With an industrial RO system from Arbortech Corporation, plants having an incoming water source that is of lower quality will benefit from the lower level of total dissolved solids (TDS), not just lower hardness.  The advantage of utilizing a properly-selected industrial RO as opposed to a DI water unit is seen in the savings realized by producing ultra-pure water in-house versus replacing ion exchange (IX) tanks.  Since Arbortech provides IX as a service, you can appreciate that this is an objective, not subjective opinion.

WHAT RO DOES AND CONSISTS OF...

The RO process uses membranes to remove dissolved solids, organics, pyrogens and bacteria from water.  RO also removes sodium, chloride, hardness, fertilizers, insecticides, arsenic, heavy metals and many other contaminants.  

Perhaps most importantly, RO reduces the level of total dissolved solids (TDS).

An industrial RO system typically consists of four or five stages:

1. Pre-treatment;

2. The RO system itself, including the membranes and instrumentation;

3. An atmospheric storage tank for the RO permeate;

4. A pressurization system to distribute the stored RO water to the application(s);

5. Post-treatment if required.

Pre-treatment methods are determined through review of a prospective customer's complete water analysis.  Arbortech Corporation has done extensive testing in some areas of Northern Illinois and has a good idea of what to expect from some wells.  In other geographical areas,  the well digger or local water treatment dealers may have information on expected contaminants in nearby water supplies.  Pre-treatment is crucial to the proper functioning of an RO system and required items will often vary from well to well.  The Arbortech Corporation recommends a detailed analysis of the water quality so a proper recommendation for pre-treatment can be made.  A detailed water analysis can often be obtained from local laboratories for about $150.00.  With proper pre-treatment and an appropriately conservative selection of the overall equipment package, it is not difficult to achieve RO efficiencies of up to 75%, which means that for every 4 gallons drawn into the system, up to 3 gallons may be purified and made available for use.

Pre-treatment may be as simple as a sediment filter or as intricate as implementing an anti-scalant chemical feeder for very hard water in high efficiency RO systems.  Many water supplies serving industry have already been filtered to remove total suspended solids (TSS), which are different than total dissolved solids (TDS), but in some cases initial or additional filtration must be provided to remove the suspended solids before the RO removes the TDS.

Pre-treatment helps to ensure that feed water entering the RO is free from sediment and other agents that may harmful to the RO membrane.  Carbon may be used to remove chlorine if the water source is chlorinated for disinfection, for example.

RO systems often include components like a high pressure pump to drive the process of separation, the membrane(s) and membrane housing pressure vessel, and a concentrate adjustment valve.  The high pressure pump forces water into the pressure vessel which houses the membrane where the membrane's tiny holes, as small as 0.0006 microns, make the separation.  As the water is pushed through these small openings, pollutants in the water stay behind and are washed away by water escaping the pressure vessel.

The chart here following lists typical and approximate rejection rates for common impurities by an RO system...

Numbers indicate a typical  percentage range for ions removed

An atmospheric storage tank enables the system to meet peak water demand when demand exceeds the instantaneous rate at which the RO can produce water.  For example, let's presume that a purchased RO system is capable of 800 gallons per day , or an average rate of 33.3 gallons per hour.  If the instantaneous flow required is periodic but, when needed, in excess of that 33.3 gph, say 50 gallons per hour for example, then supply directly from the RO would be insufficient.  In such a case, operating the RO around the clock, with an adequately sized storage tank, and drawing water from said tank makes up for any instantaneous shortfall.  While storage tanks can be of any size or shape and should be selected based upon the specifics of the application, our experience suggests that commonly used tank sizes store between 200 and 10,000 gallons of water.  A liquid level switch in the storage tank is used to turn the RO pump "on" when the water level in the tank drops and "off" when the tank is full.  Because these tanks are typically vented to the atmosphere, contaminants may enter, causing bacterial growth, which is often controlled with ozone or chlorine.  In other cases, water may be sterilized with an ultraviolet sterilizer after it leaves the tank.

Pressurization systems for movement of the stored water typically include a pump and a bladder tank.  Selection of the components is based upon the required water pressure and those components are case specific.

Post-treatment can incorporate many processes.  Permeate (the treated water coming out of the RO system) can be passed through a granular activated carbon tank to improve the taste and odor of the water if the application so requires.  Carbon can also remove trace amounts of hydrogen sulfide in the water.  Depending on the pH of the feed water, use of an RO can decrease the pH, sometimes leaving it slightly acidic.  Since acidic water can corrode piping and fixtures, the addition of calcium carbonate is often used to increase the pH.  Gases, such as carbon dioxide and hydrogen sulfide, will pass through the RO system.  These can be removed by post-treatment, such as aeration.

Thanks for spending the time to read the above description of industrial Reverse Osmosis.  
Detailed Report on Reverse Osmosis - Application and sizing.