Industrial reverse osmosis systems produce purified water and concentrated reject water. Excessive RO reject water increases raw water consumption, wastewater discharge and operating costs.

However, increasing the recovery rate without checking water quality may cause membrane scaling and fouling. The correct approach is to optimize pretreatment, membrane configuration and operating conditions.

What Is RO Water Recovery?

RO recovery rate is calculated as:

Recovery Rate = Product Water Flow ÷ Feedwater Flow × 100%

For example, if an RO system receives 10 m³/h of feedwater and produces 7.5 m³/h of purified water, its recovery rate is 75%, while the remaining 2.5 m³/h becomes reject water.

The suitable recovery rate depends on feedwater TDS, hardness, silica, temperature and membrane design. Learn more about how RO recovery rate affects system performance.

Why Does an RO System Produce Too Much Reject Water?

Common causes include:

• High feedwater TDS or hardness
• Poor pretreatment
• Membrane fouling or scaling
• Incorrect pressure and flow settings
• Improper membrane arrangement
• Low water temperature
• Aging RO membranes

Feedwater quality and operating data should be checked before changing the recovery setting.

How to Reduce RO Reject Water

1. Improve Pretreatment

Effective pretreatment removes sediment, chlorine, hardness and other contaminants before they enter the RO membranes.

A typical pretreatment process may include:

Multimedia Filter → Activated Carbon Filter → Water Softener or Antiscalant Dosing → Cartridge Filter

Good pretreatment reduces membrane fouling and allows the system to maintain a stable recovery rate.

2. Optimize Chemical Dosing

As the recovery rate increases, calcium, magnesium, silica and other minerals become concentrated in the reject water.

The correct antiscalant and dosage can help control scaling. Chemical selection should be based on the raw water analysis, target recovery rate and membrane operating conditions.

3. Use the Correct Membrane Configuration

A two-stage membrane arrangement can treat the concentrate from the first stage and recover additional product water.

For projects requiring higher overall recovery, possible solutions include:

• Two-stage RO
• Concentrate recirculation
• Secondary RO treatment
• Customized high-recovery membrane design

The configuration must match the feedwater quality and required capacity. A properly designed industrial RO water treatment system balances water recovery, salt rejection and membrane life.

4. Maintain Correct Pressure and Flow

Low pressure may reduce product-water output, while excessive pressure increases energy consumption and membrane stress.

Operators should regularly monitor:

• Feed and concentrate pressure
• Product and reject-water flow
• Feed and product-water conductivity
• Membrane pressure difference

Do not excessively close the concentrate valve to force a higher recovery rate. This may cause rapid scaling and membrane damage.

5. Clean Fouled Membranes

Membrane fouling reduces permeate flow and system recovery. Common warning signs include:

• Lower product-water capacity
• Increased pressure difference
• Higher conductivity
• Rising energy consumption

Suitable chemical cleaning can restore performance if the membranes are fouled but not permanently damaged.

Can RO Reject Water Be Reused?

RO reject water may be reused for suitable non-critical applications, including:

• Floor cleaning
• Toilet flushing
• Cooling-tower makeup
• Dust suppression
• Certain industrial washing processes

Reject-water reuse does not increase the recovery rate of the RO unit itself, but it can improve the factory’s overall water utilization.

Its suitability should be evaluated according to TDS, hardness, chemical content and local discharge requirements.

Information Required for System Optimization

To evaluate an existing RO system, provide:

1. Feedwater analysis
2. Product and reject-water flow
3. Feed and concentrate pressure
4. Feed and product-water conductivity
5. Membrane model and quantity
6. Current recovery rate
7. Membrane operating and cleaning history

You can also read how to understand a water quality report before optimizing the system.

Frequently Asked Questions

Is a higher RO recovery rate always better?

No. Excessive recovery can increase scaling, membrane fouling and maintenance costs. The best target is the highest sustainable recovery rate for the specific water quality.

Can RO reject water be treated again?

Yes. A secondary RO system may recover additional water, but higher salinity and scaling risks must be evaluated first.

Can I reduce reject water by adjusting the concentrate valve?

Only within the system’s designed operating range. Excessive restriction may damage the membranes.

Conclusion

To reduce RO reject water safely, factories should improve pretreatment, optimize dosing, use the correct membrane arrangement and maintain proper pressure and flow.

Contact Zhongnuo Water Treatment for an RO system assessment or a customized high-recovery water treatment solution.