Ozone Catalytic Oxidation Technology Breaks Through The Treatment Difficulties of Kitchen Waste Biogas Liquid.

Ozone catalytic oxidation technology breaks through the treatment difficulties of kitchen waste biogas liquid.

The treatment of kitchen waste biogas liquid has long been a pain point in the environmental protection field. The high salt content and difficult-to-degrade nanofiltration concentrate have left traditional methods helpless. However, the emergence of ozone catalytic oxidation technology has brought new hope. It can efficiently remove pollutants and improve water quality, promoting green treatment solutions. This article will take you through the core principles, experimental effects, and optimization strategies of this technology, revealing the solution to environmental protection challenges.

The leachate from kitchen waste anaerobic fermentation is a by-product that contains a large amount of oil, organic matter, ammonia nitrogen and salt. The water quality fluctuates greatly, making its treatment particularly difficult. Traditional processes such as oil removal by air flotation and two-stage biological treatment followed by nanofiltration can partially purify it, but the concentrated liquid produced in the nanofiltration stage accounts for 20% to 30% of the influent volume, becoming a real "roadblock". This concentrated liquid is full of hard-to-degrade organic acids and high-salt substances, with extremely low biodegradability and a BOD5/COD ratio less than 0.1, meaning that conventional biological methods are almost ineffective, and new solutions must be sought.

Facing these concentrated liquids, old treatment technologies such as simple oxidation or filtration often have poor effects and may even cause secondary pollution. This leads to low overall treatment efficiency of kitchen waste leachate, affecting the sustainability of the entire waste treatment chain. The industry urgently needs an efficient, low-cost and environmentally friendly solution to meet the growing demand for waste treatment. Ozone catalytic oxidation technology has emerged as a scientific basis for solving this problem.

The core principle of ozone catalytic oxidation

The ozone catalytic oxidation technology relies on the strong oxidation capacity of ozone. Ozone molecules can directly decompose pollutants or indirectly generate hydroxyl radicals (·OH), which react extremely fast and can attack most organic substances non-selectively. Traditional ozone oxidation has a low efficiency of only 10% to 30%, with weak mineralization ability and incomplete reactions. However, when a catalyst is added, ozone decomposes more efficiently, generating more free radicals, significantly enhancing the oxidation performance, and thoroughly decomposing stubborn pollutants into harmless substances. The final product is oxygen, with no risk of secondary pollution.

The key to this technology lies in the role of the catalyst, which promotes the decomposition of ozone at normal temperature and pressure, forming a chain reaction of highly active free radicals. Compared with direct ozone oxidation, catalytic oxidation can handle a wider range of pollutants, including refractory organic acids and nitrate nitrogen. This not only improves the COD removal rate but also enhances the biodegradability of wastewater, making subsequent treatment simpler and laying the foundation for the closed-loop treatment of kitchen waste leachate.

The selection and application of catalysts

In ozone catalytic oxidation, catalysts are classified into homogeneous and heterogeneous types. Homogeneous catalysts, such as metal ions, have high efficiency but are difficult to recover, leading to resource waste. Heterogeneous catalysts are more practical, including aluminum-based, carbon-based, or solid materials loaded with metals. They are easy to separate and reuse, making them suitable for industrial applications. The selection of catalysts should consider their tolerance to high-salt environments, with a TDS value greater than 0.05 to ensure stable operation under the high-salt conditions of kitchen waste biogas liquid.

Experiments show that different catalysts have significant performance differences: aluminum-based catalysts can increase the COD removal rate to 60% to 70%, while carbon-based catalysts are more efficient, reaching 80% to 90%. At the same time, the color removal rate is astonishing, generally ranging from 90% to 95%, and the pH of the effluent is stable in the weakly alkaline range of 7.5 to 7.9, which is beneficial for subsequent treatment. However, a single catalyst cannot handle all concentration stages, and it is necessary to flexibly select catalysts based on changes in water quality to maximize the effect.

Experimental Effect and Performance Verification

In actual tests, the ozone catalytic oxidation technology performed outstandingly, achieving extremely high removal rates of COD, color, and organic pollutants in nanofiltration concentrate. When using carbon-based catalysts, the COD degradation rate approached 90%, and the B/C ratio significantly increased, indicating enhanced biodegradability of the wastewater, which is convenient for reflux to the front-end biochemical system for full-scale treatment. The color removal rate remained stable above 90%, visually transforming the wastewater from dark to clear, and the pH value was maintained within the ideal range, ensuring stable system operation.

The technology's tolerance was proven reliable. In high-salt environments, catalysts such as carbon-based materials could effectively function, with a TDS tolerance exceeding 0.05. This addresses the issue of high salt content in kitchen waste leachate. The experiments also demonstrated that the oxidation process does not produce harmful by-products, and the effluent quality is stable, allowing direct use for irrigation or discharge. These data confirm that ozone catalytic oxidation is not only highly efficient but also safe and environmentally friendly, providing a reliable basis for large-scale application.

Optimization Strategies and System Design

To achieve the best results, ozone catalytic oxidation should not rely on a single catalyst. Experiments have shown that switching catalysts at different concentration stages is a wise choice. For instance, aluminum-based catalysts can be used in the initial stage to treat high-concentration pollutants, and carbon-based catalysts can be switched to in the later stage to enhance efficiency. Using a series connection, such as multiple reactors in sequence, can ensure stable system operation and significantly improve the removal rate. This optimization strategy reduces catalyst consumption, lowers costs, and avoids treatment failures caused by water quality fluctuations.

In terms of system design, the series catalytic oxidation device can be automated to adapt to the unstable characteristics of kitchen waste leachate. By adjusting the ozone dosage and contact time, engineers can precisely match the changes in pollutant concentration, ensuring a consistently high treatment efficiency. Moreover, the durability of heterogeneous catalysts reduces maintenance frequency, extends the overall system lifespan, and lowers operating costs, achieving a win-win situation in both economic and environmental aspects.

Conclusion and Future Application Prospects

Ozone catalytic oxidation technology has been proven to be an ideal solution for treating concentrated liquid from the nanofiltration of kitchen waste biogas liquid. It can efficiently degrade COD, remove color, improve biodegradability, and cause no secondary pollution. Experimental data support its stability and tolerance, especially in high-salt environments. Promoting this technology can not only solve the current pain points of waste treatment plants but also promote resource recovery and achieve full utilization of wastewater, contributing to the development of a circular economy.

In the future, with the optimization of catalyst materials and system integration innovation, ozone catalytic oxidation is expected to expand to more challenging wastewater treatment fields, such as industrial waste liquids or municipal sewage. Researchers are exploring low-cost catalysts and intelligent control technologies to further reduce energy consumption. In conclusion, this technology injects new vitality into the environmental protection industry and brings us closer to a green and sustainable future for waste treatment.

The China Statistical Yearbook 2019 indicates that the per capita consumption of fruits and vegetables in China has reached 148.2 kg, surpassing the per capita consumption of grains at 127.2 kg, making fruits and vegetables the top consumed food category. As a major producer and consumer of fruits and vegetables, China faces a less-than-optimistic situation regarding pesticide residues in these foods. In 2018, the China National Institute for Food and Drug Control's supervision and random inspection results showed that among the 2,077 batches of 16 types of vegetables inspected, 68 batches were found to be substandard. The main reasons for the substandard spinach, celery, and common cabbage were excessive chlorpyrifos, while for leeks, it was excessive iprodione [1]. Among the 1,549 batches of fruits inspected, 39 batches were found to be substandard. The pass rate of citrus fruits was relatively low, with the main reasons being excessive bromophos and triazophos [2]. Moreover, excessive pesticide residues are also a significant factor affecting the export of Chinese agricultural products such as tea, fruits, and vegetables to Japan and the European Union [3].

Ozone molecules are composed of three oxygen atoms and possess strong oxidizing properties. The stability of ozone gas is higher than that of ozone solution. At 20°C, the solubility of ozone in water is 12.07 mg/L. In water, ozone can react with water molecules to produce hydroxyl radicals, which also have strong oxidizing properties [4]. The oxidation mechanism of ozone includes direct oxidation by oxygen atoms and indirect oxidation driven by hydroxyl radicals produced by the self-decomposition of ozone molecules. The reaction rate of direct oxidation is lower than that of indirect oxidation; indirect oxidation can rapidly trigger chain reactions [5]. Among them, hydroxyl radicals can alter the molecular structure of organic pesticides, opening the benzene rings, breaking double and triple bonds in the pesticide molecules.

Furthermore, it can be decomposed and oxidize chemical groups such as nitro, amino, and methoxy groups. Ozone thoroughly alters the molecular structure of pesticides by breaking chemical bonds and oxidizing functional groups, thereby changing the properties of pesticides to eliminate their toxicity and reduce the content of pesticide residues. The decomposition products of pesticides under the action of ozone are mostly small-molecule compounds such as acids, alcohols, or amines, and are mostly water-soluble substances. Moreover, ozone does not produce secondary pollutants during the process of degrading into oxygen [6]. Therefore, the use of ozone to degrade pesticide residues on fruits and vegetables is considered a safe, effective, and environmentally friendly treatment technology [7].

The utilization of ozone can be divided into gaseous or liquid forms based on its different states. There are subtle differences in efficiency and principle between the two methods in removing pesticide residues from fruits and vegetables. This review will clarify the removal effects of pesticide residues on the surface of fruits and vegetables by ozone gas fumigation and ozone water immersion based on existing experimental results, and analyze the environmental factors that affect the effectiveness of the treatment, such as gas phase temperature and humidity, bubble size, water temperature, and pH.

The degradation effect of gaseous ozone on pesticide residues in fruits and vegetables

Gaseous ozone is in contact with the surface of fruits and vegetables through fumigation, ultimately achieving the purpose of removing pesticide residues on the surface of fruits and vegetables. A commonly used device for treating pesticide-contaminated fruits and vegetables with gaseous ozone in the laboratory is shown in Figure 1.

According to the statistics of the National Bureau of Statistics on the scale of fruit and vegetable cultivation in China, the scale of grape cultivation has been steadily increasing in the past decade, reaching 14.3141 million tons in 2020. To avoid damage from some fungi and pests during cultivation, spraying pesticides is a common preventive measure for grapes, which inevitably leads to the problem of pesticide residues. Currently, the ozone atmosphere fumigation method is commonly used to degrade the pesticide residues on the surface of fresh grapes. An experimental team compared the degradation effects of several commonly used pesticides in grape cultivation after being stored for 3 weeks under refrigeration conditions (2 ℃, relative humidity 95%) in air and an ozone environment with a concentration of 0.64 mg/m³ [9]. The experimental results showed that the residues of pyrimethanil, dimethomorph, carbendazim, tebuconazole, triadimefon, and triflumizole on grapes stored in the ozone-containing air for 3 weeks decreased by 90.7%, 63.4%, 38.5%, 80.2%, 61.4%, and 51.8%, respectively. The degradation rates were significantly higher than those of grapes stored in air, indicating that the ozone atmosphere has a significant effect on the degradation of some pesticide residues on grapes.

At the same time, other commonly used pesticides in grape cultivation, such as cyprodinil, pyrimethanil, and fenamidone, are often used to prevent gray mold and other fungal diseases. However, their susceptibility to ozone action varies. The contents of the first three pesticides decreased significantly after ozone treatment, while the changes in iprodione and fludioxonil were not obvious. Figure 2 shows the degradation effects of cyprodinil, pyrimethanil, and fenamidone on fresh grapes under three treatment methods.

The degradation rates of cyprodinil in the three experiments were not significantly different, while the other two were greatly affected by the experimental schemes. The trends of the short-term results and the long-term storage experiments of grapes were consistent, indicating that ozone fumigation can significantly reduce the residues of these three pesticides.

For fresh grapes, it is necessary to reduce their pesticide residues in a short time without affecting their sensory quality. Researchers treated grapes with 2.0 and 3.0 mg/L ozone gas for 1 hour to investigate the removal rate of chlorothalonil on the flesh and skin of grapes and its impact on grape quality. The experimental results showed that there were significant differences in the results of the two ozone concentrations. However, while the higher concentration of ozone treatment brought a higher removal rate, it also significantly affected the titratable acidity, pH, soluble solids, and color of the grapes. There were no significant differences after treatment with 2.0 mg/L ozone. Therefore, the team believed that 2.0 mg/L ozone treatment was a more appropriate choice, and it could also prevent the grapes from becoming sour during storage, thereby maintaining the fruit quality for a longer period of time [13].

The ozone atmosphere fumigation method also showed good results in the degradation of pesticide residues on other substrates. The experimental content and results are shown in Table 1.