Study On The Effect Of Combined Earthworm-rice Husk Biochar On The Composting Characteristics Of Municipal Sludge

In this paper,urban sludge was composted with earthworms and rice husk biochar to study the physical and chemical properties of sludge in different composting treatment groups and the changes of total heavy metals,effective state and forms over time during the composting process.The results of the study are as follows.(1)Earthworm avoidance experiments show that the addition of rice husk biochar to sludge is beneficial to earthworm survival to a certain extent,and earthworm adaptability increases with the addition of rice husk biochar in a certain range,with the earthworm adaptation factor reaching a maximum at 6%rice husk biochar addition.(2)The pH of each treatment group increased and then decreased,the electrical conductivity(EC)increased gradually and the total organic carbon(TOC)decreased and then increased with the composting process in all treatment groups.After the composting,the pH and total nitrogen(TN)of the sludge compost alone decreased and the EC,TOC and total potassium(TK)levels increased.The pH of vermicomposting increased and then decreased,TN losses increased and EC,TOC and TK increased further.The addition of rice husk biochar reduced the pH of the vermicomposting and the pH increased with the increase in the proportion of rice husk biochar added;the addition of rice husk biochar further reduced the EC and TOC of the vermicomposting and the pH and TOC decreased with the increase in the proportion of rice husk biochar added;the addition of rice husk biochar reduced the TN and increased the TK of the vermicomposting and both TN and TK increased with the increase in the proportion of rice husk biochar added.(3)The total amount of heavy metals Mn and Zn increased in each treatment group,while the total amount of Ni and Pb increased and then decreased with the composting process.When rice husk biochar was added to the vermicomposting,the total amount of Ni,Zn and Pb decreased,except for the increase in the total amount of heavy metals Mn,with Mn and Ni at the lowest level when 2%rice husk biochar was added and Zn and Pb at the lowest level when 8%rice husk biochar was added.(4)The effective state content of heavy metals Mn and Pb increased in each treatment group,while the effective state content of Ni and Zn decreased and then increased with the composting process.The effective state content of heavy metals in vermicomposting was higher than that in sludge composting alone,and decreased with the addition of rice husk biochar.(5)The proportion of heavy metals in the exchange and carbonate bound states decreased,the proportion of organic bound and residue states increased,and the proportion of heavy metals in the Fe-Mn oxidation,organic bound and residue states increased with the composting process in the sludge compost alone.The addition of earthworms helps to reduce the proportion of exchange and carbonate bound states of heavy metals other than Mn.Except for Mn and Zn,where the proportion of exchange state increases,Ni and Pb,where the proportion of exchange state decreases with the composting process.And the proportion of organic bound and residue states of Mn and Zn decreases,while Ni and Pb increase with the composting process.The addition of rice husk biochar to the vermicomposting was beneficial to the transformation of each heavy metal form to the organic bound and residue states,achieving passivation of heavy metals,and the best passivation effect was achieved at 8%rice husk biochar addition.(6)The single-factor contamination index method,the Nemerow integrated contamination index method and the ground accumulation index method were used to evaluate the contamination of sludge with heavy metals in each treatment group after composting.The addition of rice husk biochar to the vermicomposting helped to reduce the single-factor contamination index of heavy metals.The Nemerow composite contamination index,the ground accumulation index and the composite potential ecological risk index decreased gradually with increasing proportion of rice husk biochar addition and was minimised at 8%rice husk biochar addition.The potential ecological risk assessment found that the potential ecological risk of heavy metals Mn,Ni,Zn and Pb in each composting treatment group was in descending order:Ni>Pb>Zn>Mn.Figure[15]Table[17]Reference[129]...