Fundamental Issues of BNR process design in MBR

The implementation of biological nutrient removal (BNR) processes in MBR relies on the same principle used in conventional activated sludge (CAS). But, there are some unique factors that must be taken into consideration (Crawford, 2006; Daigger, 2010).

  • Excess DO returns to anoxic tank – The mixed liquor in membrane tank typically contains dissolved oxygen (DO) at high concentration such as 4-8 mg/L. If this mixed liquor is recycled to anoxic tank directly at a rate of 4Q (400% of influent flow rate), effectively 16-32 mg/L of oxygen is carried back to anoxic tank based on fresh wastewater volume. Considering the fact that typical municipal wastewater contains only 50-100 mg/L of readily biodegradable COD, which is barely sufficient for denitrification, loosing 16-32 mg/L of readily biodegradable COD to the oxygen carried over can hamper denitrification efficiency significantly.
  • Limited flexibility in recycle rate – It is a trend to put separate membrane tanks from aeration tanks to make system maintenance easier, where treated water in aeration tank moves to membrane tank to be filtered while the concentrated mixed liquor is sent back to aeration tank (see diagram). To avoid excessive mixed liquor accumulation in the membrane tank, mixed liquor must be recycled back to either aeration tank typically at 250-400% of influent flow rate as discussed here. Once the recycle rates are set to maintain proper MLSS in membrane tank, there is no enough room to adjust mixed liquor recycle rates to optimize nitrogen removal efficiencies. Therefore the existence of membrane limits the process flexibility by tying up two recycle rates each other.
  • Relatively low MLSS in anaerobic/anoxic tank – To mitigate the excessive oxygen carry over to anoxic tank, cascade type recycle is preferred in MBR as shown here, where the mixed liquor with the highest MLSS in membrane tank is recycled to aeration tank first and then the mixed liquor in the aeration tank is again recycled to anoxic tank. As a result, substantial MLSS gradient is developed among anoxic, aerobic, and membrane tanks. Tank size must be determined considering the MLSS imbalance.
  • Interference from coagulants – In spite of the above limitations, nitrogen removal can be achieved at a comparable level with CAS by modifying the process as discussed here. However, phosphorus removal efficiency of MBR has a fundamental disadvantage against CAS due to the lower biosolids production (or longer SRT). Nonetheless, MBR are often used to achieve extremely low phosphorus level in effluent such as 0.05-0.1 mg/L, which cannot be achieved with CAS. The strict goal in effluent phosphorus level requires a large amount of coagulant dosages. The high coagulant dosage reduces free phosphorus concentration in aeration tank excessively and diminish the growth of PAO, which can break the biological phosphorus removal mechanisms. If the biological phosphorus removal mechanisms are broken, phosphorus must be removed chemically and biological phosphorus removal would not function.
  • Mixed liquor short circuiting in small reactor – The high MLSS in MBR based BNR is responsible for the much smaller biological tanks than those of CAS based BNR. In general, smaller tanks are more vulnerable to short circuiting since the time allowed to the liquid to be mixed is shorter. In addition, the mixed liquor recirculation from membrane tank, which does not exist in CAS based BNR, makes the real liquid residence time in aeration tank even shorter. Table 1 here compares apparent HRT and real HRT in modified UCT process, where real HRT are only 1/3 – 1/8 of apparent HRT. Therefore, securing good mixing in each bioreactor is more important in MBR than in CAS. Sufficient mixing in the place, where mixed liquor is recycled to, can reduce short circuiting problems.

 

© Seong Hoon Yoon