Process Configuration, Cost Breakdown and Engineering Application Insights

Abstract

Against the backdrop of growing global water scarcity, advanced wastewater treatment and safe reuse have become a core strategy to alleviate the imbalance between water supply and demand. The dual-membrane process combining ultrafiltration (UF) pretreatment and reverse osmosis (RO) desalination has emerged as a key technical route for high-standard wastewater resource recovery. This paper systematically evaluates the technical applicability and economic feasibility of the dual-membrane process for municipal wastewater reclamation for industrial use. Through comparative analysis of mainstream desalination, pretreatment and concentrate treatment processes, a complete process train including submerged UF + RO main system and denitrifying biofilter + dissolved air flotation + activated carbon adsorption concentrate treatment system is established. Based on full-scale operational data, the total operating cost of the dual-membrane process is approximately 0.975 yuan per cubic meter of produced reclaimed water. The solution demonstrates strong economic competitiveness in water-scarce regions, and provides a reliable technical and economic reference for municipal wastewater reuse projects targeting industrial water supply.

1. Overview of Wastewater Reclamation Desalination Technologies

1.1 Water Quality Requirements for Industrial Reuse

Municipal wastewater reclamation serves multiple end uses including agricultural irrigation, urban miscellaneous use, industrial production, environmental replenishment and source water supplementation. For industrial applications such as boiler makeup water, process water, cooling water and cleaning water, desalination is the core treatment requirement, with strict controls on hardness, conductivity and chloride ions. Biologically treated municipal effluent that meets basic discharge standards requires further advanced desalination to qualify as industrial feedwater.

1.2 Comparison of Mainstream Desalination Processes

Currently, commercially mature large-scale desalination technologies include distillation, ion exchange, electrodialysis and reverse osmosis. A comprehensive comparison of technical and economic characteristics is shown in the table below.

Distillation processes are mainly economical for seawater desalination with waste heat supply, with no large-scale application in municipal wastewater reuse. Ion exchange has high investment and operating costs with limited application cases. Electrodialysis has lower pretreatment requirements but cannot remove organic matter and microorganisms, with few domestic reuse projects. Reverse osmosis technology has the advantages of reliable performance, convenient operation and management, abundant engineering cases, and continuously declining equipment and operating costs with the large-scale production of membrane elements. It is the preferred desalination technology for municipal wastewater reclamation.

1.3 Pretreatment Selection for RO Systems

Reverse osmosis membranes have strict requirements for influent water quality, and reliable pretreatment is essential to ensure stable system operation and extend membrane service life. Common pretreatment solutions include multimedia filtration and ultrafiltration. Considering operational stability and effluent consistency, ultrafiltration is the preferred pretreatment scheme.

Ultrafiltration systems are divided into pressure-type and submerged configurations. For municipal wastewater with complex components, submerged ultrafiltration membranes have stronger fouling resistance, lower transmembrane pressure difference and lower operating energy consumption, making them the preferred pretreatment solution for dual-membrane wastewater reclamation projects.

1.4 Dual-Membrane (UF-RO) Process Principle

The dual-membrane desalination process adopts a two-stage membrane separation configuration:

1. Submerged ultrafiltration serves as the pretreatment stage, effectively removing suspended solids, colloids and partial organic matter to produce high-quality influent that meets RO inlet requirements.
2. Reverse osmosis serves as the deep desalination stage, removing dissolved salts, residual organic matter, microorganisms and trace pollutants to produce high-quality reclaimed water that meets industrial water standards.

This process combination achieves stable and efficient desalination of secondary municipal effluent, and has been widely verified in large-scale reclaimed water projects worldwide.

2. Treatment Solutions for Dual-Membrane Concentrate

While the dual-membrane process produces high-quality reclaimed water, it generates concentrated wastewater with enriched pollutants and salts. Proper treatment and disposal of membrane concentrate is a critical part of ensuring project compliance and economic viability.

2.1 Disposal Routes of Membrane Concentrate

There are two mainstream disposal routes for dual-membrane concentrate in engineering practice:

1. Return to the front end of the wastewater treatment plant: Suitable for cases where the concentrate volume is small relative to the total plant capacity. This scheme saves separate concentrate treatment investment but increases the load of the main treatment process.
2. Separate treatment before discharge: Required when the concentrate volume is large enough to affect the main process performance, or when the reclaimed water system operates as an independent commercial unit. High salinity of concentrate also makes separate treatment necessary to avoid accumulation of salts in the main treatment system.

For independent reclaimed water projects with high-salinity concentrate, separate treatment is the recommended solution.

2.2 Target Pollutants and Process Comparison

Dual-membrane concentrate mainly includes RO concentrate and UF backwash wastewater. RO concentrate contains elevated COD, total phosphorus, total nitrogen and inorganic salts, while UF backwash wastewater mainly contains total phosphorus. The core control indicators for discharge compliance are COD, TP and TN, with additional consideration of the impact of high salinity on biological treatment efficiency.

COD Removal Technologies

Three mainstream processes are available for COD removal from concentrate:

• Activated carbon adsorption and regeneration: Stable treatment effect, low in-situ regeneration operating cost, but requires larger land area and higher initial investment.
• Fenton oxidation: High treatment efficiency, lower capital cost, but high operating cost and large sludge production (classified as hazardous waste in some regions).
• Ozone oxidation: Small footprint, low investment, but relatively high operating cost.

For projects with sufficient available land, activated carbon adsorption and regeneration is the preferred solution due to its stable performance and low long-term operating cost.

Phosphorus Removal Technologies

Biological phosphorus removal has limited potential for membrane concentrate, as the influent is already biologically treated effluent. Post-chemical phosphorus removal is the recommended approach. Given the low SS content in membrane concentrate, conventional sedimentation has poor floc formation and low removal efficiency. Dissolved air flotation (DAF) is preferred for solid-liquid separation after coagulant dosing, achieving more efficient phosphorus removal.

Nitrogen Removal Technologies

Total nitrogen in membrane concentrate is mainly in the form of nitrate. Mainstream denitrification processes are compared as follows.

Denitrifying biofilters have the advantages of high volumetric load, small footprint, low investment and mature application in high-salinity RO concentrate denitrification. With salinity acclimation of functional bacteria, they can achieve stable denitrification performance under TDS of 8,000–10,000 mg/L, making them the preferred nitrogen removal solution for dual-membrane concentrate.

2.3 Recommended Concentrate Treatment Flow Sheet

The recommended complete concentrate treatment process is:
RO concentrate → Denitrifying biofilter (TN removal) → Dissolved air flotation (TP removal) → Activated carbon adsorption (COD removal) → Mixed with plant effluent for compliant discharge

UF backwash wastewater directly enters the DAF and activated carbon units without denitrification treatment, as its TN content is low. This combined process achieves efficient and stable removal of all target pollutants with good economic performance.

3. Full-Cycle Operating Cost Analysis

Based on actual operational data from a large-scale dual-membrane reclaimed water plant in China, this section provides a detailed breakdown of direct operating costs for both the main treatment train and concentrate treatment train, all converted to cost per cubic meter of final RO produced water.

3.1 Cost Breakdown of Main Dual-Membrane Train

Submerged Ultrafiltration Unit

The direct operating cost of ultrafiltration includes electricity consumption, chemical consumption and membrane depreciation:

• Electricity cost: 0.009 yuan/m³, mainly for overcoming transmembrane pressure loss, calculated based on average transmembrane pressure of 3.5 m over the 7-year design service life.
• Chemical cost: 0.026 yuan/m³, covering sodium hypochlorite, citric acid, sodium hydroxide and reducing agent for maintenance and recovery chemical cleaning.
• Membrane depreciation: 0.040 yuan/m³, calculated based on 8-year service life and average flux of 28 L/(m²·h).

The total direct operating cost of the UF unit is 0.075 yuan/m³ of UF permeate. Converted to RO product water with a 75% RO recovery rate, the UF cost is 0.100 yuan/m³.

Reverse Osmosis Desalination Unit

The direct operating cost of reverse osmosis includes electricity consumption, chemical consumption and membrane depreciation:

• Electricity cost: 0.181 yuan/m³, mainly for high-pressure pump operation, calculated based on power consumption of 0.283 kWh/m³ at 25℃ average water temperature.
• Chemical cost: 0.169 yuan/m³, covering antiscalant and cleaning agents.
• Membrane depreciation: 0.122 yuan/m³, calculated based on 7-year service life and average flux of 18 L/(m²·h).

The total direct operating cost of the RO unit is 0.472 yuan/m³ of RO product water.

3.2 Cost Breakdown of Concentrate Treatment Train

All concentrate treatment costs below are converted to cost per cubic meter of final RO product water, based on 75% RO recovery rate and 90% UF recovery rate.

Denitrifying Biofilter for Nitrogen Removal

• Electricity cost: 0.053 yuan/m³, for backwashing and circulation pumping systems.
• Chemical cost: 0.454 yuan/m³ of concentrate, mainly sodium acetate carbon source, calculated based on average nitrogen removal of 20 mg/L.
• Total direct cost: 0.507 yuan/m³ of concentrate, equivalent to 0.169 yuan/m³ of RO product water.

DAF for Phosphorus Removal

• Electricity cost: 0.023 yuan/m³, for reflux pumps and mixers.
• Chemical cost: 0.053 yuan/m³ of concentrate, mainly PAC coagulant, calculated based on average TP removal of 0.7 mg/L.
• Total direct cost: 0.076 yuan/m³ of concentrate, equivalent to 0.037 yuan/m³ of RO product water.

Activated Carbon Adsorption & Regeneration for COD Removal

• Electricity cost: 0.090 yuan/m³ of concentrate, for process pumps and fans.
• Natural gas cost: 0.242 yuan/m³, for thermal regeneration of spent activated carbon.
• Make-up carbon cost: 0.076 yuan/m³, for fresh activated carbon supplement.
• Water cost: 0.002 yuan/m³, for carbon washing and boiler makeup water.
• Total direct cost: 0.410 yuan/m³ of concentrate, equivalent to 0.197 yuan/m³ of RO product water.

3.3 Overall Cost and Economic Benchmarking

The summary of total operating costs is shown in the table below.

For economic benchmarking:

• Industrial tap water price in many regions exceeds 1.3 yuan/m³, and is even higher in water-scarce areas.
• Seawater desalination operating cost exceeds 2.92 yuan/m³ for RO process, and 4.25 yuan/m³ for multi-effect distillation process.

The dual-membrane reclaimed water process has a total operating cost of 0.975 yuan/m³, which is significantly lower than alternative water sources in water-scarce regions, demonstrating outstanding economic applicability.

4. Engineering Application Analysis

4.1 Applicable Scenarios

The dual-membrane wastewater reclamation process is particularly suitable for the following scenarios:

• Water-scarce cities and regions with high industrial water demand and tight water supply
• Industrial parks requiring stable high-quality reclaimed water for production use
• Municipal wastewater treatment plants upgrading to resource recovery facilities with reuse targets
• Projects with strict effluent salinity and hardness requirements for process or boiler makeup water

4.2 Key Engineering Design Considerations

• Recovery rate optimization: Reasonably set RO system recovery rate according to raw water quality and concentrate disposal capacity, balancing water production efficiency and membrane fouling risk.
• Pretreatment reliability: Ensure stable UF effluent SDI < 3 to protect RO membranes and extend their service life, which is the key to controlling long-term operating costs.
• Concentrate treatment matching: Design the concentrate treatment scale according to the actual water recovery rate, and reserve capacity for future system efficiency adjustment.
• High-salinity adaptation: For biological treatment units for concentrate, select salt-tolerant strains and set up gradient acclimation procedures to ensure stable denitrification efficiency under high salinity conditions.
• Sludge and waste disposal: Properly design sludge dewatering and spent carbon disposal systems to meet hazardous waste management requirements.

4.3 Operation & Maintenance Optimization Practices

• Implement graded chemical cleaning strategies based on transmembrane pressure and flux decline to reduce chemical consumption and extend membrane service life.
• Adopt online nitrate nitrogen monitoring linked with carbon source dosing to achieve precise carbon source control and reduce unnecessary chemical consumption.
• Optimize activated carbon regeneration cycles according to actual COD load to reduce energy consumption and carbon loss.
• Establish membrane performance archives to track attenuation trends and arrange membrane replacement plans in advance to avoid unexpected production interruption.

4.4 Full Lifecycle Economic Value

In addition to direct operating cost savings, the dual-membrane reclamation process delivers additional full-lifecycle benefits: it reduces freshwater withdrawal and wastewater discharge fees, helps industrial users meet water consumption quota requirements, and supports compliance with increasingly strict discharge regulations. For long-term operation projects, the comprehensive economic benefit is more prominent than the single cost comparison shows.

5. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that the dual-membrane process will remain the core technical solution for high-standard wastewater reclamation in the coming decade. Its technical maturity, stable effluent quality and continuously improving cost-effectiveness make it the most reliable choice for municipal and industrial water reuse projects.

Combined with SYNERAQUA’s skid-mounted modular equipment manufacturing capabilities, dual-membrane systems can be prefabricated into standardized units, greatly shortening on-site construction periods and improving installation quality, which is especially suitable for phased construction of industrial park reclaimed water projects. Our integrated SCADA and smart water operation platform enables automatic optimization of operating parameters, precise chemical dosing and intelligent membrane cleaning scheduling, further reducing operating costs by 8–12% compared with conventional manual operation. We also continue to develop concentrate reduction and zero-liquid-discharge technologies to help clients achieve higher water recovery rates and lower disposal costs, supporting the transition of wastewater treatment plants from pure discharge facilities to comprehensive water resource recovery centers.

6. Conclusion

The dual-membrane UF-RO process is a technically and economically feasible solution for municipal wastewater reclamation targeting industrial reuse, with the following core conclusions:

1. The submerged ultrafiltration + reverse osmosis dual-membrane process has strong applicability for wastewater desalination, with mature technology, convenient operation and stable effluent quality that meets high-standard industrial water requirements.
2. For dual-membrane concentrate requiring separate treatment, the combined process of denitrifying biofilter + DAF phosphorus removal + activated carbon adsorption achieves stable removal of TN, TP and COD, with reliable performance and good adaptability to high-salinity conditions.
3. The total direct operating cost of the complete dual-membrane system including concentrate treatment is approximately 0.975 yuan per cubic meter of reclaimed water, which is significantly lower than industrial tap water and seawater desalination costs, showing excellent economic applicability in water-scarce regions.
4. This process can effectively reduce water environmental pollution and alleviate water resource shortage, and is a viable technical choice for promoting urban water resource recycling and sustainable development.

FAQ

Q1: What are the core advantages of the dual-membrane process over other desalination technologies for wastewater reuse?
Compared with distillation, ion exchange and electrodialysis, the dual-membrane process combines high desalination efficiency, simultaneous removal of organic matter and microorganisms, convenient operation, abundant large-scale engineering cases and continuously declining costs, making it the most mature and cost-effective solution for municipal wastewater reclamation.

Q2: Why is denitrifying biofilter recommended for dual-membrane concentrate nitrogen removal?
Denitrifying biofilters have high volumetric load, small footprint and low investment. Most importantly, they have mature application experience in high-salinity RO concentrate treatment. With proper bacterial acclimation, they can maintain stable denitrification performance under high TDS conditions, which is more reliable than autotrophic denitrification and more efficient than deep bed filters for concentrate treatment.

Q3: Is reclaimed water produced by the dual-membrane process more economical than tap water for industrial use?
For most water-scarce regions, the 0.975 yuan/m³ operating cost of dual-membrane reclaimed water is significantly lower than the industrial tap water price of 1.3 yuan/m³ or higher. When additional wastewater discharge fees and water quota constraints are considered, the comprehensive economic advantage is even more prominent, with a reasonable investment payback period for most projects.

Q4: What are the key factors affecting the operating cost of the dual-membrane process?
The main cost drivers are RO membrane depreciation, high-pressure pump electricity consumption, antiscalant and cleaning chemicals, and concentrate treatment costs. Optimizing pretreatment to extend membrane life, improving energy recovery efficiency, and reducing concentrate treatment chemical consumption are the core directions for further cost reduction.