Experimental And Simulation Research On Excess Sludge Disintegration By Combining Impinging Stream And Low-Temperature Thermal Hydrolysis

In recent years,with the continuous improvement of the wastewater treatment rate,the associated excess sludge is also increasing rapidly.On one hand,excess sludge contains a large number of toxic and harmful substances such as pathogenic microorganisms and heavy metals,if not treated appropriately,it will be a serious environmental pollution hazard.But on the other hand,excess sludge also contains useful organic resources,such as protein,fat and polysaccharide,which are mostly found in zooglea and bacterial cells.However,bacterial cell walls and their extracellular polymers form a natural barrier from directly utilizing these resources.Therefore,using appropriate methods to destroy zooglea and bacterial cells,transform the hard-to-degaradate solid phase organic matters into easy-to-degaradate soluble phase,is one necessary pretreatment means of sludge reduction and resource processing.In this study,the effect,mechanism and energy efficiency of combined method of impinging stream(IS)and low temperature thermal hydrolysis(LTTH)disintegrating excess sludge was resesrched.Firstly,the IS and LTTH was employed individually to treat excess sludge by single factor experiment and orthogonal test to obtain the impact of various factors.Then,based on the results of individual treatments,sludge disintegrating experiment combining IS and LTTH was conducted.The combining order was determined firstly,and then the combined method was optimized by response surface methodology(RSM).The flow field inside the nozzle was also simulated to analyse the mechanisms of sluge disintegrating in the nozzle.Single factor experiments of IS showed that,in general,two-nozzle-impinging could achieve better sludge disintegrating effect than three or four when total pressure and separate pressure settled;With the change of time,obvious effect was obtained within the initial 6 minuts,after 6 min the efficiency went slow;Pressure lifting did not continuously improve the disintegration effect,the highest growth rate of the chosen organic matters was achieved witin pressure of 10-14 MPa;Increasing the nozzle distance within a certain extent could improve the disintegration effect,in this study better result was achieved at 8 cm;The effect of low MLSS concentration was better than high concentration.The result of orthogonal test showed that the maximum disintegration rate 50.11%was achieved at the time of 12 min,the pressure of 14 MPa,the nozzle distance of 7 cm,and the MLSS concentration of 3000 mg/L.The simulation of the flow field inside the nozzle showed that,there was a strong turbulence in the nozzle,and sudden change of pressure and velocity existed in the specific part.These properties bring effects such as collision,shear and hydrolic cavitation,which could partially contribute to the disintegrating effect before the sludge impining.Single factor experiments of LTTH showed that,the concentration of SCOD,DNA and protein increased rapidly in 0-40 min,then slowed down;The disintegration effect was limited at 45℃,when the temperature rose to 55 and 65℃ the effect was enhanced obviously;MLSS concetration at 6000-10000 mg/L made for better VSS and TS removal rate;When alkali introduced,the alkali-heat synergy was superior to the separate effect of heat and alkali together.The result of orthogonal experiment showed that,the significance order was temperature>alkali>treatment time,the optimal combination was treatment time of 75 min,temperature of 70℃,and alkali of 90 g NaOH/(kg·TS).Pressure,IS-time,temperature and LTTH-time was chosen as experimental factors for combined sludge disintegration method.By means of transmission electron microscope analysis and organic matter index analysis,the combination order was determined as IS followed by LTTH.By means of RSM,with increment of SCOD concentration(SCOD+)and Disintegration Energy Ratio of increment of SCOD concentration(DER-SCOD+)serving as response values,mathematical models and the corresponding optimal operation parameters were obtained.The optimal SCOD+3279.5 mg/L was achieved at the pressure of 16 MPa,IS-time of 9 min,temperature of 66℃ and LTTH-time of 45 min;The optimal DER-SCOD+530.4 mg/(kW·h·g)was achieved at the pressure of 16 MPa,IS-time of 3 min,temperature of 66℃,and LTTH-time of 15 min.