In this module you will learn the basic principles of reverse osmosis, its effectiveness for remove dissolved solids in water, the main functions of an industrial reverse osmosis system and how you can increase efficiency while reducing complexity with an intelligent CRE pump.

Osmosis is a natural phenomenon which is the movement of water through a semi-permeable membrane from a low to a high concentration solution.

The membrane is permeable to water but rejects almost all ions and dissolved solids. Water will move until the osmotic equilibrium is reached.

But if pressure greater than the osmotic pressure is applied to the concentrated solution, the process can be reversed, hence the name reverse osmosis.

While ultrafiltration can remove suspended solids from water, reverse osmosis is able to remove the remaining dissolved contaminants, such as ions, heavy metals, and so on. The combination of both technologies are normally applied in the physical treatment, for example in water reuse.

The most popular configuration of an industrial reverse osmosis membrane consists of a flat sheet of several layers, wounded into a cylinder to increase surface area while saving space. The cylinder is then placed in a pressure vessel.

As water enters through the inlet, it flows over the membrane surface. The cleaned water or permeate is collected in a center tube while the remaining part called concentrate or rejection is collected at the end.

The relation between the volume of permeate and the volume of concentrate compared to the inlet raw water is called the rejection rate.

For Reverse Osmosis to function, considerable pressure is needed, making it an energy demanding process.

Now we will look at the minimum of components included in a basic RO system in order to function: ...the  inlet side  with the pump for increasing  the pressure of the raw water,... ...the Reverse Osmosis membrane… ... and on the other side of it, the two outlet streams, where one is the permeate outlet where the clean water comes out … and the other is the outlet for the concentrate or rejection.

Typically, a counter pressure or throttling valve is placed at the concentrate outlet to build up pressure inside the membrane. There are also two flow sensors: One on the permeate side and one on the concentrate side, used to balance the rejection ratio for the process.

When setting up the system, the concentrate valve is fully open, and there will be full flow on the condensate side but no flow on the permeate side, as there is not sufficient pressure to allow water to cross the membrane.

By adjusting the counter pressure valve, pressure can be increased up to the optimal rejection rate, which will depend on the system design, and the quality of the raw water for treatment.. in that sense, the raw water quality can contain various compounds that might damage the membranes or affect the system’s performance.

Next there is the pre-treatment phase. If no pre-treatment is done, compounds like carbonates or sulphates or dissolved salts can lead to clogging of the membrane... this is called scaling and can affect the overall performance of the system… ...or aggressive substances such as chlorine or chlorides can destroy the membrane’s material which will compromise the effectiveness of the treatment.

Activated carbon filters or softeners are normally implemented for pre-treatment, as the initial step is to remove traces of chemicals or to polish the raw water from chlorine or chlorides before entering the membrane…but an alternative pre-treatment strategy can be to treat the water with different chemicals such as antiscalants for example, using dosing pumps.

An efficient pre-treatment can save you a lot of trouble and expenses. However scaling can occur over time if, for example, raw water is stagnant in the membrane. This can be monitored by installing a sensor that measures the differential pressure between the inlet and the concentrate outlet of the Reverse Osmosis unit.

As deposits build on the membrane, this differentical pressure will rise. And when it reaches a predefined maximum, the membrane must be cleaned, for example by a process called forward flushing.

To enable forward flushing, the counter pressure valve has a bypass with a piloted valve. At normal operation this is closed, but when the membrane needs to be cleaned it is opened. The concentrate now has free passage, and this will increase the flow in the concentrate side thus flushing away the deposits. Some applications perform the forward flush with raw water and some with permeate."

As an additional safety precaution, a conductivity sensor can be added, to measure the quality of the permeate. If the membrane’s integrity is compromised allowing impurities or dissolved solids passing through the conductivity of the permeate increases and, when exceeding a set level, the system is shut down to avoid undesirable contamination in the permeate tank.

The control of the different functions and components can be complex and usually are performed by a PLC or external controllers, but there is a another way… by using an intelligent CRE pump.

And for extra security, other components can be added, such as a piloted valve to shut off the inlet water in case of malfunction, a pressure switch to prevent dry running and even a level switch to ensure that the tank is not overfilled and that the process is paused until the tank level has dropped.

All in all, in a reverse osmosis system the compact and intelligent CRE pump comes with a small footprint and high efficiency, which ensures a low price per treated cubic meter. It has a user friendly interface, measured signals are displayed digitally, and even offers the option for remote monitoring.

That concludes this module about the basic principles of reverse osmosis, and the benefits of using an intelligent CRE pump to reduce complexity.