ch2mbr

AS-MBR

1. Overview

Active sludge membrane bioreactor (AS-MBR) is a process of membrane separation of activated sludge.
This is increasingly becoming an alternative for wastewater treatment plants as improved
effluent quality often is required together with special limitations.

A major drawback for the process is however membrane fouling, which needs to be further
handled in order to create a cost-beneficial solution.

Fouling can potentially be reduced by applying biofilm (BF) instead of activated sludge.

Prospects of even more compact plants, with reduced aeration requirements, and improved
effluent quality are other driving forces for investigation of BF-MBR.

The limited literature concerning BF-MBR illustrates however varying degrees
of success and reveals less fouling and better treatment in the AS-MBR.

2. Processes

2-1 AO-MBR Process

2-2. A2O-MBR Process

2-3. CH2-MBR (Modified A2O) Process

2-4. MUCT-MBR Process

Modified UCT process is virtually same as Virginia Initiative Plant (VIP) process, but typically UCT
process is designed for the SRT of 13-25 days while UCT process is designed for 5-10days.
But, when it comes to MBR, the difference between the two processes does not exist.
The major benefit of UCT process is it does not directly return the nitrate bearing mixed liquor to anaerobic tank,
thereby ORP remains low enough to maximize phosphate release from phosphorus accumulation organisms (PAO).
However, the cascade type mixed liquor recycle to anaerobic tank causes diluted MLSS in anaerobic tank.

To compensate the MLSS dilution in anaerobic tank, tank size needs to be increased.

In modified UCT process, similar to MLE process, mixed liquor in rear end of the process is recycled to the
front end of the process using a cascade type mixed liquor recycle as shown, where the mixed liquor in membrane tank
is recycled to aeration tank, the mixed liquor in aeration tank is to anoxic tank,
and the mixed liquor in anoxic tank is to anaerobic tank. The internal recycles, QM/O, QO/AO, and QAO/AA range 2Q-5Q, 1Q-2Q, and 1Q-2Q,
respectively, depending on condition and the goals in nutrient removal.

2-5. MLE-MBR Process

Fig. 1. MLE (Modified Ludzack-Ettinger) processes.

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 de-nitrification 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 anoxic 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 required QO/AO to obtain a target nitrogen removal efficiency ( ) can be calculated as follow.
For instance, QO/AO needs to be approximately 2Q, if target nitrogen removal efficiency, R, is 0.67 according to the following equation.
QO/AO = 〔 R / 1-R ]Q

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.