## 3 Analytical Methods for Characterizing Production Waste  
The objective of this research is to promote sustainability in the industry by utilizing waste from the production process as biomass, thereby enabling both waste reduction and the mitigation of GHG emissions. The analysis of production waste was conducted in the Environmental Gas and Biofuel Laboratory at the School of Engineering, University of Talca. Six 20 kg samples were collected in the facilities in plastic containers filled with non-commercial-grade pet food: three for dogs and three for cats. The samples were ground into fragments smaller than 5 mm using an IKA-WERKE mill (model M20, serie S/N, Staufen, Alemania). They were then dried to a 10% wt % moisture content using a [PERSON] oven (model UM500, serie S/N, Shangai, China) until a constant weight was achieved. Precision weighing (0.001 g) was performed using a Mettler Toledo balance (model AB 304, Santiago, Chile). Nitrogen determination was performed by weighing 1 g of the dry sample (Kjeldahl method--AOAC 979.09 [25]--Equip Velp Scientifica, DKL Heating Digester, Europe). Following the ISO 18125 methodology [26], the higher and lower heating values (HHV and LHV) were determined by weighing 1 g of the dry sample and analyzing it with a Cal2k ECO bomb calorimeter (DDS Calorimeters, Randburg, South Africa), calibrated using certified benzoic acid (Merck, Saint Louis, MI, USA). The determination of ash content was performed in a Vulcan furnace (model A-550, serie S/N, New York, NY, USA) following the ISO 18122:2015 methodology [27]. The ash content was calculated as the proportion of ash in the sample relative to the original sample weight, expressed as a percentage. X-ray fluorescence (XRF) was employed to determine the qualitative elemental composition of the pet food samples. A sample capsule was prepared for each type of ash in triplicate using 2 g of dry, ground, and sieved sample (mesh size 1.6 mm). The equipment used was a Thermo Scientific Scientific (model Niton XL3t GOLD+, Billerica, MA, USA) analyzer, which provided real-time measurements processed in an MS Excel spreadsheet. To determine the quantitative elemental composition, an Atomic Absorption Spectrometer (AAS) by Analytik Jena (model NovAA 800, serie S/N, Jena, Germany) was used. It was equipped with a flame, graphite furnace, and hydride generation systems (HS 60). For each ash type, 2 g of sample was analyzed in triplicate. Samples were dry, ground, and sieved (mesh size 1.6 mm). Digestion was carried out using 7 mL of nitric acid (65% v%) (Winkler, Santiago, Chile) and 1 mL of hydrogen peroxide peroxide (30% v%) (Winkler, Santiago, Chile) at 60 \({}^{\circ}\)C under a gas extraction hood using a Labtech EH hot plate for 8 h. After digestion, samples were dried and reconstituted in HPLC-grade water (Winkler, Santiago, Chile) to a final volume of 50 mL. Metal standards supplied by Merck and analytical-grade reagents were used. Concentrations of metals were determined using flame and graphite furnace techniques, while arsenic, selenium, and mercury were analyzed using a hydride generator with a 4% NaBH4 modifier. To determine volatile organic compounds (VOCs), a thermal desorber coupled with a gas chromatography-mass spectrometry system (TD-GC-MS, Milan, Italy) was used. For this analysis, 2 g of each sample was placed on a burner. Using a Markes Easy VOC LP-1200 suction pump, LLantrisant, UK 10 mL of gas-phase sample was collected in adsorption tubes ([PERSON] C2-BAXX-5315 for odor and sulfur; C6/7-C30 for thiols and mercaptans, LLantrisant, United Kingdom). These were introduced into a cold trap thermal desorption injector (Markes Unity-xr, LLantrisant, United Kingdom). Gases were extracted with helium (Air Products Indura, Talca, Chile) and passed through a thermal trap set at 300 \({}^{\circ}\)C for one minute. The gases were then cooled to 20 \({}^{\circ}\)C and reheated to 300 \({}^{\circ}\)C for an additional 5 min.