JP2003200162A - Treatment method for wastewater from cationic electrodeposition coating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treatment method for stably keeping the filtering capacity of a membrane filter apparatus, in treating wastewater discharged from a final washing process of cationic electrodeposition coating using the membrane filter apparatus. <P>SOLUTION: An antibacterial agent is continuously or intermittently added to wastewater and/or a filtrate and membrane filtration is performed while suppressing the propagation of acid consuming bacteria and preventing a rise in the pH of wastewater. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating wastewater from a final washing tank in cationic electrodeposition coating with a membrane filtration device. [0002] Cationic electrodeposition coating is widely used for coating automobile bodies, automobile parts, electric products, building materials, and the like. The electrodeposition coating system comprises an electrodeposition step of electrochemically forming a coating film on an object to be coated, a cleaning step of washing off non-electrodeposition paint, etc., and a drying-baking step of curing the coating film. Generally, the washing step is roughly classified into a UF filtrate recovery washing step and a final washing step.
The former uses an UF (ultrafiltration membrane) to filter the paint in the electrodeposition tank to wash the object to be coated using a UF filtrate, which removes paint components physically attached to the object to be coated, and removes the electrode component. This is the step of collecting the liquid in the tank. The latter is a step of performing a final cleaning using pure water or industrial water, and a small amount of paint components and unusual ions that could not be washed away in the UF filtrate recovery water washing step are washed off, but after being used for washing. Water is discharged out of the process as wastewater. [0003] A large amount of water is required for final rinsing, which is a finish cleaning.
Water at a rate of 00 L / min or more is supplied as washing water, and the washed water is discharged out of the process as wastewater. Since this wastewater contains paint components and miscellaneous ions in a small amount, it is necessary not only to eventually treat the wastewater in some way but also to lose paint. The amount is equivalent to 5% of the paint physically taken out of the electrodeposition main tank by the object to be coated. In recent years, a membrane separation method has been proposed as a method for treating such wastewater. The membrane is separated into a filtrate and a concentrated solution by a membrane filtration device, and the filtrate is used as washing water, and the concentrated solution is returned to the electrodeposition tank. In some cases, a method of collecting and reusing the paint has been put to practical use. SUMMARY OF THE INVENTION The present invention relates to a method for stably maintaining the filtration performance of a membrane filtration device when treating wastewater discharged from a final washing step of electrodeposition coating with a membrane filtration device. The wastewater discharged from the final washing tank in the cationic electrodeposition coating process is applied to a membrane filtration device, separated into a filtrate and a concentrated solution, and the filtrate is circulated and used as washing water. It is an object of the present invention to provide a membrane filtration method capable of stably treating a concentrated liquid which is returned to an electrodeposition main tank and recovered and reused as a paint. [0006] As a method for stably maintaining the performance of a membrane filtration device, WO 96/07775 discloses a method of adjusting the pH of wastewater by adding an acid to wastewater applied to the membrane filtration device. The inventor has found that bacteria that consume acids without exception are present in the washing wastewater of the final washing step,
The present inventors have found that the pH of the wastewater rises during the night or on the weekend while the final water washing step and the membrane filtration device in the final water washing step are stopped, that is, due to stagnation, resulting in the present invention. In a normal case, once in a membrane filtration device,
After the wastewater from the final washing tank is received in the tank, the post-treatment is started, and the pH is adjusted by adding an acid to the tank. The pH is detected by a sensor as disclosed in JP-A-2000-263044. This is carried out on the filtrate side of the membrane filtration device due to a decrease in reliability due to dirt on the surface and a problem of maintenance management of the sensor. For this reason, it takes a long time for the pH to be adjusted to the predetermined pH for a while after the start of the treatment. Therefore, the apparatus is operated at a higher pH until the pH is adjusted to an appropriate pH, and the problem is that the processing capacity of the membrane filtration device gradually decreases due to such an operation being repeated. Was. [0008] In addition, the membrane filtration apparatus normally treats wastewater only during the operation of the coating process, and is stopped during the night or on the weekend when the coating process is stopped. If the water stays for a long time, the pH of the final washing tank rises due to the action of bacteria living in the waste water, and for a while after the treatment of the membrane filtration device is started, the waste water on the higher pH side must be treated. For this reason, the performance of the membrane filtration device has been reduced. [0009] In addition to the decrease in capacity due to an increase in pH, the progress of membrane contamination due to the propagation of bacteria in wastewater has also been a factor in reducing the capacity of the membrane filtration device. Furthermore, the amount of acid used for pH adjustment became unnecessarily large, and economic loss could not be ignored. According to the present invention, waste water discharged from the final washing tank in the cationic electrodeposition coating process is passed through a membrane filtration device, separated into a filtrate and a concentrated solution, and the filtrate is circulated as washing water. And return the concentrate to the electrodeposition tank, and in a method of collecting and reusing it as a paint, an antibacterial agent and / or a bactericide is continuously or intermittently added to wastewater and / or the filtrate,
The object has been achieved by suppressing the growth of bacteria that consume acids and preventing the pH of wastewater from increasing. The antibacterial agent and the bactericide according to the present invention may be any as long as they can suppress the proliferation of bacteria consuming acids or can kill bacteria. Silver ions, copper ions, aldehydes, phenolic compounds, chlorine Compounds and the like can be used alone or together, but (1) there is no particular problem when mixed in paints, and (2) biological treatment even when a part of the filtrate is discharged as waste water out of the final washing step. It is important to select one that has little impact on wastewater treatment. Further, the antibacterial agent must be dispersed as uniformly as possible in the entire wastewater, and is preferably a water-soluble one, and particularly preferably an aqueous solution. The antibacterial agent and the bactericide of the present invention can be added directly to the wastewater in a final washing tank or a tank for receiving the wastewater into the membrane filtration device or in the above-mentioned receiving pipe, or by using a filtrate tank or the like. Can also be added. Further, it may be added to both the wastewater and the filtrate. The time of addition of the antibacterial agent and the bactericide is not particularly limited, but when added to a place other than the final washing tank, it is preferable to add the antibacterial agent during the treatment with the membrane filtration device in order to spread the entire antibacterial agent to the wastewater. When adding to the final washing tank, it may be immediately after the process is stopped. In addition, the addition method can be added continuously in small amounts, or an appropriate amount may be added intermittently.However, more than necessary addition may affect the quality of the recovered paint and wastewater treatment. Therefore, it is preferable to add periodically while observing the state of pH change. The present invention can be applied to all cases where wastewater from a final washing tank is treated by a membrane filtration device.
In particular, it is useful for stable operation of a membrane filtration device installed in a post-washing process of an automobile body having a large capacity of a final washing tank, and ultrafiltration membrane (UF) is used as a membrane used in the membrane filtration device.
Microfiltration membrane (MF). Embodiments of the present invention will be described below. EXAMPLE 1 Waste water from a final washing tank installed in a cationic electrodeposition coating process of an automobile body is separated into a filtrate and a concentrated solution, and the filtrate is circulated as washing water, and the concentrated water is recovered as a paint. The concentrated circulating liquid (a liquid obtained by concentrating wastewater from the final washing tank applied to the membrane filtration device) was collected from the concentration circulation tank of the membrane filtration device to be reused, and the filtrate was collected from the filtrate tank, and the pH was adjusted with acetic acid. Thereafter, the mixture was placed in a 200-ml container with a stopper and kept, and a change in pH when the sample was retained was tracked. At the same time, the change in pH was monitored for a sample in which a 0.2 mol / L silver nitrate aqueous solution was added as an antibacterial agent so that the silver concentration was 5 ppm. Table 1 shows the results. The solids concentrations of the concentrated circulating liquid and the filtrate measured by collecting 10 g and drying at 105 ° C. for 3 hours were 0.17 wt% and 0.03 wt%, respectively.
wt%. [Table 1] Example 2 40 L of a concentrated circulating fluid was collected from the same membrane filtration apparatus as in Example 1, adjusted to pH 5.7 with acetic acid, divided into 20 L portions, and only one of them was used as an antibacterial agent. .2m
An ol / L aqueous silver nitrate solution was added so that the silver concentration became 5 ppm. In addition, 10 g was collected, dried at 105 ° C. for 3 hours, and the measured solid content was 1.13 wt%. Subsequently, the mixture was allowed to stand for 5 days while stirring at room temperature, and then charged into a membrane filtration device, and a polyacrylonitrile-based hollow fiber type ultrafiltration module (a hollow fiber outer diameter of 1.4 mmφ, a hollow fiber inner diameter of 0.8 mmφ, a molecular weight cutoff of 13,000, Effective membrane area 0.
2 m ² , a hollow fiber length of 255 mm including an adhesive layer) was passed under the conditions of an inlet pressure of 140 KPa, an outlet pressure of 60 KPa, and a temperature of 28 ° C., and the initial filtrate amount was measured. Next, the inlet pressure was changed to 170 KPa and the outlet pressure was changed to 130 KPa, and the operation was continued for 6 hours while returning the filtrate to the circulation tank. After 6 hours, the inlet pressure is 140 KPa and the outlet pressure is 60 KPa again.
After adjusting the amount of the filtrate, the stability of the amount of the filtrate was evaluated by the ratio of the amount of the initial filtrate to the amount of the filtrate after 6 hours of operation, and the results are shown in Table 2. The pH of the test solution to which the antibacterial agent was added was maintained at 5.7 even after 5 days, and the amount of the filtrate after passing for 6 hours and the amount of the filtrate after passing for 6 hours remained unchanged at 291 ml / min. On the other hand, the pH of the test solution to which the antibacterial agent was not added increased to 6.5 after 5 days, and the amount of filtrate at the beginning of the passage was 290.
After passing through the solution for 6 hours at a rate of ml / min, the filtrate amount was 258 ml / min and the ratio was 0.89. [Table 2] Example 3 40 L of a concentrated circulating solution was collected from the same membrane filtration apparatus as in Example 1 and divided into 20 L portions. Only one of them was treated with an aqueous solution of 0.2 mol / L silver nitrate as an antibacterial agent at a silver concentration of 5%.
ppm. 10 g of these are collected and 1
The solid content measured by drying at 05 ° C. for 3 hours was 1.09 wt%. The pH was 5.9.
Subsequently, the pH of the test solution to which the antibacterial agent was added was 5.9 after standing for 7 days while stirring at room temperature, but the test solution without the antibacterial agent increased to 6.9. The pH was readjusted to 5.9 with acetic acid. The test solution which was left for 7 days after the addition of the antibacterial agent and the 7
After standing for a day, the pH-adjusted test solution was charged into a membrane filtration device,
The change in the amount of filtrate was compared in the same manner as in Example 2. Table 3
As shown in (1), the test solution to which the antibacterial agent was added did not change at 297 ml / min after the passage of 6 hours. On the other hand, the initial filtrate volume of the test solution to which no antibacterial agent was added was 295 ml / min, and the filtrate volume after passing for 6 hours was 283 ml / min, and the ratio was 0.96. In addition,
After a lapse of 6 hours, the pH was 5.9 and remained unchanged. [Table 3] According to the present invention, when the wastewater discharged from the final washing step of the electrodeposition coating is treated with the membrane filtration device, the filtration capacity of the membrane filtration device is stabilized and the amount of acid used for pH adjustment. Can be saved.

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Claims (1)

Claims 1. A wastewater discharged from a final washing tank in a cationic electrodeposition coating process is applied to a membrane filtration device, separated into a filtrate and a concentrated solution, and the filtrate is circulated as washing water and the concentrated solution is used. A method for treating cation electrodeposition coating wastewater, comprising continuously or intermittently adding an antibacterial agent and / or a bactericide to the wastewater and / or the filtrate in a method of returning to the electrodeposition main tank and collecting and reusing it as a paint. .