What are the common types of fouling blockages in reverse osmosis/nanofiltration membranes? How to judge? How to solve it? (1)

1.   Particulate matter contamination.
Characteristics: It generally occurs in the early stage of commissioning and operation, which is manifested as a rapid increase in a period of differential pressure and normal water conductivity (if it is not serious, it is detected early and dealt with in time).
Cause: During debugging, the membrane system inlet pipe (commonly found after the security filter) did not rinse or rinse the residual welding slag or the residual sawdust or other impurities left by the PVC pipe were intercepted by the first section of the first membrane, and finally gathered on the end face of the membrane, blocking the membrane runner.
Judgment method: First determine the time of occurrence (a few days after starting) and the speed of pressure difference increase in the first stage (very quickly), and then open the membrane end cap to confirm whether there are impurity particles on the membrane end face.
Countermeasures:Clean the pipeline thoroughly without leaving particulate matter.

2.   Colloidal fouling
Characteristics: When this kind of fouling occurs, the pressure difference in the first section is generally high, the first and second membranes at the inlet end of the first section are more serious, and the membrane behind is relatively clean.   And the online cleaning effect is extremely poor, and it can only be removed and cleaned offline.   After cleaning, it can't last a few days and has to be removed and washed.   The security filter element is replaced quickly, and in severe cases, it needs to be replaced within a few hours.   When you take apart the security filter, you will find that there is slime on its walls.
Causes:
(1) The ultrafiltration broke, resulting in the fine flocculate body of the sedimentation tank at the front end penetrating the sand, and the ultrafiltration was first enriched in the security filter element and then penetrated through the filter element to block the membrane.
(2) Excessive addition of PAC or sodium metaaluminate or PAM in the sedimentation tank at the front end of the membrane leads to excessive aluminum and PAM residues in the precipitated effluent.
(3) Scale inhibitors are incompatible to produce flocs.

The incompatibility of the added scale inhibitor is common in the reaction between the scale inhibitor and the residual aluminum in the reverse osmosis into the water to form a floc blocking membrane.

How to judge:
(1) The judgment method of ultrafiltration wire breakage is as follows:
Method 1: Observe and test the cleanliness of the SDI filter membrane.   The picture below shows the initial filtration of 250ml of ultrafiltration water production membrane during the SDI test (ultrafiltration water production is too dirty, it takes a long time to filter 500ml, and then stops).
Method 2: Analyze whether the turbidity of ultrafiltration water production is abnormally increased, and the normal turbidity of water production is about 0.1NTU.
Method 3: Confirm the broken wire by bubble observation method.

(2) The method for judging the excess of PAC or sodium metaaluminate or PAM in the membrane front sedimentation tank is as follows:Whether PAC or sodium metaaluminate is excessive is mainly to detect the precipitated aluminum content, which we generally control at <0.1-0.3mg/L according to project experience, and the risk of membrane blockage surges higher than the control value.  Whether PAM is too much to look at the feel is whether there is a slippery feeling when the fingers touch and rub the precipitated water with the fingers.

(3) The judgment method of scale inhibitor incompatibility is as follows: It is mainly judged by experiments, and the experimental process is illustrated by our actual project as an example.   Take the membrane into the water and add the scale inhibitor used on site (the dosage is the same as the on-site) and PAC (converted according to the content of aluminum ions in the membrane inlet).   Then the stirring reaction is observed whether there is any precipitate formed.   At the same time, a set of blank experiments is done, but without PAC, the rest is the same.
•No PAC is added, only the scale inhibitor used on site is added, no precipitate appears, and the system is uniform.
•The addition of on-site PAC and scale inhibitor quickly produced a precipitate that appeared to be largely consistent with the contaminant characteristics on the membrane end face.
•With another scale inhibitor experiment (also added PAC for field use), no sediment was observed, and the system was homogeneous.
The above experiments prove that the scale inhibitor used in the field is incompatible with the PAC, resulting in flocculent precipitation, and another scale inhibitor needs to be replaced.

Countermeasures:
(1) If the ultrafiltration is broken, the wire plug must be repaired according to the membrane manual method.   If it breaks too much, it is recommended to replace it.
(2) When there is excessive dosing of PAC or sodium metaaluminate or PAM in the precipitation tank at the front end of the membrane, the appropriate dosage should be determined through the beaker experiment and process control objectives.
(3) If the scale inhibitor is found to be incompatible, it must be replaced.   When using a new scale inhibitor, it is necessary to determine whether it is compatible with the agent added on site through experiments.