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There are two types of filtration: batch filtration and simple continuous filtration. Batch filtration is usually used, but sometimes a simpler continuous filtration method is used. Either way, the aeration must be continuous during the filtration run.
When intermittent filtration is used, the system will be suspended for a short period of time after filtration, while the aeration is still continuous, as shown in the figure below.
Aeration when stopping filtration can effectively clean the membrane surface, and although automatic control equipment is required to start and stop filtration, intermittent filtration is still the recommended filtration method because it ensures that the membrane filtration process is more efficient and stable.
The recommended intermittent filtration cycle is 9 minutes to run and 1 minute to stop.
01 MBR membrane filtration process
There are two types of MBR membrane filtration processes, one is gravity filtration driven by head difference, and the other is filtration driven by pump pumping.
In any case, a fine grid with an accuracy of 3.0mm or less needs to be installed in front of the membrane tank, otherwise the membrane modules may be severely fouled by the garbage brought by the raw water.
It is advisable to set up a buffer tank (hydraulic homogenization tank) of sufficient capacity before the MBR process to balance the relationship between the BOD load and the amount of water to be filtered, thereby ensuring the stability of the biological treatment and membrane filtration process.
(1) Configuration of gravity filtration
Gravity filtration uses the natural head difference caused by the height difference from the liquid level of the membrane tank to the outlet of the filtered water as the driving force for filtering the produced water (as shown in the figure below).
In order to ensure sufficient suction pressure during filtration to be able to overcome the head loss of the pipe valve, the permeate should usually be designed below the level of the membrane tank (usually 3m or more below the level).
It is recommended that the design from the water collection pipe to the water outlet should be directly routed through the wall, and it is recommended that the outlet of the water production pipe can be designed in a U-shape so that the pipeline can be watersealed.
The filtration flow is controlled by an automatic control valve (permeate control valve). When the water level in the reactor reaches a low level, the filtered water is stopped, and if the high level is reached, the raw water is stopped.
In gravity filtration, the air that collects in the piping will seriously reduce the effective head, and it is necessary to exhaust the air at least once a day, otherwise the effective head will decay sharply.
An air drain should be set at the highest point of the permeate line, and an automatic exhaust valve should be installed before the exhaust port. Stop the filtration and close the permeate valve completely, then open the air drain valve for a few minutes and the air will be easily discharged.
(2) The configuration of pump suction filtration
The process of filtration using pump suction is shown in the figure below.
The permeate flow is controlled by a flow meter and a pump with an automatic regulating valve, or by a flow meter and a pump with inverter control.
In addition, if the water level in the reactor reaches a low level, the filtered water is stopped, and if the liquid level is high, the raw water is stopped.
In some cases where the pump is pumping, it is necessary to install equipment to drain the air that collects in the production pipe. Some of the more common methods are: vacuum pumps, water jets, or manual priming.
02 Necessary equipment for membrane filtration system
The main peripherals required for the membrane filtration process are as follows:
a. Fine grille
In order to protect the membrane and prevent clogging, pretreat the raw water of the membrane bioreactor with a grid of less than 3 mm.
It is recommended to use a metal grid to avoid overflow and penetration at any time.
b. Flow control device
A flow control device is set on the permeate pipeline, which can be an automatic adjustment valve interlocked with the permeate flowmeter, or a variable frequency suction pump interlocked with the permeate flowmeter.
If a row of membrane cells contains multiple series of membrane modules, it is recommended to install a flow control device on the membrane system in one row.
c. Determination and calculation of transmembrane differential pressure
The transmembrane pressure difference is obtained by measuring the pressure inside and outside the membrane (in the permeate pipeline and in the membrane pool) and calculating it, you can choose to install a pressure gauge or pressure transmitter on the permeate pipe and the membrane basin respectively, and read it directly after being calculated by PLC, or you can choose to use a differential pressure gauge.
d. Air supply devices (e.g. blowers, etc.)
The function of the air supply device is to supply air for aeration. The air supply of each module is selected according to the standard aeration volume of the module as shown in the manufacturer's technical manual.
e. Gas Flow Meter
A gas flow meter is recommended to monitor air volume. If each row of cells contains more than one series of modules, it is recommended to install at least one gas flow meter per row of membrane systems.
f. Suction pump
In the pump suction filtration mode, a suction pump is necessary. Due to the need for precise permeate flow control, the suction pump needs to be controlled by a variable frequency.
A volute pump with self-priming (centrifugal pump) or a positive displacement pump with self-priming (progressive cavity pump) is recommended.
g. Level sensor
A level gauge is necessary and is installed in the membrane tank to monitor the level of the membrane basin and to calculate the differential pressure across the membrane on the PLC.
H. Siphon destroys the tube
In the case where the pump is running and the permeate discharge is lower than the membrane tank, it may be possible that the filtration will not stop due to the siphoning phenomenon and the pump will not stop.
To prevent this siphon situation from happening, a siphon destruction pipe must be installed after the permeate pump.