Process Configuration, Energy Performance and Full Life-Cycle Economic Evaluation

Abstract

Cost reduction and energy saving are the inevitable development trends of reverse osmosis (RO) seawater desalination technology, and RO system configuration design is a critical factor determining overall operating cost and energy consumption. Taking the design conditions of a seawater desalination plant in Tianjin as a case, this paper conducts a technical and economic comparison of four design schemes, with single-stage seawater reverse osmosis (SWRO) as the benchmark, including split partial single-pass reverse osmosis (SSP), partial second-pass reverse osmosis (PSP) and split partial second-pass reverse osmosis (SPSP), through software simulation. Results show that SPSP achieves the widest water quality temperature regulation range and the best energy saving performance. Under the premise of constant product water quality, production volume and system recovery rate, SPSP provides a temperature regulation value of 0–10℃, while SSP and PSP only provide 0–3℃, independent of raw water quality and operation duration. Under identical water quality and operation conditions, SPSP consistently achieves the lowest specific energy consumption (SEC) among the four configurations. Further analysis of SPSP indicates that split ratio has a significantly greater impact on temperature regulation range than product water quality requirements: each 5% increase in split ratio raises the critical temperature by 3℃, while tightening the Cl⁻ effluent requirement from 160 mg/L to 60 mg/L only reduces the critical temperature by 3℃. Economic comparison covering capital expenditure and operating expenditure confirms that SPSP achieves an optimal balance between incremental investment and energy cost reduction, presenting more prominent economic advantages than SSP and PSP. Overall, SPSP is the design scheme with the best comprehensive technical and economic performance for seawater RO systems.

1. Introduction

After decades of development, reverse osmosis has become the mainstream technology in the seawater desalination industry, accounting for more than 60% of the global desalination market. However, the high-pressure driving requirement of RO processes results in persistently high energy consumption. Against the backdrop of carbon peak and carbon neutrality goals, further energy consumption reduction and water production cost decline have become core requirements for the development of seawater reverse osmosis technology.

The specific energy consumption (SEC) of SWRO projects has dropped from approximately 5.0 kW·h/m³ 25 years ago to around 3.0 kW·h/m³ at present, with the most energy-efficient plants achieving as low as 2.9 kW·h/m³, of which the RO system itself accounts for only 2.0 kW·h/m³. Theoretical calculation shows that the minimum thermodynamic energy limit for reverse osmosis desalination is 1.06 kW·h/m³ for 35,000 mg/L seawater at 50% recovery, indicating that there is still considerable room for energy saving in RO systems.

In addition to improving the performance of high-pressure pumps and other energy-consuming equipment, optimizing membrane system configuration and process design is another important approach to reduce RO system energy consumption, including hybrid membrane design, double-end product water collection, 16-inch membrane element configuration and three-center high-pressure system design.

Among existing optimized configurations, the split partial second-pass (SPSP) design, which collects permeate separately from the feed and brine ends of pressure vessels and sends high-salinity rear-end permeate to a brackish water RO (BWRO) system for further desalination, can effectively reduce the scale of the second-pass system and lower engineering investment. The split partial single-pass (SSP) design recycles part of the rear-end permeate back to the feed inlet to dilute raw water, improving product water quality without additional BWRO systems. The conventional partial second-pass (PSP) design sends part of the total SWRO permeate to BWRO for secondary desalination.

This paper conducts a systematic technical and economic comparison of four RO design schemes (SWRO, SSP, SPSP, PSP) under identical product water volume, quality and system recovery conditions, based on the industrial water quality requirements of a large seawater desalination plant in Tianjin, to provide reference for engineering design and scheme selection.

2. Comparison Scheme Design

2.1 Design Conditions

The raw water of the target large-scale seawater desalination plant is taken from the Bohai Bay, with a TDS of 28,000–32,000 mg/L. The product water is supplied to surrounding industrial enterprises, with a water quality requirement of Cl⁻ ≤ 120 mg/L, a design capacity of 1000 m³/h, and an overall RO system recovery rate of 44.64%.

Toray membrane elements and Toray DS2 v217166 simulation software are adopted for analysis. The annual salt passage increase is set at 7% for SWRO membranes and 5% for BWRO membranes. Fouling factors for different operation years are set as follows: 0-year operation: 0.956 for SWRO, 0.977 for BWRO; 3-year operation: 0.886 for SWRO, 0.955 for BWRO; 5-year operation: 0.840 for SWRO, 0.942 for BWRO.

The SWRO system adopts 2100 pieces of TM820M-440 membrane elements installed in 300 seven-element pressure vessels. For both SPSP and PSP schemes, the BWRO system adopts 126 pieces of TM720D-440 membrane elements installed in 18 seven-element pressure vessels.

2.2 Scheme Configuration

(1) Single-Stage SWRO (Benchmark)

Single-stage SWRO is adopted as the comparison benchmark, with a system recovery rate of 44.6%.

(2) Split Partial Second-Pass (SPSP)

When the water temperature rises to the critical value, rear-end permeate from SWRO vessels is sent to the BWRO system for further desalination. BWRO permeate is blended with front-end SWRO permeate to meet the overall product water quality requirement. The flow rate entering BWRO is set at approximately 145 m³/h to maintain consistency with other schemes.

(3) Split Partial Single-Pass (SSP)

When water temperature reaches the critical value, part of the rear-end permeate is recirculated back to the SWRO feed inlet. The recirculation flow is set at 145 m³/h, corresponding to the permeate volume of the last two membrane elements. This internal circulation raises the SWRO stack recovery rate to 48%, while the overall system recovery rate remains unchanged at 44.64%. For simulation purposes, the seven-element pressure vessels are configured as a three-stage arrangement with a 3:2:2 element ratio, with permeate from the third stage recirculated.

(4) Partial Second-Pass (PSP)

SWRO permeate is collected uniformly without split collection. When water temperature exceeds the critical value, part of the SWRO permeate is sent to the BWRO system for secondary desalination, then blended with the remaining SWRO permeate. The recovery rate and flow distribution parameters are identical to those of the SPSP scheme for consistent comparison.

3. Results and Discussion

3.1 Influence of Raw Water TDS

Critical temperature is defined as the maximum water temperature at which the system can meet the target product water quality (Cl⁻ ≤ 120 mg/L). Beyond this temperature, permeate salt content will exceed the set limit.

Simulation results under three inlet TDS conditions (28,000 mg/L, 32,000 mg/L, 35,000 mg/L) at initial operation (0 year) show that critical temperature decreases with increasing inlet TDS for all four designs, which conforms to the desalination characteristics of RO membranes: lower temperature reduces ion permeability and increases salt rejection, so higher feed salinity requires lower operating temperature to maintain the same permeate quality.

Under identical inlet TDS, the critical temperatures of different designs rank in descending order as: SPSP > SSP = PSP > SWRO. Taking 28,000 mg/L inlet TDS as an example, the critical temperature of SWRO is 25℃, SPSP reaches 35℃, while both SSP and PSP reach 28℃. This indicates that all three optimized designs improve water quality regulation capability compared with single-stage SWRO. SSP and PSP provide a temperature regulation range of only 0–3℃ relative to SWRO, while SPSP achieves a 0–10℃ regulation range, showing significantly better performance.

Notably, the temperature regulation increment of SSP and PSP remains constant at 3℃, and that of SPSP remains at 10℃ across all tested inlet TDS levels, confirming that the achievable temperature regulation range of each design is independent of raw water salinity.

For SEC performance at critical water quality conditions, SEC increases with rising inlet TDS for all designs, as higher osmotic pressure requires greater driving force to maintain production volume. Under identical inlet TDS, SEC values rank in descending order as: SSP > PSP > SWRO > SPSP. At 28,000 mg/L TDS, the SEC values of the four schemes are 3.768, 3.548, 3.533 and 3.452 kW·h/m³ respectively.

The higher SEC of SSP is attributed to the internal recirculation of rear permeate, which increases the SWRO stack recovery rate and requires higher operating pressure. PSP has higher SEC than SWRO due to the additional energy consumption of the BWRO system. Although SPSP also includes a BWRO system, its wider temperature regulation range allows operation at higher temperatures with lower overall SEC, which ultimately falls below that of single-stage SWRO.

Comprehensive analysis confirms that SPSP outperforms the other designs in both water quality regulation capability and energy saving performance under varying inlet salinity conditions.

3.2 Influence of Operation Duration

Under 32,000 mg/L inlet TDS, simulation across 0, 3 and 5 years of operation shows that critical temperature decreases with increasing operation years for all designs, caused by the annual increase in membrane salt passage. As membrane salt rejection declines over time, lower operating temperatures are required to maintain the same permeate quality.

Under identical operation duration, the ranking of critical temperatures remains consistent: SPSP > SSP = PSP > SWRO. For 3-year operation, for example, SWRO has a critical temperature of 18℃, SPSP reaches 27℃, and both SSP and PSP reach 21℃. The temperature regulation ranges of each design remain constant across all operation years, indicating that regulation capability is also independent of membrane aging and fouling status.

Regarding SEC, all designs show increasing energy consumption with longer operation time, as the fouling factor reduces effective membrane area, requiring higher operating pressure to maintain production volume. For operation periods below 3 years, SEC ranks as SSP > PSP > SWRO > SPSP. After 3 years of operation, the ranking shifts to SSP > SWRO > PSP > SPSP, as PSP gradually gains energy advantage over single-stage SWRO with membrane aging. At 3-year operation, the SEC values of the four schemes are 4.579, 4.223, 4.176 and 4.067 kW·h/m³ respectively.

Across all tested operation durations, SPSP consistently maintains advantages in both water quality regulation and energy efficiency.

3.3 Influencing Factors of Temperature Regulation Range

Further analysis is conducted on SPSP, the optimal scheme, to identify key factors affecting its temperature regulation range, with SWRO as the benchmark.

(1) Product Water Quality Requirements

Under constant system recovery and production volume (32,000 mg/L TDS, 0-year operation), stricter Cl⁻ effluent requirements reduce the critical temperature for both SWRO and SPSP, as higher water quality standards require lower operating temperatures. When the Cl⁻ requirement is tightened from 160 mg/L to 60 mg/L, the temperature regulation range of SPSP narrows from 11℃ to 8℃, showing only a limited impact.

(2) Permeate Split Ratio

Split ratio is defined as the proportion of rear-end high-salinity permeate sent to the BWRO system relative to total product water. Four split ratio schemes (14.29%, 19.61%, 24.95%, 30.16%) are compared under fixed conditions (32,000 mg/L TDS, 5-year operation, 120 mg/L Cl⁻ requirement, 44.46% total recovery, 1000 m³/h capacity).

Results show that each ~5% increase in split ratio raises the critical temperature by 3℃, while SWRO recovery only increases by ~0.13%, and SEC remains basically stable without significant change. This indicates that split ratio is the dominant factor determining the temperature regulation range of SPSP systems, and can be adjusted flexibly in engineering applications without obvious negative impact on energy consumption.

4. Techno-Economic Analysis

Economic comparison is conducted from two aspects: capital expenditure (CAPEX) and operating expenditure (OPEX), of which OPEX mainly includes chemical consumption and energy cost.

4.1 Capital Expenditure

Since all schemes maintain identical total production volume and recovery rate, the investment for water intake, pre-treatment, post-treatment and other common units remains the same. The incremental investment of PSP and SPSP compared with single-stage SWRO comes from the additional BWRO system, including one BWRO feed pump, 126 brackish water membrane elements, 18 seven-element pressure vessels, as well as supporting pipelines, valves and instruments.

The total incremental investment is approximately 1.5 million RMB, equivalent to 62.5 yuan per m³/d of capacity. According to DesalData 2023 statistics, the average engineering CAPEX for international seawater desalination plants is about 4666.48 yuan/m³, meaning the additional BWRO investment accounts for only 1.3% of total project cost, with very limited impact on overall investment. SSP requires no additional CAPEX as it only adds an internal recirculation pipeline.

4.2 Chemical Consumption Cost

SWRO systems require consistent dosage of antiscalant and reducing agent. Since all four schemes have identical SWRO feed volume, chemical costs for the SWRO section are identical.

PSP and SPSP require additional sodium hydroxide dosing for BWRO feed conditioning. With identical BWRO feed volume, the sodium hydroxide (100% basis) dosage is approximately 1 g/m³, corresponding to an incremental chemical cost of only 0.0075 yuan per cubic meter of product water, which is negligible for overall operating cost.

4.3 Energy Cost

Based on previous simulation results, SSP increases SEC by 0.2–0.4 kW·h/m³ compared with SWRO. PSP has roughly equivalent SEC to SWRO across the full operation cycle. SPSP reduces SEC by 0.1–0.3 kW·h/m³ relative to SWRO.

Calculated at an industrial electricity price of 0.7 yuan/kW·h, SSP increases energy cost by 0.21 yuan/m³, while SPSP reduces energy cost by 0.14 yuan/m³.

Table 1 Economic Indicator Comparison

Comprehensive comparison shows that SSP has no incremental investment but higher operating energy cost; PSP has no obvious energy cost increase but requires additional investment; SPSP requires moderate incremental investment but achieves continuous energy cost reduction throughout the operation cycle. Among the three optimized schemes, SPSP presents the most prominent comprehensive economic advantage.

5. Engineering Application Insights

5.1 Applicable Scenarios

Different RO design schemes are suitable for different project requirements:

• Single-stage SWRO: Suitable for projects with lenient product water quality requirements, stable raw water temperature and low investment budget.
• SSP: Suitable for temporary water quality improvement requirements with limited site space and no room for BWRO system installation, where moderate energy consumption increase is acceptable.
• PSP: Suitable for projects requiring flexible water quality adjustment with small-scale second-pass systems, especially for applications requiring graded water supply.
• SPSP: The preferred scheme for large and medium-sized seawater desalination plants with strict product water quality requirements, wide raw water temperature fluctuation range, and long operation cycles, achieving optimal life-cycle economy.

5.2 Key Design Considerations

• Split ratio optimization: Split ratio is the core parameter determining SPSP performance. Design should be based on the maximum raw water temperature and minimum required water quality, to determine the most economical split ratio while avoiding over-configuration of the BWRO system.
• Membrane element matching: SWRO and BWRO membrane types should be reasonably selected and matched to ensure stable salt rejection performance across the full temperature range and operation cycle.
• Energy recovery device configuration: For large-scale BWRO systems, matching energy recovery devices can further reduce energy consumption and improve the economic advantage of SPSP.
• Operation flexibility: Valves and control systems should be configured to enable flexible switching between full single-stage operation and partial second-pass operation according to seasonal temperature changes, minimizing unnecessary energy consumption in low-temperature periods.

5.3 Operation and Maintenance Best Practices

• Develop segmented operation strategies based on seasonal temperature changes, adjust split ratio dynamically to balance water quality compliance and energy consumption.
• Regularly calibrate membrane salt passage and fouling factor according to actual operation data, and update the critical temperature threshold in time to ensure stable effluent quality.
• Optimize chemical dosing for BWRO systems according to actual feed water quality, to avoid unnecessary chemical consumption while preventing membrane scaling and degradation.
• Establish a full life-cycle cost evaluation mechanism, comprehensively consider membrane replacement cycle, energy price changes and other factors, and dynamically adjust the operation scheme to achieve the lowest long-term operating cost.

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that optimized RO system configuration is one of the most cost-effective approaches to reduce the energy consumption and water production cost of seawater desalination projects, and the SPSP design concept perfectly balances water quality assurance and energy efficiency for large-scale desalination plants.

Our modular seawater desalination RO systems adopt standardized SPSP configuration design, with pre-engineered split permeate collection and BWRO units, which can be flexibly adjusted according to different raw water salinity, temperature ranges and product water quality requirements. Equipped with our intelligent RO operation optimization platform, the system realizes real-time dynamic adjustment of split ratio and operating parameters based on feed water temperature, salinity and membrane performance attenuation, further reducing comprehensive energy consumption by 5–8% compared with conventional fixed-parameter SPSP design. We also provide full-process technical services including scheme optimization, economic evaluation and operation guidance for seawater desalination plants, helping clients achieve the optimal balance between investment cost and long-term operating cost.

7. Conclusion

This paper takes single-stage SWRO as the benchmark and conducts a systematic technical and economic comparison of three optimized RO design schemes (SSP, PSP, SPSP), with the following main conclusions:

1. Under the premise of constant product water quality, production volume and system recovery rate, SPSP achieves the best performance in both water quality temperature regulation range (0–10℃) and energy saving effect. SSP and PSP have the same temperature regulation range (0–3℃), while PSP has better energy performance than SSP. The maximum temperature regulation increment of each design is a fixed value, independent of raw water quality and operation duration.
2. For SPSP systems, product water quality requirements have limited impact on temperature regulation range, while split ratio is the dominant influencing factor. Each ~5% increase in split ratio raises the critical temperature by 3℃, with negligible impact on SWRO recovery rate and SEC, making split ratio the most direct and effective regulation parameter in engineering applications.
3. Economic analysis shows that SPSP achieves an optimal balance between moderate incremental investment and continuous energy cost reduction throughout the operation cycle, and has more prominent comprehensive economic advantages than SSP and PSP.
4. Comprehensive technical and economic comparison confirms that SPSP has the strongest engineering application value among the three optimized schemes, in terms of both product water quality regulation capability and full life-cycle cost control.

FAQ

Q1: What is the core difference between SPSP and conventional full second-pass RO design?
Conventional full second-pass RO sends all first-pass permeate to the second-pass system for desalination, resulting in large second-pass scale, high investment and high energy consumption. SPSP only sends the high-salinity rear-end permeate of SWRO to the BWRO system, making full use of the salinity gradient along the pressure vessel. The BWRO scale is only 15–30% of the full second-pass design, which greatly reduces investment while meeting water quality requirements.

Q2: How to determine the optimal split ratio for SPSP projects?
The optimal split ratio should be determined based on the maximum raw water temperature in the hottest month, the required effluent quality standard, and the membrane performance at the end of its service life, with a certain safety margin. Oversized split ratio will lead to waste of BWRO investment, while undersized split ratio cannot guarantee water quality compliance in high-temperature periods. Dynamic adjustable split ratio design can further optimize operation economy.

Q3: Which scheme is more suitable for projects with large seasonal temperature fluctuations?
SPSP is the most suitable choice for projects with large raw water temperature fluctuations. Its wide temperature regulation range can maintain stable effluent quality throughout the year, and the second-pass system can be put into operation or shut down flexibly according to temperature changes, avoiding unnecessary energy consumption in low-temperature seasons and achieving the lowest annual average energy consumption.

Q4: Does the additional BWRO system significantly increase operation and maintenance workload?
No. The BWRO system in SPSP design has a small scale, simple process flow and basically the same operation and maintenance requirements as conventional RO systems. Since the feed is SWRO permeate with low turbidity and low pollution load, the BWRO system has slower membrane fouling rate and longer chemical cleaning cycle, which will not bring obvious additional operation and maintenance burden.