| After mechanical dehydration, municipal sewage sludge still has a moisture content of about80wt%, which cannot meet the requirements of the sludge disposal. The high moisture content of sludge has become the bottleneck of the disposal, while thermal hydrolysis is an effective sludge pretreatment method for sludge reduction. Thermal hydrolysis pretreatment can improve the dewaterability of sludge and the energy consumption is much lower compared with conventional thermal drying for avoiding the loss of latent heat of vaporization. Therefore it will have great importance to study the thermal hydrolysis characteristics of dewateted sewage sludge.In current dissertation, an experimental study of sludge thermal hydrolysis is conducted in aim to learn and develop a deep dewatering technology. On the basis of original studies, a larger experimental platform for sludge thermal hydrolysis and dewatering is established. The sludge is heated by direct steam injection of high pressure, which would be helpful for further industrial promotion and application. In this study, three kinds of sludge from Hangzhou Qige, Jiaxing Lianhe and Shanghai Shidongkou are studied. The main contents of the study include the dewatering characteristics of sludge after thermal hydrolysis, the hdrolysis characteristics of sludge solid and the migration and species distribution of heavy metals in sludge.The thermal hydrolysis experiments of different kinds of sludge show that the best thermal hydrolysis temperature is during170to190℃, and the reaction time is30min. After thermal hydrolysis, the frame filter has the best dewatering performance and so that it is recommended. The moisture content of filter cake could drop to about50wt%and the filter cake has a good natural drying performance. Centrifugalization has little effects on sludge dewatering, and the moisture content of sludge cake is between70-80wt%which is still high. Therefore centrifuge is not appropriate to dewater the sludge after thermal hydrolysis. The experimental results are similar to the results of the small electric heating experiments, and it could provide technical support for further pilot scale test and industrial application. During thermal hydrolysis pretreatment, parts of the solids in sludge are dissolved and hydrolysed. After thermal hydrolysis, the particle size and the organic part of sludge are decreased, while the organic concentration in the filtrate is increased. Because of the hydrolysis of the organic matter in sludge, the heating value of sludge on dry basis is decreased. The heating values of Hangzhou, Jiaxing and Shanghai sludge after their thermal hydrolysis at their optimum conditions lost38.4%,18.8%and22.4%respectively, which are greater than thermal drying.After thermal hydrolysis, most of the heavy metals remain in solid phase and exist in a more stable form. The concentration of heavy metals in hydrolysate is very low and the proportion of heavy metals transferred to solid phase are mostly greater than99.5%. Due to the sludge solution, the heavy metal concentration in solid phase rises in different degrees, but the thermal hydrolysis pretreatment promotes the stability of heavy metals and reduces their potential mobility, which would be helpful for further treatment of sludge disposal.On the basis of experimental results and operation experiences and also combining the existing industrial application, a continuous thermal hydrolysis and depth dehydration system is designed. The thermal hydrolysis part of this system adopts two preheating tanks, six reaction tanks and one flash tank, and it could realize the continuous operation through valves controlling. At last, the mass and energy balance of the system are analysed for an example which dealing30t/d mechanical dewatered sludge. The results show that the energy consumption is611MJ/t which is only about40%of thermal dring. The cost of thermal hydrolysis and dehydration system is lower than60RMB/t, which is economically feasible. |
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