Study On Internal Circulation Membrane Bioreactor For Enhanced Nitrogen Removal

For conventional nitrogen removal processes, nitrification and denitrification are carried out in separated tanks with different dissolved oxygen concentration or in a single reactor where aerobic and anoxic conditions are formed sequentially. Such processes usually need the recirculation of nitrified wastewater or the addition of external organic carbon and alkalinity. The investment and operation cost is high, and the land occupied is large. As to these problems, a novel MBR named internal circulation membrane bioreactor (ICMBR) is developed, in which the aeration not only supplies the required oxygen, but is utilized for the circulation of nitrified wastewater. It carries out the nitrogen and carbon removal of domestic wastewater in a single treatment unit.Simulated domestic wastewater is used in this research. Through nearly one year continuous experiment, the carbon and nitrogen removal capacity of the reactor under various operation conditions and the operating parameters are optimized. With the reactor operating at a hydraulic retention time (HRT) of 12h, the solid retention time (SRT) of 45d, the temperature at 22-25℃, the pH at 7.0-8.0, the aeration rate at 80L/h, the ammonia solid loading rate below 0.040 kgNH₄⁺-N/(kgMLSS·d), the ammonia concentration of the effluent was mostly under 8.0mg/L, the NH₄⁺-N removal efficiency is about 89%, the total nitrogen (TN) removal efficiency is about 78%, the COD concentration of the effluent keep below 40.0mg/L at most time, and the COD removal efficiency is more than 90%. The nitrogen balance experiments show that most of nitrogen is removed as gas, so it achieves the aim of nitrogen removal. When the ammonia loading rate of the influent increase to 0.050 kgNH₄⁺-N/(kgMLSS·d), the system become unsteady acutely, the ammonia and nitrite accumulate easily in the ICMBR in a short time.The results of experiment imply that there is obvious simultaneous nitrification and denitrification (SND) process in the reactor. The study investigates it from the process mechanism, impact factors and dynamics. The micro-environment in sludge floc and the macro-environment of the reactor jointly cause that the SND process occurs in the system. The C/N ratios in the influent and the aeration rates have great influence on the effect of SND. When the C/N ratio is at 10 and the aeration rate is at 80L/h, the optimal effect of nitrogen removal achieves through the SND process. The study establishes the dynamic model of SND and further investigates the SND phenomenon from quantification aspect.The activated sludge concentration increases slowly. VSS/SS decreases with the course of operating time, from 0.869 to 0.752. The sludge volume index (SVI) alters with the variations of operating conditions. The average nitrification activity of microorganism in the ICMBR is 0.0373 kgNH₄⁺-N/(kgMLSS·d), and higher than the conventional activated sludge system. The denitrification activity is 0.0266kgNO_x^--N/(kgMLSS·d), lower than the anaerobic condition. From the varieties of TMP, it shows that ICMBR can reduce the membrane fouling compare to conventional MBR.