Reverse Osmosis (RO)

The typical Reverse Osmosis separation process is based on an ionic diffusion process, where a feed solution is pumped across a semi-permeable membrane that freely passes water, but rejects most solutes.

Pressure drives fluids through the RO membrane when the osmotic pressure is overcome. Depending on the application and its components transmembrane pressures (TMPs) can range from 150-600+ psi or 10-40 bar, The membrane retains those compounds in a process stream called retentate. The fluid that passes through the membrane is referred to as permeate. The pressure required to drive this process is dependent on the concentration of the feed – in other words, the more concentrated the stream, the higher the pressure required to overcome osmotic pressure.

As the membranes separate process streams, particles can agglomerate at the membrane surface, causing what is referred to as fouling. This phenomenon can slow down the flow rate and/or increase pressure across the membrane, if the process is not optimized. Additionally, harsh cleaning chemicals can degrade the integrity of a membrane, as well. Maintaining proper operational best practices, such as backwashing in some membrane formats and/or cleaning procedures in others, can prevent fouling. Automation can also help detect this by monitoring flow and/or pressure drop across the membrane.

Reverse Osmosis systems are very robust and typically require little routine maintenance.  When they do, it is recommended that qualified technicians work on these systems to maintain optimal performance.  Some of the typical maintenance tasks around Reverse Osmosis system maintenance include:

• Pre-treatment system – some membrane systems might include a pre-treatment step, such as cartridge filters, MF, and/or UF membranes. These will need to be exchanged periodically to maintain Reverse Osmosis system performance.
• Gauges and other instrumentation – as these systems typically include both manual and automated instrumentation, it is important to check and ensure these are operating correctly. In the case of automation, it is important to calibrate these instruments per the manufacturers’ recommended protocol.
• Valves, solenoids, and other wear parts – it is not uncommon for valves to become stuck and/or freeze, particularly if they are not exercised during normal operations. It is important to turn valves off and on periodically, as well as check to ensure solenoids are operating correctly, to maintain system reliability.
• Element replacement – when a membrane has gone past its useful life, it will need to be replaced. Loss of performance will typically manifest itself in reduced rejection or clarification results and/or through changes in flow rate.  Ensure that technicians are qualified to replace elements, so they don’t damage elements upon installation, and also ensure proper seals to prevent system leakage.

Otherwise, Reverse Osmosis systems will typically have an on-going cleaning protocol, which maintains the health and reliability of the membrane system.  These cleaning chemicals will ensure proper flow and rejection, as well as prevent unwanted microbiological contamination – ensuring optimal performance and sanitary conditions.

Reverse Osmosis concentrates most compounds other than water at very high levels, although there are some exceptions to this such as ethanol. These materials include, but are not limited to:

• Low concentrations of proteins, bacteria, etc. from previous unit operations
• Metal ions
• Dissolved salts
• Sugars
• Low molecular weight organics

Reverse Osmosis membranes are constructed of polymeric materials. The specific polymers typically involved a polysulfone (PS) substrate with a by a thin film such as polyamide coated on top.

The primary form factor for Reverse Osmosis is spiral-wound (alternating layers of flat sheet, feed spacer, and permeate carrier), which is predominantly driven by the adoption of RO technology in water treatment systems.

Feed – The process stream that enters the membrane for clarification and/or fractionation

Flux – The rate of extraction of permeate, which is typically measured in LMH (liters per square meter of membrane surface per hour – l/m²/h) or GFD (gallons per square foot of membrane surface per day – gal/ft²/day)

Fouling – The deposition of solids on the surface of a membrane

Permeate – The liquid stream that passes through the membrane (aka filtrate)

Retentate – The liquid stream that is rejected by the membrane

Concentration Factor – The ratio of initial feed volume to retentate or concentrate, which is an indication of target volume reduction achieved by membrane filtration (see previous section for calculation)