| Management of sewage sludge has become a worldwide problem. The traditional method of sludge disposal, including landfill, incineration and agricultural use and so on. Recently, many countries including China have developed a series of regulations to forbid the land filling practices due to land consumption and landfilling leachate pollution. Incineration can cause air pollution due to emissions and requires high energy. Therefore, the most effective way of sludge disposal is land use. However, land use without any disposal will adversely affect the soil and its microorganisms, thereby affecting plant growth. It is necessary for sewage sludge to undergo an additional stabilization treatment. Earthworms in terrestrial ecosystems plays a very important position to the stability of the sludge by the earthworm treatment. The use of earthworm in sludge management has been termed as vermistabilization. Vermicomposting is a decomposition process involving interactions between earthworms and microorganisms. During vermicomposting process, important plant nutrients are much more soluble and available to plants than those in the primary sewage sludge.In this paper, investigations were made to explore the effect on sludge physicochemical properties, nutrient content, enzyme activity, heavy metal and its forms after treatment by earthworms (ie, earthworm Eisenia faetida). while the leaching and pot experiments, of sludge and vermicompost applied after the soil nutrients and heavy metals in the soil of the vertical distribution of different proportions after applying sewage sludge vermicompost plant enzymes, chlorophyll and other plant physiological characteristics of the changes of sludge vermicompost on plant accumulation of heavy metals the impact of the changes revealed law of its formation mechanism. Effects of vermicompost substrate on capsicum seedlings was also studied. The main results are as follows:The parameters such as pH, organic carbon, Kjeldahl nitrogen and conductivity decreased after vermicomposting. The contents of NO3--N increased with the time after vermicomposting, while the contents of NH4+-N decreased, both then stabilized. On the whole, TP decreased after vermicomposting, while the available P increased. Potassium change was first increased and then decreased.The results showed that the activities of sludge enzyme had much change after treated by earthworms. The activities of acidic phosphatase and alkaline phosphatase increased in the early stage of vermicomposting whereas decreased in the late stage, while the activity of neutral phosphatase kept decrease in the whole stage. The other enzyme activities in sewage sludge had also changed after vermicomposting. The activities of urease increased in the early stage of vermicomposting process whereas decreased in the late stage, while the activity of protease and invertase kept decrease in the whole stage. As a whole, with time, the activities of these enzyme decreased as organic matter contents decreased. A significant decrease of the activity of protease was observed as compared to the other enzyme.The final result indicated the total concentration of heavy metals decreased after vermicomposting. It shows that Cd could be strongly accumulated by earthworm Eisenia faetida. Earthworms can accumulate heavy metals of sewage sludge. The bio-concentration factor of Cd in earthworm was higher than other heavy metals. The reduction in metal content (as compared to initial level) was in the order:32.8%(Pb)>31.5%(Zn)>28.0%(Cr)>23.9%(Cu)>22.9%(Ni)>13.9%(Cd). It is clear that heavy metal loss was caused by the presence of earthworms. The present study showed earthworms can change the bioavailability of heavy metals in sewage sludge. In most of the heavy metals except Pb, the most bioavailable fraction (exchangeable metal) have a significant decrease in the proportion of heavy metal fractions after vermicomposting. Hence, on the basis of present study, vermicomposting may be an appropriate technology for heavy metal remediation from municipal sewage sludge.It was found that the content and low molecular size fraction of DOM in sewage sludge increased after vermicomposting, and the fraction of relative low molecular weight in DOM increased by12%. The FT-IR and fluorescence spectra showed that after vermicomposting, the contents of carboxyl, polysaccharide and aromatic compounds in DOM increased. Furthermore, a few protein-like matter was found in DOM after vermicomposting. The changes in FT-IR and fluorescence spectra for DOM indicated that polycondensation and conjugation unsaturated structures decreased after vermicomposting, and simple organic molecular fraction increased.Under the leaching of sewage sludge, the content of nitrate in different soils were increased in varying degrees, the same as the casts. The application of casts increased more significantly than the application of sewage sludge. The leaching of NO3--N was most likely to result in acid soil. Sludge after leaching, the soil from top to bottom are all expressed as NH4+-N content decreased; for vermicompost, the acid soil from top to bottom performance of NH4+-N content decreased, while NH4+-N contents in middle of the neutral and alkaline were higher than in top and bottom. Overall, the leaching of sludge application in soil NH4+-N content was much higher than the vermicompost treatment, regardless of the sludge or vermicompost, NH4+-N leaching amounts of accumulation are descending:acid soil> neutral soil> alkaline soil. Sludge applied to soil after the leaching of NH4+-N was higher than that of vermicompost application. After leaching, all the phosphorus content of soil increased, and the content of phosphorus in the upper horizons in all the soil was greater than in middle and lower horizons. Whether sludge or cast, the cumulative leaching of phosphorus are descending:acid soil<neutral soil<alkaline soil. Leaching in the rain, the soil Cd content of each layer have no significant difference, and are higher than their respective soil before leaching. The distribution of Pb and Cr in soil layers was irregular.Sewage sludge and vermicompost amended soil, can make the total chlorophyll increased. Under the same proportion, the application of vermicompost increased the chlorophyll content more significant. Peroxidase activity decreased with the proportion of sludge and vermicompost. Comparing the peroxidase activity in the control, it was increased slightly in10%of the sludge and10%vermicompost amended soil, while decreased in varying degrees in other proportion. Application of sewage sludge and vermicompost caused the nitrate content of lettuce increased significantly.The lettuce nitrate content in vermicompost treatment is lower than in sludge treatment. Nitrate content in the20%of the sludge or vermicompost treatments than other treatments. Content of heavy metal in lettuce was in the order:Zn> Cu> Pb> Ni> Cr> Cd. The heavy metal content can affect heavy metals uptake and accumulation of lettuce. The bio-concentration factors for heavy metals were different in the sludge or vermicompost treatments. The application of sludge and vermicompost about20%proportion would improve the BCF of most heavy metals.Vermicompost mixed with vermiculite (8:1, v:v) could be used as substrate in nursery bed to raise capsicum seedlings and its promotive effect on seedling growth was better than the commercial substrate which is usually made from chicken feces and peat. |
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