Research On The Effect Of Exogenous Microorganisms Inoculation On The Performance Of Municipal Sludge Aerobic Composting

With the improvement of the capacity and rate of sewage treatment in China,the yield of sludge was also increased year by year.The water content of sludge was high,and the components of sludge were complex.If a large number of sludge was not treated and disposed reasonably,it would cause secondary pollution to environment.Because the composting posed little harmful effect on the environment and met the sustainable development strategy,composting was the effective way to reach the goal of resource utilization of the sewage sludge.In this study,municipal sludge,sawdust and mushroom residues were used as raw materials for composting.The effect of the additive ratios of conditioner on the efficiency of composting was investigated and the reasonable ratio of materials was determined by measuring physicochemical factors,including temperature,pH value,organic matter,water soluble organic carbon,moisture content,total nitrogen,NH₄⁺-N,NO₃^--N,total phosphorus,total potassium and germination index.Composting was conducted according to the reasonable ratio of materials for composting,and the thermophiles and white-rot fungi were added into the compost in stages.By means of high throughput sequencing technology,the variation of bacterial community structure and the impact of exogenous microorganisms inoculation on bacterial community structure during sludge composting were determined.BCR sequential extraction method was used to analyze the impact of exogenous microorganisms inoculation on transformation of heavy metals during composting process.Meanwhile,the pollution level,potential ecological risk and health risk assessment of heavy metals in the sludge and the product of sludge composting were also investigated.The main conclusions were as follows.（1）Three different ratios（10:2:1,10:3:1,10:3.5:1）of sewage sludge,sawdust and mushroom residues were used for aerobic composting respectively.After 40 days,three treatments could meet the requirements of the composting harmlessness.Heating rate of T1 treatment（sludge: sawdus: mushroom residues = 10:2:1）was the fastest,and its temperature was higher than the other two treatments.The highest temperature of T1 treatment was 68.10℃.After composting,moisture content and organic matter of T1 treatment were reduced more comparing with the other two treatments.The content of N,P,K were also higher in T1 treatment.It is suggested that increasing the additive proportion of sludge moderately could improve the quality of composting,increase the amount of processed sludge,reduce the additive quantity of conditioner and decrease the cost of sludge treatment.（2）Sewage sludge,sawdust and mushroom residues were mixed at 10:2:1 ratios for aerobic composting.Exogenous microorganisms inoculation extended the high temperature duration,decreased the nitrogen loss and accelerated the decomposition and detoxification of the compost.During the entire period of composting,the structure of bacterial community changed significantly.There was low similarity of bacterial community structure among different stages of the same composting,but high similarity was observed in different composting of the same period.The inoculation of microbial agent improved the abundance of bacterial community,increased the proportions of dominant genera in thermophilic phase,but did not change the species of dominant genera.Canonical correspondence analysis showed that pH presented the highest influence on the structure of bacterial community,and that temperature possessed positive correlation with 9 genera.（3）When sewage sludge,sawdust and mushroom residues were mixed at 10:2:1 ratios for aerobic composting,the concentrations of heavy metals in sludge,composting product with inoculation and composting product without inoculation were in the order of Zn > Cu > Cr > Pb > Ni > Cd,and the concentration of heavy metals in sludge were higher than that in the other two composting product.At the end of composting,the proportion of residual fraction of Cu was increased significantly in the composting with inoculation,suggesting that inoculation strengthened the stabilization of Cu.As for Cr,Ni,Pb and Zn,composting is bad for their stability.Through the treatment of inoculation,composting is good for the stability of Cr and Zn,and reduced the loss proportion of residual fraction of Ni and Pb.Therefore,Cr,Cu,Ni,Pb and Zn were more stable in composting product with inoculation.（4）Pollution evaluation of heavy metals suggested that sludge and the product of sludge composting belonged to light pollution and warning limits,respectively.Potential ecological risk assessment of heavy metals showed that sludge and composting product was in a relatively low level.Cd was the major contributor to the comprehensive potential ecological risk of heavy metals,and its potential ecological risks was reduced one level by composting.Health risk assessment of heavy metals suggested that the non-carcinogenic indexes of heavy metals to adults and children were 0.22,1.02 in sludge respectively,implying a potential non-carcinogenic health risk to children.The non-carcinogenic health risks of heavy metals to adults and children did not exist in composting product with or without inoculation.The pollution level,potential ecological risk and health risk of heavy metals in sludge were reduced by composting.