What Is Reverse Osmosis (RO)?


It is one of the popular and best water filtration methods available in the market today. It works by forcing water across a semi-permeable membrane, leaving contaminants behind that are then flushed down the drain. In other words, this method allows you to deionize or demineralize water by forcing it under pressures through a semi-permeable reverse osmosis membrane.


To have a better understanding of RO, it is prudent also to understand osmosis, which is a naturally occurring phenomenon. Basically, osmosis is a process where a weaker saline solution will tend to migrate to a strong saline solution. A good example is how our kidney absorbs water from our blood.

Semi-Permeable Membrane

This is a membrane that allows some molecules or atoms to pass but not others. A good comparison of this membrane is a screen door. It only allows air molecules to pass through but not pests or anything larger than the holes in the screen door.

How Does A Reverse Osmosis System Work?

As mentioned above, reverse osmosis works by forcing water through a semi-permeable membrane. High pressure is used to increase pressures on the salt side of the RO, forcing the water across the semi-permeable membrane, leaving contaminants behind that are then flushed down the drain. You are finally left with clean-tasting drink water.

What Does A RO System Remove?

RO is capable of removing almost all contaminants available in the water. That is, up to 99%+ of the dissolved salts (ions), particles, colloids, organics, bacteria and pyrogens from the feed water. But remember, an RO system is not a reliable removal of bacteria and viruses from water. Basically, RO is very effective in treating the surface, brackish, and groundwater for both small and large flow applications. 

Here is a list of impurities RO remove from water:

  • Pesticides and herbicides
  • Sediment
  • Arsenic
  • Lead
  • Fluoride
  • Chlorine and chloramine
  • Detergents
  • Nitrates & Sulfates

Benefits of A RO System

  • Reduces or remove bad taste and odors.
  • Reduce sodium.
  • Easy to install and maintain
  • Remove harmful dissolved contaminants in water.
  • More environmentally friendly than bottled water
  • Fits under the kitchen sink

Does Reverse Osmosis Wastewater?

An RO system sends water with rejected contaminants (removed salts, dissolved pollutants, and minerals) down the drain as wastewater. 

For example, 4 gallons of water exits the drain for every gallon of water produced. The wastewater helps clean the water, just like a washing machine uses water to clean clothes. 

How to Reduce Wastewater In A RO System?

Consider doing the following:

  • Using the RO reject water for artificial lakes or landscaping.
  • Add a permeate pump to a reverse osmosis system to increase its efficiency. Permeate pumps reduce the wastewater from an RO system by 75 to 80%. 
  • Choose reverse osmosis system with an automatic shut off valve. An ASO valve stops the flow of water to the drain once the storage tank is full.

Reverse Osmosis Performance and Design Calculations

There is a variety of calculations that are used to determine the performance and design of a reverse osmosis system. Here is what you need to measure accurately the performance of an RO system:

  • Temperature
  • Feed flow
  • Feed pressure
  • Permeate pressure
  • Concreate pressure
  • Feed conductivity
  • Permeate flow
  • Permeate conductivity

Salt Rejection %

Salt Rejection % = Conductivity of Feed Water – Conductivity of Permeate Water × 100
Conductivity of Feed

The above equation is used to tell how effective is RO membrane at removing contaminants. In other words, this equation tells you more about how the entire system is performing rather than telling you how each membrane is performing. Basically, a well-designed RO system with properly functioning RO membranes will reject 95% to 99% of most feed water contaminants.

Note: Higher salt rejection means the system is performing well. Low salt rejection implies that the membrane requires cleaning or replacement.

Salt Passage %

The salt passage is the inverse of salt rejection. It expresses the amount of salt in percentage passing through the RO system. Here is the equation:

  Salt Passage % = (1 – Salt Rejection %)

Note: Low salt percentage is an indicator the system is performing well. A higher percentage means, you need to clean the membrane or replace it.

Recovery %

It is the amount of water that is recovered as good permeate water or water that is not sent to drain as a concentrate.

% Recovery = Permeate Flow Rate (gpm) × 100
Feed Flow Rate (gpm)

Higher recovery percentage means you are sending less water to drain as concentrate hence saving more permeate water. But too high recovery percentage can lead result to larger problems due to fouling and scaling.

Note: Proper recover % at which an RO operate depends on what the RO system is designed to do. By calculating the percentage recovery, you can quickly determine if an RO system is operating outside of the intended design.

See the example below for more information:

If the recovery rate is 80%, then this means that for every 100 gallons of feed water that enter the RO system, you are recovering 80 gallons as usable permeate water and 20 gallons are going to drain as a concentrate.

Concentration Factor

A very crucial factor in designing an RO system, the concentration factor is related to the RO system recovery. The more water you recover as permeate, the more concentrated salts and contaminants you collect in the concentrate stream. This can lead to a higher potential for scaling on the surface of the RO membrane when the concentration factor is too high for the system design and feed water composition.

Concentration Factor = 1
1 – Recovery %

See the example below:

 If your feed flow is 100 gpm and your permeate flow is 75 gpm, then the recovery is (75/100) x 100 = 75%. 

Concentration factor

 1 ÷ (1-75%) = 4.

From the example, a concentration factor of 4 means that the water going to the concentrate stream will be four times more concentrated than the feed water is. If the feed water in the above example was 600 ppm, then the concentrate stream would be 600 * 4 = 2,400 ppm.


Flux is expressed as volume per area per unit of time. Flux is used to express the rate at which water permeates a reverse osmosis membrane. Typical units of measurement are gallons per square foot per day (i.e. GFD or GSFD) or liters per square meter per hour (l/m2/hr).

Gfd = gpm of permeate × 1,440 min/day
# of RO elements in system × square footage of each RO element

 Mass Balance

(Feed flow1 x Feed Conductivity) = (Permeate Flow x Permeate Conductivity)

 + (Concentrate Flow x Concentrate Conductivity)

1Feed Flow equals Permeate Flow + Concentrate Flow


The above equation is used to determine if your flow and quality instrument is reading correctly or require calibration. In cases the instrumentation is not reading correctly, that is an indicator the performance data trending that you are collecting is useless.

Data to collect from an RO system to perform mass balance calculation:

  • Feed Flow (gpm)
  • Permeate flow (gpm)
  • Concentrate Flow (gpm)
  • Feed Conductivity (µS)
  • Permeate conductivity (µS)
  • Concentrate Conductivity (µS)

See the example below:

Permeate flow  5 gpm
Feed conductivity  500 µS
Permeate Conductivity 10 µS
Concentrate Flow 2 gpm
Concentrate Conductivity 1200 µS


Solution would be:

(7 x 500) = (5 x 10) + (2 x 1200)

3,500 ≠ 2,450


Then find the difference

(Difference / Sum) x 100

((3,500 – 2,450) / (3,500 + 2,450)) x 100

= 18%

A difference of +/- 5% is ok. A difference of +/- 5% to 10% is adequate. A difference of > +/- 10% is not acceptable and calibration of the RO instrumentation is necessary to ensure that you are getting reliable data. From the example above, the RO mass balance equation falls out of range and requires attention.


The Difference Between Passes and Stages in A Reverse Osmosis System

These two terms (pass and stage) are often mistaken for the same thing in an RO system. It can be confusing if you don’t get the difference. Let’s now dive deeper.

What is the difference between a 1 and 2 stage RO and 1 and 2 pass RO


  • A 1 And 2 Stage RO System

In a one-stage Ro system, the feed water enters the RO system as one stream and exits the RO as either concentrate or permeate water. While in the two-stage system, the concentrate from the first stage then becomes the water to the second stage.

The permeate water collected from the first stage is then combined with permeate water from the second stage. These additional stages increase the recovery from the system. An RO system with concentrate recycle can be utilized at the first stage to help improve the system recovery.

  • Single Pass RO Vs. Double Pass RO

The difference that exists between a single pass RO system and a double pass RO system is clear. When it comes to double pass RO, the permeate from the first pass becomes the feed water to the second pass which ends up producing a much higher quality permeate because it has essentially gone through two RO systems.

A double pass also offers the opportunity to remove carbon dioxide gas from the permeate by injecting caustic between the first and second pass.

Reverse Osmosis Pretreatment

Here are common problems an RO system experiences due to lack of proper pretreatment.

  • Fouling

Fouling of membranes is due to the emulsified or suspended materials that may be present in the feed water to the RO system. Fouling basically takes place when contaminants accumulate on the membrane surface effectively plugging the membrane. A good example of such materials is clay, silica, oil, iron, sulfur, and humic acids. These substances can be present in a very fine or colloidal form. 

Proper pretreatment minimizes the need to worry about fouling related problems. For example, multi-media filters (MMF) or microfiltration (MF) methods are commonly used to prevent fouling. In some cases, cartridge filtration will do.

  • Scaling

Scaling causes a higher energy use and a shorter life span of the membranes. This is because you have to clean the membrane often. Basically, scaling is the deposition of particles on a membrane during RO, causing it to plug. A good example of a common scale that forms on an RO membrane is calcium carbonate. The best way to prevent scaling is by using antiscalants and scale inhibitors. These are chemicals that once added to water, reduce the scaling potential of the feed water. Adding them increase the solubility limits of troublesome inorganic compounds, making it possible to achieve a higher recovery rate and run at a higher concentration factor.

The other option is using a water softener. A water softener can be used to help prevent scaling in an RO system by exchanging scale-forming ions with non-scale forming ions. It is crucial to have a 5-micron cartridge filter placed directly after the water softener just in case the underdrains of the softener fail.

  • Chemical attack

Modern thin-film composite membranes are not tolerant of chlorine or chloramines, and the result of any chemical attack is significant. For example, oxidizers such as chlorine cause irreparable damages by leaving huge holes on the RO membrane. Such an attack results in a higher permeate flow and poor quality permeate water.

  • Mechanical Damage

There are various reasons that can lead to mechanical damage, such as pre and post plumbing. The best fix is using variable frequency drive motors to start high-pressure pumps for RO systems and installing check valve(s) and/or pressure relief valves to prevent excessive back pressure on the RO unit that can cause permanent membrane damage.

Other Pretreatment Solutions for RO Systems

  • Sodium Bisulfite (SBS) Injection: SBS, which is a reducer once added to water stream before an RO at the right amount, can help remove residue chlorine.
  • Granular Activated Carbon (GAC): GAC, which is made from coal, nutshell, or wood is used for both removing organic constituents and residual disinfectants (such as chlorine and chloramines) from water.

RO Data Trending and Normalization

It is good to note the RO membranes are the heart of the RO system, and certain data points need to be collected to determine their health. These data points include flows, temperature, and system pressures.

These data points once analyzed help in normalization, so those flow variations are not interpreted as abnormal when no problem exists. The results of the analysis also help in determining when is the right time to clean the membrane or replace it.

As a general rule of thumb, when the normalized change is +/- 15% from the baseline data, then you need to take action. If you don’t follow this rule, then RO membrane cleanings may not be very effective at bringing the membranes back to near new performance.

RO Membrane Cleaning

Periodic cleaning is very crucial. Often the quality of feed water will help you determine when to clean RO membrane. As a general rule, if the normalized pressure drops or the normalized salt passage has increased by 15%, then it is time to clean the RO membranes. If the normalized permeate flow has decreased by 15%, then it is also time to clean the RO membranes.