Effect Of Earthworm Mucus Combined With Biochar Composting On The Behavior Of Heavy Metals In Sludge

Sewage treatment plant sludge as a solid waste,Contains pathogenic microorganisms,colloidal substances and metal salts and other harmful substances.However,the sludge also contains rich organic substances and nutrients such as nitrogen,phosphorus and potassium,which are conducive to plant growth and development.Composting is a process of fermentation and decomposition by microorganisms.It can kill pathogenic microorganisms and reduce the migration and transformation ability and bioavailability of heavy metals in sludge.Studies have shown that adding biochar to the composting process can reduce the loss of nutrient elements,enhance the microbial degradation of organic matter,reduce the availability of heavy metals and promote the migration and transformation of heavy metal forms.Earthworm mucus is also reported to be rich in nutrients such as nitrogen,phosphorus and potassium,which are beneficial to plant growth.It can promote the growth and development of plants and improve the enrichment efficiency of heavy metal elements.The protein contained in the mucus can also chelate or complexate heavy metal elements,the stabilization of heavy metals is obvious.However,it is unclear whether the combined composting of biochar and earthworm mucus can affect the heavy metals in the sludge.Therefore,this study first obtained earthworm mucus through different electrical stimulation methods(current:10-50mA,voltage:3-12V).Then through the analysis of the characteristics of the earthworm mucus under different electric shock modes.Choose the earthworm mucus obtained by 5V electrical stimulation and different types(bamboo charcoal,rice husk charcoal,straw charcoal)and different proportions(2%,6%,10%)of biochar combined with sludge composting.In order to explore the effect of combined composting of earthworm mucus and biochar on the behavior of heavy metals in sludge.Through research,main results were summed as follows:(1)Electrical stimulation method can obtain different quality and quantity of earthworm mucus by adjusting current and voltage.20mA current stimulation can promote the secretion of earthworm mucus to the maximum,and at the same time the total nitrogen and total sugar content in the mucus also reach the maximum(p<0.05).5V voltage stimulation can also maximize mucus secretion,and the mucus conductivity(EC),total nitrogen(TN),total phosphorus(TP),total sugar and protein content are also the highest(p<0.05).The total potassium(TK)content in earthworm mucus drops to the minimum in 30mA and 6V electric shock modes,respectively.Correlation and regression analysis found that different electric shock modes have different main influence factors on the quality of earthworm mucus.Current mainly affects the total phosphorus content of mucus,and voltage mainly affects the protein content of mucus.(2)Mucus-sludge,biochar-sludge,and mucus-biochar-sludge combined composting can increase sludge pH,EC and reduce organic matter,TN,TP content.Compared with mucus-sludge composting,biochar-sludge composting further increases pH and EC of sludge and reduces organic matter content.Among them,the most suitable addition ratio for the decomposition of three organic matter is 2%,and the 10%addition ratio has the best adjustment effect on pH,EC,TN,and TP.Compared with biochar-sludge composting,under the same composting conditions(the same type of biochar,at the same ratio),mucilage-10%straw charcoal-sludge combined composting and mucus-6%rice husk charcoal sludge composting are respectively It has the best effect on the improvement of sludge EC and the degradation of organic matter.Mucus-2%bamboo charcoal-sludge combined compost has the best effect on solidification of sludge TN and TP.(3)The total amount of heavy metals Cu,Pb,Cd,and Zn in mucus-sludge composting,biochar-sludge composting,and mucus-biochar-sludge combined composting all increased due to the concentration effect.The effectiveness of heavy metals decreases with the increase of the three kinds of biochar.In biochar-sludge composting,the concentration of Pb and Cd in the sludge is the largest with the addition of 10%bamboo charcoal,2%rice husk charcoal and 2%straw charcoal.Cu has the most significant concentration effect in the ratio of 10%bamboo charcoal,10%rice husk charcoal and 2%straw charcoal;Zn has the most obvious concentration effect when the ratio of 6%bamboo charcoal,2%rice husk charcoal and 2%straw charcoal is added;under the same conditions,Mucus-biochar-sludge combined composting has a more significant concentration effect of biochar-sludge compost.In biochar-sludge composting,2%straw charcoal treatment has the greatest decrease in the effectiveness of Cu,and 10%straw charcoal treatment has the most significant decrease in the effectiveness of Pb,Cd and Zn.Compared with biochar-sludge composting,under the same biochar and the same ratio,mucus-biochar-sludge composting can promote the inactivation of heavy metals in the sludge.Among them,the mucus and 2%rice husk char,2%Straw charcoal,6%rice husk charcoal,and 2%straw charcoal have the largest passivation ranges for Cu,Pb,Cd and Zn,respectively.(4)In sludge composting,mucus-sludge composting,biochar-sludge composting and mucus-biochar-sludge combined composting,mucus-sludge composting significantly promotes the conversion of heavy metals Cu,Pb,Cd,and Zn into relatively stable iron and manganese Bound state,organic bond state and residue state.In the biochar-sludge compost,2%,6%and 10%rice husk char-sludge compost had the best passivation effects on Zn,Cd,and Cu,respectively,and 2%straw charcoal had the best passivation effect on Pb.In the mucus-biochar-sludge combined composting,the conversion effect of earthworm mucus and 6%,10%bamboo charcoal-sludge compost on the form of Cu and Pb is better than that of mucus-compost and other biochar addition ratios and sludge compost treatment group.Comparing the passivation effects of earthworm mucus combined with three kinds of biochar on the heavy metals Cu,Pb,Cd,and Zn,it can be seen that in general,earthworm mucus-rice husk char-sludge compost has the best passivation effect on Cu and Cd,and earthworm mucus-Bamboo charcoal sludge compost has the most obvious effect on Pb passivation,and earthworm mucus-straw charcoal-sludge compost has the best form migration and transformation of Zn.Figure[16]table[20]reference[135]...