*2.3. Biochemical Methane Potential Tests to Determine de Anaerobic Degradation*

The Biochemical Methane Potential (BMP) test procedure has been developed in accordance with UNE-EN ISO 11734 [31], including a gas chromatograph for the measurement of biogas composition. In order to determine the amount of methane produced the UNE-EN ISO 11734 [31] standard uses manometric methods. The VDI-4630 [25] procedure has been used to transform the measured pressure inside the reactor into the gas flow generated.

The test conditions were the same as those used on the full scale. Digestion is performed under mesophilic conditions, 37 ± 1 ◦C, with a ratio between substrate and inoculum of 1:3 m/v, that is, for 300 mL of UASB sludge, 100 g of CrM residue are added for digestion [25].

Pressure changes inside the reactor are measured daily. Also, a sample of the gas generated in the reactors is extracted daily and analyzed in a gas chromatograph to determine the proportion of methane and hydrogen in the biogas. An Agilent (California, USA) 7820A gas chromatograph (GC) was used with a thermal conductivity detector (TCD), equipped with a Molsieve 5A-CP molecular sieve, and a PoraPLOT Q capillary column. N<sup>2</sup> is used as a carrier gas to detect H<sup>2</sup> more easily. In addition, by using N<sup>2</sup> as a gas to ensure air displacement and anaerobic conditions inside the bottles.

Twenty-one repetitions are made with the aim of obtaining reliable results with the least dispersion possible. Since the substrate is a lab prepared sample, there is no dispersion in the results due to it. The only dispersion variable is the inoculum, which, as discussed, is obtained from a UASB reactor from a WWTP. However, attempts have been made to minimize this dispersion by taking inoculum samples on the same days of the week, when the level of production was similar and the characterization of the inoculum too.
