| With the increase of sewage load, a larger number of municipal sewage sludge was produced inevitably. Treatment and disposal of sewage sludge becomes a difficult problem to wastewater treatment plants. At present, studies focus on developing a new technology for treatment of sewage sludge. In view of the problem of treatment and disposal of excess sludge, using characteristics of ultrasond effectivily, ultrasonic pretreatment was combined with sludge anoxic/aerobic digestion process. With small and pilot scale texts in laboratory, the effects of ultrasonic pretreatment on aerobic/anoxic digestion process were investigated and mechanisms of sludge reduction were discussed. Finally, demonstration project using this ultrasonic-aerobic/anoxic digestion process to treat sludge was also carried out in one of wastewater treatment plant in Guangzhou.In this paper, the effects of ultrasonic pretreatment on sludge physical and biological characteristics were firstly studied. The results indicated that the optimal conditions of ultrasonic pretreatment were as follows:ultrasonic frequency of 28 kHz, ultrasonic intensity of 0.15 W/mL, and ultrasonic treatment time of 10 min. In addtion, it was found from our study that different sludge concentrations corresponded to different optimal ultrasonic pretreatment conditions.Then the influences of ultrasonic pretreatment on aerobic/anoxic digestion process were investigated with small and pilot scale texts in laboratory. The results of small scale texts indicated that, the optimal sludge retention time is 12 d; the results of pilot scale texts showed that stability time of sludge was shortened greatly and digestion efficiency was markedly improved by ultrasonic pretreatment. The stability time of sludge with ultrasononic pretreatment was 12 d, which was shortened by 8 d compared to that of sludge with control treatment. After digestion time of 25d, TSS, VSS and TN removal efficiency reached to 38.32%,50.48% and 40.5%, respectively, which was increased by 6.72%,8.52% and 4.69%, respectively, compared to the control.Contrastive pilot scale experiments of ultrasonic-aerobic/anoxic digestion and conventional aerobic digestion were conducted. The results showed that, TSS and VSS removal efficiency of ultrasonic-aerobic/anoxic digestion achieved to 35.77% and 50.02%. which were increased by 3.90% and 8.99%, respectively, after digestion time of 25d. The digestion time for ultrasonic-aerobic/anoxic digestion was shortened by 6 d compared to conventional aerobic digestion. TN removal efficiency of ultrasonic-aerobic/anoxic digestion was 40.8%, which was much higher than that of aerobic digestion. Furthermore, ammonia and nitrate concentration in sludge supernatant of ultrasonic-aerobic/anoxic digestion were lower than that of aerobic digestion, so that the pressure of circumfluence to wastewater treatment plant was reduced.Based on the study of laboratory operation conditions, biochemical analysis and molecular biological research method were adopted to investigate the mechanisms of sludge reduction, from substance transformation, energy transmission and succession of microbial community structure, etc. The results of biochemical analysis revealed that ultrasonic pretreatment accelerated the solubilisation of the EPS and protein and enhanced enzymatic activities. At the same time, the ATP concentration in ultrasonic sludge was lower than that of the control, energy transmission in ultrasonic sludge was faster, resulting in the removal efficiency of TSS and VSS in ultrasonic sludge was markedly improved.The study of molecular biology showed that microbial and ammonia-oxidizing bacterial community structure changed in different operation process, which could reflect the dynamic succession of microbial and ammonia-oxidizing bacterial community. The sequencing indicated that 4 of predominant species in DGGE profiles of microbial communities are belonged to Bacillus, which playing an important role in sludge digestion. TSS and VSS of ultrasonic sludge were degradated faster. Among the predominant species of ammonia-oxidizing bacterial communities throught the operation,6 are denitrifiers, which playing an important role in improving the efficiency of nitrogen removal.3 of them are belonged to Pseudomonas,1 is belonged to Acidovorax,2 are belonged to Burkholderia. Pseudomonas, Acidovorax and Burkholderia. Nitrification and denitrification were improved by ultrasonic pretreatment, resulting in TN removal efficiency of ultrasonic-aerobic/anoxic digestion was higher. Real-time PCR revealed that the copy number of AOB decreased at the beginning, and then increased, which were associated with concentration of ammonia, higher ammonia content favors the growth of ammonia oxidizers in the reactor. In addition, high quantity of AOB populations can improve the efficiency of nitrogen removal.Furthermore, the mechanisms of sludge reduction were described through characterizing the process of cell lysis during ultrasonic- aerobic/anoxic digestion using green fluorescent protein.The results indicated that the sludge supernatants from aerobic phase promoted the releasing of the intracelluar GFP and the cell lysis capability of the sludge supernatant in ultrasonic-aerobic/anoxic digestion might be related to the hydrolytic enzymes secreted from the sludge.On the basis of study of laboratory operation conditions and mechanisms of sludge reduction, this ultrasonic-aerobic/anoxic digestion process was applied to treat the sludge in wastewater treatment plant, investigated the the effects of this technology on sludge reduction. The operation was carried out for 3 months, TSS, VSS and TN removal efficiency achieved to 36%,50% and 40%. Ammonia, nitrate, nitrite and TP content in sludge supernatant are very low, the corresponding loads of nitrogen and phosphorus pressure to wastewater treatment plant are reduced, so that nitrogen and phosphorus can't influence the effluent quality. The cost of ultrasonic-aerobic/anoxic digestion is 0.324-0.389 yuan/m3 wastewater, which is higher than that of aerobic digestion. However, the novel process established in this study has the features of less site area, shorter sludge retention time, more effective treatment efficiency, and so on.This research provided theoretical and practical basis for the popularization and application of the ultrasonic-aerobic/anoxic digestion technology.
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