Enhanced dry anaerobic digestion of swine excreta after organic nitrogen being recovered as soluble proteins and amino acids using hydrothermal technology

Highlights: • About 58% of TN in SE became soluble after HT treatment at 150 °C. • 21% of TN in SE could be recovered as soluble proteins and amino acids at 150 °C. • Intensive proteins and amino acids breakdown occurred at > 170 °C. • CH₄ yield was improved by 51% during dry AD of SE after N nutrients recovery. - Abstract: This study investigated the feasibility of N recovery from swine excreta (SE) as value-added soluble proteins and amino acids using hydrothermal (HT) technology for improving dry anaerobic digestion (AD) performance. The effect of temperature on characteristics and transformation behaviors of N species in SE subjected to HT pretreatment at different temperatures (110–210 °C for holding 30 min) was investigated. After HT pretreatment at 150 °C, 58% of the total nitrogen (TN) content in SE became soluble, with 19.03%, 1.55% and 0.56% of TN being converted into soluble proteins, amino acids and urea, respectively. Temperatures higher than 170 °C favored the breakdown of these proteinaceous compounds, leading to accelerated formation of ammonia. After soluble proteins and amino acids being separated by water extraction, CH₄ yield (322.13 cm³ g⁻¹ of VS_fed) from the solid residue pretreated at 150 °C was improved by 51% in comparison to that from raw excreta (213.70 cm³ g⁻¹ of VS_fed), achieving 3 days' shorter in effective CH₄ production period. HT technology could be an effective pretreatment alternative for the recovery of soluble proteins and amino acids and subsequently enhanced dry AD of SE.