How Reverse Osmosis Works

From Design to Cleaning

Reverse osmosis (RO) produces high-purity water for use in many applications, including low and high pressure boilers, manufacturing processes, cleaning, and ultrapure water systems. RO offers numerous benefits that can save water, fuel, operating costs, labor, and more. Understanding how it works will help in the selection, maintenance, and operation of RO systems.

How RO Works
RO uses a semipermeable membrane to produce high-purity water.  While water is allowed to easily pass through this semipermeable membrane, other dissolved molecules and ions are selectively prevented from passing.  Depending upon the ion or molecule, upwards to 99% can be removed. Dissolved gases will pass through an RO membrane, though.

Imagine a rectangular tank split in two by a semipermeable membrane (Figure 1). Pure water, with no dissolved solids, is added to one side of the tank (Figure 2). Given enough time, the pure water will move from one side of the semipermeable membrane to the other (Figure 3). The levels on both sides of the tank would be even.


Now imagine adding dissolved solids (e.g., calcium, magnesium, sodium, sulfate, bicarbonate, chloride) into one side of the tank (Figure 4). The natural process of osmosis is going to take place. 





What is osmosis?  Osmosis is the process where molecules of a solvent (water) pass through a semipermeable membrane from a less concentrated solution to a more concentrated solution to equalize the concentrations on both sides of the membrane. This results in the level on the more concentrated side increasing, and the level on the pure side decreasing (Figure 5). If not for gravity, the difference in levels would be even greater.

Next, imagine a tightly-fitting piston or plunger (Figure 6) that applies pressure to the higher, more concentrated solution. As pressure increases, pure water is forced back through the semipermeable membrane in the “reverse” direction of natural osmosis (Figure 7). Thus, it is called reverse osmosis (RO). 




The amount of pure water that can be removed from the concentrated solution (Figure 8) will primarily depend upon the level of dissolved solids, the pressures used, and the scale-forming dissolved solids present. Concentrating scale-forming dissolved solids beyond their point of solubility will foul the semipermeable membrane surface and decrease performance of the RO unit. 

Typically, 75-80% of water fed to an RO unit is converted to high purity water (also called permeate) with the rest sent to the drain as reject water (unless other uses can be found). An RO system operating at an 80% recovery rate produces 80 gallons of permeate for every 100 gallons of water that goes through it (only 20 gallons of reject water is sent to the drain). Effective pretreatment can allow higher recovery rates and longer runs between cleanings.

Pretreatment can be an integral part of RO performance. 

  • Carbon filters or an oxygen scavenger feed can remove chlorine which will irreversibly damage membranes. 
  • Water softeners will remove hardness which will reduce output by fouling the membrane surface. 
  • Chemical antiscalants may also be used to minimize problems associated with membrane fouling.
  • Filtration may be required to remove suspended solids which can foul the system.

The precise pretreatment required will depend upon the water quality feeding the system and the RO requirements. Having a good pretreatment program in place can help facilitate a better return on your RO investment.

RO Design
Most commercial and industrial systems use multiple membranes arranged in parallel and stages to provide the required quantity and quality of water. Concentrated water from one stage is directed into successive membrane stages for greater efficiency and usability. Processed permeate water from the first stage can also be passed through additional membrane stages to achieve a lower dissolved solids level in the finished water, if required.

RO Cleaning
As RO membranes begin to foul, system performance will decline. It is important to clean at the proper time so the RO system can be returned to full functionality.  Typical types of cleanings include caustic and acid cleanings. Waiting too long to clean can have a negative impact on future RO performance. 

Knowing when to clean requires “normalizing” the data to adjust for the effects of pressure and temperature changes over the runtime of the RO. Water treatment providers and membrane manufactures offer software programs to help users normalize their RO data to appropriately target when cleanings are warranted. 

A cleaning frequency of 3-4 times a year to even a year or more between cleanings is possible, depending upon water quality, effectiveness of pretreatment equipment, operation, past cleaning history, and so forth.

Using RO with Low Pressure Boilers
Using RO pretreatment to provide high-purity makeup water to a boiler system may reduce blowdown and makeup requirements which ultimately saves:

  • Fuel
  • Water
  • Chemicals

Of this list of three, fuel savings is typically the greatest value.  A full economic and mass balance analysis should be conducted to see if RO is the appropriate technology for a boiler system.

Reverse Osmosis is an accepted, well-tested technology for producing high purity water for low and high pressure boilers, manufacturing processes, cleaning, ultrapure water systems, and more. Like any technology, its appropriateness for the application must be thoroughly evaluated for success to be found. Chem-Aqua can help you determine whether RO is applicable to your system or process needs.  Contact Chem-Aqua today!

Written by:  James McDonald, PE, CWT

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