Size can be a very important parameter in the rate of biogas production rather than for the ultimate biogas production from a given substrate. Therefore, the size of the substrate particles is considered to be fundamental for kinetic studies (Sanders 2001; Vavilin & Angelidaki 2005) rather than for the BMP determination. However, some literature (Palmowski & Muller 2000; Perez-Lopez et al. 2005) showed how the BMP determination can be influenced by this parameter and a standardization of the particles size can be recommended to increase the reproducibility of the BMP test for some heterogeneous substrates (PabonPereira et al. 2009).