Study On The Stabilization Of Heavy Metals In Aerobic Fermentation Of Municipal Sludge Enhanced By Cattle Bone Meal And The Safety Of Its Products

The stabilization treatment of heavy metals has become a hot research topic due to the fact that the resource utilization of the products of aerobic fermentation of municipal sludge is limited by heavy metals.Bone meal has been widely used in the remediation of heavy metal pollution in water and soil due to the fact that it contains hydroxyapatite.However,the application of bone meal in aerobic fermentation of municipal sludge is rarely reported.Therefore,in this study,cattle bone meal was used as additive to explore its influence on the aerobic fermentation process of municipal sludge and the stabilization of heavy metals.Furthermore,pot experiment was carried out to evaluate the safety of the application of aerobic fermentation products of cattle bone meal and municipal sludge.The main conclusions are as follows:（1）Aerobic fermentation experiments with 0%（A0）,2%（A2）,5%（A5）,10%（A10）and 15%（A15）of cattle bone meal and municipal sludge were carried out.The results showed that cattle bone meal promoted the rise of temperature in the pile and accelerated the evaporation of water and ammonia,with the highest temperatures of 72.6°C and 72.0°C in the A10 and A15treatment groups.Moreover,cattle bone meal alleviated the acidification,promoted the degradation of organic matter,increased the conductivity and effective phosphorus content of the pile to some extent,which indicated that cattle bone meal was beneficial to improve aerobic fermentation efficiency and quality of municipal sludge.（2）At the end of the aerobic fermentation experiment,the contents of Zn,Cr and Pb in the products of all experimental groups were lower than the concentration limits in the Control Standard for Agricultural Sludge Pollutants（GB4284-2018）.The results of heavy metal speciation analysis showed that the addition of cattle bone meal promoted the increase of heavy metal residue form in the aerobic fermentation of municipal sludge and reduced its bioavailability.Among them,and the proportion of residue form of Zn,Cr and Pb in the A10treatment group increased from initial 4.71%to 13.65%,57.53%to 68.77%and45.49%to 88.91%,respectively.Furthermore,the bioavailability factors of heavy metals in A10 treatment group could be reduced up to 23.30%,93.95%and 41.51%,respectively,with a better comprehensive stabilization effect of heavy metals.（3）The redundancy analysis results between speciation distribution of heavy metals and physicochemical parameters in A10 treatment group suggested that available phosphorus was positively correlated with residues form of heavy metal,and organic matter had the greatest correlation with heavy metal speciation distribution.In addition,the residue samples of A10 treatment group before and after aerobic fermentation were characterized by SEM-EDX,XPS,XRD and FTIR and the results showed that phosphorus mainly existed as the form of phosphate in the residual samples.The stabilization of Zn,Cr and Pb by cattle bone meal in aerobic fermentation was achieved by the formation of phosphate precipitates with Zn₃（PO₄）₂·4H₂O,Ca Zn₂（PO₄）₂·2H₂O,Cu Pb₂（PO₄）（SO₄）（OH）,Pb₃（PO₄）₂ and Cr₂P₂O₇.（4）Pot experiment results indicated that the soil fertility has been improved and the growth of Brassica rapa L.has been promoted to a certain extent with the appropriate application of aerobic fermentation products of cattle bone meal and municipal sludge in soil.The contents of Heavy metals Zn,Cr and Pb in soil were all safe and the availability of heavy metals remained relatively stable.Compared with the A0 control group,the contents of heavy metals in the planted Brassica rapa L.decreased to different degrees.It was noteworthy that the levels of Zn in the A10 and A15 treatment groups were 18.49 mg/kg and15.68 mg/kg,the levels of Cr were 0.35 mg/kg and 0.29 mg/kg,and the levels of Pb were 0.19 mg/kg and 0.09 mg/kg,respectively,which were in compliance with the National food safety standard Maximum levels of contaminants in foods（GB 2762-2017）.（5）In conclusion,the aerobic fermentation process was promoted and the stabilization of heavy metals was enhanced effectively with the addition of 10%and 15%cattle bone meal in aerobic fermentation of municipal sludge,and the corresponding products were safe for land application under the conditions of this experiment.