Impacts of thermal hydrolysis on anaerobic digestion and heavy metal risk in chicken and cow manure

Livestock and poultry manure are commonly used as bio-fertilizer. However, they contain organic matter and unstable heavy metals, which may adversely affect the environment if they are not appropriately treated. Anaerobic digestion is a method of treatment used to achieve a stable and innocuous state of manure, by bioconversion of organics to methane. This study investigated the effect of thermal hydrolysis on the biodegradability of livestock and poultry manure and the risk of heavy metal migration. In this study, a mesophilic condition (40 ℃) was used and the substrates were chicken and cow manure. Thermolysis technology was applied in the study. Furthermore, existing models were used to evaluate the effect of continuous application of chicken manure for 15 years on the accumulation of heavy metals in soil. Thermal hydrolysis was used to enhance organic matter hydrolysis and improve anaerobic digestion efficiency by increasing the solubility of carbohydrates, proteins and biogas production. The results show that the total content of lignocellulose, hemicellulose, and lignin in chicken manure accounted for 25.0% of the total solid (TS), while it was 57.7% in cow manure. After thermal hydrolysis treatment, the lignocellulose degradation rate in chicken manure was about 5.3%, with minimal change in the cellulose content before and after thermal hydrolysis, estimated at 10.4% before and 10.6% after thermal hydrolysis. The degradation rate of hemicellulose was 9.7%, and that of lignin was 16.4%. The degradation rate of lignocellulose in cow manure was approximated at 19.2%, with a higher degradation rate of cellulose, hemicellulose, and lignin than in chicken manure (21.7%, 15.9%, and 20.5%, respectively). In addition, the results showed chicken manure specific biogas yield from volatile solid (VS) increased from 482 to 542 L/kg (12.4%) after thermal hydrolysis treatment while, for cow manure, the specific biogas yield increased from 321 to 421 L/kg (31.2%) after thermal hydrolysis treatment. In addition, Methane (CH₄) content in biogas decreased after thermal hydrolysis treatment. Methane content in chicken manure decreased from 60.2% to 57.6%, and cow manure decreased from 58.7% to 56.6%. The highest to lowest content of heavy metals in both manure was Zn, Cu, Cr, respectively. The Zn, Cu and Cr contents in raw chicken manure were 321.36, 52.04 , and 14.23 mg/kg, while the Zn, Cu and Cr contents in cow manure were 135.79, 40.23, and 15.38 mg/kg, respectively. The total amount of heavy metals in both manures were complying with the guidance of "Control standards of pollutants in sludge for agricultural use (GB 4284-2018)". Meanwhile, these heavy metals can also meet the relevant requirements for heavy metals in organic fertilizers in Regulation (EU) 2019/1009 of the European Parliament and of the Council. The translocation risk of heavy metals in chicken manure could not be reduced solely through thermal hydrolysis. Thermal hydrolysis cannot reduce the translocation risk of heavy metals in soil. However, after anaerobic digestion, heavy metals will transform into more stable residue states. After 15 years of applying chicken manure and cow manure, there was no significant accumulation of heavy metals in the soil. If the loss of heavy metals was not considered, the content of Zn increased from 23.25 to 31.02 mg/kg after 15 years of continuous application, but the concentration was within the standard requirements, while the content of other heavy metals did not increase significantly. It could be concluded that during these 15 years, potential ecological risk index was at a low risk level. Conclusively, thermal hydrolysis and anaerobic digestion could improve the biodegradability of livestock and poultry manure and reduce the risk of heavy metals.