Grass for biogas production: The impact of silage fermentation characteristics on methane yield in two contrasting biomethane potential test systems J. McEniry a , E. Allen b , J.D. Murphy b , P. O’Kiely a, * a Animal & Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, Co. Meath, Ireland b Bioenergy and Biofuels Research Group, Environmental Research Institute, University College Cork, Ireland article info Article history: Received 14 September 2012 Accepted 5 September 2013 Available online 29 October 2013 Keywords: Grass silage Additive Preservation Aerobic stability Anaerobic digestion abstract Grassland biomass is likely to be harvested and stored as silage to ensure a predictable quality and a constant supply of feedstock to an anaerobic digestion facility. Grass (Phleum pratense L. var. Erecta) was ensiled following the application of one of six contrasting additive treatments or a 6 h wilt treatment to investigate the effects of contrasting silage fermentation characteristics on CH 4 yield. In general, silage fermentation characteristics had relatively little effect on specific CH 4 yield (from 344 to 383 Nl CH 4 kg 1 volatile solids). However, the high concentrations of fermentation products such as ethanol and butyric acid following clostridial and heterofermentative lactic acid bacterial fermentations resulted in a numerically higher specific CH 4 yield. While the latter fermentation products of undesirable microbial activity have the potential to enhance specific CH 4 yield, the numerically higher specific CH 4 yield may not compensate for the associated total solids and energy losses during ensiling. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Grassland biomass can be an excellent feedstock for biogas production and will likely be the dominant feedstock for on-farm anaerobic digestion in temperate Northwest Europe [1,2]. In order to ensure a predictable quality and a constant supply of grass to an anaerobic digestion facility, it is most likely to be harvested and stored as silage [1]. The main objective of ensilage is the efficient preservation of the energy content of a crop and this is achieved by the combination of an anaerobic environment and the bacterial fermentation of sugar. The lactic acid produced in the latter process lowers the pH and prevents the proliferation of spoilage microor- ganisms [3]. However, fermentation under farm conditions is not a controlled process and silage fermentation characteristics will depend on the nutrients fermented and the microorganisms responsible [4]. Silage which has undergone a desirable fermenta- tion is generally characterised by a low pH, high lactic acid content and low concentrations of butyric acid and ammonia-N [5,6]. Furthermore, the ensiled energy is almost completely recoverable in a closed lactic acid dominant fermentation [7]. In contrast, and despite the negligible loss of energy, the production of ethanol by yeast during fermentation is undesirable because no acidification occurs [8]. Similarly, under sub-optimal ensiling conditions a sec- ondary clostridial fermentation may lead to considerable total solids (TS) and energy losses due to extensive production of CO 2 and H 2 from the fermentation of lactate and hexose sugars [3]. A range of fermentation products formed during ensiling can influence specific CH 4 yield. For example, the specific CH 4 yield of some silages has been reported to be higher than for the original parent material due to the proportionately greater loss of TS than energy during the formation of fermentation products such as ethanol and 1,2-propanediol [9e11]. Similarly, a more hetero- fermentative lactic acid bacteria (LAB) fermentation with higher concentrations of acetic acid has been reported to enhance CH 4 production [12,13]. However, in all these cases, the potential losses occurring during fermentation must also be taken into account in order to make a more complete assessment of the overall effects of silage fermentation. However, in general, only a limited number of studies [10,12,14] have provided information on the impact of grass silage fermen- tation characteristics on CH 4 production. Thus, the objective of this study was to investigate the effects of contrasting grass silage * Corresponding author. Tel.: þ353 46 9061100; fax: þ353 46 9026154. E-mail addresses: josephmceniry@gmail.com (J. McEniry), padraig.okiely@ teagasc.ie (P. O’Kiely). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.09.052 Renewable Energy 63 (2014) 524e530