Difference between ultrafiltration membrane backwashing and reverse osmosis membrane backwashing

Difference between ultrafiltration membrane backwashing and reverse osmosis membrane backwashing

Regarding the backwashing (reverse flushing or cleaning) of RO membranes, there are not too many points of disagreement per se, and basically we all have a consensus. But a friend said that the RO membrane system backwash problems, so I think today it is necessary to share with you again about the basic concept of RO membrane backwash, and venture to guess the reasons for the spread of the above erroneous views may be widespread.
First of all, the old rules we through the reverse osmosis membrane reverse cleaning of the basic definition and operation of the process to understand the main points of RO backwash and may bring problems.


First, reverse osmosis membrane reverse cleaning
(1) reverse osmosis membrane reverse cleaning of the basic definition of forward cleaning: the historical design of the CIP system is the cleaning fluid from the pressure vessel inlet end of the inflow, after passing through the membrane element from the pressure vessel thick water end of the flow, this flow direction and RO system is the same as the normal operation of this cleaning method is usually common and effective. Reverse Cleaning: However, reverse cleaning has been found to be beneficial in some cleaning situations. When there is heavy biological, colloidal, or particulate contamination, the contaminants can become heavily enriched in the first membrane at the influent end. It is very difficult to break up these contaminants and flow them through the entire membrane element in the pressure vessel. In most cases, however, it is easier to elute the contaminants by reversing the flow of the cleaning solution (from the thick end to the inlet end; Figure 2 has the inlet and thick end labeled the other way around).

(2) Reverse osmosis membrane cleaning considerations
①Prioritize the removal of end scale
When doing backwash cleaning, we must first remove the insoluble salt scale at the end of the membrane system. Because the edge of the salt crystals of the scale is very sharp, if the reverse cleaning is not removed, the salt crystals are likely to directly damage the membrane surface, which will bring more serious damage than the conventional cleaning (forward cleaning). Therefore, when the system is scaled, generally the first conventional (forward) cleaning, prioritizing the removal of scaling at the end of the system.

②Setting of cleaning flow rate limit
In the conventional direction cleaning, the end of the reverse osmosis membrane element by the thrust ring support protection, will not be squeezed deformation. However, when cleaning in the reverse direction, the first membrane is not protected by the thrust ring. Therefore, we recommend limiting the cleaning flow rate in the reverse direction. 8-inch membrane elements are routinely cleaned at 136-182 L/min, and we recommend that the reverse cleaning flow rate be limited to 2/3 of the normal cleaning flow rate, i.e., 91-121 L/min. If the differential pressure is very high and the contamination is very severe, the cleaning flow rate should be reduced to 1/3 of the normal flow rate, i.e., 45-61 L/min, to minimize the possibility of membrane deformation. For membrane elements with 34 mil inlet screens, the purge flow rate is somewhat higher than for conventional membrane elements (see table below). Start at a lower flow rate and increase slowly based on the actual differential pressure. When the contaminant is removed and the differential pressure is reduced, the flow rate can be increased slowly, and then reverse cleaning at a conventional flow rate is the final step.

Note: At no time should the membrane elements be cleaned from the production side as this will cause back pressure on the membrane elements and irreversible damage to the membrane elements.

Other Precautions
Not all RO membrane systems can be simply retrofitted for reverse cleaning. The best solution is to design the system to be cleaned from any direction when it is built. Note that the requirement is for bi-directional cleaning, and the system cannot be designed for reverse cleaning only. A second option is to design the system so that it can be flushed in the reverse direction (concentrate to influent). While not as effective as reverse cleaning, this helps to remove contaminants from the front end of the RO system. The third option was to remove the heavily contaminated first membrane, remove the concentrate seal, change the orientation of the membrane, and then install the concentrate seal into the other end (previously the concentrate end). In this way the severe contamination can be more easily removed by conventional forward cleaning, and in many cases operating in this manner can also aid in the removal of contaminants. The limits of cleaning fluid concentration, pH, temperature, etc. used for reverse cleaning are the same as for conventional cleaning.

Summary: After reading the above basic points of reverse osmosis membrane backwashing, we find that in certain circumstances (when there are serious biological, colloidal, particulate contamination, pollutants can be in the feed end of the first membrane on a large number of enriched), reverse osmosis membrane backwashing with the regular washing can really improve the state of the membrane element, of course, the relevant design and process should strictly adhere to the above specifications.
Among them, we should pay special attention to the above content on the “back pressure” statement: at no time from the water side of the cleaning membrane element this will cause the membrane element back pressure, so that the membrane element by irreversible damage. Why do many people think that backwashing a reverse osmosis membrane creates back pressure? Because many people interpret the backwashing of reverse osmosis membranes as similar to the backwashing of ultrafiltration membranes (a wild guess). Ultrafiltration membrane backwashing is precisely the production of water as the inlet to the backwash, while the inlet and the thick water/discharge port as the backwash of the lower and upper outlet, respectively. Imagine, if a friend for granted will reverse osmosis membrane backwash understanding of this state, will inevitably come to the conclusion that reverse osmosis membrane backwash will cause back pressure on the membrane element, and damage to the membrane structure.