Aluminum is a very active metal. In order to prevent surface oxidation or defects caused by the surrounding environment, aluminum profiles need to be surface-treated prior to use. Through the treatment, a protective film can be formed on the surface of the aluminum profile, and the protective film plays an important role in appearance, corrosion resistance, mechanical strength, and service life. Treatment of aluminum surfaces requires the consumption of large amounts of water, and usually 50 to 80 tons of water per ton of aluminum surface treatment. The waste water produced by aluminum surface treatment is mainly acid-alkali waste water, which contains a large amount of Al3+, SO42-, and a small amount of Ni2+, Sn2+, Cr3+, F-, etc. The waste water is usually treated by acid-base neutralization. The wastewater is generally acidic and must be neutralized with alkali. During the neutralization process, the formation of hydroxides such as Al3+, Ni2+, Sn2+, and Cr3+ is removed during the adsorption and precipitation process; and the anions such as SO42- and F- are coagulated. The cations such as Ca2+ and Fe2+ precipitated and were partially removed. The main water-consuming links for surface treatment of aluminum profiles are post-washing processes for the processes of degreasing, alkaline etching, neutralization, and oxidation. The water quality requirements for the above-mentioned processes are not too high. The pH of the water after washing, neutralization, and oxidation is required. Above 2, alkaline washing water after the pH need to be less than 12, the turbidity is less than 50NTU, Al3 +, SO42- were lower than 500, 2000mg / L to meet the requirements. Therefore, the author uses the coagulation + microfiltration membrane separation and combination process to treat aluminum surface treatment wastewater in order to achieve the purpose of reuse.

In this study, the optimal coagulant, coagulant aid and dosage of the combination process were determined by coagulation and sedimentation test, and then the effect of microfiltration membrane operation factors on the membrane filtration performance and the overall operational effect of the combined process were examined. And conducted a technical and economic analysis. The combined process can achieve the goal of water conservation and pollution reduction, with social, environmental and economic multiple benefits.

1, test materials and methods

1.1 Test water

The equipment used for the test was placed directly next to the integrated wastewater conditioning pool of the aluminum profile surface treatment production line of an aluminum profile plant in Foshan City, Guangdong Province (the wastewater from the plant enters the conditioning tank in addition to wastewater containing a single type of pollutants such as chromium and nickel, and the remaining wastewater Directly into the adjustment pool), the wastewater used is directly taken from the integrated wastewater conditioning pool, and its water quality: Al3+500~600mg/L, SO42-2000~3000mg/L, turbidity 300~500NTU, pH3~5.

1.2 Test device

Aluminum Surface Treatment Wastewater Treatment and Reuse Technology

The effective volumes of the coagulation tank, inclined tube sedimentation tank and filter tank are 30, 50, and 80L, respectively. The microfiltration membrane is made of polyethylene hollow fiber membrane manufactured by Mitsubishi Corporation of Japan. The membrane pore diameter is 0.1 μm, and the membrane diameter is 0.27 mm. 0.42mm, membrane area 2.0m2. The membrane module is placed directly in the filter tank and an air aeration tube is provided below it to function as a flush membrane module. Membrane modules are equipped with solenoid valves at both ends of the outlet pipe to control the system's outlet and anti-aeration with a time controller.

1.3 Test methods

1.3.1 Coagulant screening test

In order to select a suitable coagulant and coagulant, the coagulation effects of various commonly used coagulants were compared through sequential batch tests at the exploratory stage. According to the characteristics of aluminum surface treatment wastewater and follow-up wastewater treatment program needs, the test selected turbidity, Al3 +, SO42- removal rate and flocculation settling performance as a measure to find the best coagulant and dosage.

Take a few 1000mL beakers, add 500mL of comprehensive adjustment tank wastewater (water temperature and room temperature are around 28 °C), add the measured coagulant, add NaOH powder to adjust the pH to 8.0, and then add the measurement aids to each beaker Coagulating agent PAM; placed on a six-speed stirrer, stirring at 360r/min for 1 min, then stirring at 150r/min for 2 min; allowing the supernatant to stand for 30 min to test the effect of flocculation and sedimentation.

1.3.2 Coagulation + microfiltration membrane separation combination test

The coagulant and coagulant aid solution respectively enter the suction line of the inflow pump through the metering valve, enter the coagulation tank after being rapidly mixed with the raw water through the impeller, enter the inclined tube sedimentation tank after being slowly mixed, and the supernatant after the separation of the sludge and water Into the filter tank, under the suction pump suction pumped through the membrane to obtain filtered water. Inlet and outlet valves work alternately under the control of a time controller. According to the content of suspended solids in the effluent of the coagulation system, the sludge at the bottom of the filter tank is excluded once every 6 to 8 cycles.

2. Results and Discussion

2.1 coagulation sedimentation test

In this study, 25 kinds of coagulants including FeCl2, FeCl3, Fe2(SO4)3, AlCl3 and Ca(OH) were selected. In order to increase the density of flocs, PAM was selected as a coagulant. Coagulant dosing concentrations were 100, 150, 200, and 250 mg/L, respectively; Coagulant dosing concentrations were 2, 4, 6, and 8 mg/L, respectively. After a comparative experiment, the optimal dosage and flocculating effect of various coagulant/coagulant were obtained.

Aluminum Surface Treatment Wastewater Treatment and Reuse Technology

The coagulation effect of Ca(OH)2/PAM is obviously better than other coagulants. When the mass concentration of Ca(OH)2 was 200 mg/L and the mass concentration of PAM was 4 mg/L, the effluent turbidity after coagulation and sedimentation was 50 NTU, and the effluent Al3+ and SO42- were lower than 18, 1500 mg/L, respectively. Shows that the use of a single coagulation and sedimentation treatment of aluminum surface treatment wastewater, barely able to achieve degreasing, alkaline corrosion, neutralization, oxidation and other processes for water quality requirements, but the amount of coagulant dosage, and the water quality is not stable enough.

The single coagulation and sedimentation treatment is carried out by adding a sufficient amount of coagulant and coagulant aid so that the flocs that are formed by the colloidal particles suspended in water have good sedimentation performance can be removed. Using the coagulation + microfiltration membrane separation and combination process, the microfiltration membrane separation technology can achieve a solid-liquid separation level of the order of 0.1 μm, and the coagulant and coagulant added do not destabilize the colloidal particles only by compressing the double-layer. Yes, there is no need to form particles that depend on gravity for sedimentation. This not only reduces the amount of coagulant and coagulant, but also increases the range of contaminants that can be removed compared to single coagulation. The experiment did not change the Ca(OH)2 dosage (200 mg/L), but the coagulation aid PAM was only added half the dose (2 mg/L). The processing effect.

2.2 Effect of anti-aeration of microfiltration membrane on membrane filtration performance

In this study, the average membrane flux was used as an evaluation index of membrane filtration performance. In order to reduce the degree of membrane fouling and maintain the stability of the membrane filtration performance, a cyclic anti-aeration operation was used to remove the cake layer deposited on the membrane surface. The pressure of anti-aeration was 0.15 MPa, and the time for each counter aeration was 3min. The results of membrane filtration performance change under different operating cycle conditions show that the anti-aeration of the membrane cycle can largely blow off the mudcake layer deposited on the membrane surface, reduce the membrane fouling, recover the membrane flux, and effectively maintain the membrane filtration. The stability of the performance; using the filter 30min, anti-aeration 3min operation, the cycle of water production can be stabilized at about 40L / (m2 ̇ h).

2.3 Coagulation + microfiltration membrane separation process performance

Ca(OH)2 and PAM were used as coagulants and coagulants, and the dosing mass concentration was 200 mg/L and 2 mg/L, respectively, and they were run with 30 min filtration and 3 min anti-aeration. The average membrane flux was controlled at 40 L/. (m2 ̇h) stable operation for 2 weeks. Treatment of Synthetic Aluminium Extrusion Surface Wastewater by Coagulation + Microfiltration Membrane Separation Process .

Aluminum Surface Treatment Wastewater Treatment and Reuse Technology

The turbidity of the final effluent after filtration by the microfiltration membrane is about 10 NTU, which fully satisfies the requirements for turbidity of the water for the washing step after degreasing, alkaline etching, neutralization, and oxidation.

Although the system influent SO42-fluctuation is large (mainly caused by irregularly discharging part of the degreasing or light-emitting bath solution), the final effluent SO42- is stable at about 1000 mg/L. The resulting calcium sulfate precipitates and aluminum hydroxide precipitates to SO42- Adsorption may be the main way to remove sulfate in this system; after the treatment process of coagulation + microfiltration membrane separation, the effluent Al3+ is stable at about 8 mg/L. After the combination process, the effluent of the system fully meets the requirements of the water for the washing steps of the degreasing, alkaline etching, neutralization, and oxidation processes on the Al3+ and SO42-.

3, technical and economic analysis

Compared with the conventional process, the coagulation + microfiltration membrane separation and combination process for aluminum surface treatment wastewater has the advantages of stable water quality, small engineering footprint, capital construction investment, high degree of automation, and convenient operation and management [3], and Water can be reused.

Taking the daily treatment and reuse of 1000t waste water as an example, a technical and economic analysis of the process of separation and combination of coagulation and microfiltration membranes was carried out: infrastructure investment was 150,000 yuan, equipment cost (film price 150 yuan/m2) was 186,000 yuan, depreciation expense (film life is 3a, others are 20a) is 0.192 yuan/m3, energy consumption is 0.4 yuan/m3 (electricity fee is 0.8 yuan/(kW ̇h)), pharmaceutical cost is 0.133 yuan/m3, labor cost is 0.15 yuan /m3, the total operating cost of the system is 0.875 yuan/m3. Compared to the current water cost of the company of 2.5 yuan/m3 (including 1.3 yuan/m3 for tap water, 0.4 yuan/m3 for waste water treatment, and 0.8 yuan/m3 for sewage), the economic benefits are obvious.

4. Conclusions The process conditions for the treatment of synthetic waste water from surface treatment of aluminum profiles using a coagulation + microfiltration membrane separation and combination process are as follows: Ca(OH)2 as coagulant, PAM as coagulant, Ca(OH) The mass concentrations of 2 and PAM were 200 mg/L and 2 mg/L, respectively. The microfiltration membrane separation system was operated with 30 min filtration and 3 min anti-aeration. Under this condition, the system operated stably for 2 weeks. Although the influent water quality changed greatly, the effluent turbidity was stable at about 10 NTU, the Al3+ and SO42- were stable at about 8 mg/L and 1000 mg/L, and the effluent water quality completely satisfied the degreasing and alkaline eroding. Water requirements for water washing after processes such as, neutralization, and oxidation. Compared with the conventional treatment process, the coagulation + microfiltration membrane separation and combination process has obvious advantages in technology and is economically feasible.