It has been known that membrane fouling is significantly affected by dissolved oxygen (DO) levels, if it is below a critical level, e.g. 1-2 mg/L. No upper limit of safe DO range is known, but excessive aeration that might be necessary to obtain high DO can break down flocs and cause accelerated membrane fouling.
The experimental result shown in Fig. 1 suggests there is a direct correlation between membrane fouling and DO, where TMP almost immediately increases when DO starts to drop. In this experiment, vertically mounted polyethylene membranes with 0.1 micron pore sizes were used at 20 LMH. By switching air to nitrogen gas, biological system switched to the anoxic mode, but membrane scouring could be performed constantly. When DO was raised again after this experiment, the supernatant COD, which is a direct indicator of SMP, decreased significantly when DO increased from 0.2 mg/L to 1.0 mg/L, but it decreased only marginally with further DO increases.
a) TMP profile
Fig. 1. Variations of: (a) TMP profile and (b) DO concentration (Modified from Kang, 2003).
In other side-by-side tests, same phenomenon was observed (Lee, 2005). In this study 0.4 micron vertically mounted hollow fiber membranes were used at low DO (<0.1 mg/L) and high DO (>3.0mg/L). For low DO reactor, 0.9 L/min nitrogen and 0.1 L/min air were supplied to scour membrane surface as well as to oxygenate the reactor. The high DO reactor was supplied with 1.0 L/min air. As shown in Fig. 3, TMP increasing rate was much higher in the low DO reactor although total gas flow rates were identical for both reactors.
Fig. 3. Comparison of filtration characteristics in two MBR with low DO (<0.1mg/L) and high DO (>3.0mg/L) (Jin, 2006)
© Seong Hoon Yoon