Short Communication Demand-driven energy supply from stored biowaste for biomethanisation Peter Aichinger a,c , Martin Kuprian b , Maraike Probst b , Heribert Insam b , Christian Ebner c,⇑ a University of Innsbruck, Institute of Infrastructure, Technikerstraße 25d, 6020 Innsbruck, Austria b University of Innsbruck, Institute of Microbiology, Technikerstraße 25d, 6020 Innsbruck, Austria c AlpS – Center for Climate Change Adaptation, Grabenweg 68, 6020 Innsbruck, Austria highlights  Biowaste storage supported substrate hydrolysis leading to higher biomethane yields.  Storage led to biowaste acidification and accumulation of volatile fatty acids.  Fast methane production: >95% was produced within the first days. article info Article history: Received 28 April 2015 Received in revised form 29 June 2015 Accepted 30 June 2015 Available online 7 July 2015 Keywords: Anaerobic digestion Sludge Biogas Renewable energy Wastewater treatment abstract Energy supply is a global hot topic. The social and political pressure forces a higher percentage of energy supplied by renewable resources. The production of renewable energy in form of biomethane can be increased by co-substrates such as municipal biowaste. However, a demand-driven energy production or its storage needs optimisation, the option to store the substrate with its inherent energy is investigated in this study. The calorific content of biowaste was found unchanged after 45 d of storage (19.9 ± 0.19 kJ g 1 total solids), and the methane yield obtained from stored biowaste was comparable to fresh biowaste or even higher (approx. 400 m 3 Mg 1 volatile solids). Our results show that the storage supports the hydrolysis of the co-substrate via acidification and production of volatile fatty acids. The data indicate that storage of biowaste is an efficient way to produce bioenergy on demand. This could in strengthen the role of biomethane plants for electricity supply the future. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The share of renewable energy is steadily increasing which is reflected by increased numbers of wind, solar, and biogas plants (Hinrichs-Rahlwes, 2013; Hahn et al., 2014). Both wind and solar technologies are weather dependent and struggle to supply energy on demand (Hahn et al., 2014; Schaber et al., 2012). Energy in form of biomethane on the contrary, can be stored in tanks or bags or can be directly injected into the compressed natural gas (CNG) grid (Holm-Nielsen et al., 2009; Ruben et al., 2012) and is seen as a high priority option today. A current trend in wastewater treatment plants is to co-digest energy-rich substrates such as the source-separate collected organic fraction of municipal solid waste (OFMSW or biowaste). The reason is that many plants do not max out their capacity in digestion volume or combined heat and power. Biowaste is a municipal organic waste stream consisting of kitchen and garden residues that is collected source-separately in Europe. It was produced in an amount of >240 Mio tons in 2012 according to EurObserv‘ER (2014). For co-digestion it is of high value because it is ubiquitously available, cheap, and has a high biomethane potential of 500–700 L methane kg 1 organic dry content (Nieto et al., 2012; Kymäläinen et al., 2012; Sosnowski et al., 2003). Besides the use of biowastes as a co-substrate, mono-digestion or co-digestion of OFMSW with other substrates is also possible. Often, it is useful to regulate biogas production of wastewater treatment plant (WWTP) digesters. For WWTP operating close to energy self-sufficiency it is reasonable to shift energy production to periods with high energy demands or high feed-in tariffs to avoid expensive electricity purchases or to avoid cheap electricity sale when production exceeds the on-site demand. Production is also advised when gas production exceeds the capacity of gas storage or combined heat and power units (CHP) to avoid gas flaring. It is our full scale experience that biomethane production from energy-rich substrate can onset http://dx.doi.org/10.1016/j.biortech.2015.06.147 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +43 512 3929290; fax: +43 512 39292939. E-mail address: Ebner@alps-gmbh.com (C. Ebner). Bioresource Technology 194 (2015) 389–393 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech