Reverse Osmosis (RO)A Polymeric Membrane Solution
Reverse Osmosis Overview
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What is Reverse Osmosis?
Reverse Osmosis refers to a classification of membranes where separation range is categorized by rejection characteristics of a known solute, traditionally sodium chloride (NaCl). Typical ranges of rejection are 96-99.8%. Like NF, separation with RO is largely based on diffusion of dissolved species through the membrane and overcoming osmotic pressure of the process fluid. The membrane configuration is usually cross-flow. For numerous process applications, polymeric RO membranes are produced via application of a thin film (e.g. polyamide) to a polysulfone (PS) UF substrate.
Examples of solids that will not pass through the membrane into permeate include proteins, virus, bacteria, and suspended solids greater than 0. 1-1 nm, or with a molecular weight cut-off (“MWCO”) of <10 Da. Any remaining high value materials left in a process stream would typically be concentrated by RO, making this format attractive as a final filtration step.
How do you ensure performance from Reverse Osmosis membranes?
Variables to monitor performance
There are several variables which are used to monitor RO system performance. These include the following operational parameters:
- Feed flow, pressure, and conductivity
- Permeate flow, pressure, and conductivity
- Retentate flow, pressure, and conductivity
- Other – parameters such as protein concentration and COD are utilized in process applications to measure performance. However, since these tests typically require offline measurement and/or more advanced analytical procedures, conductivity is oftentimes used as a proxy.
Calculations to validate performance
Calculations can also be performed to understand rejection and passage. Simplified formulas are provided below:
Rejection % = (Feed Water Conductivity/Process Variable – Permeate Conductivity/Process Variable) / (Feed Water Conductivity/Process Variable)
Note that conductivity is an effective way to get a quick read on RO performance, but ultimately, measuring the specific rejection of the process variable (e.g. protein) is the best way to measure true process performance.
Passage % = 1- Rejection %
Passage is simply the inverse of rejection. Maintaining high rejection and low passage is the ultimate goal of RO system performance. When this rises, it typically means there is some process issue that requires addressing via CIP, mechanical inspections, or potentially replacement.
Recovery % = (Permeate Flow Rate / Feed Flow Rate) * 100
While recovery is a typical calculation used to measure water treatment performance (e.g, polishing) , it is also valuable in process applications to understand how much process stream is being recovered on a % basis. It is also helpful in understanding how well the system is concentrating the process stream.
Concentration Factor % = 1 / (1 – Recovery %)
Since concentration is typically the main goal of RO in process applications, this is a good way to validate the effectiveness of your application. As is the case with other variables, changes in performance over time should be monitored to ensure optimal system performance.
Benefits of Reverse Osmosis
When properly designed and operated, RO can offer several benefits over traditional separation process:
With advances in element construction and system design, substantial surface area can be designed into a membrane solution vs traditional filtration technologies.
Lower energy consumption
These systems consume far less energy than thermally driven processes.
Ease of operation
RO membrane operations are well understood, and control systems can ensure smooth, safe separation operations.
Minimal negative effect on quality
RO would be able to remove materials without the risk of thermal degradation like evaporative processes.
Industrial Applications of Reverse Osmosis
RO is used broadly across process industries, most namely dairy, food ingredients, biotechnology/life sciences, beverages, and automotive manufacturing operations. Some of the key applications across these industries include the following:
- UF permeate processing (lactose concentration)
- Milk production (concentration)
- WPC and WPI production (solids concentration)
- Polishing (purification of COW water and other process streams)
- Sugar/sweetener processing (concentration)
- Other fermentation processes (concentration, water recovery)
- Polishing (condensate purification)
- Cell mass removal (downstream processing of bulk fermentation)
- Juice production (concentration)
- Final rinse (purification)
- Utility water (purification)
FAQs on Reverse Osmosis
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
- 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)