Enhancement Of Nitrogen Removal In Smbr At Low Temperatrue By Low Intensity Ultrasound

As the impact of low temperature, the biological treatment of urban sewage infrigid area has been facing with poor removal efficiency. MBR has the potentialadvantages of treating low temperature urban sewage. It has been used in the waterreuse and wastewater treatment, and many practical projects in the world have beensuccessfully applied in. So investigating the performance of submerged MBRtreating low temperature urban sewage and developing measures to strengthen itseffect are of great significance.In the study, the wastewater at low temperature was treated by an submergedmembrane bioreactor (SMBR). The best ultrasound-enhanced indexes and theperformance of the reactor in removing nitrogen after ultrasound radiated werestudied. And membrane fouling with ultrasound radiated was also investigated.Furthermore, the mechanism of ultrasound enhancing the biological nitrogenremoval at low temperature was mainly investigated.A lab-scale SMBR with effective volume of 28L operated at 10±1℃, andcontrolled HRT at 2.4h. The sewage was prepared artificially. The effectiveness ofSMBR treating low temperature urban sewage in start-up period was studied. Theresults showed that in treating low temperature urban sewage, the start-up period ofSMBR was as long as 27d.Low temperature had little effect on total COD removalrate, which can be maintained over 90%, while it had larger effect on NH₃-N removal.In the prophase of start-up period, NH₃-N had very low removal rate. It didn't raiseup to the perfect level until nitro-bacteria started adapting the low temperature. Whenthe COD removal rate reached its steady state, NH₃-N biological removal and totalremoval rates were 59.7% and 64.9%, but they were still unstable. The biologicalremoval rate was 69.6%, stable at 65% to 70%.The ultrasound can improve the performance of SMBR treating low temperaturesewage at short HRT. Samples Irradiated by ultrasound within 48h were examinedevery 2 to 6 hours to study the strengthening effects on NH₃-N degradation underdifferent combinations of ultrasonic parameters. Results showed that ultrasound withpower densities of 0.21W/L at irradiation time of 20min and 0.28W/L at 15min,energy consumptions of 4.2W·min/L, were the optimum parameters. Under which,the improvement of NH₃-N biological removal efficiency was up to 32%. Nitrogencompounds in the effluent were mainly NH₃-N. NO2-N and NO3-N contents were verylow. The membrane cleaning cycle extended 2d when ultrasound radiation time wasreduced by half, meaning ultrasound can reduce membrane fouling at low temperature. With the system running, the total amount of EPS increased. And themain component was protein, which means protein was the reason of membranefouling. After ultrasound radiation, Sludge filamentous bulking didn't happen during theexperiment.At last, the mechanism of ultrasound enhancing the biological nitrogen removalwas investigated. Changes of cell membrane permeability can explain therelationship between ultrasound parameters and increased NH₃-N removal ratesbetter than dehydrogenase activity increasing. When the power densities were0.14W/L and 0.21W /L, cell membrane permeability increased with the extension ofirradiation time and the increase of power densities. But if energy input wasexcessive, the membrane holes became so large that cells were unable to completeself-healing, changing cell membrane permeability irreversibly, and consequentlyinhibiting the improvement of metabolism.