| In recent years the effect of low intensity ultrasound (US) irradiation on improving the activity of biological cells or enzymes has been confirmed by a great number of studies, which leads to opening up a new field of ultrasound application. The application of the bio-effect caused by the low intensity ultrasound to intensify the biological process in waste water treatment is only in the primary stage of investigation both at home and abroad and related studies have not been advanced in depth yet. In this thesis the experimental study on application of low intensity ultrasound irradiation with frequency of 21kHz ~ 40kHz to enhancing the activity of the sludge is introduced, the factors influencing the activity of the sludge and the mechanism of ultrasound action are explored. In this study the effect of active sludge pretreated by low intensity ultrasound on the bio-degradation of organic pollutant in wastewater was investigated. The influence of low intensity ultrasound irradiation on the structure of microbe community in sludge has been revealed. A technology of sludge aerobic digestion enhanced by low intensity ultrasound irradiation is suggested. An empirical formula for the relationship between input energy of ultrasound and the digestion rate has been deduced. The conclusions of this study are as follows:①The activity of sludge could be effectively enhanced by US irradiation with frequency of 21kHz ~ 40kHz under certain range of power input and irradiation time, for example, after pretreatment of the sludge by low intensity US with a power input of 40W (power density 0.05W/mL)and frequency of 28kHz for 10 min, its OUR, proteases activity and dehydrogenases activity were increased by 129%, 23.7% and 24.6% respectively and the intensifying effect was a continuous process. Test results show that the power, frequency and irradiation time of the ultrasound as well as the concentration and property of the sludge have significant influence on the effect of the ultrasound on enhancing the activity of the sludge. For different biological enzymes the same irradiation parameters of the ultrasound may produce promoting or inhibiting effect. Therefore, the intensifying effect of the ultrasound irradiation on the activity of the sludge may exist only in a narrow range of its parameters.②After pretreatment of activated sludge by low intensity US, the normal synthesis metabolism of the sugars and protein of activated sludge during the wastewater purification may be disturbed, which leads to obvious variation of the sludge extracellular polymeric substances (EPS) composition, for example, when the sludge pretreated by US at the frequency of 21 kHz and 28 kHz, power input of 20 W and 40 W (power density 0.02W/mL and 0.04W/mL) with exposure time of 10 min, the polysaccharose concentration in the sludge EPS is less than that in the control one and consists of 77.6%~90.4% of the latter, but the protein content is more and consists of 110.0%~133.5% of the control sample.③The sludge, pretreated by US with proper irradiation parameters and then mixed with wastewater, can enhance the removal rate of organic substances in waste water. An important phenomenon has been observed that when the sludge was pretreated by US at the frequency of 28 kHz, power input of 40 W with exposure time of 2 min, after 6h of operation, the supernatant SCOD removal rate of the reactor was increased by 8.2% compared with that of the control one, but the yield coefficient of the sludge was 0.489 gMLSS/gCOD, smaller than 0.554gMLSS/ gCOD for that of the control one, a phenomena similar to the "uncoupling" mechanism of the sludge was observed, which has certain meaning for sludge reduction during wastewater purifying. However, the US pretreatment made the flocculation effect of the sludge worsen, therefore, the separating unit in the active sludge technology should be improved (e.g. using film separation) to ensure the quality of the effluent in process.④The experiment results of sludge aerobic digestion with low intensity ultrasonic irradiation show that when the excess sludge was irradiated by low intensity ultrasound intermittently at a power density of 0.045W/mL or 0.09 W/mL with exposure duration of 10min~20min each time, the Kd of its digestion process was increased by 40%~90%. The US irradiation is able to improve the aerobic digestion efficiency of the sludge and the stabilization time for the excess sludge aerobic digestion can be shortened by 3~7 d. The influence of acoustic energy input on the digestion rate is quite similar to that of temperature on the enzyme reaction and there existed optimal energy input and maximal reaction velocity and the ultrasound irradiation can improve the sedimentation of the digestion mixture.⑤The experiments results show that the mechanism of enhancing biological activity of the sludge by the low intensity US is possibly attributed to the energy effect, mechanical effect and cavitation effect, which can improve the liquid-solid mass transfer of the sludge, enhance the activity of biological enzyme and increase the biodegradable substance in mixture. On the other hand, free radicals in high concentration were generated by ultrasonic cavitation, which can inhibit or even damage the biological activity of the cells. Therefore, the effect of low intensity US irradiation on the sludge activity is a synthesized performance of enhancing and inhibiting action. The ERIC- PCR fingerprint diagram for the microbe population structure indicated that the low intensity ultrasonic pretreatment cannot alter the prevailing sludge group composition rapidly, but the biological metabolism of some microbe groups might be depressed which result in change of the quantity balance of the populations, and the whole metabolic characteristic of sludge would be varied. .
|
PDF Full Text Request