| Sludge disposal has become an acute problem in sewage treatment plants. Though incineration, composting, bricking making, cement production industries can use sludge as feed materials, landfill disposal is still considered to be an alternative due to its low disposal costs.Sludge is a biosolid and biodegradable. It will be stabilized gradually and steadily as soon as being placed in a landfill. It is considered that a recycle of "landfill - sludge landfilling - sludge biodegradation - aged sludge excavation - re-landfilling of fresh sludge " should be possible, if the stabilization process is explored fully and the time for the sludge stabilization is drawn.However, the operation of landfilling of sludge is found quite difficult as it contains over 80% moisture and extremely low strength and permeability. The objectives of this work were to transform the geotechnical properties of sewage sludge by mixing additives such as aged refuse excavated from long-term closed refuse landfill, coal ash, demolition wastes, and soil, followed by the exploration of the landfill stabilization process of the original and transformed sludge. Both bio-sludge generated from a biological treatment plant of sewage and chemical sludge generated from a primary treatment plant of sewage using AlCl3 as main coagulant were tested.This study investigates the geotechnical properties of the dewatered sewage sludge both generated from chemically enhanced primary treatment and biological treatment in Shanghai. The result shown that void ratio and compressibility of chemical sludge was lower and shear stress and bearing capacity was better than that of bio-sludge. Moisture has significant effect on geotechnical properties of sludge, and has negative relevant with shear stress and fiction angle. Sludge could meet the requirement of shear stress and bearing capacity no less than 50kPa and 25 kPa respectively when its moisture content decreased to 64%.side slope of 12.6°may be constructed without causing sliding problems for chemical sludge during landfill operation. The malodor could decrease to 3rd grade. Consolidation coefficient under 50 kPa was calculated to be 0.010726 cm2/s and the compressive coefficient under 50～100 kPa was 1.16.It was found that the geotechnical properties of sludge for landfill operation should meet the requirements of the following specifications: bearing capacity over 50kPa, vane shear strength over 25 kPa, permeability of over the magnitude of 10-6～10-5cm/s, and moderate odor. In this case, the sludge should be mixed with the additives and ratios of additives to sludge should be over 6:10、9:10、7:10 and 9:10 for coal fly ash, aged refuse, demolition wastes and soil, respectively.The annual production rate of leachate from fresh sludge with moisture of 80% was about 44L/t. The biodegradability of leachate decreased and annual production of leachate increased to 287 L/t in the presence of 33% aged refuse in the mixtures. Meanwhile, it was observed that the presence of the additives can accelerate the stabilization of sludge. Pollutants concentrations in leachate from mixtures with additives were always lower than those of the fresh sludge.A field lysimeter with 36 t of fresh sludge was established and its long-term stabilization process was monitored. It was found that pollutants concentrations in the leachate from the field-test lysimeter were significantly lower than those of the laboratory lysimeters. Moreover, it seemed that the degradation in the field lysimeter was faster. After 498 days of biodegradation, the organic matters decreased by 67.1 %、61.6%、30.5%and 51.4% for the laboratory-scale lysimeters of bio-sludge, bio-sludge+aged refuse, chemical sludge and field test lysimeter of chemical sludge, respectively. The biodegradation of the bio-sludge was observed to be faster than the chemical sludge.The humidification of sludge in the lysimeters was studied. It was found that humus, humic percentum and humic index slightly increased during the stabilization process. Molecular weight of humic acid decreased in early days revealing its mineralization process, and increased with time in metaphase and anaphase showing its maturation process. Fulvic acid decomposed in prophase and composed in metaphase and anaphase. Humic percentum decreased in the sequence of the laboratory-scale lysimeters of bio-sludge+aged refuse, bio-sludge, field test lysimeter of chemical sludge, and chemical sludge, while humic index decreased in the sequence of the laboratory-scale lysimeters of bio-sludge, bio-sludge+aged refuse, field test lysimeter of chemical sludge, and bio-sludge.Biological toxicity faded during the stabilization of sludge, slow in prophase and rapaid in metaphase and anaphase, according with logarithm formula. The germinating index of aged refuse+biosludge, biosuldge, chemical sludge and field-test chemical sludge can reach to 80% at 520 d、612 d、650 d and 670 d of placement, respectively. Mixing of aged refuse could reduce the toxicity of the biosludge and increased the germinating index.Settlement of simulated landfills could be classified into the initial stress settlement occurred mainly in the first year and secondary degradation settlement in the following years. The predicted ultimate settlement of the chemical sludge in field-test lysimeter, chemical sludge, biosludge and aged refuse+biosludge in the laboratory scale lysimeters were evaluated to be 12.33%、20.54%、13.77%, and 26.57% of the initial sludge height, and the corresponding settlements in first year were 98.9%、94.4%、97.4% and 98.9% of the ultimate settlement, respectively. 99.99% of the ultimate settlement may reach at 870 d, 1170 d, 995 d, and 762 d of the placement, respectively. Consolidation coefficient of field-test lysimeter was calculated to be 0.00215 cm2/s.The equations among the settlements, pollutants concentrations in leachate （COD, NH3-N, etc）, chemical compositions in the sludge （organic matters, BDM, VM, TS, moisture, etc） and the placement time, were established. Taking the leachate discharge standard and the BDM in soil as reference, it was estimated that resultant biodegraded sludge can be regarded as the stabilized sludge or aged sludge which can be excavated and recycled after around 3.2 to5.8 years placement, when the BDM in the sludge decreased from around 30～60% in sludge to 4.76% in soil and COD to 100 mg/L in the leachate. If the aged sludge was to be used for planting, 1.4 to 1.8 years at lest would be needed. |
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