Transformation Behavior Of Heavy Metals In The Sludge Dewatering Process And Its Solidification

Three novel composite conditioners based on skeleton builders (called as F-L, F-S andS-F) have been proposed for deep dewatering of sewage sludge, in order to solve the presentproblem in sewage sludge treatment and disposal. This thesis is focused on the migration anddistribution behavior of heavy metals in the dewatered sludge cake and the filtrate in deepdewatering of sewage sludge with three composite conditioners, geo-property of the sludgecake and landfill adaptability, and the solidification/stabilization of heavy metals in thedewatered sludge cake with solidification reagents, which provides a basis for further sludgetreatment and disposal. The major research results and achievements are listed as follows:1. Influence of skeleton-builder-based composite conditioners on the distribution andspeciation of heavy metals in sludgeTo study the behavior of heavy metals in dewatering, their contents were analyzed inraw sludge, three different conditioned sludge, dewatered sludge cake and filtrate,respectively. The results show that1) the content of heavy metals in the dewatered sludgeand filtrate after conditioning meet the requirement of landfill or discharge into urban sewernetwork;2) the filtrate can recirculate into wastewater treatment plant for central treatment;3) heavy metals still dominantly concentrate in the dewatered sludge cake after conditioningin three novel composite conditioners.2. Study on engineering property in dewatered sludge cake and discussion on schemeof solidification.The engineering property and mechanical strength of the dewatered sludge cake weretested and the possibility was discussed when using red mud, flyash, blast-furnace slag andlime as curing agents.The results show that permeability coefficient of the sludge cake meet the disposalstandard ruled by Ministry of Environmental Protection of People’s Republic of China afterdewatering in three novel composite conditioners. And, eventhough the basic engineeringproperty of dewatered sludge is enhanced, the insufficient mechanical strength needs curingagents to guarantee basic landfill.In system F-L sludge cake, the addition of RM, GBFS and FA increase the plasticity,decrease the maximum dry density and increases the permeability coefficient to a degree, which meets the landfill requirements. Uncalcinated red mud(FL-RM) proves to be the bestcuring agent and the solidification scheme is that the dosage should be10%of dry sludge.In this dosing scheme, the optimum water content is40.9%, the maximum dry density is1.131g/cm3and the permeability coefficient is1.17×10-6cm/s. The unconfined compressionstrength and direct shear strength are586.7kPa and45.4kPa after3days maintenance.In system F-S sludge cake, the addition of Lime, GBFS and FA increases the plasticityand decreases the maximum dry density. Lime is the best curing agent comparing withGBFS and FA and the solidification scheme is that the dosage should be10%of dry sludge.In this dosing scheme, the optimum water content is46.8%, he maximum dry density is1.076g/cm3and the permeability coefficient is1.57×10-6cm/s. The unconfined compressionstrength and direct shear strength are480.1kPa and38.4kPa after3days maintenance.The effect of curing agents in system S-F is not as good as expected and needs furtherstudies.3. Leaching properties of heavy metalsSample FL-RM and FS-Lime with reliable solidification ability were chosen for TCLPleaching test and prismatic leaching test. The results show that the heavy metal content oflandfill leachate is below the upper limit regulated in the samples after4days maintenance.In summary, The thesis discusses the selection of following curing agents, and red mudand lime used as the medium in system F-L and F-S dewatered sludge cake give a referencefor further sludge treatment and disposal.