Single-phase and two-phase anaerobic digestion of fruit and vegetable waste: Comparison of start-up, reactor stability and process performance Rangaraj Ganesh a , Michel Torrijos a,⇑ , Philippe Sousbie a , Aurelien Lugardon b , Jean Philippe Steyer a , Jean Philippe Delgenes a a INRA, UR50, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne F-11100, France b Naskeo Environnment, 52 rue Paul Vaillant Couturier, F-92240 Malakoff, France article info Article history: Received 19 November 2013 Accepted 21 February 2014 Available online 26 March 2014 Keywords: Fruit and vegetable waste Single-phase process Two-phase process Solids recirculation Process stability Energy yield abstract Single-phase and two-phase digestion of fruit and vegetable waste were studied to compare reactor start- up, reactor stability and performance (methane yield, volatile solids reduction and energy yield). The single-phase reactor (SPR) was a conventional reactor operated at a low loading rate (maximum of 3.5 kg VS/m 3 d), while the two-phase system consisted of an acidification reactor (TPAR) and a methano- genic reactor (TPMR). The TPAR was inoculated with methanogenic sludge similar to the SPR, but was operated with step-wise increase in the loading rate and with total recirculation of reactor solids to con- vert it into acidification sludge. Before each feeding, part of the sludge from TPAR was centrifuged, the centrifuge liquid (solubilized products) was fed to the TPMR and centrifuged solids were recycled back to the reactor. Single-phase digestion produced a methane yield of 0.45 m 3 CH 4 /kg VS fed and VS removal of 83%. The TPAR shifted to acidification mode at an OLR of 10.0 kg VS/m 3 d and then achieved stable per- formance at 7.0 kg VS/m 3 d and pH 5.5–6.2, with very high substrate solubilization rate and a methane yield of 0.30 m 3 CH 4 /kg COD fed. The two-phase process was capable of high VS reduction, but material and energy balance showed that the single-phase process was superior in terms of volumetric methane production and energy yield by 33%. The lower energy yield of the two-phase system was due to the loss of energy during hydrolysis in the TPAR and the deficit in methane production in the TPMR attributed to COD loss due to biomass synthesis and adsorption of hard COD onto the flocs. These results including the complicated operational procedure of the two-phase process and the economic factors suggested that the single-phase process could be the preferred system for FVW. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction High-rate anaerobic reactors for wastewater treatment have been in use for quite some time yet there is a need for further re- search into reactor design and new avenues for the treatment of solid wastes. Given the growing demand for energy recovery and efficient disposal of solid waste, such research is vital. Single-phase anaerobic systems, in which all three reactions of hydrolysis, acetogenesis and methanogenesis take place simulta- neously in a single reactor have been the preferred reactor design for the majority of waste (Lissens et al., 2001; Bouallagui et al., 2005). However, the operation of such systems at a high OLR and for waste with large biodegradable organic content such as fruit and vegetable waste (FVW) becomes difficult as this type of waste undergoes rapid acidification resulting in the inhibition of metha- nogenic activity (Mata-Alvarez et al., 1992; Callaghan et al., 2002; Bouallagui et al., 2009). The maximum loading rate reported for single-phase digestion of FVW was within 3.6 kg VS/m 3 d(Verrier et al., 1987; Mata-Alvarez et al., 1992; Bouallagui et al., 2003; Lin et al., 2011). Two-phase systems, in contrast, have the advantage of buffering the OLR in the first stage, allowing a more constant feeding rate to the methanogenic second stage (Bouallagui et al., 2005; Koutrouli et al., 2009; Ghosh et al., 2000). Higher loading rates in the range 5.7–7.7 kg VS/m 3 d were reported for the two- phase digestion of FVW (Verrier et al., 1987; Mtz.-Viturtia et al., 1995; Dinsdale et al., 2000; Rajeshwari et al., 2001; Bouallagui et al., 2004). The different types of oxido-reductive activities, pH optima and growth rates of acidogens and methanogens have been capitalized by phase separation to increase process efficiency http://dx.doi.org/10.1016/j.wasman.2014.02.023 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +33468425185; fax: +33468425160. E-mail address: michel.torrijos@supagro.inra.fr (M. Torrijos). Waste Management 34 (2014) 875–885 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman