereas enhanced acidic conditions (pH < 6.0) at the basal portion of the reactor comprised a determining factor to hinder sulfate reduction. SRB were able to handle H 2 S and acetate concentrations as high as 232 mg L 1 and 3111 mg L 1 , respectively, after establishing an effective acidogenic/sulfidogenic environment, indicating that the proposed system has the potential to be used as the first stage in the anaerobic processing of sulfate-rich wastewater streams. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Anaerobic treatment of sulfate (SO 4 2 )-rich wastewaters often faces drawbacks associated with sulfide production by sulfate- reducing bacteria (SRB), which includes direct cell inhibition and the precipitation of essential metals by dissolved sulfide, as well as operating problems due to the corrosion of metallic structures (Koschorreck, 2008; Lens et al., 1998; Maree et al., 2004; McMahon and Daugulis, 2008). Undissociated sulfide (H 2 S) characterizes the most toxic form of sulfide, as the molecules easily permeate the cell membrane of microorganisms and further affect the enzymatic metabolism by denaturing native proteins (Koschorreck, 2008; McCartney and Oleszkiewicz, 1993). Reduced methane production levels comprise the primary impact of the establishment of sulfi- dogenic conditions in anaerobic systems, directly reducing the bioenergy recovery potential from biogas. In addition to the toxic character of sulfide, SRB may also outcompete methanogenic archaea (MA) when using common substrates (e.g. hydrogen and acetate), consuming electrons from methane production (Chen et al., 2008). Different operating strategies have been proposed to minimize negative effects of sulfide over anaerobic reactors, such as gas stripping and enhanced precipitation by metallic salts. The appli- cation of an inert gaseous stream (e.g. nitrogen) aims to reduce dissolved sulfide concentrations in the liquid phase by stripping, producing a H 2 S-rich biogas (Maree et al., 2004; McMahon and Daugulis, 2008). Similar operating conditions are also observed in jet-loop reactors, in which biogas recirculation is used to increase * Corresponding author. E-mail addresses: carolgilg@sc.usp.br (C. Gil-Garcia), leandro.godoi86@gmail. com (L.A.G. de Godoi), ltfuess@alumni.usp.br (L.T. Fuess), mdamianovic@sc.usp.br (M.H.R.Z. Damianovic). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman https://doi.org/10.1016/j.jenvman.2017.11.043 0301-4797/© 2017 Elsevier Ltd. All rights reserved. Journal of Environmental Management 207 (2018) 303e312