Design and Thermophysical Performance of a Power-Driven Electrochemical–Disc-Tube Reverse Osmosis Integrated System for High-Energy Propellant Wastewater Treatment

Unsymmetrical dimethylhydrazine (UDMH), owing to its high specific impulse, hypergolic characteristics, and favorable thermochemical properties [1], [2], [3], [4], has long been employed as a representative high–energy-density energetic material and a key propellant in aerospace propulsion systems, including various rocket engines and power units [5]. In addition, UDMH [6], together with its associated oxidizer dinitrogen tetroxide, exhibits satisfactory storage stability and engineering applicability under ambient conditions, which has led to its extensive use in strategic ballistic missile propulsion systems as well as launch missions involving satellites, space stations, and deep-space probes [7], [8], [9]. These high-power, high–energy-density application scenarios have resulted in a sustained increase in the consumption of UDMH-based propellants over their entire life cycle, and the generation of highly hazardous wastewater during production, operation, and decommissioning has gradually emerged as a critical factor constraining the safe and reliable operation of engineering systems [10], [11], [12].

Despite its irreplaceable strategic value in aerospace and defense propulsion engineering [13], wastewater containing UDMH is characterized by extreme toxicity and strong reducing properties, posing substantial risks to personnel safety, equipment integrity, and the long-term stability of engineering systems [14]. To address the safe disposal of UDMH-contaminated wastewater, a variety of treatment approaches have been reported, including chemical, physical, biological, and hybrid processes [15]. From an engineering systems perspective, however, single treatment technologies generally exhibit inherent limitations with respect to treatment efficiency, controllability of energy consumption, operational stability, and long-term techno-economic performance. Such limitations make it difficult to satisfy the requirements for continuous, high-load operation under practical engineering conditions involving highly toxic and high-concentration UDMH wastewater. Consequently, the development of combined treatment processes based on multiple physical and chemical mechanisms and coordinated operation of different units has become an increasingly important trend in the engineering treatment of UDMH wastewater [16].

Existing studies have largely focused on the degradation pathways of UDMH from the viewpoints of reaction mechanisms or functional materials. Gao et al. [17] constructed TiO₂ nanorod/CdS heterojunctions on fluorine-doped tin oxide substrates using hydrothermal and successive ionic layer adsorption and reaction methods, achieving efficient UDMH degradation through the visible-light sensitization effect of CdS. Wang et al. [18] prepared immobilized hybrid catalysts by combining chitosan–silica with non-noble metal ions and systematically examined degradation efficiency, chemical oxygen demand (COD) removal, and the formation and control of N-nitrosodimethylamine during UDMH wastewater treatment. Meng et al. [19] further integrated catalytic wet peroxide oxidation with vacuum ultraviolet irradiation to realize rapid mineralization of high-concentration UDMH wastewater. While these studies have provided important theoretical insights into the reaction behavior and degradation mechanisms of UDMH, their primary emphasis has remained on material performance or reaction pathways, with comparatively limited attention to system-level engineering issues such as power input constraints, energy distribution characteristics, thermo–mechanical coupled stability, and equipment integration.

By contrast, electrochemical oxidation (ECO) has demonstrated considerable engineering potential for the treatment of highly hazardous organic pollutants, attributable to its high redox potential, controllable reaction process, and compact system configuration. From an engineering thermophysics standpoint, this process constitutes a typical power-driven electrochemical operation, in which the operating state is jointly governed by current density, volumetric energy input, and the generation and transfer of heat during reaction. Under continuous electrical power input, ECO enables deep oxidation of organic contaminants while maintaining a balance between treatment efficiency and operational reliability at the engineering scale. Systems centered on ECO are therefore increasingly regarded as a promising option for the engineered treatment of highly toxic wastewater, as they can satisfy discharge requirements while retaining relatively simplified process flows and stable operation. Boron-doped diamond (BDD) electrodes, characterized by high electrochemical stability and thermochemical inertness, have shown favorable performance in the degradation of various refractory organic pollutants. Nevertheless, systematic engineering investigations addressing power matching, structural reliability, and long-term operational stability of BDD-based electrochemical systems for UDMH wastewater treatment remain limited.

Engineering practice indicates that effluent turbidity and concentrations of certain intermediate products may fluctuate following treatment by ECO alone, potentially compromising system stability and downstream equipment safety. As a result, coupling a high-pressure membrane separation process downstream of the ECO unit has become an effective approach for improving overall treatment performance and operational stability through coordinated multi-unit operation. Disc-tube reverse osmosis (DTRO) membrane systems, featuring high structural strength, a wide pressure tolerance range, and relatively low specific energy consumption, exhibit clear advantages in the advanced treatment of high-concentration, highly toxic wastewater. These membrane systems demonstrate strong adaptability to variations in influent water quality and operating conditions, enabling stable effluent quality over a broad operational range. From a system integration perspective, high-pressure membrane separation units show favorable matching with upstream high-power electrochemical processes in terms of pressure stability, structural reliability, and energy consumption per unit throughput.

Against this background, the construction of a power-driven engineering system model coupling ECO with DTRO membrane separation, together with model-driven design and verification of the reactor, membrane unit, and pipeline system under constraints of power input, energy controllability, and high-load operational stability, is of significant importance for advancing the engineering application of UDMH wastewater treatment equipment. The present work considers an integrated, mobile treatment system with a processing capacity of 50 L/h. Through the combined use of multiphysics modeling and structural simulation, key components are systematically designed and validated, and experimental investigations are conducted to assess treatment performance under continuous high-load operating conditions.

The remainder of this article is organized as follows. Section 2 presents the structural design and parameter selection of the electrochemical reactor, together with verification of engineering reliability through static and coupled simulations. Section 3 addresses the design and modeling of the downstream DTRO membrane unit and pipeline system, with an evaluation of their structural and operational characteristics under different working conditions. Section 4 reports silver-ion simulated wastewater experiments and pilot-scale tests using actual UDMH wastewater to systematically assess the overall treatment performance and operational behavior of the integrated system. Finally, the main conclusions are summarized. The overall research framework is illustrated in Figure 1.

As the high–power-density core unit of the integrated “ECO–DTRO” treatment equipment, the electrochemical reactor operates under continuous electrical energy input and fluid-driven conditions. Its engineering design not only governs reaction efficiency but also directly affects the overall energy consumption and long-term operational stability of the system. Consequently, reactor design must satisfy electrochemical reaction requirements while accounting for the coupled constraints among energy input, fluid transport, and structural response.

This section addresses the engineering requirements for practical application of the electrochemical reactor by combining process modeling with structural modeling. Systematic hydraulic modeling and optimization of pump parameters, mixing process modeling and selection of stirring intensity, and multiphysics coupled simulations are conducted to analyze and validate the structural response of the reactor under static loading and operational vibration. The objective is to ensure structural safety and stable operation while avoiding unnecessary power redundancy and energy loss, thereby providing a reliable engineering and modeling basis for subsequent experimental validation and system integration.

During continuous operation of the electrochemical reactor, the influent pump serves as the primary source of mechanical energy input. Its parameter selection directly determines the power demand associated with fluid transport and has a significant influence on overall system energy consumption and operational stability. Therefore, under the constraints of the designed treatment capacity and pipeline configuration, rational determination of pump parameters based on a simplified hydraulic model is required to prevent additional energy consumption arising from excessive head or flow redundancy.

Based on the system treatment capacity and influent conditions, the required flow rate was first calculated. Given a reactor volume of 200 L and a designated filling time of 5 min, the required influent flow rate can be obtained from Eq. (1):

The calculated flow rate is 2.4 m³/h, where Q denotes the flow rate, V represents the reactor volume, and t is the filling time. To ensure completion of the influent process within the specified time while maintaining stable operation, the rated pump flow rate should be slightly higher than 2.4 m³/h.

On this basis, the pump head requirement was estimated. The total pump head consists of the static head and hydraulic losses, where the static head is primarily determined by the vertical height difference between the suction inlet and the terminal elevated tank. With an outlet pipe diameter of 25 mm and a design flow rate of 3.5 m³/h, the corresponding internal flow velocity can be calculated using Eq. (2):

The resulting flow velocity is 7.1 m/s, where V₁ is the inlet velocity and S_m is the cross-sectional area of the pipe. For a 90° elbow in the pipeline, the local resistance coefficient is taken as 0.294, and the corresponding local head loss can be estimated using Eq. (3):

In Eq. (3), h denotes the local head loss, $\xi$ is the local resistance coefficient, and g is the gravitational acceleration. Substitution yields a local head loss of approximately 0.756 m per 90° elbow. The frictional loss along the pipeline is calculated as 0.124 kPa/m. Considering a total pipeline length of approximately 5 m, the corresponding frictional head loss is about 0.06 m, which is negligible relative to the static head.

Taking into account the pump suction head (not exceeding 4 m), the reactor height (approximately 1 m), and local head losses, the net required head is approximately 5 m. After adding hydraulic losses, the required pump head should be no less than 6 m. Based on these hydraulic calculations, a Yangtze River pump with a rated flow rate of 3.5 m³/h and a rated head of 10 m was selected. This configuration satisfies the influent requirements and operational stability while avoiding unnecessary power redundancy associated with excessive head or flow capacity.

The selected influent pump, determined through quantitative hydraulic modeling, enables appropriate control of mechanical energy input while ensuring reliable process operation, thereby providing an engineering basis for reducing overall system energy consumption.

During reactor operation, the stirring system constitutes another important source of mechanical energy input. Its operating parameters directly influence mass transfer efficiency at reaction interfaces and overall system homogeneity, and also represent a significant component of continuous operational energy consumption. Consequently, selection of stirrer parameters requires rational control of stirring intensity to satisfy reaction kinetics and mass transfer demands while avoiding excessive rotational speeds that would lead to inefficient energy use.

A mixing dynamics model was established based on reactor dimensions and operating conditions to estimate the required stirring intensity. A conventional impeller-type stirrer was employed, which typically operates at rotational speeds below 100 r/min in engineering applications. Considering the spatial scale of the reactor and flow characteristics, the design rotational speed was set to 88 r/min. The corresponding stirring power was calculated using Eq. (4):

where, n is the rotational speed; D is the impeller diameter (m), with a maximum value of 0.078 m based on reactor geometry; $\rho$ is the liquid density, taken as 1 g·cm^-3 for the wastewater; and N_p is the power number of the impeller, which is 0.696 for the selected impeller type. Substitution of these parameters yields a stirring power of approximately 0.55 kW.

Considering that practical stirrer efficiency typically ranges from 0.7 to 0.8, the rated power should be slightly higher than the theoretical value to ensure stable operation under design conditions. Accordingly, the rated power was estimated as 0.55/0.75 $\approx$ 0.73 kW, and a stirrer motor with a rated power of 0.75 kW was selected.

This model-driven parameter selection ensures adequate mixing and mass transfer while enabling rational control of mechanical energy input, thereby avoiding unnecessary energy consumption associated with excessive speed or power and supporting stable, continuous operation of the electrochemical reactor.

To ensure long-term and stable operation of the electrochemical reactor under the combined effects of electric fields, fluid flow, and mechanical driving forces, a structural mechanics model of the reactor was established. Both static and dynamic simulations were performed. These simulations were used not only to verify structural safety but also to inform structural optimization, thereby reducing the risk of instability or additional energy consumption caused by structural deficiencies during operation.

Within the electrochemical reactor, multiphysics coupling occurs among electric field distribution, fluid flow, mass transfer, and the resulting structural stresses and deformations. Under engineering operating conditions, these processes interact with one another, and structural stability is a prerequisite for maintaining electrochemical reaction efficiency, continuity of fluid transport, and proper operation of downstream membrane separation units. Accordingly, the structural simulations conducted in this study aim to ensure that the reactor body provides a stable and reliable operating environment for these coupled processes, thereby supporting sustained system operation from an engineering standpoint.

In addition, under high current density electrolysis, significant Joule heating and reaction heat release occur within the reactor. Temperature control influences electrode polarization behavior and mass transfer processes, making the reliability of the reactor structure and cooling components a critical factor in maintaining thermal stability during operation.

Finite element analysis was carried out to evaluate the structural strength and deformation of the electrochemical reactor under static liquid loading and vibration induced by stirring. The analysis procedure consisted of the following steps.

(1) Import of reactor geometry: Static structural analysis was selected in ANSYS, and the reactor geometry created in SolidWorks was imported in *.stp format.

(2) Material properties: The reactor vessel was modeled as stainless steel with an elastic modulus of 193 GPa, Poisson's ratio of 0.31, and density of 7750 kg/m³. The electrode plates were modeled as BDD with an elastic modulus of 400 GPa, Poisson’ s ratio of 0.36, and density of 2340 kg/m³. Cooling tubes were modeled as titanium alloy with an elastic modulus of 96 GPa, Poisson's ratio of 0.36, and density of 4620 kg/m³.

(3) Mesh generation: A mesh size of 20 mm was applied to critical components such as electrode plates, while default meshing was used for other parts except the larger vessel body.

(4) Load and boundary conditions: Static liquid pressure corresponding to a liquid height of 620 mm from the vessel bottom was applied to the inner wall and internal components. The bottoms of the vessel support legs were constrained with fixed boundary conditions.

(5) Solution and output: All components were included in the analysis, and deformation and stress distributions were obtained. The finite element mesh is shown in Figure 2.

When static liquid pressure corresponding to a 620 mm liquid height was applied, the resulting stress and deformation distributions are shown in Figure 3. With 200 L of wastewater in the vessel, the overall stress distribution was relatively uniform, with no pronounced local stress concentration. The maximum stress was 8.2302 MPa, mainly located in the vessel cover supporting the stirrer and at the cooling tube interfaces. This stress level is far below the yield strength of 304 stainless steel (310 MPa), indicating a large safety margin under static loading conditions, with safety factors exceeding 37 and 60, respectively, fully satisfying static strength requirements.

Based on these simulation results, the support layout of the cooling tubes was further evaluated and optimized to reduce potential material redundancy while maintaining structural safety.

Under static loading, the maximum deformation was 1.6112 mm and occurred at the furthest end of the cooling tube. This deformation is attributed to the relatively large span and weaker constraints of the cooling tube and falls within the normal elastic deformation range of a flexible structure. Given that the primary function of the cooling tube is heat transfer and that it is connected to the vessel via flanged joints, this deformation does not compromise sealing performance or heat transfer efficiency, nor does it cause interference with internal electrodes or the stirrer. Overall, the reactor exhibits limited deformation and good stiffness and geometric stability under full static liquid loading, providing a stable spatial environment for electrochemical reactions.

Further analysis was performed by applying a rotational speed of 88 r/min to the stirrer with 200 L of wastewater in the vessel to evaluate stress and deformation under operating conditions. The corresponding results are shown in Figure 4. The dynamic stress distribution closely matches that under static loading, with a maximum stress of 8.2303 MPa, indicating negligible change relative to the static case. Stress concentrations remain localized in the vessel cover supporting the stirrer and at the cooling tube interfaces.

Similarly, the maximum deformation remains 1.6112 mm at the furthest end of the cooling tube, with no noticeable amplification. This indicates that, at the selected design speed, mechanical vibration induced by stirring does not significantly affect overall structural strength or deformation. This behavior is attributed to the low stirring speed, corresponding to an excitation frequency of approximately 1.47 Hz, which is far below the natural frequencies of the reactor structure, thereby avoiding resonance. In addition, the overall structural stiffness, together with reinforced connections at the vessel cover flange and pipe interfaces, effectively suppresses vibration transmission.

Structural simulations demonstrate that the reactor effectively avoids resonance risks under normal operating conditions, and that vibration induced by stirring has a negligible influence on structural strength and deformation. As a result, fatigue safety and structural reliability are ensured during long-term continuous operation. This structural stability also provides a foundation for reliable long-term heat transfer by the cooling system under high current density electrochemical conditions, contributing to the suppression of accumulated Joule heating and reaction heat release and thereby maintaining thermal stability during high-load operation.

Following the model-driven design of the electrochemical reactor, this section focuses on system integration of the downstream membrane treatment and piping units. Reverse osmosis operates under sustained high-pressure conditions, and the selection of membrane modules, anti-fouling strategy, and piping layout affect not only separation performance but also the system power demand, operational energy consumption, and long-term stability. Accordingly, a membrane-separation process model and a coupled pressure–structure model for the piping network were established to support membrane element selection, definition of anti-fouling measures, optimization of the piping layout, and assessment of vibration-related risks, thereby ensuring coordinated and reliable operation at the system level.

Considering the wastewater characteristics, treatment objectives, and energy constraints associated with high-pressure reverse osmosis operation, a flux and fouling-prediction model was used to guide membrane selection and system configuration. A Dow BW-series polyamide composite reverse osmosis membrane was adopted as the desalination module. This membrane provides a broad applicable pH range, supports chemical cleaning, and maintains stable separation performance under long-term operation. The desalination rate can reach 95–99%, enabling permeate reuse.

Based on the influent salinity level, the minimum element size 4040 seawater-desalination membrane was selected. The membrane parameters are listed in Table 1.

For a single element, the permeate flow is 0.16 m³/h and the recovery is 15% (permeate/feed), corresponding to a feed flow rate of approximately 1.0 m³/h and a reverse osmosis processing time of about 1 h. The operating osmotic pressure is approximately 20–30 bar, placing the membrane unit among the principal energy-consuming components of the overall system.

The detailed technical specifications of the selected polyamide composite membrane are provided in Table 2.

Given the required operating pressure of 20–30 bar and a feed flow of approximately 1.0 m³/h, a Nanfang Pump CDMF3-31 was selected as the reverse osmosis booster pump. Its specifications are given in Table 3.

This pump selection provides stable and continuous pressure driving under the high-pressure requirements of the membrane unit, supporting sustained flux over long-term operation and limiting the risk of increased energy consumption or instability caused by pressure fluctuations.

To control membrane fouling, multiple measures were incorporated in the reverse osmosis unit based on the fouling model. First, a cartridge safety filter with a 5 $\mu$m rating was installed at the reverse osmosis inlet to remove suspended particles larger than 5 $\mu$m, thereby reducing the risk of feed-channel blockage and physical damage to the desalination layer. In addition, given that UDMH wastewater is typically generated from cleaning processes using groundwater or tap water, the potential presence of hardness ions in the raw water was considered.

During reverse osmosis operation, water permeates the membrane under pressure, while dissolved salts become progressively concentrated on the feed side. As the concentration factor increases, hardness ions may exceed their solubility product, leading to scaling risk. To mitigate this, an antiscalant dosing system was installed upstream of the reverse osmosis unit. The imported antiscalant delays crystal growth and precipitation, suppressing deposition of sparingly soluble salts on the membrane surface. The dosing concentration was optimised according to site-specific water quality characteristics.

In addition, to limit attachment and accumulation of contaminants and sparingly soluble salts, an automatic flushing programme was implemented based on the model-predicted fouling rate. During operation and at shutdown, low-pressure, high-flow flushing with reverse osmosis permeate was applied to remove deposits and maintain long-term flux stability.

These anti-fouling measures serve not only to stabilise permeate quality but, more critically, to suppress fouling and scaling-driven pressure rise over time, thereby controlling the long-term energy demand of the membrane unit and maintaining system efficiency and reliability during continuous operation.

The shell thickness of the membrane treatment unit was calculated using Eqs. (5) and (6):

where, t_m is the shell thickness; P is the pressure; d is the inner diameter; n is a coefficient (taken as 4.8); and S is the stress coefficient (taken as 48.3 MPa). The shell thickness is then obtained as:

The overall structure of the membrane treatment unit is shown in Figure 5. The selected membrane element has a diameter of 40 mm and a length of 1000 mm. The shell length is approximately 1200 mm, with a shell diameter of 100 mm and a thickness of 5 mm. Both ends are fixed to the prototype via frustum-shaped connections. The three inlet/outlet ports have an inner diameter of 28 mm with a wall thickness of 4 mm.

The membrane element separates the shell cavity into two regions. The two lower ports connect the annular space between the membrane and shell. Pressurised wastewater enters through the lower right inlet; under pressure, approximately 15% of the feed permeates the membrane, while the remaining $\sim$85% concentrate exits via the lower left outlet and returns to the electrochemical reactor for recirculation. The permeate meets the discharge requirement and leaves the membrane treatment unit through the right-side outlet connected to the inner side of the membrane.

This structural design satisfies the pressure-bearing and strength requirements and provides mechanical support for long-term high-pressure operation, reducing the likelihood of operational fluctuations or increased energy consumption due to structural deformation or failure.

The hydrazine wastewater treated by the prototype exhibits pronounced corrosivity and is subjected to internal pressure variations during operation. Consequently, the piping network requires adequate corrosion resistance, pressure-bearing capacity, and long-term operational stability. Considering material strength, corrosion resistance, and engineering feasibility, stainless steel was adopted throughout the piping system to provide sufficient safety margin and service life under long-term continuous operation.

Wastewater first enters the reactor through the rear inlet via external piping. After degradation under stirring, the effluent exits through the front outlet and passes to the downstream filtration unit. Two filters are arranged in series, with nominal ratings of 20 $\mu$m and 5 $\mu$m, respectively, to remove suspended solids, reduce the fouling load on the membrane unit, and prevent physical damage to the membrane structure by coarse particles. The filtered wastewater is then delivered to the membrane unit by the booster pump.

After membrane separation, the effluent is divided into two streams: the concentrate is recirculated to the electrochemical reactor to maintain salt balance and reaction conditions, while the permeate meets the discharge standard and is released through the system outlet. These flow paths form a closed treatment loop within the prototype. The piping arrangement supports stable pressure distribution and limits additional energy losses associated with frequent start–stop operation or sharp pressure fluctuations.

During piping design, wall thickness was determined using a coupled internal-pressure–structural-stress model to ensure adequate safety under rated operating pressure while avoiding excessive design that would increase material usage and cost. The wall thickness of straight pipes was calculated using Eq. (7):

where, t is the wall thickness; D is the outside diameter; P is the design pressure (taken as 3 MPa); S is the allowable stress (130 MPa for stainless steel); Y is a coefficient (0.4); W is the high-temperature strength reduction factor for welded joints (1); and $\Phi$ is the longitudinal weld joint factor (1). The thickness of bends or elbows was calculated using Eq. (8):

where, t_w is the wall thickness. For the inner side of a bend or elbow, the coefficient I was calculated using Eq. (9):

For the outer side of a bend or elbow, I was calculated using Eq. (10):

where, I is a coefficient (approximately 1 from calculation); R is the bend radius, and other parameters are consistent with the straight-pipe calculation.

According to the pressure levels at different locations within the prototype, pipe outside diameters of 16 mm, 20 mm, and 25 mm were selected. The corresponding minimum wall thicknesses were calculated as t₁₆ = 0.182, t₂₀ = 0.229, t₂₅ = 0.286. Considering manufacturability and safety margin, appropriate standard wall thicknesses were adopted for modelling and fabrication.

Pipe lengths were determined according to the relative positions of functional components, installation clearances to the prototype enclosure, and overall layout requirements. As the internal piping structure is relatively complex, the piping network was divided into several functional sub-units for modelling and verification to reduce local stress concentrations and vibration risks. These sub-units include the influent line, effluent line, filter connection lines, upstream piping of the membrane unit, and the external discharge line.

This segmented modelling and parameter-selection strategy supports pressure safety while reducing vibration sensitivity of the piping network under high-pressure pumping and membrane separation, thereby improving overall system reliability and energy-efficiency stability.

To ensure structural integrity of critical components under static loading and dynamic excitation, finite element simulations were conducted for the piping network, including static structural analysis and random vibration analysis, to verify reliability under operating pressure and transport-induced vibration. Loads and boundary conditions were defined according to practical operating scenarios. The simulation results and assessments for key pipe segments are summarised below.

These simulations were used not only to confirm structural safety but also to assess dynamic stability under high-pressure pumping and membrane operation. Vibration and deformation of the piping network influence pressure stability within the system and may indirectly affect the power demand and fluctuations in energy consumption.

(1) Influent pipe

As shown in Figure 6, this section of piping is arranged horizontally within the prototype and is supported by fixed constraints at both ends, mainly subjected to a uniformly distributed vertical load. The simulation indicates that the maximum stress and strain occur near the mid-span region, consistent with bending behaviour of a simply supported beam under uniformly distributed load. The maximum equivalent stress is 3.99 MPa, which is far below the allowable stress of stainless steel (130 MPa), corresponding to a safety factor of 32.6. The maximum deformation is only 0.0019 mm. This deformation level does not affect sealing performance, joint reliability, or internal fluid transport, thereby avoiding pressure fluctuations or additional pumping power losses attributable to local structural deformation.

(2) Reactor effluent pipe

Figure 7 shows the simulation results for the reactor effluent pipe under a vertical uniformly distributed load. Fixed constraints were applied at three ports, forming a statically determinate support condition. The contour plots indicate stress and strain concentration in the mid-span region of the right horizontal segment. The maximum equivalent stress is 4.51 MPa and the maximum deformation is 0.0099 mm. This distribution is consistent with the bending response of a continuous beam supported at multiple constraint points, where the bending moment reaches a maximum near the span centre, resulting in peak stress and strain. The maximum stress remains well below the allowable stress of stainless steel (130 MPa). The deformation is less than 0.01 mm and is therefore not expected to impair joint sealing or flow transport characteristics.

(3) Filter connection piping

As shown in Figure 8, this section connects the reactor outlet to the filter inlet and constitutes a critical transition segment for wastewater transport. The layout is L-shaped, with fixed constraints applied at both ports. Under a vertical load along the Z direction, the maximum stress and strain are concentrated at the outer side of the upper-right bend. The maximum equivalent stress is 6.28 MPa and the maximum deformation is 0.0100 mm. The concentration arises primarily from geometric discontinuity at the elbow, where section changes and the outer material experiences tensile stress under bending, making it a local weakness. Nevertheless, the stress level remains far below the allowable stress (130 MPa). The deformation is at a micrometre scale and is not expected to affect sealing, connection stability, or the inlet conditions of the downstream filter.

(4) Upstream piping of the membrane treatment unit

As shown in Figure 9, this segment is located at the front end of the membrane unit and is connected to adjacent devices through fixed constraints at both ends. Under vertical loading, the maximum stress and strain occur at the transition between two pipe diameters. The maximum equivalent stress is 1.08 MPa and the maximum deformation is 0.0016 mm. The concentration is attributed to abrupt changes in cross-sectional area, which redirect and redistribute internal force paths, representing a typical geometric discontinuity effect. The stress level is substantially below the allowable stress. The deformation is minor and does not compromise sealing at connections or introduce additional loads to supports or adjacent components.

(5) Discharge piping

Figure 10 presents the simulation results for the discharge piping downstream of the membrane unit, which transports treated permeate out of the prototype. Both ends are fixed to the membrane outlet and the external discharge interface. Because the straight segment is relatively long, its mechanical behaviour under vertical uniformly distributed load is comparable to that of a simply supported beam. Accordingly, the maximum stress and deformation occur in the mid-span region of the longest straight segment. The maximum equivalent stress is 49.3 MPa and the maximum deformation is 0.499 mm, representing the most pronounced response among the analysed segments. This behaviour results from the large mid-span bending moment under end constraints, leading to higher bending stress and deflection. Although 0.499 mm remains within the elastic range, sufficient clearance from surrounding structures is required to prevent contact or interference under full-load conditions. Based on the simulation results, this long-span segment warrants focused attention; therefore, an auxiliary mid-span support was added in the design to control deformation and reduce stress levels.

(1) Filter connection piping

Figure 11 shows the random vibration analysis for the connection piping between the 20 $\mu$m filter and the reactor effluent pipe. The upper and lower ports were fixed, and the first modal frequency is 1325.5 Hz, indicating high stiffness. The dynamic response indicates pronounced deformation concentration in the curved region of the larger-diameter segment. This behaviour is attributed to reduced local stiffness associated with the bend geometry, and to local amplification of vibration modes caused by sectional discontinuity. As a structurally weaker segment within the connection system, the bend may experience higher alternating stresses under sustained random vibration, potentially affecting sealing integrity at the adjacent filter interface. Accordingly, vibration-isolation brackets or constraint devices were incorporated near the bend during installation to increase local stiffness and suppress displacement amplitude, ensuring structural reliability and sealing integrity under complex vibration conditions.

Figure 12 shows the random vibration analysis of the connection piping between the 20 $\mu$m and 5 $\mu$m filters. The first modal frequency is 2289.3 Hz, indicating high overall stiffness. Similar to Figure 11, the maximum deformation is concentrated at the bend. Under random vibration excitation, bends are prone to localised response and displacement concentration because of geometric discontinuity and reduced local stiffness. Therefore, additional constraints or damping devices near the curvature were considered to increase local stiffness and reduce deformation amplitude, supporting stable long-term operation of the filtration system under dynamic environments.

(2) Upstream piping of the membrane treatment unit

As shown in Figure 13, the first upstream pipe segment before the membrane unit is fixed at both ends and contains a diameter transition. The maximum modal frequency is 3070.4 Hz, and the directional deformation plot indicates that the largest deformation also occurs at the diameter-transition region. In practice, additional vibration mitigation should be applied at this connection region to limit dynamic displacement and maintain sealing stability.

(3) Discharge piping

Figure 14 presents the random vibration response of the discharge piping downstream of the membrane unit. Owing to its relatively large dimensions, the results are shown in subplots. The first modal frequency is 122.2 Hz (Figure 14b), which is comparatively low and indicates higher global flexibility and increased sensitivity to external excitation. The deformation distribution (Figure 14a) shows that the maximum displacement occurs at the mid-span of the uppermost horizontal long pipe segment, while other long segments also exhibit notable dynamic response. This behaviour is consistent with beam-type vibration characteristics: long-span pipes, with low stiffness and low natural frequency, are prone to bending resonance under broadband random vibration, leading to larger alternating displacements and stresses. Over time, such responses may cause fatigue damage or joint loosening, compromising discharge reliability. Based on the dynamic risk assessment outputs, the support scheme for the discharge piping was optimised by adding elastic brackets, thereby improving reliability for mobile application scenarios.

In summary, finite element analyses under static loading and random vibration conditions indicate that the designed piping structure satisfies strength and fatigue-safety requirements while maintaining favourable dynamic stability. The results show that appropriate support and constraint arrangements effectively suppress vibration responses in long-span segments, thereby reducing pressure fluctuations and avoiding additional energy consumption attributable to structural vibration. These simulations provide structural-level assurance for maintaining stable, low-fluctuation power demand during high-pressure pumping and membrane separation operation.

(4) Piping section of the concentrate return line to the reactor

Figure 15a and Figure 15b below respectively show the vibration deformation and equivalent stress distribution in another section of the return pipeline. This pipe segment exhibits a first-mode frequency of 394.8 Hz, which is relatively low, indicating significant structural flexibility and susceptibility to external excitation. Simulation results reveal that maximum vibration deformation occurs in the vertically oriented pipe section with a longer span. The low natural frequency suggests this section may couple with common transportation or environmental vibration frequencies, thereby amplifying dynamic responses. Under random vibration, continuous oscillation of the long-span vertical piping generates alternating stresses at connection points, posing fatigue risks. This may also affect the stress state of connected membrane units and reactor interfaces, threatening system sealing integrity and functional stability. Therefore, a damped vibration-reduction structure is adopted.

(5) The membrane treatment unit housing

As a critical wastewater treatment unit following the reactor, the stability of this section's shell directly impacts treatment outcomes. The shell is fixed at three inlet and outlet ports. According to simulation results in Figure 16, the shell exhibits a maximum modal frequency of 1697.8 Hz, with the greatest directional deformation occurring in its central section. During actual operation, ensuring sufficient shell stability is essential, necessitating the addition of fixed supports at the location of maximum deformation.

The silver-ion tests were conducted not only to examine the applicability of the “ECO + DTRO” process to different contaminant matrices, but also to analyse the effects of conductivity adjustment on current efficiency and the associated system power demand, thereby providing reference information for subsequent high-load treatment of hydrazine-containing wastewater.

To further evaluate the generality of the “ECO + DTRO” configuration for heavy-metal contamination and to characterise system behaviour under controllable conditions, simulated wastewater containing silver ions was used for auxiliary validation. Silver is straightforward to quantify, and its electrochemical behaviour exhibits similarities to that of certain organic contaminants, which makes it suitable for examining electrode activity, reaction kinetics, and the coupled effect of membrane separation.

The experiments focused on the influence of reaction time and electrolyte addition on the removal of silver ions and COD, including: (1) ECO without electrolyte addition; (2) comparative tests with sodium chloride added to increase conductivity; and (3) evaluation of the coupled effect with downstream DTRO membrane treatment.

As indicated by the results in Figure 17 and Figure 18, in the absence of added electrolyte, ECO provided sustained and stable COD removal from the silver-containing simulated wastewater. The COD concentration decreased from 62 mg/L to 15 mg/L after 180 min, corresponding to a removal efficiency of 75.8%. This behaviour is consistent with time-dependent oxidation driven by strongly oxidative species generated in situ on BDD electrodes (e.g., ·OH), which promote progressive degradation of organic constituents. Subsequent DTRO treatment showed clear polishing capability. When the effluent after 180 min of ECO was subjected to membrane filtration, COD further decreased from 15 mg/L to 8 mg/L. This confirms that DTRO effectively retains and separates low-molecular-weight intermediates and residual organics formed during electrochemical treatment, demonstrating the complementary roles of ECO and DTRO in deep contaminant removal. These results support the feasibility of the coupled electro-oxidation–membrane separation process model.

As shown in Figure 18, sodium chloride exerted a pronounced strengthening effect on the ECO process. With NaCl addition, a COD removal of 83% was achieved within 30 min, compared with 32% without electrolyte under the same reaction time. This effect can be attributed to two concurrent mechanisms. First, chloride ions (Cl^-) can be anodically converted on BDD to reactive chlorine species (e.g., HClO/ClO^-), which possess strong oxidative capacity and act in concert with ·OH, thereby expanding the oxidation pathways and accelerating transformation. Second, increased conductivity reduces ohmic resistance, enabling higher current density and improved current utilisation under a given applied voltage, which accelerates electrochemical conversion. From a system standpoint, these effects imply that, for a specified treatment target, operation can be achieved with lower applied voltage and/or shorter reaction time, reducing electrical energy consumption per unit throughput.

As shown in Figure 19, during ECO without added electrolyte, the silver-ion concentration decreased progressively from 1.09 mg/L to 0.63 mg/L after 180 min, corresponding to a removal efficiency of 42%. This removal is attributed mainly to electrochemical reduction occurring at the cathode, where a fraction of Ag⁺ can be reduced to metallic silver and deposited. In addition, reactive oxygen species generated during electrochemical treatment may induce changes in silver speciation or promote complexation with other constituents, indirectly altering solubility and distribution. Sodium chloride addition produced a substantial strengthening effect on silver removal, achieving 80% removal within 30 min. The primary mechanism is rapid chemical precipitation of sparingly soluble AgCl through the combination of Cl^- with Ag⁺. In parallel, the increased conductivity improves current utilisation and cathodic reduction capacity, while anodically generated reactive chlorine species may further contribute through coupled oxidation–precipitation pathways. These results indicate that adjustment of the wastewater matrix composition, such as chloride addition, can selectively strengthen removal of specific heavy-metal ions.

DTRO exhibited pronounced retention of silver species. After 180 min of ECO, subsequent DTRO treatment increased the overall silver removal from 42% to 93%. This suggests that silver-containing species formed during electrochemical treatment (e.g., fine AgCl precipitates, colloidal silver, or silver associated with organic fragments) exhibit increased effective particle size or reduced permeability, enabling efficient interception by the DTRO membrane.

A UDMH-containing wastewater stream collected from a field site was used to validate treatment performance. The wastewater contained UDMH and nitrogen oxides, and the designed treatment capacity was approximately 50 L/h. Electrochemical degradation tests were conducted in a batch reactor using UDMH wastewater with total dissolved solids adjusted by sodium sulfate. After start-up, the reactor was operated under continuous stirring and constant-current electrolysis. To characterise degradation behaviour, samples were collected from the outlet at predetermined time points for subsequent analysis. DTRO tests were performed in a dynamic mode: wastewater was introduced after the booster pump was started, and pump flow rate and membrane operating pressure were adjusted. Sampling and measurements were carried out at regular intervals to record permeate flow rate, membrane pressure variation, and concentrate total dissolved solids. After stable operation of the prototype was achieved, the influence of the DTRO concentrate recirculation ratio on the electrochemical reaction system was examined.

As shown in Figure 20, pilot-scale results obtained under field conditions were as follows. With a wastewater volume of 200 L and a BDD electrode area of 1.32 m^-2, operation at 900 A (approximately 70 mA·cm^-2) for 3 h reduced COD from 5050 mg/L to 2500 mg/L. Over the same period, UDMH decreased from 2227 mg/L to 33 mg/L, monomethylhydrazine decreased from 7.9 mg/L to 1.54 mg/L, and hydrazine decreased from 0.9 mg/L to 0.196 mg/L. This operating condition corresponds to a current-density range typically associated with effective ECO on BDD electrodes, indicating that reaction kinetics and mass-transfer processes remained well matched without evidence of pronounced polarisation growth or excessive non-productive current consumption. In addition to high removal efficiency, stable operation was maintained, providing experimental support for continuous high-load treatment with controllable energy demand.

During 0–1 h, COD exhibited an initial increase followed by a decrease, whereas UDMH showed the fastest degradation rate within this period. This behaviour indicates that, in the early stage of reaction, UDMH decomposition generated intermediates that are more readily oxidised by dichromate during COD measurement, resulting in an apparent COD increase. Both the initial and final wastewater were weakly acidic. In most cases, BDD-based treatment shows higher removal of ammonia nitrogen and total nitrogen under weakly alkaline conditions than under acidic conditions. The UDMH removal reached 99.2%. The experimentally observed time profiles of pollutant concentrations are consistent with trends predicted by the simplified reaction-kinetics-based model, supporting the reliability of the core reaction model. This agreement indicates stable system operation within the selected current density and reaction time window, without abrupt increases in power demand caused by mass-transfer limitation or polarisation escalation, and provides experimental evidence for energy-controllable operation under high-load conditions.

At 900 A (approximately 70 mA·cm^-2), the system maintained stable reaction rates and consistent pollutant-removal trends, indicating that polarisation did not develop uncontrollably within this current-density interval and that power input remained within a manageable range. These results indicate the presence of an engineering operating window for highly toxic, high-concentration nitrogen-containing organic wastewater in which treatment effectiveness and energy-demand stability can be balanced, providing a basis for subsequent optimisation of operating conditions in terms of specific energy consumption per unit throughput.

An integrated, mobile treatment unit based on a coupled ECO–DTRO process was designed and validated for the treatment of UDMH wastewater. With emphasis on the structural reliability, operational stability, and engineering feasibility of a power-driven system, key components were developed through model-driven design, verified by simulation, and assessed experimentally. The main conclusions are as follows.

(1) A system-level design model covering hydraulic transport, mixing, structural mechanics, and multiphysics coupling was established, and an optimised electrochemical reactor employing BDD electrodes was developed. Simulations indicate that stresses in critical components under static loading and dynamic excitation remain well below allowable material stresses, with deformations maintained within controllable limits. The model-guided structural design ensures structural integrity and dynamic stability during long-term continuous operation, providing reliable mechanical support for maintaining stable power input and reaction conditions under high current density.

(2) Selection and system-integration design of the downstream DTRO membrane unit were completed, forming a closed treatment loop. Silver-ion simulated wastewater experiments confirmed the complementary removal performance of the “ECO + DTRO” configuration across different contaminant systems. The results indicate that adjustment of solution conductivity (e.g., electrolyte addition) can markedly strengthen electrochemical reaction efficiency and, while maintaining treatment effectiveness, reduces ohmic resistance and improves current utilisation. These effects contribute to improved system-level energy efficiency and more effective use of power input.

(3) Field pilot tests were conducted using actual UDMH wastewater. Under operation at a current density of approximately 70 mA·cm^-2 for 3 h, the unit achieved a UDMH removal of 99.2%, with substantial reductions in COD, monomethylhydrazine, and hydrazine. The observed pollutant trends are consistent with predictions from the reaction-kinetics-based model, indicating stable operation within this operating window without evident uncontrolled polarisation growth or abrupt increases in energy demand. These results support the engineering feasibility of achieving high-load treatment with high removal efficiency under controllable power input. In addition, within this operating interval, no degradation behaviour attributable to mass-transfer limitation or intensified polarisation was observed, indicating that fluctuations in power demand can be maintained within acceptable bounds during continuous operation, with potential for scale-up and long-term deployment.

Overall, the integrated “ECO + DTRO” unit enables efficient removal of hydrazine-related contaminants while forming an engineering system characterised by stable power input, controllable energy performance, and structural reliability through structural optimisation, process integration, and matched operating parameters. The results provide a practical process option for engineered, mobile treatment of highly toxic hydrazine wastewater.