This is a question that we get quite often, especially from home owners who are considering the use of a Zeta Rod to treat the water in their home.
The short answer is: "depends".
Before we go into more detail to the answer, we should look at how an RO system is and how it works.
First the definition of osmosis: movement of a solvent through a semi-permeable membrane into a solution of higher solute concentration that tends to equalize the concentration of solute on both sides of the membrane.
Basically what this means is that nature will find a way to move systems that are out of equilibrium, into equilibrium. Osmosis is one of such ways. In nature the wall of a cell can act as a semi-permeable membrane (a semi permeable membrane is one that will allow the passage of water only). When the concentration of salts (the solute) is higher inside the cell than in the environment surrounding the cell, the difference in concentration (osmotic pressure) will cause solvent (water) to pass through the cell's wall and in this way balance the amount of salts within the cell to its environment.
Osmosis is a little counterintuitive. Another example would be this: consider that you have two columns of water separated by a semipermeable membrane (fig 1). Column A has water with a low salt concentration and column B has water in it with a high salt concentration. At the initial time both columns have the same amount of water.

After a period of time, water from column A will permeate through the membrane to column B in order to try to balance the concentration of solute in the water. Once the solutions have reached equilibrium, the difference in height will determine the osmotic pressure.
If at this time you're thinking: "hold on a minute, this seems to be backwards fro what my RO unit does!". You are correct, this is osmosis, what your RO unit does is reverse osmosis.
If we go back to the initial drawing, but this time we apply pressure, through a piston for example, to column B. We would have to apply a pressure equal or larger to the osmotic pressure of the solution to force water from Column B to Column A. And what we would have in Column A is known as the permeate, which is very high purity water. The left over in Column B is referred to as concentrate.
So a reverse osmosis system forces the water that has a high concentration of salts across a semi-permeable membrane by applying enough force to overcome the osmotic pressure of the water. Every RO unit will have a permeate, in your home application this will be the drinking water, and a concentrate, in your home this will be the water that goes to the drain.
RO units produce very high quality water, it removes practically all salts and any pollutant that may be in the water.
The primary problem with RO units is membrane fouling. The membranes are prone to mineral scaling and biofouling (bacteria). As the membrane fouls the efficiency of the RO diminishes until it gets to the point where replacing the membrane, or cleaning it is necessary.
The Zeta Rod is an excellent pretreatment option for RO units, for it will prevent the membranes from fouling, thus extending the life of your RO membranes, and reducing the cost of maintaining and operating the RO unit.
SO, going back to the original question of the post, It depends on what function is the RO unit performing.
If you have an RO unit because you have a private well and there may be some contaminants in the water, then the RO should stay and the Zeta Rod will be of great help to the unit by keeping the membranes from fouling.
However, if you have an RO unit just to prevent scaling in your dishes, sink, faucets, etc. Then the Zeta Rod can be a suitable replacement for the system.
Zeta Corporation has treated RO units ranging in size from small under the sink units to large municipal plants. This Case Study shows the benefits that the Zeta Rod provided to a Pepsi Cola bottling plant's RO unit.
So if you have an RO unit at home, installing a Zeta Rod system will greatly benefit your RO operation and help you with scaling issues in the rest of your home.
For any questions or comments, please contact Zeta Corporation at 888-785-9660, or by email to info@zetacorp.com.