The kinetic behaviour is represented by a zero order model when the inoculum concentration (Xo) is higher than the amount of biomass produced during the activity test (YSX·(So-S)). Therefore, the inoculum concentration should always be high compared to that of the substrate (in term of volatile solids) and the substrate to inoculum (S/I) ratio
should be recognised as one of the major parameter affecting the results of anaerobic assays (Neves et al. 2004).