Since formaldehyde is widely used in industrial production, completely restricting its use is not realistic; therefore, it is essential to treat the wastewater generated from formaldehyde production. Researchers both domestically and internationally have conducted extensive studies on technologies for treating formaldehyde-containing wastewater. The main methods for treating formaldehyde wastewater include oxidation, biological treatment, air stripping, condensation, and lime treatment.

　　Formaldehyde is a colorless gas with a strong, irritating odor. It is readily soluble in water, alcohols, and ethers. A 35–40% aqueous solution of formaldehyde is known as formalin. Formaldehyde is an important organic raw material, primarily used in the plastics industry, synthetic fibers, leather processing, pharmaceuticals, dyes, and the production of wood adhesives, among other applications.

　　Hazards: Formaldehyde is highly toxic to humans and warm-blooded animals. It can irritate the skin, easily causing dermatitis, as well as respiratory irritation, allergies, abnormal lung function, liver dysfunction, and immune system abnormalities. If humans consume water contaminated with formaldehyde over a prolonged period, it may lead to dizziness, anemia, and various neurological disorders. Given the wide range of industrial applications for formaldehyde, completely banning its use is unrealistic; therefore, it is essential to treat wastewater containing formaldehyde generated during production.

　　The national standard "Integrated Wastewater Discharge Standard" (GB 8978-1996) stipulates that the formaldehyde content in secondary discharge standards shall not exceed 2 mg/L. However, formaldehyde can directly react with proteins, DNA, and RNA within microbial cells, leading to microbial death or inhibition of their biological activity. When the concentration exceeds 200 mg/L, microbial activity is almost completely suppressed; therefore, high-concentration formaldehyde is unsuitable for biological treatment. Moreover, formaldehyde solutions are true solutions, making coagulation processes ineffective as well.

　　Researchers both domestically and internationally have conducted extensive studies on the treatment technologies for formaldehyde wastewater. The main methods for treating formaldehyde wastewater include oxidation, biological treatment, air stripping, condensation, and lime-based methods.

　　Oxidation method

　　The Fenton reagent oxidation process is a widely studied method for treating formaldehyde wastewater, both domestically and internationally. The reagent is a strong oxidizing agent composed of H2O2 and Fe2+, which primarily relies on highly active hydroxyl radicals (·OH) to oxidize and degrade organic pollutants in the wastewater, achieving complete degradation of these organic substances within a short period of time.

　　Wet oxidation method

　　Under conditions without any external catalyst, wastewater containing formaldehyde can selectively convert its organic and inorganic carbon into CO2 and H2O at temperatures ranging from 180 to 315°C and pressures from 2 to 15 MPa, without generating nitrogen oxides, sulfur oxides, hydrogen chloride, or fly ash. When the CuO-ZnO/Al2O3 catalyst is added externally, the reaction time is shortened, and the temperature and pressure can be reduced to 130–250°C and 1–5 MPa, respectively. According to relevant studies, the removal rates of formaldehyde and CODCr can both reach over 90%.

　　Photocatalytic oxidation method

　　Photocatalytic oxidation is an emerging environmental technology that has gradually developed since the 1970s. It leverages the property of semiconductor oxide materials, which, when exposed to light, have their surface energy activated. This process can effectively oxidize and decompose organic compounds, reduce heavy metal ions, kill bacteria, and eliminate odors—particularly in the case of the most commonly used TiO2 system. In 1974, Honda et al. first discovered that TiO2 could split water into H2 and O2 under illumination conditions. As a result, photocatalytic oxidation quickly found applications in wastewater treatment, especially for the degradation of various organic compounds that are difficult for biological processes to break down. Studies have shown that photocatalytic oxidation can efficiently treat low-concentration formaldehyde-containing wastewater, achieving removal rates of over 90%.

　　Chlorine Dioxide Method

　　Chlorine dioxide (ClO2) is an excellent bactericidal, disinfectant, bleaching agent, and high-efficiency oxidizing agent. Its effective chlorine content reaches as high as 263%, which is 2.6 times the oxidizing capacity of chlorine gas. Use of chlorine dioxide for sterilization and disinfection poses absolutely no carcinogenic or teratogenic risks. It has been classified by the World Health Organization (WHO) as a Class AI product, placing it at the top of the list of safe disinfection methods. Yue Qinyan et al. studied the effects of reaction time and pH value on the treatment of formaldehyde-containing wastewater. When the formaldehyde concentration in the wastewater was 8.25 mg/L, after reacting for 30 minutes, the formaldehyde removal rate could reach as high as 80%. The optimal conditions were found to be neutral pH.

　　Ultrasonic/H2O2 Oxidation Method

　　Hydrogen peroxide (H2O2) is a commonly used oxidizing agent that can be employed either alone or in combination to treat formaldehyde-containing wastewater. According to studies by Yan Bing et al., the combined use of this approach can achieve a formaldehyde removal rate of over 80%; however, the reaction rate is relatively low. The degradation reaction follows first-order kinetic behavior, and the reaction rate increases as the initial concentration decreases. Alkaline conditions are favorable for the progress of the reaction.

　　Biological treatment method

　　Biological treatment of formaldehyde wastewater typically employs a combined approach of anaerobic hydrolysis and acidification followed by aerobic biological treatment. It is generally believed that formaldehyde wastewater exceeding 200 mg/L has inhibitory and lethal effects on various microorganisms and bacterial strains. Therefore, formaldehyde wastewater with concentrations greater than 200 mg/L—even up to several thousand mg/L—cannot be directly subjected to biological treatment and must first undergo pretreatment to reduce the formaldehyde concentration to a safe level at which microorganisms can effectively degrade it, typically below 50 mg/L, before being further treated by biological methods to remove CODCr.

　　Stripping method

　　This method leverages formaldehyde’s properties—its high solubility in water, low boiling point, and easy volatility—to pre-treat formaldehyde in production wastewater by stripping it out using steam. This step reduces the load on subsequent treatment processes and improves overall treatment efficiency. After stripping, the volatilized formaldehyde gas can be recovered and used as a raw material for production, where it is formulated into a 37% formaldehyde solution. However, this method is suitable only for wastewater with extremely high formaldehyde concentrations (above 5,000 mg/L). The treatment process cannot reduce the formaldehyde concentration below 200 mg/L on its own; therefore, it must be combined with other pretreatment methods, and it also consumes relatively high energy. Moreover, in many cases where production facilities already have separation processes in place to recover formaldehyde, further repeated treatment becomes economically inefficient.

　　Condensation method

　　The condensation method, also known as the urea method, primarily relies on the reaction between urea and formaldehyde under acidic conditions to produce a methylurea precipitate. The basic procedure involves adjusting the pH of formaldehyde wastewater to around 2 using hydrochloric acid and then adding an appropriate amount of urea according to a specific ratio, which can achieve a formaldehyde removal rate of over 80%. However, like the stripping method, this approach is suitable only for formaldehyde wastewater with extremely high concentrations and cannot meet the requirements of subsequent biochemical treatment processes. This method is mostly used in laboratory research, and its industrial-scale application still awaits practical implementation.

　　Lime method

　　When formaldehyde is heated under alkaline conditions, it undergoes a resinification reaction. This principle can be applied to treat formaldehyde-containing wastewater; the most commonly used catalyst is Ca(OH)2. In the presence of lime, formaldehyde polymerizes to form hexoses. Although this method does not reduce the CODCr value, the resulting sugar compounds are non-toxic to microorganisms and even promote their growth, which is highly beneficial for subsequent biological treatment. The main procedure involves adjusting the pH of formaldehyde wastewater to an alkaline level (pH 11–12) using sodium hydroxide, adding lime at a mass concentration ratio of 0.1 relative to formaldehyde, and maintaining the temperature around 70℃. Studies have shown that this process can achieve a formaldehyde removal rate of over 99%. The effectiveness of formaldehyde removal in this method is primarily influenced by two factors: theoretically, the greater the amount of lime added and the higher the reaction temperature, the faster and more complete the reaction will be.