Reverse Osmosis (RO) systems are an important part of your pretreatment program. These systems require regular maintenance to maintain their efficiency and effectiveness. This troubleshooting guide was written to help determine what causes common issues with Reverse Osmosis (RO) performance. Changes in permeate flow, permeate conductivity, and differential pressures are all indications of different causes. RO membranes remove 95 to 99 % of the dissolved salts, organics, bacteria, suspended solids, and any contaminant that has a molecular weight greater than 200. These contaminants accumulate in the membranes and develop into performance issues.
The four main categories of problems are fouling, scaling, mechanical and chemical attack.
Fouling – This is a very common issue that effects membrane performance. Fouling is any suspended solid that accumulates on the membrane surface or in the membranes feed channel plugging the membrane and reducing the permeate flow. This includes: Inorganic particulate (dirt, sand, clay, silt) and Microorganisms (bacteria or living organisms) known as "Biofouling." Biofouling is one of the most common and can be one of the toughest to treat since membranes cannot tolerate disinfectants like chlorine. The feed water will contain no disinfectants which allow the microorganisms to grow and form biofilms. You will typically see fouling issues on the front end of the RO system resulting in higher incoming pressures and lower permeate flow.
Scaling – Scaling is similar to fouling as it is plugging the membranes and reducing the flows, but it is caused by dissolved solids that become more concentrated until they exceed their solubility limits. Every ion has a limit to the amount that can be dissolved in water before it will start to form a crystal and precipitate. The two most common scale compounds are calcium carbonate and calcium sulfate. You typically see scaling issues on the end of the RO system (last stages) resulting in higher differential pressures and lower permeate flow.
Mechanical – Damage to membranes can occur when the primary pump “hard starts” causing water hammer that make the membranes leak. High differential pressures can cause the membranes to telescope and o-rings to fail. Likewise, if the backpressure on the RO is high this will cause damage to the membranes. By adding a soft start or variable frequency drive motors on the primary pump it can decrease the pressure shock on the membranes. Check valves can also be added to the permeate and concentrate piping to prevent excessive back pressure. You typically see higher permeate conductivity as feed water is leaking into the permeate.
Chemical Attack – This is the chemical destruction of the membranes surface as thin composite membranes cannot tolerate oxidizing agents. Chlorine, disinfectants, and other cleaning chemicals can destroy the membrane pores and cause irreparable damage. You typically see higher permeate flow and higher permeate conductivity when there is chemical attack.
Good record keeping by recording system data and trending the data is the most effective tool for discovering and identifying problems. The data collected the first few weeks of start-up or membrane change will give you baselines to compare to future data. At a minimum you should record; membrane feed pressure, concentrate pressure, interstage pressure, feed conductivity, permeate conductivity, feed flow, concentrate flow, permeate flow, and feed temperature. This data will allow you to calculate some indicators on the RO performance.
% Salt Rejection
This calculation tells you overall how well the membranes in the RO system are removing the incoming contaminants. A properly functioning RO will reject 95 to 99% of contaminants, which is measured by conductivity meter. The purer the water, the lower the conductivity. This calculation is useful in identifying chemical attack, mechanical issues, and general wear on the membranes.
This calculation is useful for monitoring and troubleshooting scaling and fouling of the membranes. When membranes start to get plugged with fouling or scale the water passing through them becomes restricted causing more friction and increase pressure drop. The pressure differential can be measured across the whole system, each stage or possibly each membrane housing. Using this data, you can troubleshoot if it is fouling or scaling causing the issue. Generally, fouling occurs in the front of the RO as the particles plug the membranes. Scaling normally occurs towards the last membranes of the RO or last stage as it can exceed the saturation limits. Various differential pressures can be measured allowing you to locate where issues may be occurring. Below is a graph of differential pressure and you can see the decrease when the membranes were cleaned.
Typical Pressure Trends
Normalized Permeate Flow (NPF)
This calculation needs to be normalized as several factors effect the permeate flow: feed pressure, feed conductivity, and feed temperature. Water temperature is directly proportional to pressure. As water temperature decreases it becomes more viscous and permeate flow will decrease or membrane pressure increases, as it requires more pressure to push water through the membranes. And when the water temperature increases the permeate flow will increase. Performance data needs to be normalized so flow changes are not interpreted as an issue. Note: this calculation is more complicated, and most membranes manufactures have data normalization software available. When membranes are new the NPF is at it's maximum and over time as the membranes start to plug up, more pressure is required to produce the same amount of permeate. The normalized permeate flow starts to decline. It can also be used to identify chemical attack or mechanical issue like an o-ring failure. If the water can pass through the membrane easier you will see the NPF increase.
TCF is the temperature correction factor (see membrane manufacturers for program)
This guide will help you identify common problems found with RO systems. However, there are many other factors to consider when operating and diagnosing RO’s that were not discussed in this blog. These include incoming water quality, anti-scalants, pretreatments, membrane autopsies, etc. These issues should be discussed with your water treatment company or a trained RO specialist.
If you have concerns or questions regarding your RO system, feel free to reach out to Watertech and one of our technical engineers will be in touch.
Senior Territory Manager
Rob Pierick has worked for Watertech of America for 18 years and has over 20 years’ experience providing industrial water treatment solutions for boiler, cooling, wastewater applications and healthcare water management, including water testing, Legionella testing and remediation. Rob is a Certified Water Technologist (CWT) through the Association of Water Technologies. The CWT designation involves rigorous testing, peer review, and an ethics declaration. It must be renewed every five years to maintain this designation. Rob holds a BS from the University of Wisconsin Platteville.