| Sewage sludge from wastewater treatment plants (WWTPs) has been an increasingly serious environmental problem in China as municipal wastewater discharge and treatment rate grow rapidly in recent years. With comprehensively reviewing the state of the arts of international and national sludge treatment technologies, it is found that complicated treatment process and high construction & operation cost are the most important influencing factors for too low sludge treatment rate at present. So it is necessary to develop new-typed simple and efficient sludge treatment technology with thickening and digestion integrated, which can not only simplify the treatment process, but also lower the sludge treatment cost. To solve the practical problems, a pilot-scale experimental study of a two-phase integrated sludge thickening and digestion reactor (TISTDR) was built in Chongqing Jiguanshi (WWTP) based on the bench-scale experimental study having been carried out previously. The study aspects include: fluidal condition analysis and optimization of bench-scale TISTDR with application of computational mechanics software FLUENT based on the previously developed bench-scale TISTDR; Design of pilot-scale TISTDR and its fluidal in accordance with fluidal similarity principle; and systematic study of operational performance and kinetics characteristics of TISTDR with pilot scale using the returned sludge from the secondary sedimentation tank of Chongqing Jiguanshi WWTP as subject. The main results are as follows:①Physical model of the TISTDR was established based on FLUENT software. A tetrahedral non-structural mesh method for mesh densification of the structural parts such as inlet sludge pipe, water drainage pipe, outlet sludge pipe, internal recycle pipe and methane recycle pipe and mesh refinement of the near wall was established. A computational mechanics mathematical model of the TISTDR was defined and the boundary condition and initial condition accordant to the TISTDR was established, which provided theoretical basis for optimization design of the pilot-scale TISTDR.②The fluidal field was simulated and optimized by applying FLUENT. It has found that the bench-scale TISTDR had some problems such as inlet sludge short flow in the outer reaction chamber and uneven recycle in the inner reaction chamber. Improvement measures to the these problems were proposed as: change the place of sludge feeding pipe, set the connecting hole at top of the divider between the outer chamber and the inner chamber, adjust the size of connecting hole, adjust the internal recycle pipe to become a unity, and adjust the height of the internal recycle pipe outlet and the size of the internal recycle pipe. With regard of the need of adjusting multiple structural parts to create ideal fluidal status, the optimization combination pattern of various structural parts was investigated by using L1 6( 45) orthogonal experimental method and computational mechanics indexes as performance indicators such as weighted average of velocity variance, total kinetic energy and average intensity of turbulence, which provided basis for the optimization design of the pilot-scale experiment facility.③A pilot-scale TISTDR experiment facility was designed based on the simulation and optimization results of fluidal status and a pilot-scale TISTDR experimental system was built with consideration of the real setting of the experimental base. By using methane backflow as stirring method, the effects of gas backflow rate on the fluidal status in the inner and outer reaction chamber were investigated emphatically. It has been found that when the backflow rate increased from 0 L·min-1 to 12 L·min-1 gradually, the fluidal status of the reactor developed from piston flow alike to mixed flow alike, and when the backflow rate of methane was 12 L·min-1, the tendency of completely mixed flow was the strongest. The study further found that pumped-in feeding pattern have created a non-negligible initial velocity, which would produce shock on the fluidal status of outer reaction chamber and would be adverse to the sludge thickening. However, pumping-in returned sludge would be helpful to improve the formation of completely mixed flow in the inner reaction chamber.④The pilot-scale TISTDR was started successfully with a step-by-step culture. The results indicated that under the condition of 35±2°C and dosing rate of 10%, the pilot-scale TISTDR was stable for various parameters after operated for 55 days. Acidification phenomena were not observed in startup period, indicating that selecting surplus sludge of the secondary sedimentation tank with relatively low organic content was safer for the TISTDR startup. In analyzing the degradation process of sludge substrate in the anaerobic system, NH3, released from the sludge substrate in the diphase anaerobic digestion of surplus sludge, played an important role in maintaining the level of pH and alkalinity as well as the stability of the reactor operation.⑤The treatment performance of the pilot-scale TISTDR was further studied. The results indicated that when the sludge dosing rate was 24% and the temperature inside of the reactor was kept at 33°C~35°C, the gas yield of sludge would be the highest, reaching about 340L/kgVSS and with CH4 content over 60%. After treatment, the sludge VS/TS reduced for about 22% and the treated sludge have lower organics content, 21%. With a water content of 90%, the treated sludge had a volume of only 1/10 of that of raw sewage sludge, proving a significant digestion effect.⑥The influencing effect of temperature on sludge treatment performance was preliminarily studied. The results indicated that when the temperature inside of the reactor was kept at 33°C~35°C, the reactor was operated well and the gas yield was about 300 L/d; while when the temperature decreased to 32°C, the gas yield began to decrease quite significantly with a range of 1/3; and when the temperature decreased to 30°C, the gas yield decreased sharply with a range over 2/3; if the temperature further decreased to 28°C, gas generation stopped. The operational energy consumption was analyzed also. The results indicated that energy consumption was directly correlated to the temperature of raw sewage sludge. When above 25°C, the energy consumption was 15kwh/m3, and when below 18°C, the energy consumption exceeded 40 kW·h/m3.⑦The kinetics of organics treatment of TISTDR was analyzed finally. The kinetics parameters such as K s andνmax were identified based on Michaelis–Menten equation and experimental data. By investigating the relationship between organic degradation rate and methane generation rate under different sludge dosing rate conditions, the coefficient of methane generation was obtained as y=0.3614mL/mg, which was close to the theoretical value of 0.35 mL/mg, proving that the design of the developed pilot-scale facility was reasonably and it had a good operational condition.The above results of study provided systematical support both theoretically and technically and are of great importance for advancing the engineering of TISTDR technology.
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