Study on the Mechanism and Control Strategy of Advanced Treatment of Yeast Wastewater by Ozone Catalytic Oxidation

Abstract

In this paper, the yeast wastewater secondary treatment effluent using catalytic odor oxidation treatment, using an orthogonal reaction experiment to determine the best reaction conditions, and the online monitoring of the pH, oxidation-reduction potential (ORP), and liquid ozone concentration monitoring, to the catalytic odor oxidation reaction, chemical oxygen demand (COD), and color removal effect were analyzed. The results showed that the optimal reaction condition for the advanced treatment of yeast wastewater by catalytic ozonation was accomplished with manganese dioxide used as the catalyst and a catalyst dose of 6 g·L⁻¹, pH of 12, and catalytic ozonation reaction time of 20 min. The COD was effectively reduced from 880 mg·L⁻¹ to 387 mg·L⁻¹ under this condition, the chroma was reduced from 700 times to 40 times, and these two parameters of the effluent could meet the standard of GB25462-2010. The real-time monitoring system showed that the whole reaction can be divided into two processes. The first 14 min was the indirect reaction of ozone and then the direct oxidation reaction of ozone. This process was further verified by the change trend of COD and the amount of ozone depletion by COD removal. The average ozone consumption levels of the two stages were 1.97 and 4.91 mgO₃·mgCOD⁻¹. This system can effectively monitor the reaction of the catalytic odor oxidation in the complex system to guide the effective use of ozone in practical engineering applications.

1. Introduction

The production of yeast mainly uses molasses waste liquid in sugar production as a growth carbon source and sodium chloride, magnesium sulfate, and ammonium phosphate as nutrients to produce yeast. Residual organic matter and new organic matter produced in the growth and metabolism enter the wastewater, resulting in a large amount of high-concentration organic wastewater because yeast cannot fully utilize organic matter in waste molasses. Yeast wastewater is characterized by complex water quality, high pollutant concentration, high chromaticity, difficult-to-biodegrade substances, and high salt content. There are various methods of wastewater treatment [1,2,3], but yeast wastewater is usually treated by the secondary biological treatment process of “anaerobic + aerobic” and supplemented by chemical precipitation as an advanced treatment [4,5,6]. However, after the biological treatment with a hydraulic retention time of 5–7 days, the effluent chemical oxygen demand (COD) is still as high as 800–900 mg·L⁻¹, and after the coagulation and precipitation treatment with iron or aluminum salts, with a coagulant dosage as high as 1 g·L⁻¹, the effluent COD is still higher than 500 mg·L⁻¹, which cannot reach the standard for water pollutant discharge in the yeast industry (GB25462-2010). The COD is less than 400 mg·L⁻¹ [7,8]. The chroma in yeast wastewater is gradually accumulated by the chroma in the waste molasses in the process of sugar production and yeast fermentation [9,10]. It mainly consists of phenols and amino nitrogen compounds. The polyphenols are oxidized into brown pigment under the action of enzymes, and the polymer brown melanin is synthesized by the decomposition of the reducing sugar base [11,12]. Yeast wastewater is brown and black, with a chromaticity of about 6000 times when combined with the pigment of the above substances, such as the thermal decomposition of sugar into dark brown caramel pigment and the reaction of phenols with iron to produce dark compounds. In the biological treatment of yeast wastewater, the anaerobic process has a good chroma removal effect on the wastewater, the influent chroma is about 3000 times, the anaerobic effluent can generally reach 400–600 times, and the chroma removal rate is more than 80%. However, after the aerobic treatment, the chroma increases, and the aerobic effluent chroma is generally 600–800 times because at the anaerobic stage, and the chromogenic groups of some polyphenolic compounds are reduced but not completely decomposed into small molecules, which temporarily do not show color. At the aerobic stage, after the oxidation of these chromogenic groups, they restore the former chromogenic groups and show color again. Such polyphenolic compounds are biodegradable substances but are difficult to be completely decomposed and removed in the process of the biological system. Finally, it is discharged from the biological system with an aerobic effluent [13]. At present, the advanced treatment method of yeast wastewater is mainly a chemical method, which uses a flocculant for COD removal and decolorization treatment. Given the chemical precipitation method, the large amount of coagulant dosing and its resulting chemical sludge substantially increase the cost of yeast wastewater treatment [14].

Ozone is a strong oxidant, and its application in the field of wastewater treatment has been extensively studied. Ozone can change the structure of refractory substances in water and convert them into biodegradable substances, thus improving the biochemical properties of wastewater [15,16,17,18,19]. Using metal oxides as catalysts to form catalytic ozonation systems [20,21,22,23] or homogeneous catalytic ozonation systems formed by combining UV and hydrogen peroxide with ozone, respectively [24,25,26,27,28], both systems enhance ozonation by catalytic methods to produce hydroxyl radicals and other radicals with stronger oxidizing properties, thus improving the removal of refractory substances and chromaticity [29,30,31]. Ozone can destabilize particulate matter in water, and the use of ozone as a micro flocculation technique for pretreatment processes can achieve the goal of reducing the amounts of pharmaceuticals added to coagulation processes and sludge treatment and increasing the removal rate of coagulation processes [14,32,33]. Ozone or the use of free radicals with strong oxidizing properties generated by ozone can also directly react with toxic substances to remove or reduce the toxicity of wastewater [34,35,36,37], thus allowing biological treatment. Several scholars have conducted studies on the kinetics of ozone oxidation for various characteristic pollutants [38,39,40]. However, the above studies focus on wastewater with a relatively simple composition, which greatly reduces the efficiency of ozone oxidation due to the complex water quality of actual production wastewater and the complicated reaction processes between ozone and pollutants and intermediates. Exploring the ozone oxidation reaction in complex water quality systems and controlling it are important to achieve the efficient treatment of complex water quality systems. Although ozone can play an evident role in the treatment of all kinds of wastewater, the investment and operation costs of ozone equipment are very high. In China, except for disinfection with ozone, other fields of wastewater treatment are still in the research stage.

In this paper, the COD in the secondary treatment effluent of yeast wastewater is used as the targeted parameter, the optimal catalytic ozonation conditions are determined by orthogonal tests, and the ozonation monitoring system is established to study the catalytic ozonation reaction. The change law in the complex system and the reaction process is evaluated by the ozone utilization rate and other indicators. The chroma removal of wastewater during catalytic ozonation is also investigated, the catalytic ozonation reaction control strategy is proposed for the catalytic ozonation-based theoretical basis, and the control method for effective ozone addition for the industrial application of advanced treatment of yeast wastewater based on catalytic ozonation is provided.

2. Materials and Methods

2.1. Materials

Industrial-grade oxygen with a purity of 99.2%, aluminum trioxide, nickel trioxide, manganese dioxide, titanium dioxide, and analytical purity were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All solutions were prepared with deionized water, and the pH was adjusted with dilute sulfuric acid and sodium hydroxide solutions. The COD testing agent was Hash reagent 21259-25, with a range of 20–1500 mg·L⁻¹. Yeast wastewater was taken from the effluent of the secondary sedimentation tank of a yeast plant in Hebei (referred to as wastewater later), which has the characteristics of high concentration of organic pollutants, high salinity, high chromaticity, and complex water quality, and can be used as a representative of a complex system water quality. The main water quality indices are detailed in

Table 1. Composition of wastewater.

Num.	Parameter	Numerical Value	Unit
1	COD	880	mg·L−1
2	Biochemical oxygen demand (BOD5)	41	mg·L−1
3	Bicarbonate	182	mg·L−1
4	Carbonate	<0.10	mg·L−1
5	Total dissolved solids (TDS) (TDS)	13,300	mg·L−1
6	pH	8.10	-
7	Chroma	700	times

.

2.2. Main Reaction Device and Online Monitoring System

The experiment uses the device shown in Figure 1 for the catalytic ozonation of yeast wastewater. All experiments were carried out under normal temperature and pressure. The catalytic ozonation is completed in the ozone contact oxidation tower, which is a bubbling reactor with a height of 120 cm, an inner diameter of 5 cm, and a maximum volume of 2 L. The material is organic glass. The ozone in the experiment uses oxygen as the air source and is produced by an ozone generator. The ozone enters through the microporous aeration plate at the bottom of the ozone contact oxidation tower, and the exhaust gas is collected at the top of the ozone contact oxidation tower and discharged into a tail gas absorption bottle containing potassium iodide solution.

The catalyst used in the experimental process was in powder form, which can be mixed with the wastewater and pumped into the reactor to be mixed with the wastewater fully during the ozone aeration. The catalysts used in the experiments were chemically pure grade, and the details are shown in

Table 2. List of catalyst characteristics.

Catalyst Type	Molecular Weight	Purity Quotient
MnO2	86.94	≥85.0
Ni2O3	165.39	70.0–75.0
Al2O3	101.96	≥90.0
TiO2	79.87	≥98.0

.

In this paper, an online monitoring system is designed to monitor the parameters such as gas phase ozone concentration, pH, ORP, and liquid phase ozone concentration in and out of the contact oxidation tower during the reaction in real time at the second level and uploaded to the data acquisition system through signal conversion. In the reaction, the mixed liquid is pumped from the upper part of the contact tower into the constantator and the pH, ORP, and liquid phase ozone concentration are measured. At the same time, the other end of the constantator is pumped back to the bottom of the contact oxidation tower. Through the circulation, the online monitoring is realized, and the mixing effect is enhanced.

The ozone output of the ozone generator is 7 g·h⁻¹ at the oxygen source input, and the maximum ozone concentration is 25~50 g·m⁻³, with an adjustable ozone production concentration.

2.3. Analysis and Test Instruments

In this research, a UV–visible spectrophotometer (DR6000, HACH, Loveland, CO, USA) was used to analyze the COD concentration. Gas phase ozone concentration meter (Ozone Monitors UV-500, Guangzhou Limei, Guangzhou, China) online monitoring of ozone concentration in the contact tower was performed using liquid phase ozone concentration meter (CL7685, B&C Electronics Srl, Carnate, Italy) to monitor the ozone concentration in wastewater online. A pH/ORP instrument (PH7685, B&C Electronics Srl, Carnate, Italy) was used for online pH monitoring. Real-time water samples were collected with an SZ7263 three-hole constant-current device for the pH, ORP, and liquid phase ozone concentration, and a data acquisition module (FS-1208, MAC DAQ) was used for real-time data acquisition. Chroma was detected by the water-quality determination of colority dilution level method (HJ 1182-2021).

2.4. Data Acquisition Conversion

The online monitoring system configures the meter signal output as 4–20 mA current signal and the data acquisition module FS-1208 signal input as −10V/+10V. Therefore, one 500 Ω resistor is connected in series at each group of meter output to convert the current signal to voltage signal. The voltage data values collected by the module at different moments and the data values measured by the meter are counted. Five groups of data are randomly selected, the following functional relationship can be obtained. The function relationship between pH and voltage is y = 1.7792x − 3.7441 (R² = 1). The function relationship between ORP and voltage is y = 124.94x − 253.5 (R² = 1). The function relationship between liquid phase ozone concentration and voltage is y = 2.55x − 5.168 (R² = 0.9990). The function of inlet ozone concentration and voltage is y = 21.684x − 31.352 (R² = 0.9995). The function of outlet ozone concentration and voltage is y = 25.661x − 53.295 (R² = 0.9998). The sampling time interval of the module is 3 s, and the Trace DAQ software version 6.0 stores the voltage values collected during the measurement time period in the computer, which can be transformed by calculation to derive the values of the process parameters.

The calculation method of instantaneous ozone depletion is shown in Equation (1):

$$C=\frac{(c_{G1}-c_{G2})\times Q_G-c_L\times V_L}{V_L}$$

(1)

where C is the instantaneous ozone depletion, mg·L⁻¹; c_G1 is the ozone concentration at the inlet end, mg·L⁻¹; c_G2 is the ozone concentration at the outlet, mg·L⁻¹; Q_G is the gas flow rate, L·min⁻¹; c_L is the liquid phase ozone concentration, mg·L⁻¹; and V_L is the volume of liquid involved in the reaction, L.

3. Results and Discussions

3.1. Orthogonal Test Results and Optimization

To determine the best catalytic ozonation reaction conditions, the orthogonal test method is used. Given that ozone is spilled or dissolved in water during the reaction, the reaction process cannot precisely control the amount of ozone participating, and the reaction time is selected as the investigating factor instead of the ozone dosage in the test. The ozone reactor is charged with a mass concentration of 60 mg·L⁻¹ and a gas volume of 2 L·min⁻¹. The test setup considers four factors, namely, the type of catalyst, the ozone dosage, the pH of the reaction environment, and the catalyst dosage, and four levels of each factor are selected. The orthogonal test setup and its results are shown in

Table 3. Results of orthogonal experiment.

	Catalyst Type	Reaction Time (mg·L−1)	pH Value	Catalyst Dosage (g·L−1)	COD Value (mg·L−1)
1	MnO2	200	5	3	807
2	MnO2	300	8	6	682
3	MnO2	400	10	9	465
4	MnO2	500	12	12	387
5	Al2O3	200	8	9	703
6	Al2O3	300	5	12	763
7	Al2O3	400	12	3	508
8	Al2O3	500	10	6	467
9	Ni2O3	200	10	12	680
10	Ni2O3	300	12	9	647
11	Ni2O3	400	8	6	708
12	Ni2O3	500	5	3	743
13	TiO2	200	12	6	735
14	TiO2	300	10	3	689
15	TiO2	400	8	12	723
16	TiO2	500	5	9	730
K1	2341	2925	3043	2747	10,437
K2	2441	2781	2816	2592	-
K3	2778	2404	2301	2545	-
K4	2877	2327	2277	2553	-
k1	585	731	761	687	-
k2	610	695	704	648	-
k3	695	601	575	636	-
k4	719	582	569	638	-
R	134	130	192	51	-

.

Table 3 shows the factors affecting the COD removal efficiency in the advanced treatment of catalytic ozonation of yeast wastewater are ranked as pH > catalyst type > reaction time > catalyst dosage. Therefore, the optimal reaction conditions for catalytic ozonation in yeast wastewater advanced treatment are manganese dioxide as catalyst, catalyst dosage of 6 g·L⁻¹, pH of 12, and catalytic ozonation reaction time of 20 min.

3.2. Catalytic Ozonation Reaction Parameter Study

To observe the optimal catalytic ozonation determined by orthogonal experiment and determine the most economical and effective ozonation dosage, the comparative study of the experimental process is carried out based on the reaction conditions preferred by the orthogonal test, using deionized water as a control group to react with the variation of catalytic ozonation parameters. The monitored parameters are pH, ORP, and liquid phase ozone concentration.

3.2.1. pH

The initial pH of the reaction system with deionized water is 6.50, and as the reaction proceeds as the pH slowly increases to close to 7.00, after which it remains stable, as shown in Figure 2. This result may be because the deionized water dissolves some carbon dioxide in the air during the injection of the reaction tower, so a weakly acidic environment is formed in the water, which lowers the pH of the deionized water. However, with the ozone injection, the carbon dioxide in the water is blown off during the aeration, which leads to the increase in pH [41].

For the wastewater-based reaction system, NaOH is used to adjust the pH from 8.10 to 11.85 at the beginning of the experiment. In the first 16 min, pH decreases rapidly as the reaction proceeds until it decreases to 7.60 and then stabilizes. As the reaction proceeds, hydroxide ions trigger the decomposition of ozone, and the reaction is shown in Equation (2).

$${\mathrm{O}}_3+\mathrm{O}{\mathrm{H}}^{-}\to ·{\mathrm{O}}_2{}^{-}+·\mathrm{H}{\mathrm{O}}_2$$

(2)

The hydroxide ion in the water causes the decomposition of ozone, but the water also contains HCO₃⁻ alkalinity, which can capture ·OH. The reaction is shown in Equation (3):

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+·\mathrm{H}{\mathrm{O}}_2\to ·{\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{C}{\mathrm{O}}_3^{-}$$

(3)

In the initial process of the reaction, the pollutants in the water and the alkalinity of HCO₃⁻ consume the hydroxyl radicals produced continuously, accelerate the decomposition of ozone, and continuously consume the hydroxide ions in the water, which may be the main reason for the rapid decline of pH. From the change in pH, the whole reaction can be divided into two stages from the 16th min, the change in pH in the first stage decreases substantially from 11.85 to 7.60, and the latter stage stabilizes at 7.60. The main reason is that under the condition of this reaction, the hydroxide in the solution is largely consumed by the dissolved ozone, and the reaction should be mainly the indirect reaction of ozone. The pH of the reaction after 16 min does not change, so the reaction should be dominated by the direct reaction of ozone.

3.2.2. ORP

ORP is a comprehensive reflection of the concentration of various oxidizing substances and reducing substances that can have oxidation-reduction reactions in water and can be considered an ORP energy in water. The higher the concentration of oxidizing substances in water is, the higher the ORP [42].

The initial ORP of deionized water is 367 mV, which rapidly increases to the upper limit of measurement of 1000 mV with the addition of ozone, as shown in Figure 3. This result is probably due to the continuous production of hydroxyl radicals and dissolved ozone in the water [32].

No remarkable change is observed in the ORP during the first two minutes, which is probably due to the hydraulic delay from the beginning of the reaction to the data detected in the cross flower. Although the ORP increases at the fastest rate from the second minute to the sixth minute of the reaction, the delay is substantial compared with the reaction rate. The main reason may be that the water condition of the reactor is not good enough, resulting in poor mass transfer effect of ozone in water. In addition, the maximum ORP that can be reached in the deionized water system cannot be determined for the time being due to the limitation of measuring instrument conditions.

The wastewater comes from the secondary effluent with an initial ORP of −132 mV, and the increase in ORP is stable in the first 14 min. At this stage, the decomposition reaction of ozone and the direct reaction of pollutants coexist. Under the effect of hydroxyl and manganese dioxide catalytic ozonation, the hydroxyl free radical chain reaction of ozone dissolves in the water. A part of the direct reaction of ozone and the pollutants in the water are in the water reducing agents, and the oxidizing substance increases. The system ORP increases gradually after 14 min of rapid ascension, and then the change flattens. The maximum ORP during the reaction is 857 mV. The ORP of the system in the whole reaction can be considered the result of the superposition of oxidizing substances and reducing substances. The failure to reach the maximum value as in the deionized water comparison test indicates that substances in the system at this stage cannot be oxidized by ozone and hydroxyl radicals.

3.2.3. Liquid Phase Ozone Concentration

The change in ozone dissolved in water is shown in Figure 4. When ozone is added to deionized water, the concentration of ozone in water rapidly increases from 0 to 8.20 mg·L⁻¹. In the first 6 min, the liquid phase ozone concentration increases rapidly and then reaches a stable state. This result is consistent with the trend of ORP.

However, in the catalytic ozonation of wastewater, no dissolved ozone is in the early stage of the reaction, indicating that the ozone dissolved in wastewater reacts rapidly with pollutants, catalysts, and hydroxide. After 12 min of reaction, liquid phase ozone concentrations begin to appear and slowly increase, leveling off at about 1.50 mg·L⁻¹ from the 16th minute. As the level of liquid ozone does not reach 8 mg·L⁻¹ in the comparison test, substances that consume liquid ozone still exist in the system. The reaction enters a slow process, which may be due to the decrease in the concentration of pollutants that can react directly with ozone in the system; the hydroxyl radical is produced by ozone and the catalyst, and the reaction between hydroxyl radical and pollutant plays a major role [43,44].

3.3. Study on Ozone Consumption and COD Removal Effect

The online monitoring and data conversion system can be used to obtain the change in ozone consumption in the reaction through Equation (1), as shown in Figure 5. The overall change trends of COD and ozone consumption in the reaction are similar, and the overall trend shows a gradual decline with the reaction. At the beginning of the catalytic ozonation reaction, the ozone consumption increases sharply, with a maximum value of 69 mg·L⁻¹, and gradually decreases with the progress of the reaction, with a minimum value of 36 mg·L⁻¹. At the early stage of the reaction, the direct reaction and decomposition reaction of ozone with pollutants are carried out synchronously, and the consumption of ozone is large. At the later stage of the reaction, the direct reaction of ozone and the decomposition reaction with hydroxide are weakened, and the ozone consumed by the catalytic reaction is the main ozone consumption. The COD in the wastewater gradually decreases, the decrease trend becomes smooth after 14 min, and the minimum value is 387 mg·L⁻¹, which meet the requirement of the Yeast Industry Water Pollutant Discharge Standard (GB25462-2010) that the COD is less than 400 mg·L⁻¹.

Every 2 min in the reaction is taken as a statistical stage, and the catalytic ozonation is divided into 10 stages on average. The utilization rate of ozone in each stage is shown in Figure 6. When the reaction time is less than 15 min, the utilization rate of ozone in the catalytic ozonation is very high. The average value of the ozone consumed by removing COD per unit mass is 1.97 mgO₃·mgCOD⁻¹. In the last five minutes of the reaction, the average value of ozone consumed by COD degradation is 4.91 mgO₃·mgCOD⁻¹.

3.4. Study on Ozone Consumption and Color Removal Effect

The orthogonal test finds that each catalytic ozonation test has a very evident chroma removal effect, the effluent chroma can reach less than 100 times, and the chroma removal rate can reach 85%. Among them, the three groups numbered 3, 4, and 10 have the best effluent color effect, which can reach 40 times, which is better than the discharge requirement of 50 times in “Discharge Standard of Water Pollutants for Yeast Industry” GB25462-2010. The pigment formed by saccharine reaction (Maillard reaction) during yeast fermentation and caramel pigment added artificially during product preparation are the main reasons for the high color of yeast wastewater. Both substances are mixtures of complex macromolecular compounds with conjugated double bonds. Ozone can react with benzene rings, double bonds, and co-yoked carbonyl groups in the chromogenic group. Moreover, due to the excellent properties of dehydrogenation, oxidation, and isomerization of transition metals, under the synergistic action of ozone, the chromogenic group can be destroyed to achieve the decolorization effect.

Figure 7 shows that the removal effect of chroma is clearly better than that of COD, but their change trends are the same. Moreover, the substance causing the chroma contributes to the measurement of COD. However, the degradation of pigment substances is only the change in molecular structure and has not been completely removed from the water as pollutants.

3.5. Comprehensive Analysis of Control Strategy for Process Monitoring Data

The whole reaction is divided into two stages from the 14th minute. At the first stage, the COD removal is 450 mg·L⁻¹, accounting for 91.28% of the total removal rate. No liquid phase ozone is in the system all the time, the pH decreases, and the reoxidation potential increases gradually. The average ozone consumption is about 1.97 mg·L⁻¹. At this stage, the pH decreases steadily, and ORP gradually increases, but no liquid phase ozone appears. According to Equation (3), dissolved ozone continuously reacts with hydroxide ions and generates hydroxyl radicals. Hydroxyl radicals constantly react with pollutants. At this stage, the gradual increase in ORP can imply that reducing substances are constantly removed by oxidizing groups such as hydroxyl radicals generated in the system.

In the second stage, the most apparent sign is the emergence of liquid phase ozone, and the ORP value substantially increases. The main reason for liquid phase ozone is that the hydroxide ions added to the system are consumed. At this stage, the reaction of the system is mainly the direct reaction of ozone, and the removal effect of COD decreases, which also leads to the decrease in the utilization rate of ozone. The reason why liquid phase ozone cannot reach higher levels as in deionized water is that most ozone is still directly reacting with pollutants. At the 18th minute, the liquid phase ozone concentration in the wastewater system is about 1.41 mg·L⁻¹. Compared with the deionized water system, the corresponding time point of this liquid phase ozone concentration is 30 s at the second minute. The ORP values corresponding to these two moments are 790 and 481 mV. Therefore, the ORP at this time is only 1.5 mg·L⁻¹ liquid phase ozone and part of the oxidizing-free group.

3.6. Analysis of Control Strategy for Catalytic Ozonation

Compared with direct oxidation of ozone, catalytic ozonation has noticeable advantages in the COD removal effect [45]. The effect of indirect catalytic ozonation is more evident under the experimental conditions determined by orthogonal reaction. The real-time monitoring of pH, ORP, and dissolved ozone in the process finds that with the change in COD, the process parameters also change regularly. From the point of view of the change in reaction parameters, the whole reaction can be divided into two stages, the rapid change stage and stationary stage. At the rapid change stage, the pH decreases gradually, and the system ORP and the liquid phase ozone concentration are low. At the stationary phase, pH is low, and the ORP and liquid phase ozone concentration are high. From the perspective of time points, the pH is flat at the 16th minute, the ORP reaches the maximum at the 16th minute, and the liquid phase ozone concentration approaches the maximum at the 16th minute. Therefore, the 16th minute can be used as the cut-off point of the reaction.

In terms of the removal effect, for the catalytic ozonation reaction, the reaction between ozone and wastewater can also be divided into two stages of high efficiency and low efficiency. The 15th minute is the cut-off point. Before the cut-off point, the ozone utilization rate is high, and the COD removal effect is apparent. After the cut-off point, the ozone utilization rate decreases, and the COD removal is slow, only from 436 mg·L⁻¹ to 387 mg·L⁻¹.

In the catalytic ozonation, the efficient period of COD degradation is mainly composed of the direct and indirect reaction of ozone. The indirect process includes metal oxide catalyst and hydroxy joint action, and the reaction pH change is most evident. During this period, liquid ozone concentration is not detected, which suggests that the process is the quick response of ozone. In the second stage, the removal of COD is mainly via the indirect reaction of ozone, and the concentration of liquid phase ozone appears, which is in line with the characteristics of a slow reaction of ozone oxidation. During this period, the ORP and liquid phase ozone concentration reach the maximum value, whereas the pH reaches the minimum value.

In the advanced treatment of yeast wastewater by catalytic ozonation technology, the online monitoring system is used to monitor the pH, ORP, and liquid phase ozone concentration in the reaction in real time. The liquid phase ozone concentration can be used as a marker to judge the rapid reaction stage and slow reaction stage of the ozonation reaction. The ORP can assist in characterizing the degree of wastewater COD degradation. The change in pH fluctuates greatly in this experiment, reflecting the degree of ozonation reaction from one aspect. However, in the reaction without regulating the pH, the fluctuation range is relatively small.

In the catalytic ozonation reaction with pH adjustment, when the changes in these three process parameters are flat, the efficient period of ozone reaction has ended, and the continued addition of ozone is not conducive to the economic efficiency of yeast wastewater treatment.

The catalytic ozonation has a clear chroma removal effect. When the experimental conditions achieve the best COD removal effect, the effluent chroma can also achieve the standard discharge. Therefore, it is not a focus of research.

4. Conclusions

In this experiment, the effect of heterogeneous catalytic ozonation is strengthened by adjusting the pH. Manganese dioxide is used as a solid catalyst to treat yeast wastewater effectively and deeply. The effluent COD decreases from 880 mg·L⁻¹ to 387 mg·L⁻¹, and the removal rate reaches 56%. Under the same test conditions, the chroma is reduced from 700 times to 40 times. The effluent COD can meet the requirement of COD less than 400 mg·L⁻¹, and the effluent chroma can achieve 40 times in current industry discharge standards.

The online monitoring system designed in the experiment can reflect the changes in process parameters such as pH, ORP, and liquid phase ozone concentration in time. The catalytic ozonation can be marked as a high-efficiency stage and low-efficiency stage by using the changes in process parameters above. The high-efficiency stage lasts for 15 min. The COD removal rate is 50.50%, and the average ozone consumed by removing the unit mass of the COD is 1.97 mgO₃·mgCOD⁻¹. The low-efficiency stage lasts for 5 min, the COD removal rate is 11.20%, and the average ozone consumption of removing the unit mass of the COD is 4.91 mgO₃·mgCOD⁻¹.

Under the synergistic action of the catalyst, regulating the system pH can effectively promote the indirect ozone oxidation reaction. The device inside the constant current parameter change slightly lags behind the actual data of the contact oxidation tower internal instant levels, but the overall trend is consistent with the reactor internals. During the indirect reaction, the decreasing trend in pH and the redox potential are stable. When comparing the reaction conditions of catalytic odor, the treatment effect of catalytic odor in a complex system can be investigated by the rate of change in these two indices, which can be verified by the removal of pollutants. In practical application, the economy of ozone use can be maintained by the index of liquid phase ozone concentration. The process determined in this experiment is better than the current coagulation and precipitation in terms of hydraulic residence time, agent injection cost, and subsequent treatment of waste. In practical engineering, the operation cost of ozone equipment investment and construction and the use of ozone is higher than the cost of coagulation and precipitation, but through advanced, effective monitoring means and optimized reaction conditions, it is bound to improve the effective utilization and continuous promotion of ozone.