Study On The Mechanism Of Low-Intensity Ultrasound To Enhance The Anaerobic Biological Treatment Of Wastewater

At present,urban sewage is mainly treated by aerobic biological treatment,but high energy consumption and high mud yield have become problems that cannot be ignored.Anaerobic biological treatment not only has low energy consumption and low sludge volume,but also generates clean energy such as methane.Realizing the application of anaerobic biological treatment in low-strength sewage is an effective way to transform sewage treatment,but problems such as poor sludge activity and low treatment efficiency have severely restricted its application.Low-intensity ultrasonic irradiation of anaerobic sludge can accelerate cell metabolism and growth,promote mass transfer and increase enzyme activity,but its mechanism of promoting anaerobic sludge activity and its effect on the anaerobic stage still need to be explored in depth.Therefore,the paper combines low-intensity ultrasound with anaerobic biological treatment,optimizes the ultrasonic parameters through response surface methodology,and further explores the key enzyme activities of anaerobic sludge after ultrasonic irradiation and the characteristics of extracellular polymeric substance（EPS）,etc.The analysis of the different rate-limiting effects of the acid-producing fermentation stage,the hydrogen-producing acetic acid stage and the methane-producing stage in the ultrasonic-enhanced anaerobic treatment process,initially revealed the mechanism of low-intensity ultrasound enhanced anaerobic biological treatment of sewage.The main content and results are as follows:First of all,using dehydrogenase（DHA）activity as the evaluation index,through single factor experiment and response surface method optimization experiment,the influence of acoustic energy density,irradiation time and ultrasonic sludge concentration on anaerobic sludge activity is explored.The results show that ultrasonic sludge concentration has the greatest impact on DHA activity,followed by sound energy density,and irradiation time has the least impact.The best operating conditions for ultrasonic enhanced anaerobic sludge activity are:acoustic energy density of 0.10 W/m L,irradiation time of 10.2 min,and ultrasonic sludge concentration of 10.0～10.5 g/L.On the basis of optimizing the ultrasonic parameters,it is found that long-term low-intensity ultrasonic irradiation improves the efficiency of the anaerobic baffled reactor（ABR）in treating low-strength sewage.During the long-term operation of ABR,ultrasound irradiation increased the activity of DHA and coenzyme F₄₂₀ in each compartment,and promoted EPS secretion.At the same time,ultrasonic irradiation makes the floc structure of the sludge in each compartment of the ABR looser,effectively promoting mass transfer,and thereby reducing the content of dissolved organic matter in the water body,and the effluent quality is better.Based on the optimization of the above-mentioned ultrasonic parameters,the effect of ultrasonic irradiation on the activity of anaerobic sludge was further analyzed by low-intensity ultrasound coupled with anaerobic sequencing batch reactor.The results showed that ultrasonic irradiation increased the activity of DHA and coenzyme F₄₂₀ in anaerobic sludge.The activities of DHA and coenzyme F₄₂₀ in the ultrasound group were 28.91μg/（mg VSS·h）and 0.041μmol/g VSS,respectively.Compared with the control group,it increased by 30.0%and 24.2%respectively.At the same time,low-intensity ultrasonic irradiation can increase the secretion of protein in EPS and promote the conversion and degradation of polysaccharides by microorganisms.Under the action of periodic ultrasound,the total amount of protein in the ultrasound group was 36.52±2.87 mg/g VSS,which was an increase of 17.24%compared with the control group,but the total amount of polysaccharides was 32.53±1.74 mg/g VSS,which was a decrease of 10.36%compared with the control group.In the infrared spectra of anaerobic sludge soluble EPS（S-EPS）,loosely bound EPS（LB-EPS）and tightly bound EPS（TB-EPS）,there are near 3400,1654 and 1033～1123 cm^-1.Obvious absorption peaks once again prove the existence of proteins and polysaccharides in S-EPS,LB-EPS and TB-EPS.The shift of the fluorescence peak and the increase of fluorescence intensity in the three-dimensional fluorescence spectrum indicate that low-intensity ultrasonic irradiation increases the functional group content of anaerobic sludge EPS,thereby promoting sludge activity.Finally,by exploring the changes in the conversion and degradation of substrates in the acid-producing fermentation stage,hydrogen-producing acetic acid stage and methane-producing stage under different sound energy density and irradiation time conditions,the effect of low-intensity ultrasound on each anaerobic stage is preliminarily clarified.The results show that under the action of different sound energy densities,the optimal sound energy density threshold in the methane production stage is lower,and the conversion rate is slower,which makes the rate-limiting effect of this stage on anaerobic digestion more significant.Under the effect of different irradiation time,the optimal irradiation time range of the hydrogen production and acetic acid production stage is the narrowest,and the substrate conversion and degradation rate is lower than the acid production fermentation stage and the methane production stage.So that this stage becomes the rate-limiting step of anaerobic digestion.In summary,by exploring the influence of low-intensity ultrasound on the activity and anaerobic stage of anaerobic sludge,it lays the foundation for the application of ultrasonic technology in anaerobic biological treatment,and is of great significance for improving the efficiency of anaerobic biological treatment of low-strength sewage.