Towards optimization of the total solid content in high-solid (dry) municipal solid waste digestion H. Benbelkacem a , J. Bollon a , R. Bayard a , R. Escudié b , P. Buffière a,⇑ a Université de Lyon, INSA-Lyon, Laboratory of Civil and Environmental Engineering, 9 rue de la Physique, F-69621 Villeurbanne Cedex, France b INRA, UR050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne F-11100, France highlights  Pilot scale results for high solid anaerobic digestion are presented.  The influence of the total solid content was investigated in continuous mode.  At 22% inlet total solid, solid segregation took place.  At higher solid content, good mixing was achieved but poorer methane production.  An optimum for total solid in the reactor can be found between 20% and 22%. article info Article history: Received 10 November 2014 Received in revised form 17 February 2015 Accepted 10 March 2015 Available online 17 March 2015 Keywords: Anaerobic digestion High solids Total solid content Biological activity Mixing abstract The influence of the total solid content on the performance of high-solids waste digestion has been inves- tigated on a 50 L reactor. The feedstock was residual municipal solid waste. The reactor has been operated in semi-continuous mode during 3 periods at various total solid content in the inlet (22%, 26% and 30%). Steady-state results showed that the biogas and methane production was slightly lower at 30% TS in the inlet, with an increase of the volatile fatty acid concentration. Nevertheless, solid segregation occurred more likely at low inlet TS (22%). Referring to the volumetric biological activity, it appeared that the opti- mal TS in the reactor is closed to 20–22%. In our case, this would correspond to an inlet TS concentration of 26–28%. At such TS content, the segregation effect would be limited and the biological activity is kept at a high level. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Due to the European Landfill Directive (2006/12/CE), the total quantities of disposed biodegradable waste has to be strongly reduced by 2016. Biodegradable waste means any waste able to be decomposed aerobically or anaerobically. The separate collec- tion of biodegradable waste have permitted to develop biological treatment options, like anaerobic digestion and/or composting. However, the separate collection is not so common in EU countries, where sorting only concerns glass and recyclable plastics, paper and cardboard. In this case, the remaining waste is called residual municipal solid waste (MSW). As an alternative to landfilling, resid- ual MSW can undergo mechanical biological treatments (MBT) and anaerobic digestion is one of the techniques that enables to comply with the directive, since a large amount of biodegradable organic matter is converted into biogas during the process. Anaerobic digestion (AD) technologies are thus particularly attractive for the treatment of biowaste and of residual MSW. Common technologies are referred to as ‘‘wet’’ and ‘‘dry’’ processes, depending on the total solid (TS) content (10–15% for wet, 24–40% for dry, as defined by Luning et al. [1]). There have been few investigations about the operation of dry digesters (also called high-solid digesters) up to now, even if they represent today more http://dx.doi.org/10.1016/j.cej.2015.03.048 1385-8947/Ó 2015 Elsevier B.V. All rights reserved. Abbreviations: AD, anaerobic digestion; BMP, biochemical methane potential L CH4;STP kg VS   ; CELL, cellulose (% VS ); COD, chemical oxygen demand on solid material mg O2 g 1 FW   ; FW, fresh waste; HEM, hemicellulose (% VS ); HRT, hydraulic retention time; MSW, municipal solid waste; MS, mechanically sorted; N-NH 3 , ammonia nitrogen (g N kg FW ); OFMSW, organic fraction of municipal solid waste; OLR, organic loading rate kg VS m 3 R d 1   ; RES, residual (% VS ); SOL, soluble fraction (% VS ); SS, source sorted; STP, standard temperature and pressure; TKN, total Kjeldahl nitrogen (g N kg FW ); TS, total solid (% wS ); TS in , incoming total solid (% FW ); TS out , outcoming total solid (% FW ); TS reactor , total solid inside the reactor (% FW ); TVFA, total volatile fatty acids (g COD L 1 ); VFA, volatile fatty acids (g COD L 1 ); VS, volatile solid (% FW ); VS out , outcoming volatile solid (% FW ); a, segregation index (a GB for glass beads, a PB for polyamide beads). ⇑ Corresponding author. Tel.: +33 472 43 84 78; fax: +33 472 43 87 17. E-mail address: pierre.buffiere@insa-lyon.fr (P. Buffière). Chemical Engineering Journal 273 (2015) 261–267 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej