RESEARCH REPOSITORY This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination. The definitive version is available at: https://doi.org/10.1016/j.biortech.2017.08.157 Mohottige, T.N.W., Ginige, M.P., Kaksonen, A.H., Sarukkalige, R. and Cheng, K.Y. (2017) Bioelectrochemical oxidation of organics by alkali-halotolerant anodophilic biofilm under nitrogen-deficient, alkaline and saline conditions. Bioresource Technology, 245 . pp. 890-898. http://researchrepository.murdoch.edu.au/id/eprint/38599/ Copyright: © 2017 Elsevier Ltd It is posted here for your personal use. No further distribution is permitted. Accepted Manuscript Bioelectrochemical oxidation of organics by alkali-halotolerant anodophilic bi- ofilm under nitrogen-deficient, alkaline and saline conditions Tharanga N. Weerasinghe Mohottige, Maneesha P. Ginige, Anna H. Kaksonen, Ranjan Sarukkalige, Ka Yu Cheng PII: S0960-8524(17)31471-2 DOI: http://dx.doi.org/10.1016/j.biortech.2017.08.157 Reference: BITE 18766 To appear in: Bioresource Technology Received Date: 13 July 2017 Revised Date: 23 August 2017 Accepted Date: 25 August 2017 Please cite this article as: Mohottige, N.W., Ginige, M.P., Kaksonen, A.H., Sarukkalige, R., Cheng, K.Y., Bioelectrochemical oxidation of organics by alkali-halotolerant anodophilic biofilm under nitrogen-deficient, alkaline and saline conditions, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech. 2017.08.157 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Invited submission to Bioresource Technology (Special Issue “Biotechnology for Waste Conversion”) Bioelectrochemical oxidation of organics by alkali-halotolerant anodophilic biofilm under nitrogen-deficient, alkaline and saline conditions Tharanga N. Weerasinghe Mohottige1,2, Maneesha P. Ginige1, Anna H. Kaksonen1,3, Ranjan Sarukkalige2, Ka Yu Cheng1,4* 1CSIRO Land and Water, Western Australia, Australia 2Department of Civil Engineering, Curtin University, Western Australia, Australia 3School of Pathology and Laboratory Medicine, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia 4School of Engineering and Information Technology, Murdoch University, Western Australia 6150, Australia *Corresponding author. Tel: +61 8 9333 6158; Fax: +61 8 933 6499. E-mail address: [email protected] (Ka Yu Cheng) 1 Abstract This work aimed to study the feasibility of using bioelectrochemical systems (BES) for organics removal under alkaline-saline and nitrogen (N) deficient conditions. Two BES inoculated with activated sludge were examined for organics (oxalate, acetate, formate) oxidation under alkaline-saline (pH 9.5, 25 g/L NaCl) and N deficient conditions. One reactor (R1) received ammonium chloride as an N-source, while the other (R2) without. The reactors were initially loaded with only oxalate (25 mM), but start-up was achieved only when acetate was added as co-substrate (5 mM). Maximum current were R1: 908 A/m3 (organic removal rate (ORR) 4.61 kg COD/m3.d) and R2: 540 A/m3 (ORR 2.06 kg COD/m3.d). Formate was utilised by both anodic biofilms, but the inefficient oxalate removal was likely due to the paucity of microorganisms that catalyse decarboxylation of oxalate into formate. Further development of this promising technology for the treatment of alkaline-saline wastewater is warranted. Key words: Alumina; Bayer process; alkaliphilic; halophilic; oxalate 2 1. Introduction Alumina (aluminium oxide, Al2O3) is produced from bauxite ore in a process known as the Bayer process, which involves mixing the ores with caustic solutions under elevated temperature (Meyers, 2004). Many bauxite ores, particularly those in Australia contain a wide range of organic compounds, which can detrimentally affect the Bayer process by (1) reducing the caustic in the Bayer liquor, and (2) reducing aluminium trihydroxide (Al(OH)3) precipitation rate (Den Hond et al., 2007). These organics are generally considered as unwanted waste components for the alumina industry. Therefore, effective removal of organic compounds from the process water of the alumina refining circuit is essential. Amongst all the organics present in Bayer process liquor, sodium oxalate (Na2C2O4) is the key detrimental compound (Power et al., 2012). As the Bayer liquor is typically recycled within the refinery circuit, sodium oxalate concentration in the liquor would gradually reach a level that triggers the aforementioned problems. Preferably, the oxalate from the Bayer circuit should be permanently destroyed in an economical and environmentally sustainable manner (McKinnon and Baker, 2012). One option to destroy oxalate from the Bayer circuit is by adopting microbial oxidation processes such as aerobic bioreactor processes (McKinnon and Baker, 2012). Although effective in terms of oxalate destruction, aeration requirement in aerobic processes would incur significant energy demand. Further, aerobic bioreactor processes do not normally facilitate recovery of resources (e.g. energy and caustic) (Rabaey et al., 2010). Hence, alternative treatment processes that allow energy saving and resources recovery should be considered. Although not yet embraced by the alumina industry, bioelectrochemical systems (BES) have been widely explored as an advanced microbial oxidative process for removing 3 organic contaminants and recovering resources from wastewaters (Rabaey et al., 2010; Rozendal et al., 2008). Recently, the use of BES for oxalate removal has been demonstrated by Bonmati et al. (2013). They successfully established an oxalate- degrading bioanode with both acetate and oxalate as the carbon and electron sources, and found that the anodic biofilm could oxidise oxalate under neutral pH and non-saline conditions (coulombic efficiency 34%) (Bonmati et al., 2013). In fact, alkaliphilic microorganisms have been shown as being able to generate anodic current in BESs (Badalamenti et al., 2013; Liu et al., 2010). For instance, Liu et al. (2010) showed that a Gram-positive alkaliphile (Corynebacterium sp.) could utilise formate as an electron donor at pH 9. Badalamenti et al. (2013) also recently reported two pure haloalkaliphilic strains (Geoalkalibacter ferrihydriticus DSM 17813 and Geoalkalibacter subterraneus DSM 23483) that could produce current under either alkaline (pH 9.3) (8.3 A/m2) or saline (17 g/L NaCl) (3.3 A/m2) conditions with acetate as an electron donor. Since the oxalate waste from alumina refineries is both saline (up to 25 g/L of sodium chloride) and alkaline (>pH 13), it would be desirable to develop a BES process for oxalate removal under both alkaline and saline conditions. Besides, as Bayer process water is typically deficient in nitrogen (N), external dosing of N into aerobic oxalate removing bioreactors is commonly practiced since N is an essential element for microbial growth. However, this not only incurs additional costs, but excessive dosage can lead to undesirable build-up and release of ammonia (particularly under alkaline conditions), causing odour issues. Since the required conditions (presence of organic carbon, absence of inorganic N and low concentrations of dissolved oxygen (Gauthier et al., 2000)) for biological N fixation are ubiquitous in the Bayer process water, it would be worth testing if anodophilic microorganisms could grow in an alkaline-saline 4 BES without external supplementation of N. Omission of this requirement would be attractive in terms of reducing treatment costs. In light of the above, this study aimed to examine the use of BES for the treatment of synthetic liquor mimiking Bayer process water in its alkalinity (pH 10) and salinity (25 g/L NaCl) under N-deficient conditions. The specific research questions were: (1) Can anodic biofilms be established using activated sludge as a microbial inoculum to remove oxalate under alkaline and saline conditions? (2) Can the supply of ammonium chloride as N source be omitted to establish an anodic biofilm for organics removal under alkaline- saline conditions? (3) Can the established biofilm be used to remove organic compounds present in Bayer liquor, such as acetate, formate, succinate and malonate under alkaline and saline conditions? These questions were answered for the first time by operating two parallel laboratory-scale, dual-chamber BES reactors, with one supplied with ammonium chloride (NH4Cl) as an external N source and the other not. Activated sludge was selected as the microbial inoculum considering its board microbial diversity and availability from sewage treatment plants (McKinnon and Baker, 2012). There is also a previous report of using activated sludge as a start-up inoculum for a full scale aerobic oxalate degrading bioreactor (McSweeney, 2011). An alkaline (pH 10) and saline (25 g/L NaCl) media mimicking the alkalinity and salinity of a typical Bayer process water was continuously loaded into the two reactors for a period of >350 days. The effects of various process parameters, such as anode potential, hydraulic retention time (HRT) and anolyte pH on the BES performance (anodic current,