The anaerobic systems start-up for solid waste treatment is a fundamental step, especially for those with two phases. It is necessary to know both the waste characteristics and the inoculum conditions. The objective of this work was to study the inoculum-substrate ratio (ISR) influence as a previous step of the start-up of an anaerobic system for the solid waste digestion. During this research spent grain was chosen as residue, working at three different concentrations (7; 13 and 20% w/v), studying the ISR effect in terms of anaerobic degradability (AD) and specific methane productivity (SMP). The initial acetoclastic activities (A0) were calculated based on the equation which describes the methane accumulation during each test. The model constants were also calculated and were adjusted to the experimental data. The results showed that in general the ISR variation has less impact on AD than on SMP. While maximum AD were reached in those tests with high ISR, the greatest values of SMP were with the lowest values of ISR ratio. A low ISR caused a slow hydrolysis, although the methane production was fast. So, during the start-up of a two-phase anaerobic system an elevated ISR would not be necessary in order to reach a good AD and a good intermediate products production, because the hydrolysis and the VFA production must be optimised in the first phase of these systems. While in conventional systems, where phases are together, it is much better to optimise the methane production. The ISR and the SMP indicated which inoculum percentage would be interesting based on the objective of the whole system: methane or intermediate compounds (VFA) production. All this information is important during the conventional anaerobic reactors operation because these tests would show which ISRs avoid inhibition.

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