Separations 2023 , 10 , 148
9of 15
3.4. Water Quality Indicators under Optimal Process Conditions The pulp and paper mill wastewater was treated under the experimental conditions determined according to the single-factor experiment; the conditions were a reaction time of 8 min, an ozone concentration of 16 mg/L, a pH 9, and a catalyst filling ratio of 7.5%. Table 2 shows the water quality indicators of wastewater before and after treatment. It can be seen that the BOD 5 /COD(B/C) value increases from 0.26 to 0.33 after ozonation treatment. In contrast, the B/C value is 0.37 after catalytic ozonation treatment, which means that catalytic ozonation treatment can improve biodegradability more effectively. Furthermore, the contents of NH 3 -N, TN, and TP change slightly after different treatments, which can be inferred that these treatments are insufficient to effectively remove NH 3 -N, TN, and TP. However, ozonation treatment and catalytic ozonation treatment have good reduction effects on the chromaticity and SS (suspended solid). After ozonation treatment, chromaticity decreases from 3481 to 184, and after catalytic ozonation treatment, chroma decreases from 3481 to 153. Compared with ozonation, the reduction efficiency of COD and SS by catalytic ozonation is better, and the reduction efficiency of COD and SS by catalytic ozonation reaches 52% and 65%, respectively.
Table2. Water quality indicators of wastewater before and after treatment.
COD mg/L B/C NH 3 -N mg/L TN mg/L TN mg/L Chromaticity SS mg/L
Wastewater
953 715 453
0.26 0.33 0.37
24.23 23.05 25.03
13.55 12.31 11.97
0.61 0.63 0.67
3841
2439.4
Ozone oxidation
184 153
982.2 849.4
Ozone-catalyzed oxidation
In general, ozone treatment cannot effectively remove nitrogen and phosphorus, but the reduction of COD, chroma, and SS is efficient. 3.5. FT-IR Analysis of Effluent Water Quality in Different Systems Figure 7 shows the Fourier transform infrared spectra measured before and after the treatment of the ozonation system and catalytic ozonation system. The absorption peak at 616 cm-1 may be due to the presence of halides [25]. The absorption peak at 1116 cm − 1 represents the O-H in-plane bending vibration of phenols or alcohols and the C-O-C stretching vibration of ethers. The absorption peaks at 1465 cm − 1 and1567 cm − 1 are caused by the C=C stretching vibration on the benzene ring [26]. The peaks generated at 1410 cm-1 and 1663 cm − 1 represent the presence of amide in the water, which may come from the printing ink residue in the paper mill wastewater. The absorption peak at 1796 cm − 1 can be inferred as the characteristic peak of the carbonyl group, which is the primary source of chromaticity of paper mill wastewater. Furthermore, it can be seen from Figure 7 that the absorption peak here disappears after treatment, which also explains the reduction of the chroma of wastewater. A broadband and strong absorption spectrum of O-H stretching vibration at 3450 cm − 1 indicate that the raw water may contain carboxylic acids, phenols and alcohols [27]. The Figure 7 shows the peak formed by the C-H stretching vibration of methyl at 2965 cm − 1 [28]. Most of the absorption peaks of the samples treated by initial ozonation and catalytic ozonation are the same. After being treated by the ozonation system and the catalytic ozonation system, their absorption peak intensities are reduced, which indicates that they have a good effect on removing organic matter from papermaking wastewater. The absorption peak intensity of wastewater treated by catalytic ozonation at 3450 cm − 1 , 1410 cm − 1 , 1465 cm − 1 , 1567 cm − 1 , and 1663 cm − 1 decreases more than that of wastewater treated by ozonation, indicating that catalytic ozonation has a relatively strong degradation effect on organic substances in papermaking wastewater. In addition, a new benzene ring substitution peak was formed at 785 cm − 1 , indicating that new aromatic organic compounds were produced during ozonation.
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