热水解对鸡粪和牛粪厌氧消化及重金属风险的影响

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

为探究热水解预处理对鸡粪和牛粪在厌氧消化过程中产气效率的提升作用以及对消化残渣中重金属形态变化及潜在生态风险的影响。该研究以鸡粪和牛粪为处理对象,采用热水解处理技术,开展了连续中温厌氧消化试验。考察了热水解对畜禽粪污可生化性和重金属迁移风险。该研究还评估了连续施用厌氧消化鸡粪和牛粪15 a后土壤中重金属的积累量和Hakanson潜在生态风险指数。试验发现,鸡粪中木质纤维素含量占干固体总量的25%,牛粪为57.7%。热水解处理能够增强牛粪中木质纤维素的水解,热水解后木质纤维素的降解率为19.2%;但热水解对鸡粪的木质纤维素水解影响不明显,降解率仅为5.3%。鸡粪挥发性固体(volatile solid,VS)产气率为482 L/kg,热水解后产气率提升了12.4%;牛粪产气率为321 L/kg,热水解后产气率提升了31.2%。仅通过热水解不能降低鸡粪中重金属的迁移风险;而经过厌氧消化后,除Hg和Ni外,其他重金属迁移风险均降低到了低风险水平。牛粪经过热水解和厌氧消化处理后重金属向残渣态转换,但迁移风险没有明显改善。连续施用鸡粪和牛粪15 a后,土壤未出现的重金属积累,且15 a内总潜在生态风险等级处于低风险。该研究结果可为鸡粪和牛粪的能源化、无害化处理以及土地利用应用提供了理论依据和数据指导。

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.