Removal Of Heavy Metals From Biogas Residues And Their Potentially High Value Utilization

Since the 20th century,due to the rapid development of livestock and poultry breeding industry,a large number of livestock and poultry manures have been produced.These manures contain a large number of heavy metal that have not been digested and absorbed by animals.If these manures are not treated properly,they will have great potential risks to the environment and cause secondary pollution.Composting process is the main to passivate heavy metals and realize the harmless of biogas residue.As an important part of terrestrial ecosystem,earthworm is a very high-quality source of amino acids.Earthworm can also enrich heavy metals in the environment to achieve the purpose of heavy metal reduction.At the same time,Trichoderma,as a useful rhizosphere growth promoting fungus,can directly or indirectly promote the growth of plants,and improve the sporulation ability of Trichoderma is a major research hotspot.In this paper,the biogas residue from a biogas project of a large-scale pig farm in Jiangsu Province was used as raw material for composting experiment to analyze the speciation change characteristics of heavy metals;the earthworms enriched in heavy metals were hydrolyzed to remove heavy metals to explore the removal efficiency of heavy metals;meanwhile,the earthworm hydrolysate was used to promote Trichoderma sporulation experiment to explore the effect of earthworm hydrolysate on the growth of cucumber seedlings The effect of solid fermentation Trichoderma on straw was observed by synchrotron radiation infrared microscopy and scanning electron microscopy.The main results are as follows:(1)The results of high temperature composting of biogas residue showed that the high temperature composting process could concentrate the total amount of heavy metal Cd,and the speciation distribution before and after composting was quite different.Specifically,the proportion of exchangeable Cd decreased significantly,while the proportion of residual Cd increased significantly.The results showed that composting could passivate heavy metal Cd and reduce the availability of heavy metal Cd.(2)Earthworms enriched in heavy metals were treated by alkaline hydrolysis,and the removal rate of heavy metal Cd was up to 92%.Through single factor experiment,the optimum conditions of earthworm alkaline hydrolysis were obtained as follows:temperature,100℃;time,60 min;KOH concentration,0.4 mol/l;material ratio(fresh weight of earthworm and volume ratio of alkaline liquor),1:1.The results of HPLC analysis showed that there were 17 kinds of common amino acids in earthworm hydrolysate,and the total content was up to 30.1 g/L.(3)The optimization experiment of Trichoderma spore production was carried out by adding hydrolyzed amino acid of earthworm,and wheat straw was used as fermentation substrate.The results showed that the spore production of Trichoderma could reach 4.8×10~8CFU/g.The optimal solid-state fermentation conditions were as follows:the amount of hydrolyzed amino acid was 20%;The mass ratio of straw to biogas residue was 3:1;The water content of the matrix is 60-70%;The culture period was 7-8 days.Synchrotron radiation infrared microscopy combined with scanning electron microscopy showed that Trichoderma could effectively degrade the lignocellulose components in straw when cultured with straw.(4)Compared with the control group,the treatment of earthworm hydrolysate could significantly promote the growth of cucumber seedlings.The optimum dilution ratio of earthworm hydrolysate was 300～500 times.Under this dilution ratio,the dry and fresh weight of cucumber seedlings were increased by 66.9%,33.6%,plant height was 61.5%,and stem diameter was 28.7%.The results showed that earthworm amino acid hydrolysate could be used as amino acid liquid fertilizer to promote plant growth.In conclusion,composting can effectively passivate heavy metals,reduce toxicity,enrich heavy metals with earthworms,and has a wide application prospect as a high-quality amino acid raw material.In this study,heavy metals were removed from biogas residue by earthworm composting combined with earthworm alkaline hydrolysis process,and the effect of earthworm hydrolysate rich in amino acids on Trichoderma spore production was studied.The results provide a new idea to solve the problems of difficult removal of heavy metals and low high value-added degree in the process of biogas residue composting.