2.1. Origin and Characterization of Apple Pomace, Manure, and Inoculum

AP was provided by the Regional Research and Development Service of Asturias (SERIDA) (Asturias, Spain). The AP was a solid fresh waste obtained after apple pressing for cider production. It was transported to the ITACyL in plastic containers and kept frozen for further use. The AP presented a content of total solids (TS) of 268.7 ± 14.5 g kg⁻¹ and a content of VS of 265.4 ± 14.4 g kg⁻¹.

The SM used was centrate collected after on-farm centrifugation from a pig farm located in Narros de Cuéllar (Segovia, Spain). The collected manure was put in plastic containers, transported to the ITACyL laboratory in Valladolid (Spain), and stored in the laboratory at 4 °C for further use. The mean characteristics of the SM were pH of 7.2 ± 0.1, 41.8 ±7.1 g TS L⁻¹, 30.6 ± 4.5 g VS L⁻¹, 139.1 ± 94.9 g total chemical oxygen demand (TCOD) L⁻¹, 36.2 ± 2.2 g soluble chemical oxygen demand (SCOD) L⁻¹, 4762 ± 62 mg total Kjeldahl nitrogen (TKN) L⁻¹, and 3607 ± 308 mg total ammoniacal nitrogen (TAN) L⁻¹.

The inoculum used was obtained from an anaerobic digester of the municipal wastewater treatment plant in Valladolid (Spain). The inoculum had a concentration of 10.9 ± 0.1 g VS L⁻¹.