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RO Pretreatment & Post Treatment

Pre and post treatment for industrial RO systems

The RO system is a simple machine, but what happens before and after an RO system is complicated and dynamic.

Puretec has the expertise to make sure that the feedwater to your RO system is conditioned to protect the RO system from damage and the post-RO equipment and service to bring your RO permeate to the highest quality possible for the end-user.


It’s important to condition the feedwater before it reaches the RO membranes to extend the life of the system. If an oxidizer, such as chlorine, encounters the RO membranes, then irreversible damage will occur. Furthermore, by reducing suspended solids in the feedwater and adding scale inhibitors, you can delay RO membrane cleaning and also increase the RO recovery rate, thereby saving you water, energy, time, and money.

At Puretec, we understand the critical importance of maintaining the efficiency and longevity of your industrial reverse osmosis (RO) systems. RO operations face challenges, primarily due to the buildup of biological or mineral contaminants, known as “fouling” and “scaling”. These issues not only decrease production but also increase operational costs and energy consumption. There is also the threat of oxidizing RO membranes by exposing them to an oxidizer like chlorine that is common in feedwater. Let’s dive deeper into what causes these problems and how Puretec can help you combat them.

Understanding Fouling & Scaling

Fouling: This occurs when suspended contaminants in the water deposit on the membrane surfaces, obstructing water flow and reducing efficiency. Fouling tends to happen in the front end (1st stage) of an RO system.

Scaling: Scaling happens when mineral salts concentrate enough to precipitate and deposit on the membranes, further hindering performance. Scaling tends to be more severe in the back end of an RO system (2nd stage).

Both fouling and scaling lead to increased osmotic pressure and lower permeate flow, which can strain the system, leading to higher energy use and potentially damaging the membranes prematurely.

Causes of Fouling and Scaling

Contaminants that contribute to these issues can be broadly categorized into:

Fouling Agents: Including colloids (clays, flocs), biological elements (microorganisms, bacteria, viruses), and organics (oils, humics, biopolymers).

Scaling Agents: Predominantly minerals like sulfates, calcium, magnesium, carbonates, and silica.

Methods for addressing fouling and scaling before an RO system

Fouling can be minimized by using multi-media filters, microfiltration, ultrafiltration, and cartridge filters to reduce suspended solids and turbidity.

Scaling can be delayed by using specially formulated anti-scalants that inhibit scale formation on the membrane surface. Softening by ion exchange prior to the RO system is sometimes also used. This reduces hardness in the water to reduce scaling. Lowering the feedwater pH has also been used as a method for preventing certain types of scale. However, this results in excess CO2 that reduces ion exchange capacity if there are mixed beds after the RO.

Problems Caused by Chlorine in RO Feedwater

Chlorine is commonly used in water treatment as a disinfectant due to its effectiveness in killing bacteria, viruses, and other pathogens. However, its presence in feedwater for Reverse Osmosis (RO) systems can cause thin-film composite (TFC) membranes to quickly deteriorate. This will result in lower rejection rates of salts, an increase in permeate conductivity and ultimately, membrane failure.

Choosing the right method for chlorine removal depends on various factors, including the chlorine concentration in the feedwater, the size of the RO system, and the specific requirements of the water treatment process. Regular monitoring of feedwater chlorine levels and system performance is crucial to ensure the effectiveness of the chosen chlorine removal strategy and to protect the RO membranes and system integrity.

Methods for Removing Chlorine from RO Feedwater

To protect RO membranes from chlorine damage, it’s essential to remove chlorine from the feedwater before it reaches the RO system. Several methods are available for chlorine removal:

Activated Carbon Filtration – This is one of the most common and effective methods for removing chlorine. Activated carbon adsorbs chlorine and organic compounds from the water. They need to be replaced periodically to maintain effectiveness.

Sodium Metabisulfite Dosing – Sodium metabisulfite is a reducing agent that can neutralize chlorine by converting it into chloride ions. This chemical is dosed into the feedwater in precise amounts before the water enters the RO system. The dosing rate must be carefully controlled to avoid under or overdosing. If you under dose sodium bisulfite, it can result in oxidation damage to the membrane. If you overfeed sodium bisulfite, then it will become a nutrient for certain bacteria, resulting in biofouling of the RO membranes.

Optimizing RO Operations with Pre-Treatment

To ensure your RO system operates at its peak, identifying and implementing the right pre-treatment processes is key. Based on a thorough analysis of your water sources, Puretec offers tailored solutions including:

Filtration: Targets colloids, some organics, and biological contaminants. – Anti-Scalant: Specifically designed to prevent mineral scaling.

Coagulation: Effective against organics, colloids, and certain minerals and biological pollutants when used in conjunction with multi-media filtration.

Monitoring for Maximum Efficiency

Beyond pre-treatment, monitoring the system’s pH, temperature, and recovery rate is crucial for preventing fouling and scaling. Adjusting these parameters can significantly reduce the concentration of problematic substances, thus maintaining system efficiency and prolonging membrane life.

Puretec's Commitment

At Puretec, we’re deeply committed to ensuring the optimal performance and longevity of industrial Reverse Osmosis (RO) systems through our comprehensive pre and post-treatment services. Understanding the criticality of water quality in industrial applications, we harness advanced technologies and tailored solutions to protect and enhance your RO systems. From the outset, our expert team employs a meticulous approach to pre-treatment, utilizing filtration, water softening, chemical dosing, and other methods to prevent fouling, scaling, and membrane damage. Following RO treatment, our post-treatment services kick in to refine water quality to the highest purity levels, incorporating deionization, UV disinfection, high purity piping materials, loop design, final filtration, and system sanitizations to meet your specific requirements. At Puretec, we don’t just offer solutions; we forge partnerships, working closely with our clients to monitor, maintain, and optimize their water treatment systems, ensuring they achieve maximum efficiency, reliability, and compliance with industry standards. Our commitment to excellence in both pre and post-treatment services underscores our dedication to your success, providing peace of mind and unparalleled water quality that supports your operations.

RO Post-Treatment

Post-treatment in reverse osmosis (RO) systems, especially those designed to produce high purity water, is a critical phase that ensures the final water quality meets the specific requirements for its intended use. High purity water is essential in various industries, including pharmaceuticals, semiconductors, power generation, and laboratory research. The goal of post-treatment is to further refine the water quality by removing any remaining contaminants and stabilizing the water to prevent damage to equipment or processes downstream. Here’s what is typically involved in the post-treatment process for high purity water systems:

Storage and Distribution

RO permeate needs to be stored before being distributed throughout the post-treatment system. Different types of storage tanks are available for different applications. Most are also equipped with vents that have hydrophobic vent filters to prevent contamination from entering the system and they also need drains to allow for effective draining, sanitization and flushing.

Polishing RO Permeate with DI Tanks (Ion Exchange)

After RO treatment, water often passes through deionization or polishing units to remove the last traces of dissolved solids. Mixed bed ion exchange tanks are commonly used for this purpose, capable of producing ultrapure water with very low conductivity levels.

Ultraviolet (UV) Disinfection

UV treatment is a non-chemical method for disinfecting water by inactivating bacteria, viruses, and other microorganisms. It’s particularly important in high purity water systems to ensure microbial levels are kept to an absolute minimum. UV treatment can also break down trace levels of organic compounds that may have passed through the RO membranes.

Sub-Micron Final Filtration

In some high purity systems, sub-micron (0.22 or 0.1 micron) is used as a post-treatment step to remove any remaining particulate matter, bacteria, or pyrogens just before the final point of use.

pH Adjustment

The pH of RO water can be quite low due to the removal of buffering ions. In certain applications, it may be necessary to adjust the pH to a neutral range to prevent corrosion in piping and storage systems or to meet process requirements.

Total Organic Carbon (TOC) Reduction

Even low levels of organic compounds can be detrimental in high purity applications. Additional treatment steps, such as advanced oxidation processes (AOPs), can be used to reduce TOC levels to parts per billion (ppb) or even parts per trillion (ppt).

Oxygen Removal

Dissolved oxygen can be problematic in high purity water, leading to corrosion in power generation and semiconductor manufacturing processes. Oxygen removal can be accomplished using vacuum degasification towers or chemical dosing with reducing agents like sodium sulfite.

High Purity Piping Materials

Piping materials like Polyvinylidene fluoride (PVDF), Polypropylene (PP), or electropolished Stainless Steel 316L, known for their smooth surfaces, chemical resistance, and minimal leachability, to reduce bacterial adhesion and biofilm formation. Along with high purity piping, we also employ sanitary fittings and connections in certain applications that are designed for easy sanitization, with minimal crevices where bacteria can accumulate.

Continuous Recirculation Loop Design

Implementing a loop design that allows for constant recirculation of water, ensuring there are no periods of stagnation. This constant movement helps inhibit bacterial growth and maintain water quality. We also design the piping layout to minimize dead legs, areas where stagnant water can accumulate, to less than 1.5 times the pipe diameter. This reduces zones where bacteria can grow undisturbed.

System Sanitization and Maintenance

The system should be designed for ease of cleaning and sanitization. Regular maintenance and sanitization routines are crucial to prevent microbial contamination and ensure the integrity of the high purity water.

Automated Monitoring and Control Systems

Post-treatment systems have integrated sensors and control systems to continuously monitor such things as flow rates, pressure, temperature, conductivity, and resistivity, allowing for real-time adjustments and alerts to maintain optimal system performance.

Managing high purity water systems involves a comprehensive approach that extends beyond the RO unit itself. Each post-treatment step is designed to target specific contaminants or water quality parameters, ensuring that the final product meets the stringent requirements of the application. Regular monitoring and maintenance of the entire system are essential to ensure consistent water quality and system performance.