| The sludge production has increased obviously following with the large-scaleconstruction and commissioning of urban wastewater treatment plant. The basicprinciple of sludge treatment and disposal are reduction, harmlessness, stabilization andsafety. Development of low carbon and environmental sludge treatment and disposalprocess is a research focus in sewage treatment industry at home and abroad. Usingsewage sludge and straw (expansion agent) as raw materials, Vermicomposting wasapplied to stabilization and harmless treatment of sewage sludge by employing Eiseniafetida with aerobic composting as pretreatment process in this study. The change ofsludge property and its influencing factors were investigated and vermicompost qualitywas analyzed, too. Then the land application test for vermicompost was carried out witha pilot-scale. The variation of total content, chemical speciation and migration ability ofsoil heavy metals and soil enzyme activity in different vermicompost application ratewere studied through hierarchical cluster and categorical principal components analysis.A reasonable application rate for vermicompost was proposed based on thecomprehensive evaluation result of heavy metal pollution level and its potentialecological risk. The principal conclusions were obtained as follows.①Aerobic composting could reduce sludge water content and organic mattercontent from73.0%and44.6%to52.1%and35.6%, respectively. The volume andpathogen content of sludge has decreased obviously. These changes have createdfavorable conditions for vermicomposting. Results showed that vermicomposting coulddecrease compost pH from7.4±0.2to6.5±0.1, TOC from205.8±7.6g/kg to154.7±13.1g/kg; and also increase Electric Conductivity(EC) from1.16±0.10ms/cm to1.40±0.12ms/cm, and TN, TP and TK from13.1±0.7、12.5±0.3and2.7±0.2g/kg to17.9±1.3、17.3±1.4and3.7±0.4g/kg, respectively. The growth rate of these nutrients issignificantly higher than that of control group. The content of available nitrogen,phosphorus and potassium in vermicompost increased substantially. In addition,Germination Index(GI) of vermicompost could reach more than85%. Thevermicompost, whose pathogens and heavy metal contents could meet the allowablelimit for landscaping land improvement, has a carbon-nitrogen ratio equal to8.7±1.3.Environmental Scanning Electron Microscope(ESEM) analysis showed thatvermicompost has loose structure, high porosity and abundant granular structure. It is seemed that vermicomposting of sewage sludge could obtain a good harmless effect andenhance its recycling value significantly.②The significant effect of earthworm stocking density, material water contentand their interaction on vermicomposting was in the order of stocking densities> watercontent> interaction. The first two factors play a dominant role in process among them.The optimal processing conditions for vermicomposting of sewage sludge were:stocking density was2.5kg/m~2and moisture content was60%.③Relative contents of heavy metals in vermicompost were higher than initialmixtures due to mass loss with decomposition and mineralization of organic matter byEisenia fetida and microorganism but its total content had no significant change. Theheavy metals content level was in the order of Zn>Cu>Pb>Cr>Ni>As>Cd>Hg. Mostheavy metals were found exist in more stable forms, including oxidizable speciation andresidual speciation. Therefore, vermicomposting could effectively reduce bio-toxicity ofheavy metals through passivation or curing. Eisenia fetida showed a strong enrichmenton Ni and Cd and its average corresponding enrichment coefficient were2.18and1.46.While Eisenia fetida exhibits a stronger absorptive capacity on Zn and Cu and itsaverage absorbing capacity reached to472.37and103.95mg/kg, respectively.④An application rate of20-60t/hm~2of vermicompost would decrease soil pH,but increase TOC、TN、TP and TK from25.7±0.6,1.5±0.01,0.5±0.01and14.3±0.4g/kg to2.7±6.6、2.0±0.2、0.7±0.1and16.4±0.8g/kg, respectively. The humic acid,phosphate, ammonium salt, sylvite and other inorganic ions contained in vermicompostwere main factors which improve soil fertility.⑤Application of vermicompost could significantly enhance whole activity levelof soil enzyme. Including catalase and invertase activity, which rise from9.5±0.1and1.6±0.1mL/g to9.5±2.3and2.0±0.2mL/g; and urease, alkaline phosphatase andaryl sulfatase activity, which rise from100.0±7.7,16.2±2.2and18.5±2.4μg/(g.h) to115.5±15.3,28.7±6.2and31.8±4.7μg/(g.h). The soil nutrient increase and microbialactivity enhanced were main factors which improved soil enzyme activity. It has beendemonstrated that soil enzymes have a higher aggregation and changes in aryl sulfataseactivity could characterize variation of most oxidoreductase and hydrolase activity withan application rate of20-60t/hm~2. The catalase activity was obviously inhibited by Cuand alkaline phosphatase activity suffered certain degree of inhibition originated fromCd, while Zn showed activation on aryl sulfatase, polyphenol oxidase and catalaseactivity. These influences would become more and more evident with the increase of vermicompost application rate.⑥An application rate of20-60t/hm~2of vermicompost would increase content ofsoil heavy metals while they were still within the allowable limit of secondary standardof Soil Environmental Quality Standard. The content of weak acid extraction andreducible speciation had no significant change. Migration ability of heavy metals inplant was in the order of Zn>Pb>Cu>Ni>Cd>As>Cr>Hg; while these elements withhighly biological toxicity, such as Hg、Cd and As have a weak migration ability andtheir migration coefficients were lower than0.4. Soil heavy metals have strongaggregation and changes in Zn and Cu content could characterize variation of mostheavy metals.⑦Potential ecological risk degree of soil heavy metals was in the order ofCd>Hg>Cu>As>Pb>Ni>Zn>Cr after application vermicompost a year, but overall riskcould still maintained a lower level and this was similar to that of control group. It hasbeen confirmed that Cd was the major contribution factor on potential ecological riskdue to its higher background value. The tested soil may suffer slight heavy metalpollution when application rate of vermicompost exceed50t/hm~2. It is suggested thatvermicompost is suitable for alkalinity or alkalescence soil but should not be used forplants in food chain. Finally, an application rate equal to20-30t/hm~2was proposed asreasonable dosage based on comprehensive risk assessment. |
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