Reverse Osmosis (RO) is a water treatment technology that separates dissolved contaminants from water using specially-designed membranes. 

RO membranes are semi-permeable which only allow “pure” water to permeate through them while removing the vast majority of dissolved solids from the feedwater stream. RO applications can include drinking water production, power generation, steam boiler pretreatment, wastewater treatment, and the manufacturing of beverage, semiconductor, and pharmaceutical products.

While RO systems provide reliable technology for water purification, they must be closely monitored to ensure the appropriate water quality and to reduce total operating costs such as energy, premature membrane replacement, lost production, and labor. The membranes are the heart of an RO system and require regular monitoring so potential problems can be caught early before they become severe and expensive to plant operations.


To monitor an RO system’s operating conditions and performance, data must be collected, including:

  • Permeate Flow (GPM)
  • Concentrate Flow (GPM)
  • Feedwater Temperature (°F)
  • Feed Pressure (PSI)
  • Concentrate Pressure (PSI)
  • Permeate Pressure (PSI)
  • Feedwater Conductivity
  • Permeate Conductivity

Impacts on Performance

RO membrane performance is not only impacted by scale deposit formation, microbiological fouling, and degradation but also by changes in temperature, pressure, and feedwater dissolved solids concentration. 

For example, warmer temperatures allow a membrane to pass more water and dissolved solids, increasing the permeate flow rate. Higher pressures can also enable membranes to produce more permeate flow. Changes in the feedwater dissolved solids, as measured by conductivity, can cause changes in both the permeate quality and flow rates.

The challenge with accurately evaluating operational data is it is constantly changing due to factors in ways that can be deceiving. During the warmer seasons, an RO may still be able to produce the same amount of permeate even though the membranes are fouling. During cooler seasons, the RO may still be able to have the required permeate flow with higher feedwater pressures, even though the membranes are fouling as well. If an RO were judged by its ability to produce permeate alone, the fouling signs could easily go unnoticed.

Data Normalization

To properly evaluate membrane performance, the data must be normalized to compensate for changes in pressure, temperature, and feedwater dissolved solids. Through normalization, RO operating data can be interpreted to understand the health of RO membranes. This provides an “apples to apples” comparison of how the system is working today compared to when the membranes were new (or newly cleaned) taking into account the impact of different pressures, temperatures, and feedwater dissolved solids. Following data normalization, reliable information can be interpreted, and better decisions can be made for membrane maintenance and system operations.

Three key RO normalization performance indicators are utilized for an “apples to apples” data evaluation include:

  • Normalized Permeate Flow
  • Normalized Salt Rejection
  • Normalized Pressure Differential

These indicators are the cornerstone of a successful RO monitoring and maintenance program.

Normalized Permeate Flow Rate (NPF): Indicates how well the RO membrane produces permeate water. NPF is often considered the most critical monitoring parameter, as it best reflects changes in the RO Membrane performance. If the NPF drops 10% to 15% below the baseline value (the reading at start up with new membranes or when membranes were replaced or cleaned), this indicates fouling or scaling of the RO membranes, and the RO membranes should be cleaned. If the normalized flow rate increases, membrane degradation can be suspected.

Normalized Salt Rejection (NSR): Indicates how well the RO system is at removing dissolved solids from the feedwater and reflects RO permeate quality. It measures the % of total dissolved solids (TDS) in the concentrate versus the feedwater, with 97-99% often considered good.

Normalized Pressure Differential (NPD): Indicates the resistance to water flow through the RO membranes. This measures the difference between the membrane feed and the concentrated pressure. An increase in the pressure differential can provide an early warning sign that something is blocking the water flow, such as scale, biofilm, or even physical debris.


The two graphs below show real-life RO data. The first is the permeate flow read straight off the flowmeter. Overall, it appears to be steady, with no obvious signs that anything is impacting the membranes. The second graph is the same real-life data but normalized to correct for pressure, temperature, and feedwater dissolved solids changes. As you can see, it paints an entirely different story where gradual drops in NPF can be seen due to membrane fouling along with increases in NPF when membrane cleanings were conducted.

permeate flow


With data normalization, regular monitoring, and appropriate maintenance practices, RO systems can efficiently provide sustainable water quality for many industrial, manufacturing, and process applications. 

As a global leader in custom-designed RO water treatment programs, Chem-Aqua has the experience, knowledge, and technology to effectively protect your RO equipment assets. 

Contact Chem-Aqua for assistance today!


Written by: Robert Lynch