MLE is the most commonly used biological nutrient removal (BNR) process in MBR, which is primarily targeting nitrogen removal. In the original MLE-MBR process, mixed liquor is recycled from membrane tank to anoxic tank as shown in Fig. 1a. But, the excess dissolved oxygen (DO) transferred from membrane tank (4-8 mg/L) to anoxic tank can interrupt denitrification process. This is a significant issue especially when readily biodegradable COD in the wastewater is not sufficient.
In order to reduce the impact of recycled oxygen, cascade type mixed liquor recirculation (Fig. 1b) has been devised. In this arrangement, the mixed liquor in membrane tank is recycled to aeration thank and in turn it is recycled to anoxic tank. Since the mixed liquor with 1-2 mg/L DO is recycled to anoxic tank instead of 4-8 mg/L DO, maintaining low ORP in anoxic tank is readily possible without consuming too much of readily biodegradable COD in feed wastewater. One potential drawback of this cascade type arrangement is that maintaining high MLSS in anoxic tank is harder than the traditional arrangement (Fig. 1a) since the mixed liquor with lower MLSS is transferred to anaoxic tank.
In the traditional MLE-MBR process shown in Fig. 1a, QM/AO has to be set at 2Q – 5Q for the purpose of preventing MLSS accumulation in the membrane tank, which somewhat limits the process flexibility. For instance, even if mixed liquor recycle can be slowed down due to the low nitrogen level in wastewater, mixed liquor must be recycled at 2Q-5Q to maintain low MLSS in membrane tank. On the other hand, the modified MLE with a cascade type recycle, QO/AO can be controlled independently from QM/O depending on process goals.
The benefit and the drawback of traditional and modified MLE-MBR process are summarized in Table 1.
Table 1. Comparison of traditional and modified MLE-MBR.
|Traditional MLE-MBR (Fig. 1a)||Modified MLE-MBR (Fig. 1b)|
The required QO/AO to obtain a target nitrogen removal efficiency () can be calculated as follow. For instance, QO/AOneeds to be approximately 2Q, if target nitrogen removal efficiency, R, is 0.67 according to the following equation.
MLE process is not primarily for phosphorus removal, but, some extra phosphorus removal can occur beyond the level achievable without anoxic tank depending on the ORP and solids retention time (SRT) in anoxic tank. Inorganic coagulants can be added if further phosphorus removal is required.
Fig. 1. MLE (Modified Ludzack-Ettinger) processes.
The relative MLSS in each tank can be calculated using the following spreadsheet calculator.
© Seong Hoon Yoon