Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization J. Fernández-Rodríguez a,⇑ , M. Pérez b , L.I. Romero a a Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, Spain b Department of Environmental Technology, Faculty of Sea and Environment Sciences, University of Cádiz, Spain highlights  Anaerobic digestion of organic fraction of municipal solid waste has been studied.  The OFMSW comes from non-selective collection systems.  Thermophilic temperature, dry condition and semicontinuous regime have been selected.  The Solid Retention Time (SRT) was progressively decreased to optimize the process.  The methane yield and the organic material removal in each SRT were analyzed. article info Article history: Received 4 November 2013 Received in revised form 31 March 2014 Accepted 18 April 2014 Available online 2 May 2014 Keywords: Dry anaerobic digestion Thermophilic Methane SRT OFMSW abstract The thermophilic temperature has showed operational advantages in the Anaerobic Digestion process (AD). The increasing of the rate of hydrolysis, which suppose an acceleration of the overall process, and a higher hygienization of the final digestate are one of them. Dealing with OFMSW (Organic Fraction of Municipal Solid Waste) from non-selective collection systems involves additional difficulties due to their heterogeneity. The AD process of OFMSW is conducted usually in the range of 20–35% TS (dry AD). The main objective of this study was to determine the optimum Solids Retention Time (SRT) for the dry thermophilic AD of OFMSW operating in semi-continuous regime. Thus, the following SRTs have been tested: 15, 10, 8, 6, 5, 4 and 3 days. The main results have showed that the process is feasible in SRTs lower that those found in literature. Probably the history of the reactor, joined to the lower organic content in the waste, can be a key factor and the progressive decreasing of SRT is a strategy that permits to maintain stable conditions for lower SRT. The best operating conditions for thermophilic semicontinuous systems ranged in 8–5 days, with a yield of 0.33–0.34 LCH 4 /gVS added and a concentration of Volatile Fatty Acids (VFA) around 100 mg HAc/L. The process becomes unstable for SRTs lower than 4 days, showing a clear decrease in methane produc- tivity (lower than 0.2 LCH 4 /gVS added ) and an accumulation of VFA (more than 500 mg HAc/L). In general, SRTs below 4 days were not suitable for single stage of dry AD of OFMSW. Ó 2014 Elsevier B.V. All rights reserved. 1. Background In the last years, waste production has increased at global scale [1]. The high amounts produced today require management mea- sures in order to minimize its impact on the environment. It is esti- mated that the 40–45% is the organic fraction, known as Organic Fraction of Municipal Solid Waste (OFMSW) and it can be valorized both energetically as agronomically by Anaerobic Digestion (AD). The AD or biomethanization is a biological treatment capable to remove the organic matter in absence of oxygen, generating meth- ane and a digestate with similar characteristics to the compost pro- duced aerobically [2]. The process is based on four steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis and it is conditioned by operat- ing variables such as feed rate, the solids content and temperature. Based on the temperature range, different studies have been car- ried out in order to compare the performance of the mesophilic (35 °C) versus thermophilic (55 °C) range of AD of OFMSW [3]. Each one presents advantages and disadvantages. The operation can be carried out at thermophilic range, which is more effective due to http://dx.doi.org/10.1016/j.cej.2014.04.067 1385-8947/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Current address: Department of Chemistry and Soil Science, Faculty of Science, University of Navarra, Spain. Tel.: +34 948 425 600x806271. E-mail addresses: juanafernandez@unav.es, juana.fernandez@uca.es (J. Fernández-Rodríguez). Chemical Engineering Journal 251 (2014) 435–440 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej