Experimental study on the treatment of high COD metal degreasing cleaning wastewater by coagulation and two-stage Fenton oxidation Open Access

Due to the excellent physical and chemical properties of metals, they have played a crucial role in industrial development, with demand increasing annually. Before processing metals and their products, it is necessary to remove surface grease to prevent interference with subsequent processing steps. This cleaning process generates large volumes of intermittent metal degreasing wastewater, which accounts for approximately 85% of the total wastewater produced in the metal processing cycle. The annual output of this wastewater reaches 3.4 billion tons, representing 20% of the total industrial wastewater output (Zhou et al. 2014). This wastewater contains organic pollutants such as heterocyclic compounds, phenolic compounds, long-chain alkanes, and linear alkylbenzene sulfonates. Its specific composition varies based on the cleaning agents, metals, and rust inhibitors used (Abouelela et al. 2021). The chemical oxygen demand (COD) content typically exceeds 10,000 mg/L (Li et al. 2021), and it is characterized by complex components, a high concentration of refractory pollutants, and low biodegradability (5-day biochemical oxygen demand (BOD₅)/COD ≤ 0.2) (Günes et al. 2019; Joseph et al. 2021), making its treatment highly challenging.

Currently, research on treatment technologies for metal degreasing cleaning wastewater is limited, but conventional techniques applicable to similar wastewater (containing surfactants that are difficult to degrade) can provide useful references. Jangkorn et al. (2011) used fresh alum and alum sludge as coagulants to treat wastewater with high organic and anionic surfactant content, achieving removal rates of approximately 92.80% for COD and 90.00% for anionic surfactants. Not only high removal rates were achieved, but the usage of fresh alum was also reduced considering the economic benefits. Joao et al. (2021) treated industrial wastewater containing detergents with a Fenton ultrasonic method using H₂O₂ and iron nails and achieved a detergent removal rate of 99.40%. Thus, the coagulation method is efficient and simple to operate, and it can be used on a large scale which is a promising technology for treating and reducing hazardous waste. However, its effectiveness on complex cleaning wastewater with a COD concentration above 20,000 mg/L requires further research. Advanced oxidation degrades pollutants in wastewater by generating hydroxyl radicals which are widely used in industrial wastewater pretreatment and the advanced treatment of wastewater, particularly the removal of recalcitrant organic substances (Xiaocong et al. 2023). As a form of advanced oxidation, the Fenton oxidation method is characterized by strong oxidation capacity, mild reaction conditions, a simple process, and ease of control, and it is widely applied in the treatment of recalcitrant industrial wastewater (Bobo et al. 2016; Li et al. 2018). However, a single physicochemical process often cannot directly decompose and remove large molecular pollutants from metal degreasing cleaning wastewater in practical applications; it requires pretreatment processes to alter the chemical structure of the pollutants, breaking down large molecules into smaller ones before further advanced treatment processes.

At present, there are few reports on methods for treating metal degreasing cleaning wastewater with COD concentrations exceeding 20,000 mg/L. To address this challenge, this research proposes an efficient treatment process that combines coagulation with two-stage Fenton oxidation for high COD metal degreasing wastewater. Coagulation is employed as the primary treatment, followed by two-stage Fenton oxidation to further remove residual contaminants. The effectiveness of this method was evaluated by monitoring COD removal to improve the effluent quality to meet discharge standards. This approach offers a viable technological solution for the treatment of metal degreasing wastewater in various industries.

The metal degreasing cleaning wastewater used in this study was taken from a machinery processing enterprise in China. The wastewater was milky white, and its main water quality characteristics are shown in Table 1.

The reagents, drug purity, and manufacturers used in this study are shown in Table 2.

The names, models, and manufacturers of the instruments used in this study are listed in Table 3.

Water samples were collected for analysis at the end of each reaction. The determination of COD was conducted according to HJ/T 399-2007 ‘Water Quality Determination of Chemical Oxygen Demand Fast Digestion-Spectrophotometric Method’ (People's Republic of China (PRC) Environmental Protection Bureau 2008). Prior to the determination of dissolved organic matter (DOM), water samples were pretreated with a filter membrane and then diluted to ensure that UV254 was less than 0.1. Subsequently, a three-dimensional fluorescence spectrometer was used for characterization (Xi et al. 2021).

As the dosage of coagulants increased, the COD removal rate initially rose, reaching a peak at a dosage of 5 g/L, before declining. This pattern can be explained by the fact that increasing the coagulant dosage enhances interactions between oppositely charged particles in the wastewater, facilitating the formation of a double layer (comprising an adsorption layer and a diffusion layer) during coagulation, which reduces the repulsive forces between colloids (Wang et al. 2021). Additionally, the metal salts in the coagulants form hydroxides in the wastewater, which exhibit a strong capacity to adsorb and capture pollutants, promoting the formation of flocs. However, when the dosage becomes excessive, the surface charge of the colloids may change, increasing repulsive forces between particles, hindering floc formation, and ultimately reducing pollutant removal efficiency.

In the treatment of metal degreasing wastewater, charge neutralization plays a critical role in disrupting the water–oil interface and promoting floc aggregation, making it essential to the coagulation process (Lin et al. 2023). However, the charge neutralization effect of PFC is relatively weak and not its primary mechanism of action, which may explain its lower effectiveness in treating metal degreasing wastewater (Wei et al. 2009). In contrast, PAC produces highly stable, positively charged Al₁₃ and Al₃₀ species during the reaction process, which exhibit stronger charge neutralization and adsorption capacities, especially under alkaline conditions, allowing for more effective removal of complex organic substances (Libing et al. 2020). The lower content of Al₁₃ and Al₃₀ in PAFC compared to PAC may account for PAFC's relatively lower efficiency.

As the dosage of coagulant aid increased, the COD removal rate initially rose, peaking at 8 mg/L of CPAM, before declining. CPAM, a linear high-molecular organic compound with high charge density and multiple functional groups, enhances pollutant removal by promoting the adhesion of more colloidal particles through charge neutralization and adsorption-bridging mechanisms (Zhao et al. 2018). However, when the dosage becomes excessive, too many dissociated cationic groups from the flocculant adsorb onto the surface of colloidal particles, leading to a decline in pollutant removal efficiency. This decline occurs for two reasons: first, the altered surface charge of the colloidal particles weakens the charge neutralization effect; second, an excess of cationic groups can hinder the formation of larger flocs, thereby reducing removal efficiency (Fang et al. 2023). Furthermore, since CPAM itself is a high-molecular organic compound that is difficult to degrade, an excessive amount may not fully interact with pollutants and could dissolve directly into the wastewater. This not only increases the COD load but may also complicate subsequent wastewater treatment processes.

As the pH increased, the COD removal rate from the wastewater initially rose and then declined, reaching its highest efficiency at a pH of 8. This variation is due to the different hydrolysis products and mechanisms of aluminum salts under varying pH conditions (Song et al. 2021). In acidic conditions, aluminum salts primarily form complex ions like Al(OH)²⁺, which operate mainly through charge neutralization. At a pH of 7–8, aluminum salts hydrolyze to form ⁠, which exhibits strong charge neutralization and sweep-flocculation capabilities. When the pH exceeds 8, aluminum salts primarily hydrolyze to Al(OH)⁴⁻, which also acts through charge neutralization and sweep-flocculation. However, formed at a pH of 7–8 carries a higher positive charge than Al(OH)⁴⁻, resulting in stronger charge neutralization. This enhanced neutralization more effectively counters the negatively charged pollutants in the wastewater, facilitating the formation of larger flocs and improving pollutant removal efficiency.

Figure 4(a) demonstrates that the COD removal rate in wastewater initially increases and then decreases as the rapid stirring time is extended, reaching its peak at 3 min. At the same stirring speed, increasing the stirring time promotes more frequent collisions between colloidal particles, encouraging the formation of compact, shear-resistant flocs, which improves pollutant removal efficiency. However, floc growth is limited, and excessive rapid stirring can compromise their structure, making them looser and leading to the formation of smaller aggregates. This, in turn, negatively impacts pollutant removal performance (Nti et al. 2021).

Figure 4(b) shows that the COD removal rate from wastewater initially increases and then decreases as the rapid stirring speed rises, with the highest removal rate occurring at 350 rpm. Increased stirring speed enhances the uniform mixing of wastewater and coagulant, facilitating coagulant hydrolysis, promoting colloidal particle destabilization and adsorption, and increasing the likelihood of particle collisions. However, when the stirring speed becomes too high, the flocs are exposed to excessive shear forces, which hinder the formation of larger flocs and reduce pollutant removal efficiency.

Figure 4(c) illustrates that the COD removal rate from wastewater initially increases and then decreases as the slow stirring duration is extended, with the optimal removal rate occurring at 15 min. Prolonging the slow stirring phase enhances the ability of the flocculants to adsorb and bridge smaller floc particles, promoting aggregation and improving pollutant removal. However, excessive stirring can destabilize the floc structure, diminishing its effectiveness in removing pollutants.

Figure 4(d) shows that the COD removal rate in wastewater initially increases and then decreases as the slow stirring speed rises, with the highest removal efficiency achieved at 200 rpm. Increasing the slow stirring speed promotes effective collisions between colloidal particles, enhancing the adhesive action of coagulant aid molecules on the flocs, which improves their compactness and settling properties. However, if the stirring speed becomes too high, it can disrupt the adsorption and bridging function of the coagulant aid, weakening its ability to adhere to the flocs and thus reducing pollutant removal efficiency (Liu et al. 2021).

In summary, the optimal conditions for COD removal from metal degreasing wastewater include the addition of 5 g/L PAC and 8 mg/L CPAM. The wastewater should undergo rapid stirring at 350 rpm for 3 min, followed by slow stirring at 200 rpm for 15 min, with an initial pH of 8. Under these conditions, the maximum COD removal efficiency achieved is 74.08%.

As the dosage of H₂O₂ increases, the COD removal rate from wastewater initially rises and then declines, peaking at a dosage of 4 mol/L. The increased H₂O₂ provides more hydroxyl radicals (·OH), which primarily oxidize the pollutants, enhancing their removal. However, when excessive H₂O₂ is added, it undergoes side reactions with the hydroxyl radicals, leading to the depletion of these radicals and, consequently, a reduction in COD removal efficiency (Cheng et al. 2021).

As the dosage of FeSO₄ increased, the COD removal rate in the wastewater initially rose and then declined. The highest COD removal rate was achieved at a dosage of 40 g/L. The increase in FeSO₄ dosage raises the concentration of Fe²⁺ in the solution, which continuously catalyzes H₂O₂ to produce hydroxyl radicals (·OH), effectively enhancing the degradation of pollutants. However, when the FeSO₄ dosage is too high, the excess Fe²⁺ promotes the rapid generation of a large number of hydroxyl radicals in a short time, leading to side reactions between H₂O₂ and ·OH, thereby reducing the pollutant removal efficiency of the hydroxyl radicals.

As the initial pH increased, the COD removal rate from the wastewater initially rose and then declined, with the highest removal rate observed at an initial pH of 2. At this point, 5.05 mL of 2% H₂SO₄ was required to adjust the pH. The initial pH was found to influence both the decomposition rate of H₂O₂ and the state of iron in the solution. Under low pH conditions, the formation of hydroxyl radicals (·OH) occurs at a faster rate (Yildiz et al. 2023). However, as the pH increases, more H₂O₂ decomposes into water, reducing its effectiveness in degrading pollutants. At low pH levels, FeSO₄ exists as Fe²⁺; as the pH rises, Fe²⁺ gradually converts to Fe(OH)₂, lowering the efficiency of pollutant degradation.

As the effluent pH increased, the COD removal rate from the wastewater remained relatively constant, with the residual COD concentration stabilizing between 1,130 and 1,200 mg/L. The rise in pH facilitated the precipitation and flocculation of Fe²⁺ and Fe³⁺, contributing to further pollutant removal from the wastewater. In this study, adjusting the effluent pH to 8 or 9 resulted in nearly identical COD removal rates and residual concentrations. Considering economic factors, a pH of 8 was selected, requiring 2.06 g of NaOH for pH adjustment.

As the oxidation time increased, the COD removal rate from the wastewater initially rose before stabilizing, reaching its peak at 60 min. The Fenton oxidation process involves a series of chain reactions, and extending the oxidation time allows these reactions to progress more fully. Additionally, metal degreasing wastewater contains a significant amount of high-molecular-weight pollutants that are difficult to degrade and require sufficient time for decomposition and oxidation. Increasing the oxidation time ensures more thorough degradation of these pollutants. However, once the reaction between H₂O₂ and Fe²⁺ approaches completion, further increases in oxidation time do not significantly improve the COD removal rate, leading to its stabilization.

As the settling time increased, the COD removal rate in the wastewater initially rose before stabilizing, reaching maximum efficiency at 90 min. At this point, Fe(OH)₂, Fe(OH)₃, and other suspended particles had largely settled. Extending the settling time beyond 90 min did not result in any significant improvement in treatment efficiency.

In summary, the optimal COD removal performance was achieved with a dosage of 4 mol/L H₂O₂, 40 g/L FeSO₄, an oxidation time of 60 min, an initial pH of 2, an effluent pH of 8, and a settling time of 90 min. Under these conditions, the COD removal rate reached 95.07%. However, the effluent COD concentration remained at 1,040 mg/L, indicating that additional treatment would be required.

As previously mentioned, the combined use of coagulation and two-stage Fenton oxidation has proven effective in removing COD from metal degreasing wastewater. However, further analysis is necessary to evaluate the removal efficiency of different organic compounds. This can be achieved using excitation–emission matrix (EEM) fluorescence spectroscopy to analyze the fluorescent components of the wastewater pollutants (Zhu et al. 2022).

The experimental results indicate that the optimal conditions for treating metal cleaning wastewater through coagulation are as follows: the addition of 5 g/L PAC and 8 mg/L CPAM, with an initial pH of 8. The process involves rapid stirring at 350 rpm for 3 min, followed by slow stirring at 200 rpm for 15 min. For primary Fenton oxidation, the best conditions are the addition of 4 mol/L H₂O₂ and 40 g/L FeSO₄, with an oxidation duration of 60 min, an initial pH of 2, an effluent pH of 8, and a settling time of 90 min. The optimal reagent ratio for secondary Fenton oxidation is 1:0.5. Using coagulation as a pretreatment step effectively reduces the high concentrations of pollutants in metal degreasing wastewater, which primarily consists of negatively charged anionic surfactants and oils. Coagulation works by hydrolyzing PAC and CPAM in the wastewater, producing cationic groups that neutralize charges, destabilize colloids, and remove pollutants through entrapment and adsorption, achieving a COD removal rate of 74.08%. However, coagulation is less effective in removing aromatic proteins and small-molecular-weight hydrophilic dissolved pollutants. The multistage Fenton oxidation process addresses this limitation by efficiently degrading aromatic proteins and dissolved organic pollutants.

This research is supported by Jilin Science and Technology Development Plan (20230303007SF).

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

The authors declare there is no conflict.