Reverse osmosis (RO) is widely used in industrial water treatment for its ability to produce high-purity water. But every drop of clean water produced by an RO system leaves behind a more concentrated stream—commonly known as RO reject or brine. Managing this reject stream efficiently is just as important as running the system itself, especially when you're dealing with rising water costs, stricter environmental norms, and sustainability goals.

The good news? With smart planning and the right technology, you can minimize both the volume and impact of RO reject—cutting down on waste, costs, and compliance headaches.

Understanding the Problem with RO Reject

RO systems typically recover 70–85% of the feedwater, leaving behind 15–30% as reject. This reject is rich in dissolved salts, organics, and other contaminants removed during the filtration process. If not managed correctly, it can lead to:

Higher wastewater disposal costs
Regulatory compliance issues
Environmental damage (especially in zero-discharge zones)
Lost opportunity to reuse valuable water

That’s why reverse osmosis design today must include a solid plan for managing the reject stream—not just treating it as an afterthought.

1. Optimize Recovery Rates from the Start

The first step in minimizing reject is to recover as much clean water as possible without damaging the membranes. Modern RO systems can be fine-tuned to recover up to 90% of water through:

Multi-stage RO designs
Pressure-boosting pumps
Advanced membrane technologies
Nanofiltration pretreatment

By combining nanofiltration and reverse osmosis, many industries are able to reduce hardness and organics before the water hits the RO system—allowing higher recovery with less fouling.

2. Reuse the Reject Stream Internally

Instead of sending RO reject straight to the drain, look for ways to reuse it within the facility. Depending on its composition, reject water can be used for:

Cooling tower blowdown makeup
Floor or equipment washdown
Landscaping or flushing (after minimal treatment)

This approach helps reduce the demand for freshwater and cuts back on disposal volumes—both of which save money over time.

3. Pair RO with ZLD or Evaporation Systems

For facilities aiming for Zero Liquid Discharge (ZLD), RO is just the beginning. The reject can be sent to evaporators, concentrators, or crystallizers to recover even more water and convert the remaining waste into solid form. Though the capital cost is higher, this approach eliminates discharge completely and can help meet strict environmental norms.

4. Use Brine Minimization Technologies

Emerging technologies such as:

High-recovery RO
Electrodialysis reversal (EDR)
Forward osmosis (FO)

...are being used to squeeze more water from the reject stream. While still evolving, these tools offer new ways to reduce brine volume while improving water reuse potential.

5. Analyze and Treat for Selective Recovery

Some reject streams contain valuable minerals or metals that can be recovered and reused. With selective treatment and chemical dosing, certain industries (like mining or battery recycling) are finding ways to turn waste into a resource.

Final Thoughts: Waste Less, Save More

Effective reverse osmosis doesn’t stop at clean water production—it also includes smart reject management. By optimizing recovery rates, reusing reject water where possible, and integrating advanced recovery technologies, industries can dramatically reduce waste, meet environmental goals, and save on long-term operational costs.

Don’t let RO reject go to waste. Treat it like the valuable resource it can be.

RO Reject Management: Strategies for Minimizing Waste and Costs