||In this study, the wastewater generated from etching process of the Printed Circuit Board (PCB) was treated by a process including both acidification and coagulation/sedimentation and then followed by the catalytic wet air oxidation (CWAO) over different catalysts (either Pt/SiO2·Al2O3 or Pt·X/γ-Al2O3) process in series. Although the initial chemical oxygen demand (COD) concentration of the wastewater is as high as 7740-12700 mg/L, the effluent of the pretreatment process was measured to have COD value in ranges of 3050-4260 mg/L. Several re-action parameters, such as reaction temperatures (200-260℃), oxygen partial pressures (0-3 MPa), and two kinds of catalysts were performed experimentally to investigate the COD reduction of the wastewater during the CWAO process. Both reaction temperature and variety of catalyst are found most effectively on the COD reduction. However, the effect of oxygen partial pressure on the COD reduction is just in little. Results showed that the COD reduction during the CWAO over the Pt·X/γ-Al2O3 catalyst process is the most significant, which with a tow-step re-action and both the two reactions do obey first-order reaction kinetics. A change from a higher reaction activity of the CWAO reaction to a slower one implies a decrease of the reaction rate. |
On basis of our experiments data, the effective operating conditions of CWAO for the COD reduction was observed to be at temperature of 260℃ under oxygen partial pressure of 2.0 MPa and at a retention time period of 60 min. The COD conversion was calculated as high as 75%; however, it could be enhanced up to 78% and 91%, respectively, when the CWAO was conducted in presence of the Pt/SiO2·Al2O3 and Pt·X/γ-Al2O3 catalysts, respectively. It can be seen that the organic compound of the wastewater was mineralized most completely (with a COD/TOC ratio of 3.7±0.2) after the CWAO over the Pt·X/γ-Al2O3 catalyst process. Furthermore, a higher COD/TOC ratio of 3.9±0.3 was achieved when the Pt/SiO2·Al2O3 catalyst was in presence of the CWAO process, and the primitive WAO process had the highest COD/TOC ratio of 4.8±0.4. The experimental data showed that both a higher reaction temperature (≧260℃) and an application of catalyst are more important factors for the min-eralization of the organic compound of the wastewater during the CWAO process.
In our investigation, BOD5/COD ratio has been used to assess if the WAO and/or the CWAO process treatment yield products more amenable to biodegradation. The BOD5/COD ratio was 0.68-0.93 when the reaction temperature was above 220℃ and the retention time was as long as 60 min. Unfortunately, the BOD5/COD ratio of the effluent from the CWAO process came out a lower value (0.45-0.65) though it was under the same reaction conditions. It is probable that the biodegradable portion of the organic compounds of the wastewater were decomposed easier during the CWAO process than during the WAO process. In addition, it was found that the products of the wastewater was decomposed partially into CO2 and into some low molecular weigh acids, such as formic acid, acetic acid, propionic acid, etc.
The activation energy with respect to COD was calculated to be 38.42 kJ/mole and 83 kJ/mole, respectively, for the first-step reaction and for the second-step reaction, respectively, of the WAO process. It was al-so calculated that the first-step reaction of the CWAO over the Pt/SiO2·Al2O3 catalyst process has activation energy of 18.25 kJ/mole and 25.76 kJ/mole is for the second-step reaction. However, 16.05 kJ/mole and 49.61 kJ/mole are calculated for the first-step and the sec-ond-step reactions, respectively, of the CWAO over the Pt·X/γ-Al2O3 catalyst process. It can be seen that the application of both the Pt/SiO2·Al2O3 and the Pt·X/γ-Al2O3 catalysts has a significant effect on reducing the activation energy of the WAO.
It was observed that the total COD conversion of the wastewater is as high as 96% and the BOD5/COD ratio of the effluent has been en-hanced up to more than 0.6. The combination of both the CWAO over the Pt·X/γ-Al2O3 catalyst and the biological treatment is a promising tech-nique for the PCB’s wastewater treatment to fit the wastewater control regulation in Taiwan, which requests the COD value of the wastewater discharged should be less than 120 mg/L.