Pilot Experimental Study On Treatment Of Low Organic Sludge In Integrated Thickening And Digestion Reactor

As China's social and economic development and urbanization levels and the enhancement of people's environmental awareness, urban sewage treatment rate in the increase year by year, a sewage treatment plant sludge in the production increased dramatically. For many sewage plants, the sludge treatment mode is simply thickening, dewatering and return to the environment finally.For many medium and small town wastewater treatment plants, anaerobic digestion system is behind the development of sewage treatment system. If the sludge was not appropriatly treated,organic contaminant of the municipal sludge will lead to the second pollution in the role of plurosion and rain wash. Aiming at this case, it is necessary to develop new-typed simple and efficient sludge treatment technology for medium and small town.However, most researchers focused on the anaerobic digestion of sludge with high VS content, and not much has been reported on the anaerobic digestion of sludge with a low VS content. In order to improve the effect of municipal sludge reduction and stabilization, a pilot-scale reactor for sludge thickening and digestion was operated under the condition of the mesophilic anaerobic digestion,which was developed by optimization design on the basis of lab experiment and integration of Sludge Gravity Thickener,high-speed digester,IC,UASB and EGSB technologies. The effective volume of reactor was 1805L with design sludge feeding rate 30%.The study aspects mainly include: the hydrodynamic behavior of pilot-scale reactor, the systematic study of started-up and operational performance, kinetics characteristics of pilot-scale reactor. the main results are as follows:The results of hydrodynamic behavior indicated 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. While further increasing, mixed flow alike started to lower. 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.The pilot-scale reactor 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 reactor 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 reactor startup.The operation performance were studied when the sludge feeding rate was 20%, 30%, 40%, 50% respectively. The results indicated that when the sludge dosing rate was 24% and the temperature inside of the reactor was kept at 33℃³5℃, the gas yield of sludge would be the highest, reaching about 340L/kgVS , average gas production up to 231.3L/kgVS, concentration of methane up to 62.66%.After treatment, organic matter content(VS/TS) down to 21.2%, the sludge VS/TS reduced for about 21.6% ,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.The 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℃³5℃, the reactor was operated well and the gas yield was about 300 L/d; while when the temperature decreased to 32℃, the gas yield began to decrease quite significantly with a range of 1/3; and when the temperature decreased to 30℃, the gas yield decreased sharply with a range over 2/3; if the temperature further decreased to 28℃, 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℃, the energy consumption was 15kWh/m³, and when below 18℃, the energy consumption exceeded 40 kWh/m³.The kinetics of organics treatment of the pilot-scale reactor was analyzed finally. The kinetics parameters such as K_s andν_max 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=0.362mL/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 .During the test,inside of the pilot-scale reactor,pH value was around 7.0,VFA was 150²50mg/L,alkalinity was around 1500mg/L,this indicated that the pilot-scale reactor had the ability to against shock loads and could ment the requirement of anaerobic digestion.