| With the increase of processing ratio for urban sewage, excess sludge treatment and its disposal has become a new environmental topic. The anaerobic digestion technology is valid in stabilization and quantity-reducing for excess sludge. However, for the insufficient production of digestion gas, sludge digestion process cannot achieve enough energy from itself. Therefore, energy consumption becomes the main issue, which restricts the application of anaerobic digestion technology in the sludge treatment.Sludge biodegradability can be effectively enhanced with high-intensity ultrasonic pretreatment. For the excess sludge with high organic content, even if part feed sludge is pretreated with this technique biogas production from the anaerobic digestion process can be noticeable enhanced. However, the organic content of excess sludge is generally low in our country. Biogas production is difficult to have significant improvement with the same pretreatment method and anaerobic digestion process. The biogas production cannot make effective feedback to the energy consumption of the sludge digestion process yet. On the purpose of reducing energy consumption for the sludge treatment process, bioactivity of inoculated sludge was improved and specific energy consumption for sludge disintegration was decreased. On the condition of semi-continuous feed, technological parameters for the anaerobic digestion of total disintegrated sludge inoculated by low-intensity sonicated sludge were also studied. In this paper, the main results are as follows:(1) The effects of low-intensity ultrasonic on dissolved matters from sludge and microbial colonies were studied. Furthermore, optimized parameters were achieved. Anaerobic bioactivity of sludge under different dosage of low-intensity ultrasonic was also researched. Experimental results show that SCOD/TCOD value increased after low-intensity ultrasonic pretreatment, many insoluble substances converted to solutes, which can help promote sludge hydrolysis reactions. But the increase of SCOD came from the disintegration of sludge floc and microbial cells in the process of ultrasonic threatment. It will hold back the sludge bioactivity in anaerobic digestion. With optimization of the ultrasonic parameters, power density of 0.25 W/mL, ultrasonic intensity of 0.35 W/cm2 and sonication time of 15 min were proved effectively enhanced the sludge anaerobic digestion performance. Compared with the sludge sample without low-intensity ultrasonic, its R(VS/TS) 20 value increased by 51%. The achieved sludge under the ultrasonic conditions was thought suitable to be used as inoculum for the digestion of disintegrated sludge.(2) Under power ultrasonic, the dissloution rule of organic matters from sludge was studied. Combining degradation characteristics of organic matters in disintegrated sludge under optimized inoculation condition, reasonable sludge disintegration degree and efficient power ultrasound parameters were obtained. In addition, the effects of aeration and sludge flow mode on cavitation and mechanical effect in sludge sonication were also analyzed. Through effective auxiliary measures, the efficiency of sludge sonication was further improved. Experimental results show that the obtained sludge disintegration degree under 1200 W,2 min power sonication is appropriate, whose SCOD/TCOD value is 10.9 %. Aeration cannot strengthen ultrasonic cavitation or enhance sludge disintegration. Circulation flow mode of sludge gives full play to the mechanical effect of sonication. The SCOD of disintegrated sludge is remarkable improved by 13.6% under this mode.(3) The effects of low-intensity sonicated inoculum on anaerobic digestion of disintegrated sludge in semi-continuous experiments were discussed, including characteristics analysis for the organic acids and biogas. Reasonable inoculation methods for low-intensity sonicated sludge were adopted to improve the anaerobic digestion performance of disintegrated sludge. Furthermore, benefits from group technology of ultrasonic pretreatment for anaerobic digestion of sewage sludge were analyzed. Experimental results show that the low-intensity sonicated inoculum cannot adapt the semi-continuous anaerobic digestion in a short time. It resulted in acidification. Its effects would not disappear entirely after 20 days'uninterrupted service, including the unstable digestion process and low biogas production rate. However, the low-intensity sonicated sludge, with 8 d culture period under no feeding condition, could adapt the semi-continuous anaerobic digestion as a result of the biomass improvement of methane bacteria and flora balance. With the method, the sludge digestion process started faster and made an increase for the stable biogas production by 19.6%.(4) The applicability of the group technology of ultrasonic pretreatment and its comprehensive benefit was analyzed. As the results shown, with the group technology of ultrasonic pretreatment, digested sludge got more fully degradation. Its biological stability was better. Setteability and dewatering performance of the digested sludge was obviously improved. It is beneficial to the follow-up processing for sludge disposal. Energy consumption for sludge disintegration is 26.3 kWh/m3. With the group technology of ultrasonic pretreatment, biogas production from sludge digestion can meet the energy demand of both sludge heating and sludge drying, which accounts for 86.7% energy consumption for sludge treatment. Therefore, the pretreatment technology is helpful to realize the extensive utilization of biogas. Moreover, with the group technology of ultrasonic pretreatment, net energy consumption of sludge treatment process is lowest. It accounts for 53.6% net energy consumption in sludge treatment process without pretreatment. Compared with the sludge treatment process with single sludge sonication pretreatment, its net energy consumption decreases by 17.1%. Hence, energy-saving effect of the group technology is obvious.
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