1. Introduction
Research into the development of alternative energy sources has been increasing as a result of the non-renewable nature of fossil energy sources and recent environmental challenges (Alburquerque et al., 2012). Production of biogas through anaerobic digestion of organic waste materials is on the frontline of this alternative energy research. The major products of anaerobic digestion are biogas and digestate. Digestate comprises microbial biomass, semi-degraded organic matter and inorganic compounds, and therefore can be used as soil conditioners on farmlands (Alburquerque et al., 2012). It contains more readily available nutrients than the undigested products which make it better for crops fertilization (Goberna et al., 2011; Garfi et al.,
2011; Lansing et al., 2010).