| With the acceleration of urbanization, the quantity of sewage sludge generated from the wastewater treatment is increasing rapidly. The sewage sludge, with high moisture content and solid-liquid mixing characteristic, can only be dewatered to 80% (wt) by mechanical methods. Further removal of moisture requires thermal drying or other drying technologies. However, thermal drying processes involve high levels of energy consumption, which increase the cost of disposal considerably and the dried sludge are mashed again by water. Besides, the thermal drying process is of no use to stabilize the heavy metals involved in sludge. For the purpose of energy-saving and resource recycling, this paper proposed an ecomomic and effective composite modifer, in which the active of slag is stimulated fully with several activators, for sludge drying and stabilization. With lower moisture content and a reduction in hydrophilicity and leachability of heavy metals, the sludge drying using the composite modifer proved a harmless stabilization treatment for sewage sludge. Besides, the inorganic components of sewage sludge are adjusted through the addition of composite modifier, thus it can replace part of clay as raw materials for sludge ceramsite production, an effective approach to utilize sewage sludge.Taking the sewage sludge produced by Wuhan city wastewater treatment plant for the research object, the physicochemical properties and pollution characteristics of the sludge was analyzed and the drying and stabilization effect of quicklime and Portland cement were studied. Meanwhile, reference to the utilization of HAS stabilizer in dredged sediment, other two kinds of HAS stabilizer were also used. Furthermore, an innovative composite modifier (ICM), which composed of slag, quicklime, dihydrate gypsum, active agent A and setting accelerator, was proposed and the drying and stabilization effects of it were studied as well. Besides, the drying and modification mechanisms of ICM were discussed with the analysis of XRD and SEM. Finally, experimental work was carried out for combined usage of semi-dried sludge and clay in production of sludge ceramsite, the scheme of ceramsite process was optimized and the performance as well as the internal structure of sludge ceramsite was discussed to analyze the feasibility of utilization of sludge ceramsite. The main contents of this thesis are listed as follows:(1) ICM prescription experimentAs the dosage of setting accelerator is much lower than other components, the ICM prescription experiment consists of the prescription experiment on the four components and the setting accelerator optimization test. The former was carried out by Uniform design (UD) to investigate the content of slag, quicklime, dihydrate gypsum and active agent A on sludge moisture content. Using second-order polynomial stepwise regression analysis, the regression equation was obtained and consequently an optimization model was builded. Then simulated annealing method was used to optimize the model and determine the optimum ratio of the four ICM components. Besides, the optimum ratio was further adjusted in view of economy and other factors. On the basis of the adjusted optimum ratio of four components, the setting accelerator optimization test was carried out with two kinds of setting accelerators and different dosages of them. By comparing the moisture content of stabilized sludge, the better setting accelerator and its dosage were determined. The above results indicated that the optimum ICM consisted of 40% slag, 21.09%quicklime,30.91% dihydrate gypsum,8% active agent A and additionally mixed with 1.2% setting accelerator C.(2) Experimental investigation of effect and mechanism of sludge dryingSewage sludge drying experiment was carried out to investigate the drying effect and mechanism of the optimum ICM. To evaluate the changes in sludge moisture content caused by ICM in relation to its content and duration of stabilization process, mixtures of sludge-ICM were prepared with different ICM contents and cured for different periods. Meanwhile, quicklime, as a conventional binder (or modifier), was used for comparison. On the other hand, the hydration mechanism of optimum ICM and the drying mechamism in sludge-ICM system were discussed by XRD and SEM analysis. The results indicated that comparing with quicklime, the drying effect of ICM-sludge system was much improved and ICM is more suitable for sludge semi-drying treatment with a objective moisture content of 40%-60%. XRD analysis revealed that the sludge drying process took place through the speedy formation of ICM hydration products, especially ettringite, converting most free water of sludge into crystal water.(3) Experimental investigation of the modification effect and mechanism of ICMThe leaching test and water resistance experiment of ICM-stabilized sludge were carried out to investigate the hydrophilicity variation of sludge and the leaching characteristic of stabilized products. The results of water resistance experiment indicated that the water-resistance property of ICM-stabilized sludge was much better than that of quicklime-stabilized sludge. The 3d-cured sludge, with 20% ICM, did not disintegrated after immersing for 7d in water, which revealed that the hydration reaction occurred in sludge-ICM system not only dried the sludge effectively but also changed the structure of wet sludge and improved the water resistance of sludge. Meanwhile, the results of leaching test showed that the main heavy metals from 3d-cured sludge with 10% ICM were detected at much lower concentrations than that from quicklim-based one and the limits set by the standard GB5085.3-1996.(4) Optimization of components and sinter system of ceramsiteAfter investigating the sludge drying effect and mechanism of ICM, it is proposed that replacing parts of clay with semi-dried sludge to produce sludge ceramsite. Based on the sintering theory and the trail firing, the essential technical route was first determined. And then the influence of four key factors(the weight ratio of ICM to wet sludge, clay to sludge, calcining temperature and time) on the performance indexes of ceramsite(apparent density, water absorption and compressible strength)was investigated. The results indicated that all the three indexes decreased with the increasing ICM content. In addition, with the increase of clay content and calcining temperature and time, both the apparent density and compressible strength of ceramsite increased while the water absorption increased. Through integrated analysis of all three indexes, the optimization scheme of sludge ceramsite was as follows:the weight ratio of ICM to wet sludge was 0.05 and clay to wet sludge was 2.4, the calcining temperature was 1180℃and the calcining time was 15min. (5) Performance testing and safety evaluation of sludge ceramsiteThe performance of sludge ceramsite produced by the above optimization scheme was tested and the leaching concerntrations of heavy metals in ceramsite was measured to evaluate the feasibility of producing ceramsite using semi-dried sludge and investigate the stabilization of heavy metals in ceramsite. Furthermore, the influence of sludge semi-drying and stabilization treatment on the performance of ceramsite was disucssed by the analysis of the phyase compositions and microstructure of sludge ceramsite using XRD and SEM. The experimental results revealed that all indexes of sludge ceramsite were in accordance with the standard GB/T 17431.2-1998. Meanwhile, the results showed that the heavy metals were properly stabilized in ceramsite and cannot be easily released into the environment again to cause secondary pollution. Besides, XRD and SEM analysis revealed that the the formation of aluminium phosphate in calcination process, due to the high content of phosphorus in wet sludge, increase the strength of sludge ceramsite and the formation of ettringite improved the expansion and structure of cemerasite.It could be concluded that the proposed ICM is an effective modifier for sludge drying and stabilization and replacing part of clay with semi-dried sludge for ceramsite production is practically feasible and would not cause secondary pollution. This approach is a useful disposal of sewage sludge and slag as well and it is significant in protecting the environment and utilizing resources rationally. |
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