| With the high popularity of urban sewage treatment in China and the rapid rise in the scale of treatment,the production of urban sludge has also increased dramatically.The untreated urban sludge continuously enters the environment,which not only brings a large amount of strategic and serious waste of land resources,but also generates huge disposal costs due to its huge volume,and seriously threatens the ecological safety of the surrounding environment due to the difficulty of effective management of the continuously released high concentration leachate.Therefore,the safe disposal of sludge has become an important bottleneck for wastewater treatment in China because the problem of harmless,stabilization and resourcefulness of sludge has not been effectively solved yet.The resource utilization of urban sludge is seriously hampered by the presence of many heavy metals(Cd,Pb,etc.)in urban sludge while containing a large amount of nutrients(C,N,P,K,etc.)required for plant growth.For this reason,it is of urgent and practical significance for environmental protection and resource development to conduct timely research on the efficient removal of heavy metals from municipal sludge and the adequate resource utilization of nutrient elements.The existing sludge heavy metal treatment technology only considers the treatment effect on heavy metals,and the nutrient endowment and the growth and development of plant seeds in sludge under the influence of chemicals and the nature of the sludge itself have unclear problem mechanisms and unknown degree of influence.For this reason,this study was conducted with a typical urban wastewater treatment plant sludge in Guizhou province as the research object,and the main line of research was " sewage treatment → sludge disposal high concentration of sludge leachate treatment → formation of more sludge again ".The study aims to provide theoretical basis and data support for the effective removal of heavy metals and resource utilization of nutrients in urban sludge,and finally to improve the efficient resource utilization technology of urban sludge in China.The main research results obtained are as follows:(1)The ultrasonic enhanced removal process can effectively remove some heavy metals from sludge.The heavy metal content in the sludge was effectively reduced by a single ultrasonic enhanced removal,but the content of the mobile fraction(acid extractable state,F1)was significantly increased.The best removal rates of Pb,Cd,Zn,Cu and Cr in the sludge removed by Ethylene Diamine Tetraacetic Acid(EDTA)in a single pass were 64.14%,30.00%,27.43%,18.18% and 2.12%,respectively;the best removal rates of Pb,Cd,Zn,Cu and Cr in the sludge removed by Critic Acid(CA)in a single pass were 64.14%,30.00%,27.43%,18.18% and 2.12%,respectively.The best removal rates of Pb,Cd,Zn,Cu,and Cr from the sludge removed by Critic Acid(CA)were 40.84%,26.42%,31.02%,1.1%,and 14.71%,respectively.Moreover,the proportion and content of F1 fractions of Cd,Cr,Zn,Cu,and Pb increased more significantly after the single removal of EDTA and CA.The study showed that compared with CA and EDTA strippers have better removal effect on many heavy metals in sludge,but the removal effect of heavy metals in sludge by single ultrasonic enhanced stripping is still not satisfactory.(2)The number of elutions,elution time and eluent concentration are the key parameters affecting the ultrasonically enhanced elution of heavy metals.Several experimental analyses showed that the ultrasonic time of 30 min,CA or EDTA concentration of 0.4 M or 0.2 M,and the number of elution times of 6 times were the optimal elution parameters for this experiment.Under these conditions,the cumulative removal rates of Cd,Cr,Cu,Pb and Zn by CA could reach 66.18%,30.22%,9.59%,70.89% and 50.38%;the cumulative removal rates of Cd,Cr,Cu,Pb and Zn by EDTA could reach 79.90%,23.45%,39.44%,92.73% and 52.86%,and The F1 fraction content in the sludge after 6 elutions of CA and EDTA was at a low level,and the removal of heavy metals from the sludge with multiple elutions could reach a desirable level.(3)The physicochemical properties of municipal sludge will change to different degrees after elution by ultrasonically enhanced EDTA and CA.The experimental results showed that the p H of municipal sludge significantly decreased(p < 0.05),the percentage of organic matter(OM)content significantly increased(p < 0.05),and the content of effective phosphorus(AP),effective potassium(AK),and effective nitrogen(AN)in the sludge significantly decreased(p < 0.05)after chemical elution by CA and EDTA.In terms of sludge physical properties,the triple ratio of sludge after six elutions by CA was closest to the optimal triple ratio(50:50:25),and the degree of impact on soil physical structure after land use was less.(4)Based on the results of the growth condition of Zea mexicana Schrad L.seeds,CA is more suitable as an agent for urban sludge removal for the purpose of land safe use.The experimental results showed that the germination rate and germination index of Zea mexicana Schrad L.seeds were significantly higher(p < 0.05)in the CAdetreated urban sludge.The antioxidant enzyme activities(CAT,POD,and SOD)of seeds in the CA-treated group were maintained at moderate levels throughout the seed germination period.In contrast,the seed growth conditions in the EDTA-removed municipal sludge were basically similar to those in the non-sonication-enhancedremoved sludge(CK).Based on the analysis of the principal component scores of each index in the germination experiments,the sludge after six successive removals by CA had the highest combined scores for seed growth.(5)CA has the characteristics of low removal cost,significant weight removal effect and good eco-friendliness,the removal cost of CA for harmful heavy metals in sludge is only half of EDTA(per 1 kg of sludge treated),and the removal of Pb,Cd,Zn and Cr in sludge by CA multiple times is good,and it can retain most of the nutrients in sludge,which has some land use value and has a good effect on plant seeds.It is a better choice of chemical removal agent for urban sludge because of its promotion effect on growth and development of plant seeds. |
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