*2.1. Test Samples*

The studied substrate is the residue from barley intended for malt production, hereinafter CrM. To avoid variability in the composition of the substrate, a laboratory-generated residue was used. For this purpose, barley of the *Hordeum distichon* type was used, which undergoes a malting process, keeping the grains moist for 5 days, dried at 40 ◦C for 24 h and then at 60 ◦C for one night.

As an inoculum, and therefore the source that provides methane-based and anaerobic biomass to trigger the biomethanization process, sewage sludge from a Wastewater Treatment

Plant (WWTP) from Madrid, Spain. To perform the Biochemical Methane Potential (BMP) tests it is recommended to use a stable and easily accessible inoculum [24]. Fundamental standards such as UNE-EN ISO 11734 and VDI-4630 [25] recommend using sludge from WWTP. Several authors advise it because of its accessibility and permanence of biomass [26,27], including pioneers in conducting Biochemical Methane Potential (BMP) test in 1979 [28], the following pioneers in the conduct of BMP assays [29]. In particular, in this study it is a granular sludge from an Upflow Anaerobic Sludge Blanket (UASB) reactor, from an agri-food industry sewage treatment plant. This type of sludge, along with its granule agglomeration characteristic makes it resistant to internal or process alterations [30].
