Process Optimization, Parameter Calibration and Full-Loop Resource Utilization Solution

Abstract

Aiming at the characteristics of high COD and high salinity in reverse osmosis (RO) concentrated brine, this study proposes a full-process advanced treatment and zero-liquid-discharge resource recovery scheme: chemical softening → advanced oxidation → ultrafiltration → resin softening → nanofiltration salt fractionation → high-pressure reverse osmosis → electrodialysis concentration → bipolar membrane electrodialysis acid-base regeneration. Key operating parameters of each unit were optimized to meet treatment requirements. Results show that the optimal dosage of chemical softening agents is 1.2 times the theoretical value, reducing Ca²⁺ and Mg²⁺ to 3.29 mg/L and 2.04 mg/L respectively, which meets the inlet hardness requirement of subsequent resin softening. Among three advanced oxidation processes (O₃ catalytic oxidation, Fenton oxidation, O₃/H₂O₂ combined oxidation), O₃/H₂O₂ peroxone process achieves the best performance: after 1.0 h reaction, COD removal rate reaches 66.41%, and effluent COD drops below 100 mg/L. The presence of sulfate reduces resin softening efficiency, and resin performance deteriorates significantly when total salinity exceeds 20 g/L. To prevent nanofiltration membrane scaling, the nanofiltration unit is arranged after resin softening. To avoid excessive energy consumption, the electrodialysis concentrate only needs to reach 150 g/L. Under this inlet salinity, bipolar membrane electrodialysis produces 73.2 g/L acid and 78.5 g/L alkali after 120 min reaction at 50 mA/cm², with specific energy consumption of 3.1 kW·h/kg for acid and 2.7 kW·h/kg for alkali. This process realizes deep treatment of high-salinity wastewater and full resource reuse, completely achieving zero liquid discharge, and provides technical guidance for industrial scale-up of concentrated brine treatment.

1. Introduction

Wastewater from rare earth mining, metallurgy, coal chemical and other industries contains high concentrations of ammonia nitrogen, organic matter and inorganic salts. After conventional biological treatment and RO desalination, the generated concentrated brine still retains a large amount of refractory pollutants. Direct discharge of concentrated brine causes serious damage to the natural water environment and ecological safety, and has been strictly restricted by environmental regulations in China.

With the implementation and promotion of zero liquid discharge (ZLD) policies, ZLD technologies have attracted increasing industrial attention. Conventional ZLD solutions such as multi-stage flash evaporation and mechanical vapor recompression (MVR) evaporate high-salinity wastewater with thermal energy to produce crude salts, but the obtained mixed salts usually have low purity, limited resource value and are mostly disposed as hazardous waste by landfill, resulting in high operating costs and secondary environmental risks.

Bipolar membrane electrodialysis (BMED) is an emerging resource recovery technology, which uses the unique property of bipolar membranes to split water molecules into H⁺ and OH⁻, and combines them with anions and cations in high-salinity water to produce corresponding acids and alkalis. This technology realizes on-site regeneration and reuse of acid and alkali, greatly improves the added value of ZLD products, and reduces the consumption of chemical agents in the whole water treatment system.

This study systematically investigates pollutant removal in the pretreatment stage, deep softening and salt fractionation in the advanced treatment stage, and acid-base recovery efficiency and energy consumption in the BMED stage, using RO concentrated brine as the raw water. The feasibility of BMED-based ZLD for concentrated brine is verified, and the full-process operating parameters are optimized to provide technical support for industrial application.

2. Experimental Setup and Analytical Methods

2.1 Experimental Design

The experiment adopts RO concentrated brine as raw water, and the whole process is divided into three units: pretreatment, advanced treatment, and concentration & acid-base preparation.

• Pretreatment unit: Includes chemical softening, multimedia filtration, advanced oxidation and pH adjustment. The chemical softening unit optimizes the dosage of Ca(OH)₂ and Na₂CO₃ for hardness removal. The advanced oxidation unit compares three processes to select the optimal COD removal solution.
• Advanced treatment unit: Includes ultrafiltration, resin softening, nanofiltration and high-pressure reverse osmosis, for deep softening, salt fractionation and concentration. The effects of inorganic salt type and total salinity on resin softening performance are investigated to determine the optimal process sequence.
• Concentration & acid-base unit: Includes electrodialysis (ED) concentration and bipolar membrane electrodialysis. The concentrated brine from HPRO is further concentrated by ED to improve BMED efficiency, and the effects of salinity, current density and voltage on ED and BMED efficiency, energy consumption and acid-base yield are investigated.

2.2 Materials and Equipment

The main experimental equipment and membrane materials are shown in Table 1, and the raw water quality of RO concentrated brine is shown in Table 2.

Table 1 Experimental Equipment and Materials

Table 2 Raw Water Quality of RO Concentrated Brine

2.3 Analytical Methods

• Anions (Cl⁻, SO₄²⁻) were detected by ion chromatography; cations (Ca²⁺, Mg²⁺) were detected by iCAP 6300 ICP-OES.
• Acid and alkali concentrations from BMED were measured by acid-base titration.
• Conductivity and pH were measured by Mettler Toledo S230 conductivity meter and S22 pH meter respectively.
• Removal rate, specific energy consumption (SEC) and average current efficiency (ACE) were calculated by standard formulas for electrodialysis systems.

3. Results and Discussion

3.1 Pretreatment Process Optimization

3.1.1 Chemical Softening

Chemical softening removes Ca²⁺ and Mg²⁺ hardness by dosing Ca(OH)₂ and Na₂CO₃, to meet the inlet requirement of subsequent resin softening and membrane systems. The effect of chemical dosage (0.8 to 1.7 times the theoretical value) on hardness removal was investigated.

Results show that Ca²⁺ and Mg²⁺ concentrations decrease rapidly with increasing dosage, and an inflection point appears at 1.2 times the theoretical dosage, after which the removal rate increases slowly. At 1.2 times dosage, Ca²⁺ and Mg²⁺ concentrations drop to 3.29 mg/L and 2.04 mg/L respectively. Further increasing dosage does not significantly improve softening effect, but leads to excessive chemical consumption and excess chemical sludge. Considering both removal efficiency and treatment cost, the optimal chemical softening dosage is determined as 1.2 times the theoretical value.

3.1.2 Advanced Oxidation

Three advanced oxidation processes were compared under optimal dosage conditions: O₃ catalytic oxidation, Fenton oxidation, and O₃/H₂O₂ combined (peroxone) oxidation.

Results show that Fenton and O₃ catalytic oxidation reach stable removal efficiency at 1.2 h, while O₃/H₂O₂ process stabilizes at 1.0 h. The maximum COD removal rates of O₃ catalytic oxidation, Fenton oxidation and O₃/H₂O₂ oxidation are 45.42%, 48.81% and 66.41% respectively. The synergistic effect of O₃ and H₂O₂ generates more hydroxyl radicals (·OH), accelerating organic degradation and achieving better treatment performance.

Fenton oxidation generates large amounts of iron sludge causing secondary pollution, while O₃ catalytic oxidation has lower removal efficiency. Therefore, O₃/H₂O₂ combined oxidation is selected as the pretreatment process. After 1.0 h reaction, COD concentration is reduced to below 100 mg/L, fully meeting the inlet requirements of resin softening and subsequent membrane systems.

3.2 Advanced Treatment Process Optimization

3.2.1 Influencing Factors of Resin Softening

Effect of inorganic salt type: UF permeate (containing both chloride and sulfate) and NF permeate (mainly chloride salt after fractionation) were used as resin feed respectively. Results show that resin softening efficiency is significantly higher for NF permeate with monovalent chloride salts. Sulfate ions have strong binding energy with Ca²⁺ and Mg²⁺, which compete with resin for hardness ions and reduce softening efficiency. UF feed also reaches breakthrough earlier than NF feed.

Effect of total salinity: Resin softening tests were conducted on brine with salinity of 13, 16, 20 and 30 g/L. Results show that removal rates are almost identical for 13 g/L and 16 g/L, decrease slightly at 20 g/L, and the resin completely breaks through and fails at 30 g/L. Excessively high Na⁺ concentration hinders the ion exchange of Ca²⁺ and Mg²⁺ on the resin, and the competitive adsorption effect increases with rising salinity.

3.2.2 Process Sequence Determination

Comprehensive analysis shows that both inorganic salt type and total salinity significantly affect resin softening performance. If nanofiltration is placed before resin softening, Ca²⁺ and Mg²⁺ will cause scaling and fouling of NF membranes. If RO concentration is placed before resin softening, the increased salinity will greatly reduce resin efficiency.

Therefore, when the total salinity does not exceed 20 g/L, separate salt fractionation is not required before softening. The final sequence of the advanced treatment unit is determined as: ultrafiltration → resin softening → nanofiltration → high-pressure reverse osmosis. The effluent quality of each unit is shown in Table 3.

Table 3 Effluent Quality of Each Treatment Unit

3.3 Concentration and Acid-Base Regeneration

3.3.1 Electrodialysis Concentration Performance

HPRO concentrate with salinity of ~43 g/L is further concentrated by electrodialysis before entering the BMED system, to improve acid-base production efficiency. Current density and operating voltage are the core parameters affecting ED concentration performance and energy consumption.

Voltage shows a U-shaped trend during ED operation: high initial voltage due to low initial ion concentration in the concentrate chamber; voltage decreases and stabilizes as ions migrate to the concentrate chamber; voltage rises rapidly in the later stage as ions in the dilute chamber are depleted. Higher current density leads to higher operating voltage and shorter stable operation period.

Under 28 V operating voltage, the concentrate salinity reaches 180 g/L and the dilute chamber salinity drops to 20 g/L after 30 min reaction. However, excessively high concentrate salinity will lead to a sharp rise in BMED energy consumption in the subsequent stage. Comprehensive balance shows that the optimal ED concentrate salinity (BMED inlet salinity) is 150 g/L, which achieves a good balance between concentration efficiency and subsequent energy consumption.

3.3.2 Bipolar Membrane Electrodialysis for Acid-Base Regeneration

Effect of initial salt concentration: BMED tests were conducted at initial salt concentrations of 80, 120, 150 and 200 g/L. Results show that acid and alkali concentrations increase with reaction time, and higher initial salinity leads to higher product concentration and faster production rate, due to more available migratable ions and lower solution resistance. H⁺ ions have smaller radius and are more prone to leakage through membranes than OH⁻, so the final acid concentration is slightly lower than alkali concentration.

Higher inlet salinity requires higher ED concentration energy, and will cause rapid voltage rise in the late BMED operation, increasing total energy consumption. Therefore, 150 g/L is determined as the optimal inlet salinity for BMED.

Effect of current density: Tests were carried out at current densities of 30, 40 and 50 mA/cm². Higher current density accelerates acid-base production and shortens reaction time, but reduces current efficiency due to intensified competition between H⁺/OH⁻ and salt ions. At 50 mA/cm², the target acid (73.2 g/L) and alkali (78.5 g/L) concentrations are achieved after 120 min reaction, with specific energy consumption of 3.1 kW·h/kg for acid and 2.7 kW·h/kg for alkali.

Comprehensively considering production efficiency, energy consumption and membrane service life, 50 mA/cm² is selected as the optimal operating current density for BMED under the target product concentration requirements.

4. Final Full-Process Flow

Based on experimental verification, the final concentrated brine ZLD and resource recovery process is determined as follows:

This process realizes complete zero liquid discharge of concentrated brine, and converts waste salt resources into usable acid and alkali, forming a closed-loop chemical recycling system.

5. Engineering Application Insights

5.1 Applicable Scenarios

This BMED-based ZLD process is particularly suitable for the following scenarios:

• RO concentrate treatment and zero discharge in coal chemical, coking, rare earth metallurgy and printing & dyeing industries
• Industrial parks with on-site acid and alkali consumption demand, to realize resource recycling and reduce chemical procurement costs
• Projects with strict hazardous waste disposal restrictions, where conventional evaporation mixed salts cannot be disposed of properly
• Medium and small-scale ZLD projects with brine flow rate of 100–5000 m³/d, where MVR evaporation has poor economic performance

5.2 Key Design Considerations

• Pretreatment guarantee: Sufficient hardness removal and COD removal in the pretreatment stage are critical to prevent scaling and organic fouling of subsequent NF, RO and ED membrane systems. Resin softening as a secondary safety barrier is necessary for long-term stable operation.
• Salt fractionation matching: Nanofiltration fractionation separates monovalent and divalent salts, which not only reduces the impact of sulfate on BMED performance, but also improves the purity of produced hydrochloric acid and sodium hydroxide.
• Concentration gradient optimization: Multi-stage concentration through HPRO + ED can effectively reduce the total volume of BMED feed and improve production efficiency, but the final concentration must be balanced with energy consumption to avoid excessive operating costs.
• Membrane fouling control: Regular online cleaning and regular off-line chemical cleaning protocols should be formulated for ED and BMED membrane stacks, to extend membrane service life and maintain stable current efficiency.

5.3 Operation and Maintenance Best Practices

• Adjust the dosage of softening agents and oxidants dynamically according to inlet water quality fluctuations, to avoid excessive chemical consumption while ensuring effluent quality.
• Establish regular performance testing mechanisms for ion exchange resin, and optimize the regeneration cycle and regenerant concentration according to actual operation data.
• Monitor voltage, current efficiency and product concentration of BMED system in real time, and adjust operating current density appropriately under low load conditions to reduce energy consumption.
• Develop standardized chemical cleaning procedures for electrodialysis and bipolar membrane stacks, and perform regular cleaning according to transmembrane pressure difference and current efficiency attenuation.

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that bipolar membrane electrodialysis based zero discharge technology is a transformative solution for concentrated brine treatment, which fundamentally changes the end-of-pipe disposal model of traditional evaporation ZLD, and realizes high-value resource recovery of waste salts, in line with circular economy and dual carbon goals.

Our modular skid-mounted ZLD systems integrate the full process of pretreatment, deep softening, salt fractionation, concentration and BMED acid-base regeneration, with factory prefabrication and debugging, greatly shortening on-site construction period. Equipped with our intelligent operation optimization platform, the system dynamically adjusts current density, flow distribution and chemical dosage according to real-time inlet water quality and product concentration requirements, reducing comprehensive energy consumption by 10–15% compared with conventional fixed-parameter BMED systems. We also provide full-cycle technical services including process scheme design, equipment supply, commissioning and operation guidance, helping industrial clients achieve stable zero liquid discharge while maximizing the resource value of concentrated brine and reducing total life-cycle cost.

7. Conclusion

This study systematically optimizes the full-process parameters of concentrated brine ZLD and resource recovery technology based on bipolar membrane electrodialysis, with the following main conclusions:

1. The optimal chemical softening dosage is 1.2 times the theoretical value, which reduces Ca²⁺ and Mg²⁺ to below 3.29 mg/L and 2.04 mg/L respectively. O₃/H₂O₂ combined advanced oxidation achieves 66.41% COD removal after 1 h reaction, with effluent COD below 100 mg/L, meeting the inlet requirements of subsequent membrane systems.
2. Sulfate ions and high total salinity will reduce resin softening efficiency. Resin breaks through completely when salinity exceeds 30 g/L. Comprehensively considering scaling risk and softening efficiency, the advanced treatment sequence of ultrafiltration → resin softening → nanofiltration → high-pressure reverse osmosis is determined.
3. Electrodialysis concentrates HPRO brine from 43 g/L to 150 g/L as the optimal inlet for BMED. At 50 mA/cm² current density, BMED produces 73.2 g/L HCl and 78.5 g/L NaOH after 120 min, with energy consumption of 3.1 and 2.7 kW·h/kg respectively, meeting industrial acid and alkali concentration requirements.
4. The full process realizes deep treatment of concentrated brine and on-site regeneration of acid and alkali, completely achieving zero liquid discharge, with good technical feasibility and economic value for industrial scale-up.

FAQ

Q1: What are the core advantages of BMED-based ZLD compared with conventional MVR evaporation ZLD?
The most prominent advantage is resource recovery: conventional evaporation only produces low-value mixed solid salts, most of which are hazardous waste requiring high disposal costs, while BMED converts dissolved salts into usable hydrochloric acid and sodium hydroxide, which can be reused in the production or wastewater treatment system, turning waste into resources. In addition, BMED has lower energy consumption for medium and low concentration brine, no phase change, and lower operation and maintenance requirements than evaporation systems.

Q2: Why is O₃/H₂O₂ peroxone process selected instead of Fenton oxidation?
Although Fenton oxidation has certain treatment effect, it requires dosing of iron salts and hydrogen peroxide, generates a large amount of iron-containing chemical sludge, increases subsequent sludge disposal cost, and introduces additional iron ions that will pollute the subsequent membrane system. O₃/H₂O₂ process has no secondary pollution, shorter reaction time, higher COD removal rate, and is more compatible with subsequent membrane treatment units.

Q3: Why is resin softening arranged before nanofiltration instead of after?
Nanofiltration membranes are very sensitive to hardness ions. If nanofiltration is placed before softening, Ca²⁺ and Mg²⁺ in the feed water will easily form scale deposits on the NF membrane surface, causing irreversible membrane fouling and shortening membrane service life. Placing resin softening in front of nanofiltration reduces hardness to a very low level, effectively preventing scaling of nanofiltration and subsequent high-pressure reverse osmosis membranes.

Q4: What is the suitable salinity range for this BMED ZLD process?
The process is suitable for RO concentrated brine with total dissolved solids of 10,000–50,000 mg/L as feed water. After multi-stage concentration, the optimal inlet salinity for BMED is 100–180 g/L. Excessively low inlet salinity will lead to low BMED production efficiency and high unit energy consumption, while excessively high salinity will increase osmotic pressure and cause serious concentration polarization, reducing current efficiency.