| Sludge usually refers to solid-liquid mixing of floc materials generated in the waste water treatment process. Ten thousand tons of waste water may generally produce about 10-20 tons sludge (90% moisture content) after the waste water treatment. According to the statistics, about 120 million tons of sludge was produced in China in 2014. Meanwhile, due to the sludge containing pathogenic microorganisms, parasite eggs, heavy metals, persistent organic compounds and other toxic substances, if without effective treatment and disposal, it can easily cause secondary pollution to groundwater, soil and pose direct threat to environmental safety and public health. Therefore, a safe, economical and efficient treatment and disposal of sludge is extremely urgent. Since the moisture content of sludge from secondary settling tank in the waste water treatment plant is up to 95%, while the relevant state laws and regulations on the follow-up disposal of the sludge requires the moisture content less than 60%. Thus, carrying out efficient, stable, economic research to reduce sludge moisture content is quite necessary. As the sludge component is complex, and the water in the sludge exists various combination forms with sludge particles, the binding energy varies greatly, which contributing to difficult dehydration. At the same time, research on sludge moisture distribution is rather insufficient. Currently, analytical methods for water distribution in sludge mainly include drying, centrifugal sedimentation, filter press, thermogravimetry, differential scanning calorimetry method and other methods. In actual application process, the analysis results based on the above methods may not be accurate, and their analysis process is time consuming and whose distinction is simple. Therefore, a rapid analysis method based on the low-field nuclear magnetic resonance (NMR) transverse relaxation time is proposed to measure the moisture content and water distribution of sewage sludge samples in this thesis. Compared with the above analysis methods, low-field NMR is rapid, accurate and non-destructive. Meanwhile, the thesis also conducted the experiments to improve sludge dewaterability with the pre-treatment methods of ultrasonic, thermal hydrolysis, microwave and chemical agents, and low-field NMR was applied to study the impact of different pre-treatment technical conditions on the sludge water distribution. The main contents and conclusions are as follows:(1) The moisture content and moisture distribution of different water content sludge samples were measured by the low-field NMR measurements. First the most suitable sampling frequency, echo number and sampling frequency was determined. And the 3000 mg/L copper sulfate solution was applied to improve the accuracy of moisture content and moisture distribution of sludge by NMR measurement. Low-field NMR technique and drying method were used to measure the moisture content of the sludge respectively, which turns out that the results of moisture content measured with these two methods were very close whose correlation coefficient was 0.999. And the experimental results indicated that low-field NMR techniques can be applied to sewage sludge moisture measurement. The spin-spin transverse relaxation time was employed to measure the moisture distribution of sewage sludge. The results showed that water in sewage sludge can be divided into three categories, namely, bound water, mechanical bound water and free water, and for the sludge whose moisture content is between 35.29%-96.36%, mechanical bound is dominant in the total water content of the sludge. And the calculation method of the transverse relaxation intensity of municipal sludge was put forward, and the transverse relaxation strength of municipal sludge was 43.99μm/s.(2) The relationship between binding energy of water and the water content in sewage sludge was studied. The energy required in heating process of the evaporation of sludge water can be divided into four categories:the energy to overcome the chemical bond; the energy to overcome the physical force among the intermolecular forces in sludge flocs; the energy to overcome the capillary force; the energy for water phase change. Lennard-Jones 10-4-3 formula was introduced to calculate the physical forces between water molecules and the sludge flocs, and the capillary force between the sludge flocs and the water molecules was calculated according to the saturated vapor pressure. At the same time, TG-DTA experiment was adopted to obtain the binding energy distribution of water in sewage sludge. Model data and experimental results were well matched. The results show that when moisture content of the sludge is high the binding energy between water and sludge floc can be very small. With the decreasing of moisture content, the sludge volume reduces and the binding energy between the remaining water and the sludge floc can gradually increase up to thousand kJ/kg.(3) The impact of different ultrasonic treatment frequency like 20 kHz,25 kHz,30 kHz,35 kHz and 40 kHz; the reaction time of 1 min,2 min,3 min,5 min and 10 min, on sewage sludge dewaterability and water distribution was studied. The results showed that:the ultrasonic pretreatment can help to improve the performance of sludge dewaterability. Different ultrasonic frequency has different effects on centrifuge dewatered moisture content and moisture distribution of sludge, and among those, one of the best ultrasonic frequencies is 30 kHz and the optimal duration is 2 min. In this case, the SCOD of the supernatant of the sludge increased from 87 mg/L to 1660 mg/L, which proved that in the sludge ultrasonic treatment process, a large number of organic polymer and extracellular polymeric substances in the sludge were fractured to improve dewatering performance of the sludge.(4) In the thermal hydrolysis conditions of temperature 140-230℃, reaction time 15-60 min, dosage of CaCl2 0.02-0.14 g/g DS and dosage of PAM 1-1010-6g/g DS, thermal hydrolysis pretreatment was conducted. The results showed that thermal hydrolysis pretreatment process can help to improve the sludge dewaterability and change the moisture distribution of the sludge. Range analysis and variance analysis of orthogonal experiments showed that the increase of reaction temperature significantly improves the breaking effect of sludge particles and reduces the moisture content after centrifugation dewatering process. PAM dosage has little impact on the experimental results. Low-field NMR was applied to measure the moisture distribution in sewage sludge. Compared to the raw sewage sludge sample, the moisture content of bound water and mechanical bound water is greatly reduced after thermal hydrolysis pre-treatment. The optimum conditions of Hangzhou sewage sludge thermal hydrolysis treatment is with the reaction temperature of 230℃, hydrothermal treatment 45 min, at same time, CaCl2 addition 0.14 g/g DS, and after the hydrothermal treatment, adding PAM reagent 10-6 g/g DS. In this condition, water content of the sludge significantly reduces from 88.67% to 70.68% after centrifugal dewatering process.(5) The influence of microwave pretreatment on filter press moisture content of the sludge and the moisture distribution was studied. Microwave power was set at 500 W,700 W,900 W, and microwave time of 30 s,60 s,90 s and 120 s respectively. The results showed that microwave pretreatment process can improve sludge dewaterability and water distribution of the sludge. For filter press dewatering process, due to the high pressure filter, it is able to remove higher binding energy between water and sludge particles than centrifugal dewatering process. The bound water content and the water content of sludge cake were well matched. When the microwave radiation energy was less than 0.34 kJ/mL, by causing the polar molecules to generate high frequency vibration, the double layer structure of the sludge is damaged, and the agglomeration of the sludge particles can be promoted, thus the proportion of the content of bound water in the sludge can be reduced. When microwave radiation was more than 0.34 kJ/mL, microwave radiation can make the water in the sludge boiling contributing to the microbial cell broken, releasing intracellular materials, which would significantly increase the water content, worse of the sludge dewateability.(6) By economic comparison within the three kinds of physical pre-treatment process, we find out that if the subsequent disposal process for sludge after dewatering is landfill or agricultural utilization, microwave method is the best pre-treatment process with the minimum initial investment, which is 8100 yuan/L, and the operation cost is the lowest, which is 0.11 yuan/L. While the ultrasonic pre-treatment process has obvious advantage in the operation cost if the sludge treatment process is followed by incineration.(7) The impact of chemical agents like FeCl3, CaO and sawdust on the centrifugal moisture content of sludge and water distribution was studied. The dosage of FeCl3 was between 0-11 wt.%, dosage of CaO was between 0-30 wt.% and dosage of sawdust was between 0-50 wt.%. The results showed that the appropriate amount of agents can significantly improve the dewaterability of sludge and reduce the moisture content of sludge after centrifugation by increasing the proportion of free water in water distribution of the sludge. In orthogonal experiments, the range analysis and variance analysis showed that sawdust dosage can significantly help to decrease the moisture content of the sludge after the centrifugation dewatering process. |
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