Even though applications of reverse osmosis have increased, the fundamental problems like falling rejection and reduction in performance (reduced flow or higher pressure that can be observed in membrane processes have not changed. When such problems occur, the first question to be clarified is whether there have been any changes in water quality or temperature. To make it easier to localize problems, it is important to document basic parameters such as temperature, flow, pressure, yield and conductivity. In addition, it is also helpful to measure the pressure loss, preferably between the concentrate levels.
These data should be normalized, i.e. expressed in relation to a standard situation so that it is possible to assess whether the change in performance is due to the system or changed inflow parameters. Calculation tables for this purpose are provided by the membrane manufacturers free of charge. Aside from this, it is vital to check that other facility components such as measuring equipment, anti scalant dispensing units and ion exchange systems are running smoothly.
After normalization, if there are following date deviations, it is important to investigate closely.
• 20% higher salt passage (definition: salt passage = 100% rejection [%])
• 10% reduction in flow
• 20+% of pressure loss along a pressure pipe
Frequently, small facilities record only a few measurement values, and these are generally not normalized. Where this is the case, the influence of temperature and variations in the salt load in the water should be taken into account. The rule of thumb here is that for each degree Celsius of temperature drop, flow is reduced by approx. 3%.
Investigating the problem
The investigation process depends on the problem observed. If the salt passage has increased, i.e. the rejection has worsened, this can indicate chemical or mechanical damage to the membrane or element. Reduced flow, on the other hand, is generally due to organic, biological or inorganic fouling. The following table provides an overview of the tendencies and potential reasons for a drop in performance.
It is difficult, however, to localize and then deal with the problematic point in the system.
Increased flow and deterioration of the salt rejection
When there is increased flow and a deterioration of the salt rejection, the conductivity of the permeate in all pressure pipes is checked in case any of the values are significantly elevated. If any single pipe is conspicuous, the element can be identified using the “central pipe testing method.” This involves passing a flexible hose into the central pipe of the coiling element to the end of the pressure pipe and then slowly pulling it back out, catching some of the permeate in the hose. This sample can then be tested for conductivity. Should this suddenly rise at any point, the location of the leak can be identified based on the length of hose that has been pulled out.
The leak might either be due to damage to the element itself or to the interconnector between the elements.
If all the elements demonstrate significantly increased conductivity with higher flow, this could indicate permanent damage. This can be a result of oxidation agents such as chlorine, solvents that have dissolved some of the polymer membrane, or can be caused by overpressure in the permeate, which causes delamination of the membrane.
The upstream elements are more likely to be affected by oxidation than the downstream ones. Delamination, on the other hand, is more likely to affect the downstream elements, since this is where permeate overpressure is likely to be highest. Final clarification in this case can only come through an element autopsy, as this kind of damage cannot be assessed from the outside.
Increased flow and reduced rejection are clear indicators of a leak in the system that has to be remedied. This can frequently only be achieved by replacing the elements or small parts (e.g. O-rings, interconnectors).
If there is reduced flow, or if greater pressure is required, there is soiling (fouling). Depending on how pronounced it is, this can also lead to a reduction in salt rejection. Fouling is often associated with increased pressure loss along the entire pressure pipe, as the flow channels become blocked.
While organic fouling primarily leads to blockages on the upstream side, the higher salt load on the downstream side is more likely to cause salt precipitation, i.e. inorganic fouling, or scaling. Biological fouling can spread throughout the entire system and can therefore be found at various points.
It is generally possible to determine the nature of the fouling by opening the pressure pipe. A quick inspection is usually enough to find an organic blockage, white salt crystals or biomass. Uninstalling and weighing the element will determine the degree of fouling. It must be drained for about 15 minutes before determining the weight. If, for example, an 8-inch element that normally weighs approx. 16kg suddenly weighs more than 17kg, this could be due to heavy fouling. In most cases, cleaning the element in good time can restore performance. Alkaline cleaning should be used to clear organic or biological fouling, while complexing agents are used for inorganic fouling in acidic ranges.
If initial examination does not provide clear enough results, an element autopsy can be useful. Ultimately, however, it is important to test whether the cause of the drop in performance can be eliminated so as to ensure a stable process.
Dr. Jens Lipnizki Head of Technical Marketing Membranes Liquid Purification Technologies business unit Lanxess Deutschland GmbH