The role of reverse osmosis in wastewater treatment

July 7, 2024
The growing demand for wastewater reuse is driving the pressing need for innovative RO membranes that can resist fouling and maintain stable operation with minimal downtime, ensuring that industries meet their production targets efficiently.

Could irreversible organic fouling be relegated to the past for reverse osmosis (RO) membranes?

As freshwater shortages grow globally, the demand for sustainable water recovery and reuse is becoming more urgent. Industries are increasingly turning to additional water sources, such as wastewater streams, for purified, reusable water.

RO systems stand out as a key technology in industrial wastewater treatment, processing billions of gallons annually for reuse. However, these systems frequently encounter operational challenges due to high levels of organic matter in wastewater, leading to biological and organic fouling of RO membranes and higher operating costs. The growing demand for wastewater reuse is driving the pressing need for innovative RO membranes that can resist fouling and maintain stable operation with minimal downtime, ensuring that industries meet their production targets efficiently.

Traditional limitations of RO

RO has been a pivotal solution for water purification across many industries. However, despite its widespread adoption and the expertise of its users, traditional RO systems still present serious operational challenges. Tens of thousands of industrial RO systems in use today are hindered by organic and biological fouling, which lowers the level of overall system performance and causes short membrane life.

Systems treating high-fouling wastewater streams often perform frequent chemical cleanings to maintain membrane performance. That process accelerates membrane degradation, requiring system operators to replace their membranes frequently to maintain adequate system performance and meet water requirements. This cycle of cleaning and replacement not only increases operating costs but also leads to considerable downtime — disrupting plant operations and impacting production goals.

Although “fouling-resistant” RO membranes have been available for many years, they perform similarly to standard RO membranes, and biological and organic fouling continues to be the number one challenge to consistent system operation. Solving these issues is crucial for improving system performance and reducing excess operating costs.

The hidden costs of cleaning

Though nearly all membrane system operators agree that cleaning is a headache and disruptive to plant operation, most do not know exactly what it costs to clean, as these costs vary and are considered part of a general operating budget. Cleaning costs include labor, downtime, make-up water, wastewater treatment, energy and chemicals. While specifics vary (for example, downtime cost is far more painful for a system struggling to keep up with demand, and formulated cleaners cost more than generic, bulk chemicals), an analysis of RO operators in North America found a median cleaning cost of $43.08 per 8040 element, per clean.

When also factoring that users performing alkaline cleanings at least 12 times per year are often forced to replace their RO membranes every two years — and sometimes more often in wastewater applications — one can see that the price of the membrane is a small part of the total cost of ownership.

RO reinvented

RO technological advancements have been incremental over the past 45 years. Most of these changes have been developments in manufacturing technology and the introduction of additives that resulted in higher salt rejection and lower energy operation. However, the differences between RO suppliers have remained minimal. With more companies looking to reduce their freshwater requirements by treating industrial wastewater for reuse, there is an urgent need for innovative treatment solutions now.

To meet the increasing demands and challenges faced by modern industries, a fundamental shift in the chemistry of RO membranes is necessary. ZwitterCo has achieved this innovation by integrating their novel, fouling-immune zwitterionic technology with proven, commercial RO membranes. The result is membranes that require far less cleaning to achieve complete performance restoration, setting a new standard for fouling-resistant membranes.

These membranes are designed to replace conventional and/or fouling resistant RO membranes struggling with organic or biological fouling from feed sources such as wastewater, surface water and treated effluent. Recognizing the need for easy implementation, ZwitterCo RO elements are drop-in substitutes requiring no system modifications to use. These elements are available in low energy, high rejection and high-pressure designs.

Less frequent cleaning

Some of the benefits of less frequent membrane cleaning seem obvious — others not so much. One way to consider the benefits is to group them into the following categories: performance, operational and economic. And do not forget sustainability.

Chemical cleaning causes RO membranes to degrade. Fouling not only causes irreversible performance loss (the root cause of the need for performance factors in RO company projection software), but as membranes are cleaned, they pass more solutes, resulting in higher permeate conductivity. For operators with downstream polishing technologies such as electrodeionization (EDI) or ion exchange resins, higher passage means more frequent regeneration and costs to operate.

Less cleaning has many operational benefits. Let’s face it, cleaning is a headache. Extending the time between cleanings increases uptime, helps operators meet their production targets — both volume and water quality — and avoids the need for extra staff or overtime. RO systems are designed to run smoothly with as little direct intervention as possible. Reducing cleaning frequency makes everyone’s life easier.  

Of course, the above will result in economic benefits. In addition to reducing the cost of chemicals, make-up water, wastewater treatment, energy and overtime requirements, economic benefits may come from more uptime or less unplanned downtime.  Another major benefit is longer membrane life. As described earlier, cleaning kills membranes. By reducing cleaning frequency, membranes will last longer, reducing the frequency of the large expense of replacements.

All these reductions (chemical usage, wastewater generated and sending old membranes to landfill) are good news for the environment. 

Who can this help?

RO users operating on feed waters with high organic content or high biofouling potential have the most acute need for fouling-resistant membranes. This is common for wastewater or treated wastewater effluent, surface water and industrial process streams. Some of the industries most affected include power generation, chemical process industries, refining and food and beverage.

Maintaining water volumes in power generation is vital for ensuring effective operations. In the steam cycle, water and steam losses occur naturally, usually by evaporation or process use. These losses need to be replaced by adequately treated make-up water. Just as maintaining water volumes is crucial to power generation, so is the water quality.

Power generation plants require RO membranes to efficiently purify water to use in various processes — particularly for boiler feed water. Clean, pure water is crucial to avoid scaling and corrosion, which can reduce efficiency and increase maintenance costs. Since many of these plants purify surface water for their make-up, an RO element such as ZwitterCo RO will provide stable permeate flow and quality even on feeds with high TOC. Due to the zwitterionic membrane chemistry, ZwitterCo RO elements require less frequent chemical cleanings, offering power plants reliable operation with significantly less system downtime.

Landfill leachate is an example of a wastewater application where RO is widely used to reduce ammonia, PFAS and trace volatile organic compounds (VOCs) prior to discharge. RO systems generally have multiple passes using seawater or other high-pressure membrane elements to minimize concentrate volume while meeting discharge requirements. In multi-pass processes, the permeate, or product water, from the first pass system is polished by the second pass system (and so on). With an organic-laden feed like leachate, the membranes in the first pass often foul rapidly and require frequent cleaning, sometime as often as daily.

ZwitterCo RO in a high-pressure element, designed for up to 1,200 psi operation, has been used to treat high-strength landfill leachate in the first pass of a multi-pass process. These membranes are reducing the required cleaning and are expected to double membrane life (or more). An economic analysis for this user resulted in potential savings of over $1,450 per year per 8040 element through reduced cleaning and longer membrane life.

Developing more fouling-resistant membranes has been a goal for the RO industry for decades. Though there have been incremental improvements in membrane chemistry, feed spacer designs, biocides and cleaning chemicals, and system operation and design, fouling continues to be the most common cause of system upsets and membrane replacement. This shift in material science innovation not only offers cost savings and reduced downtime, but also introduces a move towards more efficient and sustainable operational practices.


About the Author

Jon Goodman | Vice President, Commercial Development at ZwitterCo

Jon Goodman’s 35-year career in membranes reflects a passion for solving critical challenges. He leverages his expertise in sales, market management, and strategy development as he leads ZwitterCo’s Commercial Development effort. Jon previously scaled commercial operations for TriSep/Microdyn-Nadir/Mann+Hummel and FilmTec Membranes (then part of Dow Chemical). Jon is focused on accelerating growth and market adoption of ZwitterCo’s innovative membrane technology.

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