Assessment of increasing loading rate on two-stage digestion of food waste M.A. Voelklein, A. Jacob, R. O’ Shea, J.D. Murphy ⇑ MaREI Centre, Environmental Research Institute (ERI), University College Cork (UCC), Ireland School of Engineering, UCC, Ireland highlights  Single and two-stage digestion of food waste was compared at increased loading.  The methane content of the biogas increased by 14% to 71% in the two-stage system.  The two-stage system yielded up to 23% more methane than the single-stage system.  The two-stage system produced up to 404 L CH 4 kg 1 VS or 15.1 MJ kg VS 1 . article info Article history: Received 7 October 2015 Received in revised form 30 November 2015 Accepted 9 December 2015 Available online 14 December 2015 Keywords: Two-stage digestion Food waste Hydrolysis Biogas High performance reactors abstract A two-stage food waste digestion system involved a first stage hydrolysis reactor followed by a second stage methanogenic reactor. Organic loading rates (OLR) were increased from 6 to 15 g VS L 1 d 1 in the hydrolysis reactor and from 2 to 5 g VS L 1 d 1 in the methanogenic reactor. The retention time was fixed at 4 days (hydrolysis reactor) and 12 days (methane reactor). A single-stage digester was sub- jected to similar loading rates as the methanogenic reactor at 16 days retention. Increased OLR resulted in higher quantities of liquid fermentation products from the first stage hydrolysis reactor. Solubilisation of chemical oxygen demand peaked at 47% at the maximum loading. However, enhanced hydrolysis yields had no significant impact on the specific methane yields. The two-stage system increased methane yields up to 23% and enriched methane content by an average of 14% to levels of 71%. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The Zero Waste Programme for Europe promotes a circular economy (European Commission, 2015) and encourages a phase out of land filling of biodegradable waste such as the organic frac- tion of municipal solid waste (OFMSW) by 2025. Anaerobic diges- tion may be considered a beneficial treatment system for OFMSW due to direct conversion to biogas whilst simultaneously retaining nutrients in the digestate (Murphy and McKeogh, 2004). Food waste (as it has a gate fee associated with its treatment) can pro- vide the most economic source of biogas production (Murphy and Power, 2006). Biomethane potential tests (BMP) highlight the rapid degradability of commercial food waste; 95% of the 30 day BMP yield was achieved in the first 10 days by Browne et al. (2014). Commercial food waste with high degradability should be amenable to low retention times and high organic load- ing rates. Single-stage anaerobic digestion is a well-established technol- ogy for biogas production. The investment costs are relatively low and the process is well understood. However, hydrolytic and methanogenic microorganisms are optimised at differing pH (Bochmann and Montgomery, 2013). In a single-stage system the prevailing pH (7–8) favours the methanogenic archaea, leading to non-optimum growth conditions for acidifying hydrolytic bacteria. The advantage of two-stage anaerobic digestion is the spatial separation of process phases, where reactor parameters such as pH can be optimised for each phase to suit requirements of the microorganisms. The pH in the first reactor (between 4 and 6) opti- mises hydrolysis (Bochmann and Montgomery, 2013). In the upstream reactor hydrolysis and acidification break down macro- molecules into liquid fermentation products such as volatile fatty acids (VFAs) and ethanol (Bochmann and Montgomery, 2013), pre- cursors for the methanogens in the second reactor. The effluent from stage one (hydrolysis reactor) is the substrate for the http://dx.doi.org/10.1016/j.biortech.2015.12.001 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: School of Engineering, University College Cork, Cork, Ireland. Tel.: +353 21 490 2286. E-mail address: jerry.murphy@ucc.ie (J.D. Murphy). Bioresource Technology 202 (2016) 172–180 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech