| With the development of economy and society,the scale of sewage treatment in China has increased year by year,and sludge production has increased accordingly.By June 2021,the production of municipal sludge has exceeded 60 million tons,and the treatment and disposal of sludge has become an urgent environmental problem to be solved.Aerobic composting of sludge is one of the traditional ways of sludge treatment.At present,the sludge compost still faces the following questions,the sludge itself high moisture content,low C/N,need to undertake recuperating join conditioner,microbial reactions in composting process,reeking of nitrogen loss,which resulted in the reduction effectiveness,compost reaction in high temperature can kill some pathogenic microorganisms,the impact on sludge microbial community in general is not clear;Research suggests that heavy metals in the sludge,the sludge compost happens after change of morphology,part of the heavy metals to stable state,the types and degree is not clear,but the passivation of heavy metals in guangxi is high Cd background region,Cd there should not be ignored in the sludge,the sludge on the physical and chemical properties,microorganism may cause harm,further studies are needed,so after deliberation,In this paper,an appropriate amount of Cd was added to the sludge,which was explored in experiment 3.In this paper,municipal sludge of Nanning Mingyang Sewage Treatment Plant is taken as the research object,and the aerobic composting principle is used to treat the sludge,and three groups of different experiments are designed to explore the impact of the volume of the heap on the compost;Composting experiment of different excipients and different proportion of excipients;The changes of physicochemical properties,heavy metal morphology and microbial community in the reactor were induced by the addition of high concentration Cd and bacterial agent.(1)In the composting experiment with different volumes,the composting experiment was carried out with three different volumes,namely 40kg(group A and B),50kg(group C and D)and 60kg(group E),respectively.The 40 kg volume had poor warming and holding effects,which could not meet the requirements of "Stabilization Requirements of Urban Sewage Treatment" for aerobic composting temperature and time.The 50 kg volume met the requirements at temperature,and 50 kg volume reached the requirements.(2)(2)In the composting experiment of different auxiliary materials,sludge composting was matched with the proportion of three auxiliary materials,namely group E(rice bran + straw),group F(pure rice bran)and G(wood sawdust + straw),and the optimal proportion of auxiliary materials was group G(sludge: wood sawdust: Straw =45:10:5,body weight 60kg),group G had a good warming effect,and the nitrogen loss was the least.The other two groups added with rice bran showed nitrogen loss,indicating that sawdust was a better compost excipient than rice bran.(3)In the experiment of adding 3)Cd,(1)(pure sludge),(2)(adding auxiliary materials),(3)(only microbial agents),(4)(only heavy metals)and(5)(auxiliary materials plus heavy metals)were set respectively.The physical and chemical properties of the compost showed that the CEC content was obviously increased by adding microbial agents.Adding heavy metals reduced the contents of humus and CEC in the heap;In the fifth group with microbial inoculum and heavy metals,the total potassium content increased slightly,but other physical and chemical properties did not change significantly.After the experiment,the seed germination rate of five groups of piles was tested,and the GI values were 65.62%,69.75%,71.27%,50.63% and22.68%,respectively.The germination rate of group(5)(microbial inoculum+heavy metal group)was lower than that of group(5)(heavy metal only),which indicated that Cd2+ had a serious stress on seeds,and the addition of microbial inoculum had a negative effect on Cd2+(4)Composting is considered to be an effective method to passivate heavy metals in sludge.In the experiment of adding Cd,the contents of eight heavy metals such as arsenic,cadmium,chromium,copper,lead,zinc,nickel and mercury in the compost before and after composting were determined by Tessier’s five-step continuous extraction method.It is found that heavy metals cadmium,zinc and nickel have high unstable contents in the compost,which are about cadmium(64%),zinc(19.1%)and nickel(14.21%)respectively.After composting,the stable contents of cadmium,zinc and nickel rise to 94%～96%,97% and 96% respectively.The forms of arsenic and chromium do not change to passivation during composting.After composting,about2% of arsenic changes from stable state to unstable state,and the stable content of chromium in sludge is 99.23%,which decreases to about 95% after composting,while mercury,copper and lead are not significantly affected by composting.(5)The microorganisms in the experiment of adding Cd were sampled and identified according to the heating,high temperature,medium temperature and low temperature in the composting process.16 S sequencing was used for prokaryotes,and 18 S sequencing was used for eukaryotes.Species annotations were made for ASV/OTU sequences obtained,and species abundance,Alpha diversity calculation,Venn diagram and petal diagram were analyzed to obtain information about species richness and evenness within samples,and information about common and endemic ASVs between different samples or groups.The species composition of prokaryotes and eukaryotes was analyzed in detail.The data showed that there were many conditional pathogenic bacteria in the pure sludge,such as Proteobacteria and Bacteroidota,etc.After adding the conditioner,the physical and chemical properties of the sludge changed,the temperature rose,and the harmful bacteria disappeared.The increase of the abundance of beneficial bacteria such as Actinobacteriota,the aerobic characteristics of Actinobacteriota and the decomposition of organic matter can greatly improve the ability of material circulation in the soil,and can inhibit the growth of other harmful bacteria,making the bacterial community in the heap to environmentally friendly transformation.Reduce the ecological risk of sludge from the microbial level.And the addition of bacteria agent made Firmicutes replace proteobacteria to become the best and the second best species in part of the time,which further improved the composition of bacteria in the pile.Cd stress significantly reduced the content of Actinobacteriota and Firmicutes in the two groups of compost supplemented with heavy metals,and proteobacteria were more resistant to Cd stress,and became the optimal strain again in some periods.The species composition of eukaryotes showed that Cryptomycota was the dominant phylum in the early and middle stages of pure sludge compost,and Nematozoa Nematozoa Nematozoa was the dominant phylum in the late stage.After composting with auxiliary materials,the abundance of Ascomycota reached more than 90% in the high temperature period.Ascomycota rapidly became the dominant category in the high temperature period due to its cellulose decomposition ability and aerobic performance,and was also the main fungus that promoted the temperature rise of the heap.Its extreme thermophilic performance was one of the reasons for the higher temperatures of the three groups.In addition,the abundance of Ochrophyta increased in groups 4 and 5,because both Ascomycota and Nematozoa were stressed by Cd2+.Comprehensive experiment shows that adding agent used to optimize the structure of microbial communities in the pile body to environmentally friendly,but add Cd2 + in the background as a simulation of the pile body Cd,Cd itself to the environment is not only harm(seed germination rate decline),Cd are also suppresses the pile body is good in eukaryotes and prokaryotes growth,make the ecological risk of pile body stronger. |
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