Treatment process for aluminum profile wastewater

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1 Characteristics of wastewater The production process of aluminum profiles involves steps such as degreasing, alkali etching, acid washing, oxidation, sealing, and coloring of the shaped aluminum materials. After undergoing these processes, the profiles need to be washed with water. The water used for cleaning these profiles is discharged from the cleaning tanks through overflow, and this constitutes the main source of wastewater in aluminum profile factories. The production wastewater from aluminum profile factories contains not only a large amount of aluminum ions but also some metal ions such as zinc, nickel, and copper. The pH level of this wastewater varies depending on the specific production requirements, but it is usually acidic. Table 1: Water quality parameters of aluminum profile wastewater Parameter: pH, Suspended solids (mg/L), Copper (mg/L), Zinc (mg/L), Nickel (mg/L). Concentrations: 2–4, 300–10,000, 0.5–3, 1.5–4, 1.5–4. 2. Wastewater treatment process: Given that aluminum profile wastewater contains various metal ions and suspended solids, a treatment process based on neutralization and coagulation-precipitation is employed. The wastewater generated in aluminum profile production is discharged from the workshop and flows into a neutralization and adjustment tank, where air agitation is provided to balance the water quality. After the wastewater has its water quality and volume adjusted in the equalization tank, alkali is added to adjust the pH value to 6–9. It is then pumped into the sedimentation tank, with a flocculant (PAM) being added during this pumping process. In wastewater, metal ions react with alkalis to form hydroxides; under the action of flocculants, these hydroxides form larger floc particles that settle rapidly due to gravity. The clear liquid in the upper part of the sedimentation tank can be discharged directly, and the quality of this effluent meets the second-class discharge standards for Category II areas as specified in Guangdong Province’s local standards DB4426—89. 　　The sludge from the sedimentation tank is concentrated in a sludge tank, then pumped into a plate and frame filter press for dewatering, after which it is used for sanitary landfilling or comprehensive utilization. 　　3 Process Principles 　　3.1 Retention Tank 　　In the treatment of aluminum profile wastewater, retention tanks can be classified into intermittent and continuous types. For manual adjustment, the treatment tank must be divided into two sections; the retention time of wastewater in each section is 1–2 hours, and they are used alternately on a intermittent basis to facilitate manual control ； Automatic adjustment requires only one regulation tank; a pH automatic regulator is used to control the pH value of the wastewater. Since aluminum-containing wastewater contains large amounts of aluminum, and aluminum exhibits amphoteric properties in solution. When pH < 3, the main form of aluminum exists as Al(H2O)3+6 ； When pH = 7, aluminum hydroxide becomes the main form in which Al3+ exists ； When pH > 8.5, most of the aluminum hydroxide hydrolyzes into negatively charged complex anions. Therefore, during engineering commissioning, the pH value must be kept within an appropriate range to ensure that aluminum can precipitate fully in the form of aluminum hydroxide. 　　3.2 Reaction Tank The function of the reaction tank is primarily to enable thorough reaction between Al3+ and OH- in the wastewater from aluminum profiles, resulting in the formation of insoluble Al(OH)3 precipitate. Typically, vertical sedimentation tanks use vortex reactors, while horizontal sedimentation tanks use baffled reactors. 　　3.3 Coagulation and sedimentation tank: After reacting with alkali in the equalization tank, the metal ions in wastewater form insoluble hydroxides. However, since the particles formed are small, they do not settle easily under the influence of water flow; therefore, flocculants must be added to cause these particles to stick together and form larger particles, which can then be removed through solid-liquid separation in the sedimentation tank. Settling tanks can be of the horizontal or vertical type, with the latter being the most widely used. Vertical sedimentation tanks are particularly suitable for the settling of flocs; they are simple to operate and easy to manage, with the supernatant able to be discharged directly. The retention time in the sedimentation tank is 2 hours, with a surface load of 1 m3/(m2·h). 　　3.4 Sludge treatment The aluminum profile sludge discharged from the sedimentation tank has a moisture content of over 90%, and therefore requires dewatering treatment. Based on the factory’s production capacity and wastewater discharge volume, two methods—natural drying and mechanical dewatering—are selected to treat the sludge. 　　Natural drying involves placing the sludge in a drying tank and using sunlight to dry it. The advantage of this method is that it is simple and cost-effective, but it is only suitable for companies with small amounts of sludge, and it causes a lot of trouble in rainy weather ； Mechanical dewatering includes the use of centrifuges, belt filter presses, and plate and frame filter presses. However, due to the loose structure of aluminum profile sludge and its certain corrosiveness, only plate and frame filter presses yield the best results. Therefore, in engineering design, the sludge is discharged from the sedimentation tank to the sludge thickening tank using static pressure; after thickening, it is pumped to a plate and frame filter press for filtration. The moisture content of the treated sludge can be reduced to around 70%, after which the sludge cake can be transported away or utilized in various ways. 　　3.5 Key points of commissioning In the commissioning of projects for treating wastewater from aluminum profiles, the most crucial aspect is controlling the pH value of the wastewater, so that various metal ions form insoluble hydroxides, thereby achieving the best removal efficiency. Table 2 Optimal pH values for removing various metal ions, pH range of metal ions, residual concentration (mg/L), and notes: Aluminum: 5.5–8, ≤3; precipitation occurs at pH above 6.5. Copper: 7–14, ≤1. Zinc: 9–10.5, ≤1; precipitation occurs at pH above 10.5. Nickel: >9, ≤1. Based on Table 2 and the results of testing in various aluminum profile wastewater treatment projects, for typical aluminum profile wastewater, the best precipitation effect is achieved by maintaining a pH value between 7.5 and 8.5 ； For wastewater containing an excess of certain metal ions, the pH value needs to be adjusted according to the properties of those metal ions. 4 Monitoring Results and Analysis The wastewater treatment process for aluminum profiles has been put into use in over 90% of aluminum manufacturing plants in Nanhai City; the equipment is operating well, and the quality of the treated water remains stable. Table 3 shows the sampling monitoring results of some enterprises. Table 3: Monitoring Results of Wastewater from Aluminum Profile Factories
Unit: Water sample
pH, Suspended solids (mg/L), Copper (mg/L), Zinc (mg/L), Nickel (mg/L)

Xingfa Aluminum Profile Factory:
Influent: 1.86, 145, 40.55, 0.24, 2.12
Effluent: 6.29, 20.80, 0.02, 0.02, 0.08

Fengchi Wastewater Treatment Plant:
Influent: 9.97, 380, 40.40, 0.51, 1.70
Effluent: 7.94, 12.80, 0.04, 0.00, 0.26

Lianjiao Wastewater Treatment Plant:
Influent: 4.34, 295, 21.61, 3.22, 4.92
Effluent: 7.13, 7.20, 0.04, 0.15, 0.26

Emission standards in Guangdong Province: 6–9, 100, 121
Note: The above data were provided by the Nanhai Environmental Monitoring Station. The effluent standards comply with Guangdong Province’s “Industrial Waste Emission Standards” DB4426—89. 　　5 Summary 　①Engineering practice shows that this process is reasonable and feasible. 　　②Due to the amphoteric nature of aluminum in solution, aluminum ions also act as water purifiers in wastewater. Moreover, various flocculants and coagulants added contain large amounts of aluminum; therefore, no requirements are set for the aluminum concentration in the quality parameters of the treated water. 　　③The treatment process for aluminum profile wastewater has a simple principle, is easy to operate and manage, and is also suitable for the treatment of wastewater from other metals as well as acidic and alkaline wastewater. The current problem is the disposal of waste residues; aluminum sludge, even after being dehydrated using a filter press, still contains a significant amount of hydroxides, and improper disposal can lead to secondary pollution. In fact, the precipitates from aluminum wastewater contain large amounts of aluminum hydroxide; if utilized and processed to produce highly pure aluminum hydroxide as a chemical raw material, it has a wide range of applications.