Brackish-water reverse osmosis systems produce two water streams: purified permeate and concentrated reject water. In coastal desalination projects, concentrate may sometimes be discharged to the sea through a properly designed and permitted outfall. Inland projects do not have this option, making RO concentrate disposal one of the most important parts of project planning.

An unsuitable disposal method can increase operating costs, delay environmental approval or create risks to soil and groundwater. The best solution depends on concentrate flow, salinity, chemical composition, local climate, available land and applicable regulations.

 

What Is RO Concentrate?

RO concentrate—also called RO reject, brine or retentate—is the feedwater portion that does not pass through the membrane.

It contains higher concentrations of the substances rejected by the RO system, including:

•  • Dissolved salts
•  • Calcium and magnesium
•  • Chloride and sulfate
•  • Silica
•  • Bicarbonate
•  • Nitrate or fluoride
•  • Trace metals
•  • Residual antiscalant
•  • Cleaning or pretreatment chemicals, where applicable

RO concentrate is not automatically classified as hazardous waste. Its classification and permitted disposal method depend on its composition and local environmental regulations.

 

How Much Concentrate Does a BWRO System Produce?

The concentrate volume depends mainly on the system recovery rate.

The basic relationship is:

Concentrate Flow = Feedwater Flow − Permeate Flow

For example, if a brackish-water RO plant receives 20 m³/h of feedwater and operates at 70% recovery:

•  • Permeate flow: 14 m³/h
•  • Concentrate flow: 6 m³/h

Increasing recovery reduces concentrate volume, but it also raises the concentration of salts and scaling compounds inside the membrane system.

Recovery should therefore be determined through water analysis and membrane projection—not simply increased to reduce wastewater volume.

 

Why Inland Concentrate Disposal Is Difficult

Inland brackish-water projects may be far from municipal sewers, large rivers or marine discharge points. Concentrate may also have significantly higher salinity than the original groundwater.

Poorly managed concentrate can:

•  • Increase soil salinity
•  • Affect crops and vegetation
•  • Contaminate shallow groundwater
•  • Damage freshwater ecosystems
•  • Corrode drainage infrastructure
•  • Cause visible salt deposits
•  • Violate discharge permits

The disposal strategy should be evaluated before finalizing the RO plant capacity and recovery rate.

 

1. Discharge to a Municipal Sewer

Where an approved municipal sewer is available, RO concentrate may be discharged into the wastewater collection system.

This option can offer:

•  • Simple operation
•  • Limited onsite infrastructure
•  • Continuous concentrate removal
•  • Lower capital cost than advanced brine treatment

However, the local utility must confirm whether the concentrate meets its acceptance limits.

Important parameters may include:

•  • Flow rate
•  • TDS or conductivity
•  • Chloride
•  • Sulfate
•  • pH
•  • Temperature
•  • Heavy metals
•  • Chemical oxygen demand
•  • Residual treatment chemicals

Sewer discharge should not be assumed to be acceptable without written authorization.

 

2. Evaporation Ponds

Evaporation ponds use solar energy and dry climatic conditions to reduce the volume of concentrate. Water evaporates while salts remain in the pond.

This method is commonly considered in hot, arid inland areas with sufficient land.

 

Advantages

•  • Relatively simple operation
•  • Low energy consumption
•  • Suitable for remote locations
•  • Potentially effective in dry climates

 

Limitations

•  • Requires a large land area
•  • Performance decreases during rainy or humid periods
•  • Requires a suitable impermeable liner
•  • May attract wildlife
•  • Salt residues require eventual management
•  • Leakage may contaminate soil or groundwater

The pond must be sized using local evaporation, rainfall and concentrate-flow data. A general annual climate average may not be sufficient because seasonal rainfall can significantly affect pond capacity.

 

3. Deep-Well Injection

Deep-well injection places concentrate into a suitable underground geological formation isolated from usable groundwater.

This option may handle continuous flows without requiring large evaporation areas.

However, it requires:

•  • Detailed geological investigation
•  • Suitable underground formations
•  • Injection-well construction
•  • Pressure and integrity monitoring
•  • Regulatory approval
•  • Protection of freshwater aquifers

Capital cost and permitting requirements can be significant. Deep-well injection is only suitable where hydrogeological conditions and local regulations allow it.

 

4. Surface-Water Discharge

Discharge to a river, lake or other surface-water body may be possible only when authorized by the relevant environmental authority.

The assessment should consider:

•  • Concentrate salinity
•  • Receiving-water flow
•  • Seasonal low-flow conditions
•  • Aquatic ecology
•  • Mixing and dilution
•  • Temperature
•  • Chemical composition
•  • Discharge-point design

Dilution alone does not automatically make a discharge environmentally acceptable. The concentrate must meet the conditions of the applicable discharge permit.

Small freshwater bodies are particularly sensitive to increased salinity.

 

5. Controlled Land Application

In limited cases, concentrate may be used for irrigation of salt-tolerant vegetation or applied to suitable land.

This option requires careful assessment of:

•  • Soil drainage
•  • Sodium adsorption ratio
•  • Chloride concentration
•  • Boron
•  • Specific toxic ions
•  • Crop salt tolerance
•  • Groundwater depth
•  • Long-term salt accumulation
•  • Seasonal rainfall

TDS alone cannot determine whether concentrate is suitable for land application.

Improper application can gradually damage soil structure and contaminate groundwater. Agricultural and environmental specialists should evaluate the site before this method is used.

 

6. Reuse Within the Facility

RO concentrate may sometimes be reused in applications that do not require purified water.

Potential uses include:

•  • Toilet flushing
•  • Floor or yard cleaning
•  • Dust suppression
•  • Selected cooling applications
•  • Filter backwashing
•  • Compatible industrial washing
•  • Raw-water blending

Reuse feasibility depends on the concentrate composition and the water-quality requirements of the receiving process.

Using concentrate in cooling towers, boilers or sensitive equipment without proper evaluation may increase scaling and corrosion.

Reuse does not eliminate the concentrate. It reduces freshwater consumption and may postpone final disposal.

 

7. Blending with Other Approved Water Streams

Concentrate may be blended with a lower-salinity wastewater stream before permitted discharge.

Blending can help stabilize:

•  • Salinity
•  • pH
•  • Temperature
•  • Discharge flow

However, blending must not be used simply to dilute contaminants to avoid treatment requirements. The combined discharge must comply with local environmental and utility regulations.

Chemical compatibility should also be checked to prevent precipitation or pipe scaling after mixing.

 

8. Secondary RO or Concentrate Recovery

A secondary membrane system can recover additional water from the first RO concentrate.

Possible configurations include:

•  • Two-stage RO
•  • Concentrate-staged RO
•  • Secondary high-recovery RO
•  • Closed-circuit or batch-style RO
•  • Nanofiltration before additional RO

This approach can:

•  • Increase total water recovery
•  • Reduce liquid-discharge volume
•  • Produce additional usable water
•  • Reduce disposal costs where disposal is expensive

However, the second-stage concentrate will contain higher salt and scale-forming concentrations. Additional softening, precipitation, pH control or specialized antiscalant may be required.

Higher recovery is not always the lowest-cost solution when energy, chemicals, cleaning and membrane replacement are considered.

 

9. Thermal Evaporation and Crystallization

Where liquid discharge is severely restricted, thermal treatment may be used to evaporate water and concentrate salts.

Technologies may include:

•  • Mechanical vapor recompression evaporators
•  • Brine concentrators
•  • Crystallizers
•  • Spray dryers
•  • Solar-assisted evaporation systems

These processes can support minimal-liquid-discharge or zero-liquid-discharge strategies.

 

Advantages

•  • Very high water recovery
•  • Significant reduction in liquid waste
•  • Potential production of solid salt residues

 

Limitations

•  • High capital cost
•  • High energy consumption
•  • Complex operation
•  • Scaling and corrosion risk
•  • Requirement for solid-waste disposal

Thermal systems are normally considered when water is valuable, disposal is highly restricted or environmental compliance justifies the additional cost.

 

10. Offsite Transport and Disposal

For small concentrate volumes, the reject water may be collected in a storage tank and transported to an authorized treatment or disposal facility.

This approach can avoid the capital cost of onsite disposal infrastructure.

However, it involves:

•  • Storage-tank requirements
•  • Regular transport costs
•  • Spill-management measures
•  • Waste documentation
•  • Dependence on licensed contractors

It is usually more suitable for small or intermittent systems than for large, continuously operating BWRO plants.

 

Comparison of Inland RO Concentrate Disposal Options

 

How to Reduce RO Concentrate Before Disposal

The most economical strategy often begins with reducing concentrate production while keeping the RO system within safe operating limits.

Possible measures include:

Optimize the Recovery Rate

Use complete ionic water analysis and membrane projection software to identify the maximum sustainable recovery.

Improve Pretreatment

Softening, iron removal, silica control or suspended-solids removal may allow more stable operation at a higher recovery rate.

Use Multiple RO Stages

A properly designed membrane array can improve water recovery without applying excessive flux to individual membrane elements.

Install Flow and Conductivity Monitoring

Accurate instruments help identify unnecessary concentrate flow and detect changes in feedwater quality.

Maintain the RO Membranes

Fouled or scaled membranes reduce performance and may increase water losses. Monitor normalized flow, pressure drop and salt passage to determine the correct cleaning time.

 

Information Needed to Select a Disposal Method

Before choosing an inland concentrate-management strategy, collect:

1. RO feedwater flow
2. Permeate and concentrate flow
3. Design recovery rate
4. Feedwater and concentrate analyses
5. Daily operating hours
6. Seasonal temperature and rainfall
7. Available land area
8. Distance to an approved sewer or treatment facility
9. Local environmental discharge limits
10. Groundwater depth and geological conditions
11. Possible onsite reuse applications
12. Planned future plant expansion

A laboratory analysis of the predicted or actual concentrate is more useful than relying only on feedwater TDS.

 

Common Planning Mistakes

Avoid the following mistakes:

• Designing the RO plant before confirming concentrate disposal
• Selecting recovery based only on concentrate volume
• Assuming reject water is suitable for irrigation
• Using an unlined evaporation pond
• Ignoring seasonal rainfall and evaporation rates
• Discharging concentrate to soil or surface water without approval
• Focusing only on TDS while ignoring chloride, sulfate, silica and metals
• Adding a secondary RO stage without scaling calculations
• Failing to include disposal costs in the total project budget
• Assuming zero-liquid discharge produces no solid waste

 

Frequently Asked Questions

Is RO Concentrate Hazardous?

Not necessarily. Its classification depends on the source water, concentrated contaminants, treatment chemicals and local regulations.

 

Can RO Concentrate Be Returned to the Borehole?

Generally, concentrate should not be returned to the source borehole without a permitted injection system and hydrogeological assessment. It may increase groundwater salinity or contaminate freshwater formations.

 

Can Concentrate Be Used for Irrigation?

Only after evaluating soil, crop tolerance, sodium, chloride, boron, metals and long-term groundwater risks. High-TDS concentrate is unsuitable for many crops.

 

Can Higher RO Recovery Eliminate Concentrate?

No. Higher recovery reduces concentrate volume but increases its salinity. Even a high-recovery system still produces a waste stream or solid residue that requires management.

 

What Is the Best Option for a Remote Inland Site?

In dry areas with sufficient land, an engineered evaporation pond may be practical. Where land is limited or discharge rules are strict, additional recovery or offsite disposal may be necessary. The decision must be site-specific.

 

Conclusion

RO concentrate disposal should be planned at the beginning of every inland brackish-water project. The appropriate solution may involve sewer discharge, evaporation ponds, approved reuse, secondary RO, deep-well injection, offsite transport or thermal concentration.

The decision should be based on concentrate quantity and composition, not feedwater TDS alone. Local regulations, climate, land availability and long-term operating costs are equally important.

 

Zhongnuo focuses on designing and manufacturing industrial pure-water and desalination equipment. For projects requiring specialized waste disposal, geological injection or zero-liquid-discharge treatment, the concentrate-management section should be coordinated with a qualified environmental engineering provider.