Two filtration modes and configurations of MBR process design
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- Issue Time
- Dec 30,2024
Summary
MBR membrane components have two filtration methods: intermittent filtration and simple continuous filtration. Intermittent filtration is usually used, and sometimes a simpler continuous filtration method is used. Regardless of which method is used, aeration must be carried out continuously during filtration operation.
Two filtration modes and configurations of MBR process design
MBR membrane components have two filtration methods: intermittent filtration and simple continuous filtration. Intermittent filtration is usually used, and sometimes a simpler continuous filtration method is used. Regardless of which method is used, aeration must be carried out continuously during filtration operation. When intermittent filtration is used, the system will pause for a short period of time after filtering for a certain period of time, and aeration will still be carried out continuously at this time, as shown in the figure below.
Aeration during filtration stop can effectively clean the membrane surface. Although the start and stop of filtration requires automatic control equipment, intermittent filtration is still the recommended filtration method because it can ensure that the membrane filtration process is more effective and stable. The recommended intermittent filtration cycle is: 9 minutes of operation and 1 minute of stop.
MBR membrane filtration process
MBR membrane filtration process includes two types, one is gravity filtration driven by water head difference, and the other is filtration driven by pump suction. In any case, a fine grid with an accuracy of 3.0mm or less needs to be installed in front of the membrane pool, otherwise the membrane assembly may be seriously blocked by garbage brought in by the raw water. It is recommended to set up a buffer tank (hydraulic homogenization tank) of sufficient capacity before the MBR process to balance the relationship between BOD load and filtered water volume, thereby ensuring the stability of biological treatment and membrane filtration process.
Sample Content
(1)Configuration of gravity filtration
Gravity filtration uses the natural water head difference generated by the height difference between the membrane pool liquid surface and the filtered water outlet as the driving force for filtering water production (as shown in the figure above).
In order to ensure sufficient water suction pressure during filtration and overcome the head loss of the pipeline valve, the design of the water outlet should usually be lower than the membrane pool liquid level (generally 3m or more below the liquid level). It is recommended that the design of the water collection pipe to the water outlet adopts direct wall-penetrating pipes, and it is also recommended that the outlet of the water production pipe be designed to be U-shaped so that the pipeline water seal can be performed. The filtration flow is controlled by an automatic control valve (water production control valve). When the water level in the reaction pool reaches the low liquid level, the filtration of water is stopped; if it reaches the high liquid level, the intake of raw water is stopped. During gravity filtration, the air accumulated in the piping will seriously reduce the effective water head. At least one air exhaust operation should be performed every day, otherwise the effective water head will decrease sharply. An air exhaust port should be set at the highest point of the water production pipeline, and an automatic exhaust valve should be installed before the exhaust port. Stop filtration and completely close the water production valve, then open the air exhaust valve for a few minutes, and the air will be easily discharged.
(2)Configuration of pump suction filtration
The process of filtration using pump suction is shown in the figure below.
The water flow rate is controlled by a flow meter and a pump with an automatic regulating valve or by a flow meter and a pump with variable frequency control. In addition, if the water level in the reaction tank reaches a low level, the filtration of water is stopped; if it reaches a high level, the raw water is stopped. In some cases of pump suction operation, some equipment needs to be installed to discharge the air accumulated in the water production pipe. Some common methods are: vacuum pump, water ejector or manual perfusion.
Necessary equipment for membrane filtration system
The main peripheral equipment required for the membrane filtration process is as follows:
a. Fine grid
In order to protect the membrane and prevent clogging, please use a grid of less than 3mm to pre-treat the raw water of the membrane bioreactor. It is recommended to use a metal grid and avoid overflow and penetration at any time.
b. Flow control device
A flow control device is set on the water production pipeline, which can be an automatic regulating valve interlocked with the water production flow meter, or a variable frequency suction pump interlocked with the water production flow meter. If a column of membrane pools contains multiple series of membrane components, it is recommended to install a flow control device on a column of membrane systems.
c. Determination and calculation of transmembrane pressure difference
The transmembrane pressure difference is obtained by measuring the pressure inside and outside the membrane (the pressure in the water production pipeline and in the membrane pool) and calculating it. You can choose to install a pressure gauge or pressure transmitter on the water production pipe and the membrane pool respectively, and read it directly after PLC calculation, or you can choose to use a differential pressure meter.
d. Air supply device (such as blower, etc.)
The function of the air supply device is to supply air for aeration. The air supply volume of each membrane module is selected according to the standard aeration volume of the membrane module shown in the manufacturer's technical manual.
e. Gas flow meter
It is recommended to use a gas flow meter to monitor the air volume. If each column of membrane pools contains multiple series of membrane modules, it is recommended to install at least one gas flow meter on each column of membrane system.
f. Suction pump
In the pump suction filtration mode, a suction pump is necessary. Due to the need for precise water production flow control, the suction pump requires variable frequency control. It is recommended to use a volute pump (centrifugal pump) with self-priming function or a positive displacement pump (screw pump) with self-priming function.
g. Liquid level sensor
The liquid level meter is necessary and is installed in the membrane tank to monitor the membrane tank liquid level and to calculate the transmembrane pressure difference on the PLC.
h. Siphon breaking pipe
In the case where the pump is running with suction and the product water outlet is lower than the membrane tank, the filtration may not stop due to the stop of the water pump due to the siphon phenomenon. To prevent this siphon situation from occurring, a siphon breaking pipe must be installed after the product water pump.