| The combined employment of coal fly ash and sewage sludge for land applicationcan provide a beneficial way for their disposal. Due to the contrasting physical andchemical properties and nutrient contents, land application of both wastes, as amixture, can improve the quality and fertility of soil and production of plant.However, there are also some limitations. Coal fly ash and sewage sludge containsvariable amounts of certain toxic trace elements (e.g. Cd, Cr, Pb, Ni, B and Mo) andhigh soluble salt, and this property may affect the application of coal fly ash andsludge. Therefore, reducing the availability of heavy metals in coal fly ash andsewage sludge is one of the major hot points in the research of land application.According to previous study, there have been few works on improving the soilquality by using a combination of fly ash and sludge. In this study, the feasibility andthe appropriate application of sludge and fly ash, which was used to passivate theheavy metals’ availability, might be confirmed to the cultivation of Manilagrass andpakchois. The main physico-chemical properties and heavy metal contents of thetested soil, sewage sludge, fly ash and the composite soil as well as the transfer andtransformation (before and after the cultivation) of heavy metals in the artificialsoil-plant system were studied. The results could provide a new approach for thepractice of municipal sewage sludge conditioning and coal fly ash utilization. Themain results obtained of this thesis can be summarized as follows:1. Sewage sludge of Xiamen Yundang Sewage Treatment Plant contained abundantorganic substance and nutrients, such as N, P and K as well as some microelementsthat were required for the growth of Manilagrass and pakchois. The concentrations ofheavy metals in sludge and fly ash were far below the control standards for pollutantsin sludge for agricultural use (GB4284-84) and the control standards of pollutants infly ash for agricultural use (GB8173-87), respectively.2. With the increasing application of fly ash in the composites, pH value andavailable P raise significantly. The moisture content, CaCO3, total N, total P, total Kand organic matter of the artificial soil had been significantly reduced with the increasing application of fly ash. After the passivation process, the concentrations ofheavy metals (Cd, Ni, Cr, Cu, Zn, Mn, Fe) in the composites was decreased while As,V and Ti contents were increased compared with sewage sludge. In general, As、Cu、Zn and Sb predominantly existed in non-residual fraction (e.g. exchangeable,reducible and oxidizable fraction) and Cd, Pb, Cr, Mn, Fe, V, Ni were largely presentin the residual fraction in the composites and samples.Compared with sludge, increment of residual fraction of Cd, Mn, Co, Fe, V, Ti, Sbwas higher while non-residual fraction decreased because of the fly ash-sludgeamendment. Application of coal fly ash-sludge resulted in an increase of Ni, Pb, Cu,Zn concentration in the non-residual fraction and a decrease in the residual fraction.This indicated that the decrease of direct eco-toxicity and potential bioavailability toenvironment of Cd, Mn, Co, Fe, V, Ti and Sb, due to the amendment of fly ash andsewage sludge.3. Manilagrass and pakchois were cultivated with the direct application ofdewatered and dried municipal sewage sludge, collected from Xiamen YundangWastewater Treatment Plant, under pot experiment. The effect of sludge applicationon the growth and yield of Manilagrass and pakchois was determined, and the transferand accumulation of heavy metals in the plant-soil system was discussed. In thepre-experiment, appropriate application of sludge could increase the growth and yieldof Manilagrass and pakchois, which achieved maximum value when the sludgecontent was20%and5%, respectively. In the greenhouse experiment, the pots with12%and16%sludge, demoistured by the frame type filter, had the highest yield andnutrient concentrations of Manilagrass and pakchois. In addition, the pots with12%centrifugal sludge had the highest yield and nutrient concentrations of pakchois.4. In the fly ash-stabilized sewage sludge trials, the coal fly ash was mixed withdewatered sludge at the volumetric proportion of0%(F0),10%(F10),20%(F20),30%(F30),40%(F40) and50%(F50), respectively. The coal fly ash-sludge mixtureswere then mixed with the acidic soil at the volumetric proportion of1:4. The resultsshowed that fly ash-sludge indicated a positive effect on the growth of Manilagrass.The pots with14%sludge and6%fly ash mixture had the highest yield and nutrientconcentrations of both Manilagrass and pakchois, including the same result in the centrifugal sludge experiment.5. From the biological and environmental effects of stabilized sewage sludge, itwas the best management that the content of sewage sludge and coal fly ash was14%and6%, mixture was then mixed with soil at1:4(v/v). An addition of12%-16%sludge was the optimum application rate for the growth of plant when only sewagesludge was applied into soil.The present experimental results suggested an economic alternative for the disposalof coal fly ash and sewage sludge through the reutilization of valuable resources inthese solid wastes for agricultural and horticultural uses. |
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