Ozone Catalytic Reactor

Printing and dyeing wastewater is characterized by large volume, complex composition, high content of organic pollutants, and poor biodegradability, making it difficult to treat using conventional biological methods. In recent years, with the rapid development of the textile printing and dyeing industry, the discharge of new dyes, auxiliaries, and other organic substances that are difficult to biodegrade has further increased the difficulty of treating printing and dyeing wastewater. To address the pollution issues of printing and dyeing wastewater, it is imperative to seek an economical and efficient treatment technology.

Ozone, as a highly efficient oxidant, has advantages such as strong oxidizing ability, mild reaction conditions, and no secondary pollution. However, ozone oxidation alone has drawbacks such as strong selectivity, low utilization rate, and high operational costs. Studies have shown that by adding a certain catalyst, ozone can generate hydroxyl radicals (?OH) with extremely strong oxidizing power. ?OH can non-selectively mineralize organic matter in water and break down complex, toxic macromolecular organic compounds through chain scission, ring-opening, and other reactions, converting them into simple, non-toxic, or low-toxicity small molecular compounds, with a relatively fast reaction rate.

This experiment uses the secondary biological treatment effluent from the pilot-scale printing and dyeing wastewater of our company as the research object. Based on the CODcr removal rate (hereinafter referred to as COD) and the specific ozone consumption (the ratio of ozone mass consumed to the chemical oxygen demand removed in the treated water within a certain time, hereinafter referred to as the specific ozone rate R), the impact of ozone oxidation alone and catalytic oxidation advanced treatment on water quality is discussed, providing a reference for the production application of ozone catalytic oxidation advanced treatment of printing and dyeing wastewater.

1. Experimental Section

1.1 Experimental Materials

The influent for ozone catalytic oxidation treatment is the effluent from the secondary sedimentation tank of the pilot-scale biological treatment of printing and dyeing wastewater in our company, with a COD of 90–120 mg/L and a pH of 8–10.

COD measurement was performed using the rapid determination method, with the instrument model: Lianhua Technology 5B-1.

Other reagents: COD rapid determination reagent D, COD rapid determination reagent E, 20 g/L KI solution, 0.1 mol/L Na₂S₂O₃ solution, 30% H₂O₂ (analytical grade).

1.2 Experimental Setup and Process Flow
The ozone reactor has a rated production rate of 100 g/h. The ozone reactor uses a packed reaction column with a height of 150 cm, a diameter of 75 cm, and an effective treatment volume of 6 L. A titanium alloy microporous aerator is used for gas distribution.

The process flow is as follows: A packed column filled with printing and dyeing wastewater is used for closed intermittent experiments. Using an oxygen source, a certain amount of ozone is introduced through an ozone generator. By controlling different reaction times and catalyst dosages, the COD of the effluent at different reaction times is measured. Unreacted ozone is absorbed by a KI solution, and the ozone content in the tail gas is determined by the iodometric method. The actual reacted ozone is quantitatively calculated by comparing the input ozone with the ozone absorbed from the tail gas.

2. Results and Discussion

2.1 Ozone Reactor Production Rate

The ozone production rate was measured using the iodometric method. The principle is: the quantitative relationship O₃ → 2Na₂S₂O₃ can be derived. Based on the volume of Na₂S₂O₃ consumed, the ozone production rate can be calculated.

As shown in Figure 2, at oxygen inlet flow rates of 0.1 m³/h and 0.2 m³/h, the ozone production rate increases with power. Under the same power conditions, a higher oxygen inlet flow rate results in a relatively higher ozone production rate. Since the ozone generator used in this experiment has a high power rating of 1 kW and an ozone production rate of 100 g/h, considering the experimental requirements, economic rationality, and production stability, the power of the ozone reactor was set at 100 W, with an oxygen flow rate of 0.1 m³/h. Under these conditions, the ozone production rate is 50 mg/L. After five parallel tests, the relative error of the production rate was within 5%.

2.2 Analysis of Ozone Oxidation Alone Treatment

With the ozone dosage set at 50 mg/L, the impact of different ozone reaction times on effluent quality was analyzed.
As shown in Table 1, when the influent COD is controlled at 100 mg/L, ozone oxidation treatment can reduce the COD to below 60 mg/L (specifically 48 mg/L) within 10 minutes, fully meeting the water pollutant discharge limits for new enterprises in the textile dyeing and finishing industry. At this point, the COD removal rate reaches 52%, meeting the expected requirements of this experiment.

As shown in Figure 3, both the COD removal rate and the specific ozone rate R increase with reaction time. When the reaction time is 10 minutes, the COD removal rate reaches 52%, and the specific ozone rate is 2.14. Clearly, as the reaction time increases, the specific ozone rate increases, and the ozone treatment efficiency relatively decreases. At 5 minutes, the specific ozone rate is at its minimum of 1.65, indicating relatively better ozone treatment efficiency.

2.3 Analysis of Ozone Catalytic Oxidation Treatment

30% (wt%) H₂O₂ was added as a catalyst to the ozone reaction tower at dosages of 0.05, 0.10, 0.15, and 0.20 mL/L. The impact of reaction time on effluent quality is shown in Tables 2–5 and Figures 4–7.

2.3.1 0.05 mL/L

As shown in Table 2, when 0.05 mL/L H₂O₂ is added as a catalyst, the treatment effect is not significantly improved compared to ozone oxidation alone; in fact, the treatment effect is slightly reduced. After 20 minutes, the COD remains at 72 mg/L, with a removal rate of 46%. As shown in Figure 4, under the catalysis of 0.05 mL/L H₂O₂, both the COD removal rate and the specific ozone rate increase with reaction time. Compared to ozone oxidation alone, after adding 0.05 mL/L H₂O₂, the specific ozone rate slightly decreases. At 5 minutes, the specific ozone rate is 1.25, which is lower than the 1.67 under ozone-alone conditions, indicating some improvement in ozone utilization efficiency. The decline in COD removal effect may be related to the influent quality, as the influent COD reached 133 mg/L at this time, exceeding the expected limit of 120 mg/L for this experiment.

2.3.2 0.10 mL/L

As shown in Table 3, when 0.10 mL/L H₂O₂ is added as a catalyst, the treatment effect is quite significant. Within 5 minutes, the COD is reduced to 47 mg/L, with a removal rate of 49%. At 10 minutes, the COD is further reduced to 42 mg/L, with a removal rate of 55%, fully meeting the treatment requirements. As shown in Figure 3, compared to ozone oxidation alone, both the COD removal rate and the specific ozone rate are improved. At 5 minutes, the specific ozone rate reaches 0.92, indicating a significant improvement in ozone utilization.

2.3.3 0.15 mL/L

As shown in Table 4, when 0.15 mL/L H₂O₂ is added as a catalyst, the treatment effect is significantly improved. At 10 minutes, the COD is reduced to 36 mg/L, with a removal rate of 65%, fully meeting the treatment requirements. As shown in Figure 6, compared to ozone oxidation alone, both the COD removal rate and the specific ozone rate are improved. At 5 minutes, the specific ozone rate reaches 0.94.

2.3.4 0.20 mL/L

As shown in Table 5, when 0.20 mL/L H₂O₂ is added as a catalyst, the treatment effect is ideal. At 10 minutes, the COD is reduced to 31 mg/L, with a removal rate of 70%, fully meeting the treatment requirements. As shown in Figure 7, compared to ozone oxidation alone, both the COD removal rate and the specific ozone rate are improved. At 5 minutes, the specific ozone rate reaches 0.99.

2.3.5 Impact of Different H₂O₂ Catalyst Dosages on Effluent COD Removal Rate and Specific Ozone Rate

As shown in Table 6 and Figure 8, under both ozone-alone and H₂O₂-catalyzed conditions, the effluent COD removal rate increases with reaction time. When the influent quality meets the requirements (COD less than 120 mg/L), after 10 minutes of treatment, the COD removal rate can reach 50%, meeting the expected treatment requirements of this experiment. Adding H₂O₂ as a catalyst significantly improves the effluent COD removal rate, and the removal rate is proportional to the H₂O₂ dosage.

Under both ozone-alone and H₂O₂-catalyzed conditions, the specific ozone rate increases with reaction time, meaning that as the reaction time extends, ozone utilization efficiency continues to decline. Overall, as the H₂O₂ dosage increases, the specific ozone rate generally decreases, reaching its lowest point at an H₂O₂ dosage of 0.15 mL/L, indicating the highest ozone utilization efficiency during the treatment process. As the H₂O₂ dosage further increases, the specific ozone rate begins to rise, and ozone utilization efficiency slightly decreases. When the H₂O₂ dosage is 0.10 mL/L and the reaction time is 5 minutes, the specific ozone rate R reaches 0.92, indicating the highest ozone utilization efficiency. Under these conditions, the COD removal rate reaches 49%, basically meeting the experimental requirements, with relatively minimal consumption of ozone and H₂O₂, making it the most economical treatment condition.

3. Summary

(1) In the ozone reactor, the ozone production rate increases with power. Under the same power conditions, a higher oxygen inlet flow rate results in a relatively higher ozone production rate. In this experiment, considering the experimental requirements, economy, and stability, the power was set at 100 W, with an oxygen flow rate of 0.1 m³/L, resulting in an ozone production rate of 50 mg/L.

(2) When using ozone oxidation alone for treatment, the reaction can reduce the COD to below 60 mg/L (specifically 48 mg/L) within 10 minutes, with a COD removal rate of 52%. However, the specific ozone consumption rate at this point is 2.14, indicating relatively low ozone utilization efficiency.

(3) Adding H₂O₂ as a catalyst significantly improves the effluent COD removal rate, and the removal rate is proportional to the H₂O₂ dosage.

(4) Under both ozone-alone and H₂O₂-catalyzed conditions, the specific ozone rate increases with reaction time. Overall, as the H₂O₂ dosage increases, the specific ozone rate generally decreases, and ozone utilization efficiency improves. When the H₂O₂ dosage is 0.10 mL/L and the reaction time is 5 minutes, the specific ozone rate R reaches 0.92, indicating the highest relative ozone utilization efficiency. Under these conditions, the COD removal rate reaches 49%, basically meeting the experimental requirements, with relatively minimal consumption of ozone and H₂O₂, making it the most economical treatment condition.