Methane production from anaerobic co-digestion of the separated solid fraction of pig manure with dried grass silage S. Xie a,c , G. Wu b , P.G. Lawlor c , J.P. Frost d , X. Zhan a,⇑ a Civil Engineering, College of Engineering and Informatics, National University of Ireland, Galway, Ireland b Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China c Teagasc, Pig Development Department, Animal & Grassland Research & Innovation Centre, Moorepark, Fermoy, Co. Cork, Ireland d Agri-Food and Biosciences Institute, Hillsborough, Co. Down, Northern Ireland, UK article info Article history: Received 6 August 2011 Received in revised form 20 October 2011 Accepted 18 November 2011 Available online 26 November 2011 Keywords: Co-digestion Energy yield Grass silage Organic loading rate Pig manure abstract Anaerobic co-digestion of the solid fraction of separated pig manure (SPM) with dried grass silage (DGS) was evaluated in three identical continuously stirred tank reactors (CSTRs) at 35 ± 1 °C. The feedstock contained 20% DGS in CSTR1, 30% DGS in CSTR2 and 40% DGS in CSTR3 on a volatile solids (VS) basis. Organic loading rates (OLR) of 1.0, 1.5, 2.0 and 3.0 kg VS/m 3 /d were studied and it was found that the OLR affected the digester performance more than the DGS proportion in the feedstock. Tripling the OLR increased volumetric methane yields by 88% and decreased specific methane yields by 38%. At the OLR of 3 kg VS/m 3 /d, post-methane production potentials of digestates ranged from 38% to 41% of total methane production potentials of the feedstock. An energy yield estimation on a 654-sow pig unit showed that 268–371 MWh/a electricity and 383–530 MWh/a heat would be generated. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In the European Union (EU), pig farming is a major agricultural industry. In Ireland, the pig production is the third most important agricultural sector and contributes 6% to the gross agricultural output (Martin, 2007). It is estimated that 3.1 million m 3 of pig manure (PM) is generated in Ireland each year. Traditionally, PM has been used as fertiliser for grass and other crops through land spreading. However, environmental legislation, such as the EU Ni- trates Directive, has placed constraints on the land application of manures (S.I. No. 610, 2010). PM has the potential to produce bio- gas through anaerobic digestion (AD). When compared with tradi- tional land application of raw PM, AD of PM has a number of additional benefits: (i) production of renewable energy; (ii) improvement in the fertiliser value (Ward et al., 2008); (iii) reduc- tion in pathogen load and malodours; and (iv) reduction in pollu- tion potential. On mainland Europe AD systems are common. For example, in Germany there are more than 4000 on-farm AD systems and in Denmark there are more than 20 large scale centralised AD sys- tems in 2009 (Raven and Gregersen, 2007; Wilkinson, 2011). How- ever, as biomass to feed large digesters must be transported from the farms, transport costs can be very high. Reducing the cost of transportation, which is one of the important factors determining whether or not anaerobic digestion of pig manure is economically feasible, can be achieved by separating the solid and liquid frac- tions of pig manure and then only transporting the solid fraction for digestion (Raven and Gregersen, 2007). The solid fraction of pig manure produces more biogas per unit volume than raw pig manure due to its higher organic matter content. For these reasons AD of the solid fraction of separated pig manures is carried out in some countries such as Denmark (Møller et al., 2007). Grass silage, due to its high digestible organic matter content, is also an excellent feedstock for AD. Ireland has a very suitable cli- mate for grass production and has 4.3 million ha of grassland in comparison with only 280,000 ha of arable land (Hamelinck et al., 2004). Much of this grass is conserved as grass silage as win- ter forage for ruminant livestock. For AD, grass silage can be used, either as a single feedstock or co-digested with pig manure. Studies have shown that energy crops/crop residues can be co-digested with pig manure (Alvarez and Lidén, 2008; Kaparaju and Rintala, 2005; Lehtomäki et al., 2007). Compared to AD of pig manure on its own, co-digestion of manure with energy crops/crop residues can increase the biogas yield because the crops/crop residues can: (i) help maintain an optimal pH for methane producing bacte- ria; (ii) decrease free ammonia/ammonium inhibition, which may occur in AD of manure alone; and (iii) provide a better carbon/ nitrogen ratio (C/N) in the feedstock (Xie et al., 2011). Xie et al. (2009) found that grass silage has a much higher specific methane 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.11.076 ⇑ Corresponding author. Tel.: +353 (0)91 495239; fax: +353 (0)91 494507. E-mail address: xinmin.zhan@nuigalway.ie (X. Zhan). Bioresource Technology 104 (2012) 289–297 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech