I. The Neglected “Other Half”: RO Concentrate

In the daily operation of reverse osmosis systems, everyone focuses on permeate quality: whether conductivity meets the standard, whether salt rejection is stable, and whether SDI is within control limits. However, while the RO membrane produces pure water, another stream is continuously and quietly discharged—concentrate.

This is an unavoidable physical fact. Reverse osmosis is pressure-driven and allows only water molecules and a small amount of small-molecule substances to pass through the membrane, while the vast majority of dissolved salts, organic matter, and colloids are retained on the concentrate side. At a common industrial recovery rate of 75%, for every 100 tons of feedwater treated, about 25 tons of concentrate must be managed. The salts and COD in the concentrate are concentrated by 3–4 times, with conductivity typically reaching 4,000–20,000 μS/cm and CODcr between 120–400 mg/L.

If this concentrate is discharged directly, the environmental risks are significant. It has become a key consideration in project design, environmental impact assessment approval, and operational management. Starting from the sources and characteristics of concentrate, this article systematically reviews four mainstream treatment pathways—compliance discharge, concentration and volume reduction, zero liquid discharge, and resource recovery—to help readers build an overall framework for understanding concentrate treatment.

II. Understanding RO Concentrate: Where Does It Come From? How Much Is There? What Are Its Characteristics?

The “origin” of RO concentrate determines its “character.” During reverse osmosis, feedwater enters the membrane module under the drive of a high-pressure pump. Water molecules pass through the membrane wall to become permeate, while dissolved salts, organic matter, colloids, and microorganisms are retained by the membrane and discharged from the system with the flow on the concentrate side. Simply put, concentrate is the portion of the feedwater that “did not pass through the membrane.”

The volume of concentrate depends on the system recovery rate. At a recovery rate of 75%, concentrate accounts for about 25% of the feedwater; at a recovery rate of 85%, concentrate drops to about 15%, but salt concentration rises significantly. In practice, there is a clear upper limit for TDS on the concentrate side of a single-stage RO system—about 80,000 mg/L. Beyond this threshold, osmotic pressure at the membrane surface rises sharply, and the recovery rate may drop abruptly from a design value of 75% to 55–60%. Interstage booster pumps are forced to increase pressure, and scaling risk increases accordingly.

In terms of water quality, concentrate shows several distinct characteristics. High salinity is its most basic property, with TDS typically 3–5 times that of the feedwater. After concentration, hardness ions readily form calcium carbonate and calcium sulfate scale layers on the membrane surface and downstream equipment. Organic matter is also enriched, with COD and TOC concentrations rising significantly. The high-salinity environment inhibits microbial activity, making the concentrate poorly biodegradable and difficult to treat with conventional biological processes. In addition, trace characteristic pollutants that may be present in the feedwater—such as endocrine-disrupting chemicals, pharmaceutical residues, and heavy metal ions—become further concentrated, making treatment far more difficult than that of the original feedwater.

III. Why Can’t Concentrate Simply Be Discharged?

Direct discharge of concentrate faces three major risks.

Environmental risks are the most obvious. When high-salinity concentrate enters soil, it can cause salinization; when discharged into natural water bodies, it can disrupt the osmotic balance of aquatic ecosystems. The combination of organic matter and characteristic pollutants may produce combined pollution effects, with impacts far beyond those of a single pollutant.

Compliance risks are equally pressing. Concentrate discharged into municipal sewer networks must meet sewer discharge standards, and many regions have explicitly prohibited high-salinity wastewater from being discharged into urban sewage collection and treatment facilities. Discharge into natural water bodies must meet industrial discharge standards. In environmentally sensitive areas, zero liquid discharge has shifted from an “encouraged target” to a mandatory constraint. In January 2026, the Technical Guidelines for Pollution Control of High-Salinity Wastewater, issued by the Ministry of Ecology and Environment, officially came into effect. It explicitly requires key industries such as coal chemical, electroplating, and lithium battery to achieve zero liquid discharge of high-salinity wastewater by 2028. The Standard for the Construction of Eco-Industrial Parks (HJ 274—2026), implemented in the same year, further establishes “zero liquid discharge” as an operational baseline and requires a water resource recycling rate of no less than 85%.

Economic risk is the final “bill.” The cost of fines, production restrictions, or even delayed project acceptance due to non-compliant discharge often far exceeds the investment in concentrate treatment facilities itself.

Concentrate treatment is not an “optional” item that can be postponed; it is a question that must be answered head-on throughout the entire project lifecycle.

IV. Four Pathways for Concentrate Treatment

4.1 Pathway One: Compliance Discharge / Sewer Discharge

For projects with access to municipal sewers and concentrate salinity and pollutants within acceptable ranges, sewer discharge remains the lowest-cost pathway. Key actions include: removing characteristic pollutants through pretreatment, adjusting water quality and quantity to meet sewer discharge standards, and establishing routine compliance monitoring.

It should be noted that high-salinity concentrate poses a shock risk to municipal biological treatment systems. According to the Water Quality Standard for Sewage Discharged into Municipal Sewers (CJ 343—2010), the limits for chloride and sulfate in sewage discharged into municipal sewers are both 600 mg/L, while the salinity of RO concentrate is usually far higher. Therefore, before sewer discharge, it is necessary to assess the salinity limits in local standards and the actual tolerance of the wastewater treatment plant to high-salinity influent. This pathway is relatively low in cost but highly constrained by external conditions; it is suitable for projects with controllable salinity and mature sewer discharge conditions.

4.2 Pathway Two: Concentration and Volume Reduction

The goal of concentration and volume reduction is clear: reduce the volume of concentrate as much as possible before entering the expensive evaporation crystallization stage. Every unit of concentrate volume reduced lowers the investment and energy consumption of subsequent evaporation.

Mainstream technologies have different strengths. High-pressure reverse osmosis (HPRO) breaks through the concentration limit of conventional RO by increasing operating pressure, and can raise concentrate TDS to 100,000–120,000 mg/L with a recovery rate greater than 50%. Disc-tube reverse osmosis (DTRO) uses an open-channel design, offering outstanding fouling and scaling resistance; a two-stage DTRO system can achieve a salt rejection rate above 99.6% and a recovery rate of 50%. Electrodialysis reversal (EDR) uses an electric field to drive ion migration; it can concentrate TDS by 7–8 times, and because it concentrates organic matter and silica less strongly, it has strong fouling resistance. Membrane distillation uses the vapor pressure difference across a hydrophobic membrane to separate water from salts; permeate conductivity can be below 4 μS/cm, with a salt rejection rate above 99.99%.

The common challenges of these technologies are high pressure, high scaling risk, and the need to carefully balance energy consumption and membrane life. Concentration and volume reduction is a key bridging step in the overall zero liquid discharge pathway—it does not directly solve the final destination of the concentrate, but it determines the scale and cost of downstream processes.

4.3 Pathway Three: Zero Liquid Discharge

A typical zero liquid discharge route is: membrane concentration (HPRO/DTRO) → evaporation crystallization (MVR/multi-effect evaporation) → condensate reuse + salt crystal shipment or resource recovery.

MVR (mechanical vapor recompression) evaporation is currently the most widely used evaporation crystallization technology. In a high-salinity wastewater treatment project at an optical fiber products manufacturer, the MVR system had an actual treatment capacity of 48 t/d, a salt rejection rate of 99.23%, condensate meeting in-plant reuse standards, a recovery rate of 93.47%, crystalline waste salt purity of 96%, and a treatment cost of about CNY 65 per ton of water. Data from another RO concentrate project show that the evaporation stage consumed about 35 kWh per ton of water, with an annual operating cost of CNY 3.1 million, annual revenue from crystalline salt of CNY 2.05 million, and a payback period of about 4.3 years.

The cost of zero liquid discharge is clear: the investment in the evaporation crystallization unit is about CNY 30,000–50,000 per ton of water, usually accounting for more than 70% of total system investment, with operating energy costs in the range of CNY 70–100 per ton of water. The precision of salt separation pretreatment directly affects the quality of crystalline salt—if salt separation is not done well upstream, evaporation crystallization produces mixed salt, which can only be disposed of as hazardous waste, greatly reducing economic viability.

Zero liquid discharge is the “ultimate solution” for environmental compliance, but the economics must be carefully calculated.

4.4 Pathway Four: Resource Recovery

The core idea of resource recovery is to recover salts and water from the concentrate separately, transforming “treatment cost” into “resource revenue.”

Nanofiltration salt separation is the most critical step in the resource recovery pathway. Nanofiltration membranes have high permeability for monovalent ions (such as Na⁺ and Cl⁻) and high rejection for divalent ions (such as SO₄²⁻), allowing preliminary separation of sodium chloride and sodium sulfate in a single system. In the heavy-metal high-salinity wastewater zero liquid discharge project of Jinchuan Group, a coupled process of freeze crystallization + nanofiltration salt separation was adopted. The selective permeability of the nanofiltration system retained divalent ions and some organic matter while allowing monovalent ions to pass through, successfully achieving efficient separation of sodium chloride and sodium sulfate. The separated sodium sulfate concentrate produced mirabilite through freeze crystallization, which was further processed into anhydrous sodium sulfate product; the sodium chloride concentrate entered an MVR evaporation crystallization system to produce industrial-grade sodium chloride.

Another set of data is equally compelling: in a fracturing flowback fluid treatment project, the influent TDS was 40,000 mg/L. Through a salt separation and resource recovery process, annual industrial salt recovery exceeded 17,000 tons, turning wastewater treatment from a pure cost expenditure into a revenue-generating step.

Resource recovery is suitable for enterprises with clear water quality composition, sufficient scale, and market conditions for absorbing salt products. It represents a higher-order goal for concentrate treatment, but requires alignment of water quality, scale, technology, and market demand.

V. How to Choose the Right Concentrate Treatment Pathway?

There is no universal solution. The choice of concentrate treatment pathway depends on a comprehensive assessment of five dimensions:

Water quality and quantity—salt composition (mainly sodium chloride or sodium sulfate), organic matter concentration, and water volume—directly determine the feasibility of a technical route. Discharge conditions—whether sewer discharge is possible and whether local environmental requirements are becoming stricter—are the first screening threshold. Reuse targets—whether there is a reuse demand and what the reused water will be used for—affect the treatment depth required for condensate. Economics—investment scale, operating cost, energy consumption, and footprint—are practical constraints on implementation. Policy requirements—whether the project falls under mandatory zero liquid discharge and whether salt separation and resource recovery are required—determine the compliance baseline.

A simplified decision logic can be understood as follows: if sewer discharge is possible and compliant, prioritize sewer discharge; if sewer discharge is not possible but volume can be controlled through concentration reduction, adopt concentration reduction + compliant disposal; if zero liquid discharge is required, choose membrane concentration + evaporation crystallization; if resource recovery value exists, follow the route of nanofiltration salt separation + crystallization resource recovery.

Concentrate treatment solutions must be customized. Before finalizing a solution, it is recommended that a professional team conduct lab-scale and pilot-scale validation, letting actual water quality data speak rather than copying a “standard process package.”

VI. Systems Thinking: Concentrate Treatment Must Be “Designed In” Upfront

One of the most easily overlooked points about concentrate treatment is that it should not be a last-minute “patch” at the end of the process; it should be considered from the system design stage.

The quality of upstream pretreatment directly determines the complexity of the concentrate. If hardness and organic matter in the feedwater are not effectively controlled during pretreatment, they will enter the concentrate at several times their original concentration during the concentration process, creating extra trouble for downstream treatment. Setting a reasonable recovery rate is equally critical—the higher the recovery rate, the smaller the concentrate volume, but the higher the salt concentration, the greater the scaling risk, and the more difficult downstream treatment becomes. A trade-off is needed between concentrate “concentration” and “volume,” rather than blindly pursuing a high recovery rate.

In membrane system design, cascade combination is a direction worth considering: RO combined with nanofiltration for salt separation, HPRO or DTRO for deep concentration on the concentrate side, with membrane elements at each stage taking on different concentration tasks. Using the selectivity of nanofiltration membranes to separate monovalent and divalent salts can lay a solid foundation for subsequent crystallization and resource recovery.

Daoerdeng Membrane Technology focuses on the full lifecycle of reverse osmosis systems, from membrane selection and system design to the integration of concentrate treatment solutions, and is committed to providing systematic membrane technology support ranging from fouling-resistant RO membranes and nanofiltration membranes to high-pressure/DTRO concentration membranes. In the concentrate concentration and volume reduction stage, the Daoerdeng STRO pipe-network reverse osmosis membrane adopts an unobstructed feed system design and has been specifically optimized for fouling resistance and high-pressure resistance in the concentration of high-concentration, high-salinity wastewater.

VII. Conclusion: Concentrate Is a Burden, but Also an Opportunity

RO concentrate treatment is undergoing a clear pathway evolution: from the simplest compliance discharge, to concentration and volume reduction, to zero liquid discharge for compliance assurance, and ultimately to resource recovery. Each leap forward is driven jointly by policy pressure, technological progress, and cost reduction.

With the implementation of the Technical Guidelines for Pollution Control of High-Salinity Wastewater and the continued decline in membrane technology costs, zero liquid discharge and resource recovery for concentrate are shifting from an “advanced choice for a few enterprises” to a “standard requirement for more and more industries.” In the first quarter of 2026, 298 new industrial wastewater zero liquid discharge projects were filed nationwide, a year-on-year increase of 58%, making this trend clearly visible.

RO concentrate is not the end point of water treatment. It is a concentrated resource stream, waiting to be dismantled and utilized in a more systematic and economical way. From “compliance discharge” to “resource recovery,” this path is growing wider.