Acceleration Of Sludge Thermophilic Anaerobic Digestion By Ultrasonic Pretreatment

Sludge land utilization is restricted by stricter environment limitation with the development of society. Thermophilic anaerobic digestion has been emphasized in Europe and America in order to satisfy the requirements of biologysafety in the process of sludge land utilization. If some methods are applied as pretreatment to disintegrate sludge, the hydrolysis can be improved a lot, so that the residence time can be shortened, fermenter volume be reduced, methane production be significantly increased.Waste activated sludge collected from municipal sewage plant is fed into small thermophilic anaerobic digestion reactor after ultrasonic pretreatment by using ultrasonic bath or probe. The digestion time is controlled by the dosing rate. The effect of disintegration time on the biogas production and the removal efficiency of organic matters in the digestion process are studied.It has been found that the specific energy of ultrasonic bath is smaller than ultrasonic probe when reaching the same disintegration degree, and the treated sludge volume using ultrasonic bath is larger. The ultrasonic bath is selected for disintegration in subsequent anaerobic process, the ultrasonic density is 0.133W/mL, and the disintegration time is 60min and 90min, respectively.Reactors are inoculated with mesophilic digested sludge. The step-wise temperature increments are implemented. It has been found that temperature increment before 45℃has only a minor impact on digestion system. When temperature increases to 48~52℃, the performance of anaerobic digestion is reduced significantly. The biogas production and the removal efficiency of organic matters are recovered rapidly after the temperature reaches to 55℃, and better than mesophilic system.The biogas production and production rate are increased obviously in thermophilic anaerobic process after ultrasonic pretreatment compared to the reactor fed with non-pretreated sludge. The biogas production is increased with the argument of dosing rate at disintegration time of 90min. When the dosing rate is smaller than 12.5%, the function of ultrasonic disintegration on the removal efficiency of TCOD and VS is not remarkable. At the dosing rate of 12.5% and 25%, the removal efficiency of TCOD and VS are increased significantly compared to control group. The sludge mass expressed in VSS can be reduced in the anaerobic digestion of pretreated sludge. The removal efficiency of VSS is increased with the argument of dosing rate at disintegration time of 90min. When the dosing rate is 25%, the removal efficiency of VSS is 48.03%, and increased by 26.66% compared to control group. The sludge retention time for thermophilic anaerobic digestion of disintegration is reduced from 10~12d to 4d, and the digestion system performs well. At the same time, sludge reduction is realized.When the dosing rate is 25%, the biogas production, the removal efficiency of organic matters and sludge mass at disintegration time of 90min are smaller than 60min. It can be concluded that the performance of thermophilic anaerobic digestion may not be improved if the disintegration degree is too high.