Substrate should be characterised as thoroughly as possible. In particular, the total solids (TS) and volatile solids (VS), chemical oxygen demand (COD), nitrogen and phosphorus should be always determined. Then, the COD/VS ratio, which is typically consistent with the characteristics of a given substrate, can be calculated. Clearly, the determination of COD for solid heterogeneous substrates is always difficult and open to some uncertainty (Buffiere et al. 2006; Raposo et al. 2008).