Treatment of aircraft maintenance wastewater by coupling low-temperature evaporation with Fe–C micro-electrolysis-configured ozonation

COD

chemical oxygen demand

SS

suspended solids

TN

total nitrogen

TP

total phosphorus

In the context of ‘carbon peak and carbon neutrality’ proposed by various countries around the world, environmental protection has risen to a new height of importance among national strategic goals, and sewage treatment is an important link to achieving the dual carbon target (Song et al. 2022). The greenhouse gas emissions of the entire sewage treatment industry in China are roughly estimated to account for 2.95% of the national greenhouse gas emissions in 2030. Therefore, the sewage treatment industry must urgently change its direction and realize low-carbon or even zero-carbon sewage treatment (Zhang et al. 2020; Faisal et al. 2023). Organic wastewater with a chemical oxygen demand (COD) concentration greater than 2,000 mg/L is generally regarded as high-concentration organic wastewater (Han et al. 2023). The COD content of aircraft cleaning wastewater far exceeds 2,000 mg/L (Table 1), which belongs to high-concentration organic wastewater. The reduction and utilization of high-concentration organic wastewater have become the future development trend of the environmental protection industry.

At present, the adsorption method (Falahati & Karimi 2023), the Fenton advanced oxidation method (Huang et al. 2018; Changotra et al. 2019), micro-electrolysis (Asgari et al. 2019; Berhe et al. 2022), photocatalytic oxidation (Cherukupally et al. 2020), solar photocatalysis (Kuyukina et al., 2020; Altieri et al. 2022), the multiple electrolysis method (Wu et al. 2020), anaerobic biological treatment (Fan et al. 2018), and other methods show good development potential in the treatment of refractory organic compounds. These processes have their advantages and disadvantages in practical applications. The Fenton advanced oxidation process covers a large area but has high treatment costs and can easily cause secondary pollution. The adsorption method is relatively simple to operate and achieves good treatment effects, but regenerating the adsorbent is difficult, and the spent adsorbent becomes new hazardous waste. The electrolysis method uses simple equipment and is easy to operate, but it consumes a lot of power and produces a large amount of sludge (Mojiri & Bashir 2020). The anaerobic biological treatment method is suitable for organic wastewater with high volume and continuous generation but may not be appropriate for organic wastewater with low volume and intermittent operation. Additionally, some wastewaster may contain certain heavy metals and recalcitrant organic matter, which could diminish the effectiveness of biological treatment (Kimata-Kino et al. 2011). These methods have a significant effect on treating wastewater with a COD content of 4,500–27,000 mg/L, but they are not effective for cleaning wastewater with more than 50,000 mg/L COD from aircraft maintenance plants.

The low-temperature evaporation system has the advantages of a very short process chain, a high degree of automation, no dependance on treatment agents, high reuse efficiency, low waste production, and highly convenient maintenance (Biniaz et al. 2019). This system has been effectively utilized in the standard treatment of industrial wastewater, including wastewater concentration, recycling, and the treatment of special wastewater. Additionally, low-temperature evaporation technology has been applied in the concentration of laboratory wastewater, fine chemical wastewater, and landfill leachate (Wang et al. 2019). The iron-carbon micro-electrolysis technology effectively utilizes Fe and C to spontaneously generate weak currents for decomposing organic pollutants, removing heavy metals, nitrogen, phosphorus, and treating other types of recalcitrant wastewater. This approach offers advantages including environmental friendliness, low-cost operation, and broad applicability. It has found extensive use in pretreating recalcitrant wastewater in various industries such as printing, dyeing, electroplating, pharmaceuticals, and petroleum (Malakootian et al. 2020). The anode primarily comprises nanometer zero-valent iron and Fe, while activated carbon, graphite/graphene, and biochar are among the carbon materials used at the cathode. The combination of these materials with Fenton reaction, ultrasound, ultraviolet light, ozone, microwave radiation, electric fields, and magnetic fields demonstrates significant synergistic effects, resulting in enhanced release rates of Fe²⁺, thereby improving the removal efficiency of refractory organic matter. The main mechanism involves the production of large quantities of •OH through micro-electrolysis, leading to further degradation of refractory organic matter (Zhang et al. 2021).

Currently, the quantity of wastewater produced by aircraft maintenance enterprises is relatively small, and the composition of the wastewater is complex. Transporting it for disposal by enterprises possessing hazardous waste treatment qualifications imposes higher costs on these enterprises. Therefore, this paper primarily investigates the characteristics of the cleaning solution. For the first time, a low-temperature evaporation device and micro-electrolytic ozone oxidation coupled with physical and chemical treatment technology are employed to enhance the removal efficiency of various pollutants in the cleaning solution, achieve the reduction treatment of aircraft maintenance and cleaning wastewater, and fulfill the objective of on-site generation and treatment of wastewater. This part of the revision has been reflected in the paper.

Aircraft maintenance cleaning is generally divided into two cleaning processes, namely aluminum and steel parts cleaning. These processes mainly include rust removal soaking, oil removal soaking, and surfactant cleaning. The most critical step is the soaking process, which includes two parts: steel material cleaning soaking and aluminum material cleaning soaking. Steel soaking heating temperature is 70–93°C, soaking time is 10–20 min, while aluminum soaking temperature is 60–82°C, soaking time is more than 5 min. Aviation industry chemicals such as rust remover, sodium hydroxide, cleaning agent, and oil remover are added to the soaking process. Thus, a large amount of cleaning wastewater containing metal oxides, oil pollution, carbon deposition, coatings, and complex aerochemical products is produced. The main components of wastewater are ester, ketone, amine, acid, alcohol, and other organic components. These components belong to the category of hazardous waste HW09. These components are complex and have biological toxicity, causing serious harm to the factory environment.

Samples of aircraft maintenance wastewater (i.e., wastewater from engine maintenance, electromechanical maintenance, and aircraft accessory maintenance) were collected from an airline aircraft repair facility in the industrial area of Xi'an, China. Due to confidentiality concerns, specific sources are not disclosed. The characterization of the raw wastewater samples is presented in Table 1. Table 1 lists the COD, suspended solids (SS), chroma, turbidity, total nitrogen (TN), and total phosphorus (TP). Usually, the chemical and biological properties of wastewater can be maintained at room temperature for 72 h. It is worth noting that the speed of low-temperature evaporation is relatively slow, so we need to store the wastewater that cannot be treated in time in the refrigerator at 4°C for storage. The Fe–C micro-electrolytic material was purchased from Gong Yi Tenglong Environmental Protection Technology Co., Ltd. All the chemical reagents used in the experiments were of analytical grade.

The stability of the measurement system of the experimental device can be demonstrated by the reproducibility and relative consistency of the measurement results of a specific quantity at different time intervals. To verify the reliability of the experimental results of the device system, the following experimental approaches were adopted in this paper: (1) under identical working conditions, the running time of the laboratory experimental device was set to 15 h, and a group of samples was collected every 3 h, obtaining a total of five groups of samples for testing. (2) The acquired five groups of samples were considered as the research objects, and each group of samples was tested three times during the testing process (three parallel samples were taken for each group consecutively) and the average value of the three test results was taken as the ultimate test value. (3) Based on the average value of each test sample, the relative error of each measurement result was calculated and analyzed. (4) Based on the analysis of relative errors, three datasets with significant sample errors were selected to complete the mapping analysis of the pollution index. Then, using a direct statistical method to determine the standard deviations of these three datasets, the data will be used to evaluate the stability of the experimental device if the error is within 5%.

Prior to analysis, the supernatant sample was filtered through a 0.45 μm filter paper. The initial pH was measured using a pH meter (PHS-3C, Shanghai INESA & Scientific Instrument Co., Ltd). The parameters of the wastewater were analyzed according to the Standard Methods for the Examination of Water and Wastewater, 21st ed. (Kucharska et al. 2022). The three types of cleaning wastewater were derived from engine maintenance, mechanical and electrical maintenance, and aircraft accessory maintenance, respectively.

The wastewater was first treated using low-temperature evaporation. An experiment was conducted to study the removal of organic species in the distillation still. Next, an experiment was performed to investigate the removal of residual organic matter in the next process. Next, an experiment was performed to investigate the removal of residual organic matter in the subsequent process. The experimental condition for low-temperature evaporation was 37°C, and the pressure was −0.093 to −0.098 MPa. During the operation of the evaporator, samples were taken according to the aforementioned method. Each group of samples was 600 mL, and each parallel sample was approximately 200 mL, used for the determination of COD, SS, TN, TP, and chromaticity. COD was determined using the potassium dichromate method. Three parallel samples were digested and titrated simultaneously in about 1 h, and then SS, oil content, chromaticity, TN, TP, and other indicators were grouped according to personnel.

The Fe–C material exhibits a large specific surface area, high porosity, and excellent pressure resistance. When combined with ozone oxidation, the Fe–C material can achieve a synergistic effect, generating significant amounts of Fe²⁺, H₂O₂, and •OH. This represents an innovative and efficient degradation treatment technology. Applying this technology to degrade the target pollutants in aircraft cleaning wastewater is highly recommended. Aircraft maintenance wastewater degradation experiments were conducted in a 5.65 L micro-electrolysis reactor. All the reactions were initiated immediately by adding 5 L of aircraft maintenance wastewater and a certain quantity of ellipsoidal Fe–C filings. Before each experiment was conducted, the filings were pretreated with ethanol for 5 min to remove oil from their surface, washed with ultrapure water, rinsed again with 0.1 mol/L H₂SO₄ and ultrapure water, and dried in a vacuum drying oven at 105°C for 8 h. For micro-electrolytic packing, a new type of multi-component composite catalytic micro-electrolytic packing (iron and carbon in a certain proportion) was adopted. The iron and carbon used in the experiment are mixed forming materials of iron and carbon. The mass ratio of the filings to the wastewater was 1:1 in this experiment. In order to better carry out the COD pollutant removal comparative experiment, the Fe–C micro-electrolysis experiments, ozonation experiments and Fe–C micro-electrolysis-configured ozonation experiments have the same wastewater treatment capacity and are all 2.0 L/h. While the experimental apparatus was in operation, samples were collected using the aforementioned method to analyze pollutant indicators, followed by data plotting and error assessment.

The ozonization experiment involved removing the iron-carbon packing from the micro-electrolysis device and pre-ozonizing the reactor for 5 min before the experiment to meet any ozone demand in the reactor. The ozone inlet flow rate was set to 300 L/h, and the gas ozone concentration was set to 35 mg/L. Pollutant index determination was conducted according to the above experimental methods. The index determination, plotting, and error analysis were carried out. During the operation of the experimental setup, the analysis of sampled pollutants, plotting, and error analysis were conducted in the same way as in the Fe–C micro-electrolysis experiments.

The experimental conditions of iron-carbon micro-electrolysis ozonization coupling are as follows: The ozone inlet flow rate was set to 300 L/h, and the gas ozone concentration was set to 35 mg/L. During the operation of the experimental setup, the analysis of sampled pollutants, plotting, and error analysis were conducted in the same way as in the Fe–C micro-electrolysis experiments.

During micro-electrolysis treatment, the purpose of ozone addition was that ozone's direct oxidation effect on the polar hydrophilic groups of organic matter was better and made them easier to oxidize directly. At the same time, ozone oxidation was mainly indirect oxidation based on the production of free radicals under alkaline conditions, converting the ozone in the solution into •OH through a series of chemical reactions (Zhang et al. 2020).

Initially, the cleaning wastewater after pretreatment, derived from engine maintenance, mechanical and electrical maintenance, aircraft accessory maintenance, and mixed wastewater, was transported into the distillation still to remove the easily degradable organic contaminants, respectively. Subsequently, the wastewater was transferred into an ozone/micro-electrolysis reactor for further degradation to ascertain the optimal reaction time. Simultaneously, the pollution indicators in the treated wastewater from the mixed waste liquid were assessed to determine if their values fall below the discharge standards for the sewage pipeline network.

The COD removal efficiency of each sample was measured after evaporation, and the experimental results are shown in Figure 3. Figure 3 displays the dynamic changes in COD after treatment in the single low-temperature multi-effect evaporation system. After low-temperature evaporation, the COD removal average rate of the wastewater exceeded 90.8%. The evaporation process produced no obvious bubbles, and the condensate was clear and transparent. However, the COD content of the distilled condensate remained relatively high, and the mixed water sample reached 3,778 mg/L. The main reason for this phenomenon is that the vapor from the evaporation stage carried some residual liquid that increased the COD content of the condensed liquid (Mahdizadeh & Malakootian 2019). Further treatment is needed to bring the COD level up to standard.

Figure 3 presents the removal efficiencies of SS, oil content, turbidity, and chroma as a function of reaction time. Low-temperature evaporation has an obvious removal effect on the chroma, turbidity, SS, and oil content. The average removal rates are more than 90%. The turbidity chroma of the steam condensate remained relatively high, mainly because the evaporation of the gas in the late evaporation stage carried a certain residual liquid, resulting in the increase of chroma and COD content in the condensate. These organics are less volatile than water, so they concentrate together at any temperature. Due to serious emulsification, the high content of solid suspended matter and chroma, and the high boiling point of the waste oil generated after cleaning, it still exists after low-temperature evaporation, thus greatly reducing the effect of COD. Testing has shown that after undergoing low-temperature evaporation treatment, the oil content, phosphorus, and nitrogen levels essentially meet emission standards. Furthermore, chroma, turbidity, and suspended matter can be further eliminated through subsequent membrane filtration and physical methods. Subsequent Fe–C micro-electrolysis primarily aims to further remove organic pollutants with high volatility; therefore, the paper mainly discusses the COD removal effect of iron-carbon micro-electrolysis ozonation. The abstract and conclusion in the paper have been revised.

After evaporation, the TN and TP of each sample were measured, and the results are depicted in Figure 5. The figure illustrates that low-temperature evaporation significantly removes TP and TN from the wastewater, with average removal rates of 94.3 and 99.2%, respectively. The TN and TP levels after evaporation comply with Grade A of the Wastewater Quality Standards for Discharge to Municipal Sewers (GB/T 31962-2015) in China (Shen et al. 2022).

Following low-temperature evaporation treatment, the oil concentrations, phosphorus, and nitrogen in aircraft maintenance wastewater are in essential compliance with emission standards. Turbidity, SS, and chroma have been effectively eliminated and will also be removed during subsequent membrane filtration.

Based on treatment capacity considerations, the cleaning wastewater derived from engine maintenance, mechanical and electrical maintenance, and aircraft accessory maintenance is mixed at a ratio of 1:1:0.3 before adjusting its pH to 7.5 using 2.0%HCl. It is fed into a cryogenic evaporator and Fe–C micro-electrolysis ozone processor. The pH, COD, oil concentration, SS, chroma, TN, and TP were analyzed according to the analytical method in wastewater quality standards for discharge to municipal sewers. The specific water treatment effects of each unit link are shown in Table 2. It can be seen from the table that after low-temperature evaporation treatment and iron-carbon micro-electrolysis combined treatment, the COD value of the water quality index is 437 mg/L, the oil concentration is 5.2 mg/L, the SS is 56 mg/L, and the color dilution ratio is 64 dilution ratios, with a total nitrogen value of 12.5 mg/L and a total phosphorus value of 0.71 mg/L. Therefore, the combined process has an excellent effect on the removal of each index.

Apart from treatment effectiveness, system suitability depends on its operational stability during integration of various processes. Henceforth, we implemented this processing system within an industrial setting where operational data were monitored over three months following stabilization of maintenance wastewater through combined treatment methods (refer to Table 3). Throughout operations, random sampling occurred with the completion of 12 experiments at a frequency of 4 samples per month yielding these results: exceeding an average concentration rate above 92%; effluent COD averaging at 346 mg/L; SS averaging 21.9 mg/L; oil concentrations averaging 2.12 mg/L; color measuring forty dilution ratios; total nitrogen content registering 9.5 mg/L; total phosphorus content recording 0.4 mg/L with clear water output. The sampling analysis results of the treatment process indicated that certain pollutant index levels rose, particularly suspended matter. This increase was due to surface fouling of the membrane material in the membrane filtration unit during the process, leading to escalated treatment pressure and necessitating backwashing of the membrane material. Meanwhile, the operating device was equipped with automatic switching control, and the backwashing system was automatically initiated to maintain the membrane unit according to pressure variations. During operation, post-wastewater treatment pH levels, along with COD values, oil concentrations, SS, chroma, TN, and TP, all complied with Grade A standards outlined in the wastewater Quality Standards for Discharge to Municipal Sewers (GB/T 31962-2015), allowing direct discharge into urban pipe networks.

The cost calculation results for the specific scheme, considering 60 tons/year of cleaning wastewater, are presented in Table 4. The treatment capacity is 0.5 tons/h, with an equipment investment cost of $25,000 and hazardous waste treatment cost of $1,000 per ton. The annual treatment cost is approximately $6,000. After treatment by the disposal process, the concentration rate averages at 92%, resulting in a concentrated liquid production of 4.8 tons annually. The treatment cost of the concentrate is $1,200 per ton and export disposal costs amount to $5,760 per year. Other expenses include depreciation of equipment at approximately $2,500 annually over a 10-year basis; power costs for plant operation at $1,800 per ton; and maintenance and pharmaceutical materials costing around $1,400. The data for evaluating the energy efficiency of wastewater treatment are obtained from market prices provided by relevant sales enterprises in China. Prices and performance parameters of related equipment and consumables are collected through phone calls and emails, with at least three sets of data selected for each item to calculate the average price.

Table 4 demonstrates that low-temperature evaporation primarily consumes electric energy while reducing heat source requirements. Equipment investment costs can be recouped within 2–3 years using this process, which reduces disposal costs by nearly 80% while achieving the goal of minimizing hazardous waste – providing an effective method for enterprises to reduce energy consumption and overall expenditure. In conclusion, our combined process enables aircraft maintenance and cleaning wastewater treatment to meet discharge standards effectively. Additionally, the system's short process chain features high automation levels eliminating, manual care or the addition of treatment agents while generating minimal waste with convenient maintenance procedures. Therefore, it can be considered a highly energy-efficient way to dispose of clean wastewater through coupling low-temperature evaporation with ozonation in a Fe–C micro-electrolytic configuration.

Conventional sewage treatment systems are typically large and costly, making them unsuitable for handling hazardous wastewaters. Directly entrusting hazardous liquid waste to a third-party processing company would further escalate the expenses. Therefore, this study adopts a combined process of low-temperature evaporation and micro-electrolytic ozone catalytic oxidation for treating aircraft maintenance cleaning wastewater. Additionally, the unit parameters are optimized for the core process to reduce the volume of cleaning wastewater and meet the standard for treated water discharge.

The results demonstrate that the average removal rates of oil concentration, SS, chroma, TN, and TP are 94.0, 98.9, 93.8, 93.7, and 99.2%, respectively. In this system, the oxidation of ozone, the catalytic action of fillers and the electrochemical action of fillers promote each other, which significantly improves the removal capacity of pollutants. Coupling low-temperature evaporation with Fe–C micro-electrolysis-configured ozonation achieves COD degradation rates exceeding 98% across three different types of wastewater while maintaining an average concentration rate above 92%. The optimal reaction times of the cleaning wastewater derived from engine maintenance, mechanical and electrical maintenance, aircraft accessory maintenance, and mixed wastewater are 150, 120, 90, and 120 min, respectively. When employing this system to treat wastewaster, the optimal reaction time is appropriately adjusted in accordance with different treatment objects, and the COD pollution removal index of the three types of wastewaster and mixed wastewaster is achieved.

This process reduces disposal costs by nearly 80% after implementation while effectively achieving hazardous waste reduction goals – providing enterprises with an efficient method to decrease energy consumption and costs. The coupling of low-temperature evaporation with Fe–C micro-electrolysis-configured ozonation is highly recommended for widespread application in treating high-concentration organic wastewater that is difficult to degrade across various industries.

This technology will be advanced for reducing various emulsified industrial wastewater streams, including those from fine chemicals production facilities; landfill leachates; nuclear industry operations; mechanical processing plants; electroplating facilities; machining fluids; cleaning processes; fluorescent material manufacturing; and paint production, among others. The investigation has revealed that electricity accounts for a significant portion of operational costs due to its low efficiency, leading to slow evaporation rates and high energy consumption. Some cleaning wastewater experiences excessive foaming during evaporation, causing part of the vaporized solution to enter condensation, resulting in inadequately treated effluents. Future endeavors will focus on addressing these challenges through several key research areas:

• (1) Exploring alternative heat sources, such as solar energy and air-source heat pumps combined with low-temperature evaporation techniques, while also considering the integration of solar power systems for both operational and heating purposes, alongside the development of environmentally friendly integrated low-temperature evaporation devices. Additionally, adopting recycled evaporation treatments for smaller volumes can improve overall efficiency, while exploring options to utilize excess or residual thermal energy within enterprises could offer cost-effective solutions for treating highly concentrated industrial effluents.

• (2) Tailoring specific combination technologies based on unique pollutant compositions present within each type of industrial wastewater stream by analyzing their organic pollutant profiles, thus mitigating issues related to foaming or corrosion scaling through the use of defoamers or modifications to existing evaporative structures, while further investigating mechanisms behind removing organic pollutants via wastewater stream-evaporative processes, aiming toward both reduction and resource recovery goals.

• (3) Designing an innovative packed bed evaporator specifically tailored for handling high-viscosity fluids, optimizing its structural performance by increasing its effective surface area, thereby enhancing both thermal conductivity and mass transfer efficiencies.

This work was carried out with the financial support of the Key R & D Project of Shaanxi Province (2022SF-334), the Natural Science Basic Research Program of Shaanxi (2021JQ-849), and Supported by Open Fund of Shaanxi Province Key Laboratory of Environmental Pollution Control and Reservoir Protection Technology of Oilfields and Engineering Research Center of Oil and Gas Field Chemistry, Universities of Shaanxi Provence (XSYU-CCCE-2405).

Z. S. conceptualized the whole article, rendered support in data curation, and wrote the original draft preparation. L. Z. investigated the whole process. S. C. and X. F. wrote the review, edited the article, and visualized the process. W. H. supervised the article and administered the project.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.