Reverse osmosis (RO) is one of the most effective methods for purifying water, removing dissolved salts, contaminants, and impurities through a semi-permeable membrane. However, when it comes to producing potable water, the journey doesn’t end there. The water that comes out of an RO system is often not yet suitable for immediate use, consumption or storage. That’s where post-treatment processes come in RO come in.
These processes ensure that RO treated water meets specific quality, safety and performance standards required for potable applications. Whether used for drinking, municipal supply or other consumable purposes, post-treatment is essential to achieving safe, stable and high-quality water. This article explores the key post-treatment processes in RO systems and why each one matters.
1. pH Adjustment
RO membranes remove alkaline minerals, which can leave water slightly acidic. This low pH can cause corrosion in distribution systems and may affect the taste of drinking water. pH adjustment is a post-treatment process that raises the pH to a more neutral level, often using calcite filters or chemical dosing (e.g., sodium hydroxide). This step helps stabilise the water, ensuring it is safer and more palatable for consumption.
2. Rehardening (Remineralisation)
Because RO also strips out beneficial minerals like calcium and magnesium, the water may taste flat or considered too ’soft’ for certain uses, like drinking. Rehardening, also known as remineralisation, adds back essential minerals to improve taste and meet potable water standards. It’s commonly performed using calcium carbonate. This process is often combined with CO2 injection for absorption and pH control, ensuring that the water is not only safe but pleasant to drink.
3. Ultra Violet (UV) Disinfection
Although RO membranes will remove large matter such as microorganisms, the RO permeate will still be susceptible to biological contaminants – especially if water is stored in open tanks where bacteria could grow. For potable water, UV disinfection is critical as it uses ultraviolet light to kill bacteria, viruses and other pathogens, without adding chemicals. This process ensures that the water is microbiologically safe for drinking.
4. Chlorine Dosing Disinfection
Commonly RO permeate is chlorinated to a maximum of 1 mg/l as outlined by WHO Guidelines, when the RO permeate is to be stored in storage tanks. While UV disinfection sterilises the product water without chemicals but would leave the bio-matter in the water, with chlorination any microorganisms would be destroyed. Chlorination also continues to work if the water is exposed to ongoing bacterial contamination whereas UV does not.
5. Deionisation (DI)
In some high-purity applications (like laboratories or pharmaceutical manufacturing) even the trace ions left behind after RO need to be removed. Deionisation is a post-treatment process that uses ion-exchange resins or electrodeionisers (EDI) to polish the RO water to ultra-low conductivity, producing exceptionally pure water.
6. Blending
In certain situations, RO water is blended with a controlled amount of treated feedwater or remineralised water. This helps fine-tune total dissolved solids (TDS), taste, or mineral balance – especially in municipal or bottled water applications. Blending also improves stability and helps avoid overly aggressive water that may damage pipes or equipment.
Other Optional Post-Treatment Additions
Depending on the application, additional post-treatment processes in RO systems may include:
Oxygen Removal – Degassing of dissolved oxygen would benefit industrial systems requiring high purity, e.g. boiler feed and semiconductor manufacturing.
Recirculation loops – As an extra step to sterilisation, continuous loops can be implemented to prevent stagnation and further discourage bio growth.
TOC Reduction – Further treatment steps such as advanced oxidative processes can be used to reduce organic carbons to greatest level for specific applications.
Choosing the Right Post-Treatment for Your RO System
The ideal post-treatment processes in RO systems depend on the end-use of the water:
| Application | Common Post-Treatments |
| Drinking Water | pH adjustment, remineralisation, UV disinfection |
| Industrial Systems | Oxygen removal, DI, blending, chemical dosing |
| Pharmaceutical / Lab | Deionisation, UV, storage with recirculation |
Other chemical methods of disinfection include chlorine dioxide dosing.
Selecting the right post-treatment processes in RO systems depends heavily on the final water use, ensuring that every drop meets the quality and compliance standards required for its application.
While reverse osmosis is powerful on its own, post-treatment processes in RO are essential to deliver potable water that meets compliance and application requirements. From pH balancing to disinfection, these steps are indispensable in the journey from raw water to safe drinking water.
Need expert advice on the right post-treatment processes in RO for your system?
Contact Salt Separation Services today at sss@saltsep.co.uk – our experienced team is here to help with guidance for your application.