Introduction to phenolic wastewater treatment processes
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Introduction to the treatment process for phenolic wastewater – Treatment of phenolic resin wastewater. Industrial wastewater containing phenols is one of the harmful types of wastewater that requires focused treatment in China’s water pollution control efforts. Wastewater generated from phenolic resin production contains high concentrations of organic substances such as phenols, aldehydes, and resins, and it is characterized by high organic content, high toxicity, and a low pH value. There are many process methods for treating phenol-containing wastewater. Widely used ones include biochemical methods, chemical oxidation methods, extraction methods, adsorption methods, and air stripping methods. In recent years, new methods such as catalytic oxidation, liquid membrane separation, synergistic complexation extraction, and magnetized flocculation oxidation have emerged. However, in the actual treatment of phenol-containing wastewater, especially to meet discharge standards, biochemical methods remain the dominant approach. This paper employs a phenol-formaldehyde polycondensation–two-stage biological oxidation method to treat high-concentration wastewater from phenol-formaldehyde resin production, in order to recover valuable components. 1 Test Section 1.1 Wastewater Quality The wastewater is derived from the phenolic resin production process of a certain resin manufacturing company; the main organic pollutants are phenol, formaldehyde, and resin. The water quality is shown in Table 1. 1.2 Experimental Methods and Apparatus The process flow for wastewater treatment is as follows: During the polycondensation experiment, wastewater along with concentrated hydrochloric acid equivalent to 10% of the wastewater volume was added to multi-port flasks; these flasks were then placed in a constant-temperature heater for heating, with condensers used to facilitate reflux. The high-concentration phenols and aldehydes remaining in the wastewater reacted under the catalysis of the acid to form low-polymerization-degree phenolic resins. After the reaction is complete, let it stand to cool and separate into layers. During industrial testing, wastewater and concentrated hydrochloric acid equivalent to 10% of the wastewater volume were added to a 1 m3 reactor. Using the excess steam from the factory, the mixture was first refluxed at 100°C for 6–8 hours, then held at 80–90°C for 5–6 hours. After that, it was allowed to cool down naturally and the layers were separated. The device is shown in Figure 1. The aqueous layer after polycondensation is fed into the biochemical system, while the recovered resin is dried and then mixed back into the raw materials for reuse. Biochemical tests are carried out in biochemical reactors, which are of a two-stage series configuration and equipped with semi-soft packing. Reactor 1 has dimensions of φ90 mm×1800 mm, with an effective volume of 9 L ； Reactor 2 has dimensions of L450×W300, with an effective volume of 15L. The wastewater discharged after polycondensation recovery is neutralized and diluted before being fed into the biochemical reactor. After the filler had developed a biofilm, microorganisms were cultivated and acclimated in batches by gradually increasing the CODCr and phenol loads. The CODCr concentration in the influent water was set at 500–800 mg/L, with the CODCr removal rate for each batch of wastewater maintained above 80%, in order to examine the CODCr removal efficiency under an HRT of 12 hours. As the proportion of phenol-containing wastewater in the feed water increased and the phenol concentration rose, the CODCr removal rate at an HRT of 12 hours fluctuated, but remained within the range of 65% to 90%. After 4 days, the feed water is switched to diluted phenolic resin production wastewater. The following methods are used to analyze each parameter: CODCr: Rapid K2Cr2O7–FAS titration method ； Volatile phenols: bromination titration method ； pH: pH—W type acidity meter ； Chromaticity: Standard dilution method ； BOD: Standard dilution method ； Microbial biomass: gravimetric method. 2 Test Results and Discussion 2.1 Polycondensation Recovery In the laboratory, at a heating temperature of 100°C, the relationship between reflux time and the efficiency of removing volatile phenols is shown in Figure 2. The removal rate of volatile phenols increases as the reflux time prolongs. It changes rapidly between 2 and 14 hours, while the rate shows a slower change after 4 hours. The reflux time was set at 4 hours, with different reflux temperatures (when the temperature is <100°C, it is equivalent to maintaining a constant temperature); the effect of this on the removal rate of volatile phenols is shown in Figure 3. The results show that the removal rate of volatile phenols increases as the heating temperature rises; when the heating temperature is below 90°C, the removal rate of volatile phenols is low, generally being above 40% ； At a heating temperature of 100°C, the removal rate of volatile phenols increases rapidly, reaching 70.5%. The results of the tests using the production unit are shown in Table 2. In the tests of the production unit, a phenol recovery rate of 90% or higher was achieved, which may be related to heating and retention time.
2.2 Batch operation of biochemical tests The effluent obtained after polycondensation recovery is neutralized and diluted to a CODCr level of 800–1500 mg/L before being fed into the biochemical reactor; the batch operation lasts for 8 days in total. The degradation of organic matter after 12 hours of HRT for each batch is shown in Figure 4. When the CODCr concentration in the influent increased from 863 mg/L to 1338 mg/L, the CODCr removal rate decreased from 86.1% to 76.4%. For the subsequent three batches, the CODCr concentration in the influent remained stable at 1400 mg/L, and the CODCr removal rate gradually increased to 96.2% ； The concentration of volatile phenols in the influent water increased from 59.8 mg/L to 664 mg/L, and no inhibition in the degradation of CODCr and volatile phenols within the system was observed; the removal rate of volatile phenols reached 98.1%. The typical degradation curve in the later stage of batch operation is shown in Figure 5. With an inlet CODCr of 1413 mg/L and volatile phenols at 689 mg/L, organic matter is degraded rapidly at an HRT of 0–10 hours; when the HRT is 10 hours, the removal rates for CODCr and volatile phenols are 91.5% and 91.3%, respectively ； After 12 hours of HRT, the CODCr in the wastewater tends to stabilize, while volatile phenols continue to degrade slowly ； At 24 hours of HRT, the removal rates of CODCr and volatile phenols reached 97.2% and 99.2%, respectively. The degradation curves of CODCr and volatile phenols have similar shapes, and their removal rate curves almost overlap, indicating that in wastewater, volatile phenols are the dominant component of the substrate degraded by microorganisms. The degradation of organic matter in wastewater is clearly divided into two stages: the first stage is the rapid degradation phase, and the second stage is the slow degradation phase. 2.3 Continuous-flow operation of biochemical tests Continuous-flow operation employs two-stage series-connected biochemical reactors; the operating parameters are shown in Table 3, while the results are presented in Figure 6. In the first paragraph, CODCr and volatile phenols degrade rapidly ； In the second stage, CODCr and volatile phenols degrade slowly. At 12 h for HRT1 and 16 h for HRT2, the CODCr in the effluent is ≤100 mg/L and the volatile phenols are ≤1.5 mg/L ； At 16 hours for HRT1 and 21 hours for HRT2, the CODCr in the effluent was ≤50 mg/L, and the volatile phenols were ≈1 mg/L ； After 24 hours for HRT1 and 32 hours for HRT2, the volatile phenols are ≤0.5 mg/kg. Tests have shown that to achieve a low level of CODCr in the effluent (such as 100 mg/L), it is appropriate to set the HRT for the first stage at 12–14 hours and the HRT for the second stage at 16–18 hours ； To reduce the volatile phenols in the effluent to low levels (such as 0.5 mg/L), it is necessary to significantly extend the HRT. This is similar to the results of the batch tests. 2.4 Load shock test for biochemical treatment 2.4.1 Batch test When the CODCr concentration of the influent water was increased directly from 1400 mg/L to 1825 mg/L, with a volatile phenol concentration of 340 mg/L, the degradation of the wastewater is shown in Figure 7. Compared with normal batch operation (see Figure 5), the degradation curve shows a slowdown at HRT values of 0–8 hours, with a reduced degradation rate; during the HRT range of 0–10 hours, the removal rate of volatile phenol was lower than that of CODCr. After 10 hours of HRT, the removal rates of CODCr and volatile phenols increased rapidly, with the removal rate of volatile phenols being much higher than that of CODCr; after 24 hours of HRT, the removal rates reached 92.8% for CODCr and 99.2% for volatile phenols. The impact of the influent load was eliminated after 8 hours of HRT. Through 5 batches of tests, the load was gradually increased to 3065 mg/L for CODCr and 511 mg/L for volatile phenols in the influent water. The degradation performance of the wastewater is shown in Figure 8; no significant inhibition in degradation was observed in the reactor. At an HRT of 12 hours, the removal rates for CODCr and volatile phenols reached 96.4% and 99.8%, respectively, with the removal rate of volatile phenols being higher than that of CODCr at the same time. The degradation curves were similar to those observed during the stable operation phase. 2.4.2 Continuous flow test The two-stage series method is adopted, and the test parameters are shown in Table 4. The test results are shown in Figure 9. The CODCr in the influent increased from 1980 mg/L to 2967 mg/L, and the volatile phenols increased from 250 mg/L to 370 mg/L. In the first stage, the CODCr value of the effluent ranges from 138 to 450 mg/L, showing a large variation ； The CODCr value of the effluent from the second stage is 60–95 mg/L, indicating that this stage has a strong buffering capacity, which ensures stable quality of the final effluent even under load fluctuations. Compared to normal operation, no significant inhibition in biochemical degradation was observed. 3 Conclusions (1) The phenol-formaldehyde polycondensation method for recovering resin from wastewater makes use of the waste heat generated during production, thereby reducing the CODCr and volatile phenol concentrations in the wastewater, which offers certain economic benefits ； The reflux temperature was controlled at 100°C, the reflux time was 6 hours, and the holding time was about 5 hours. The removal rates of CODCr and volatile phenols from the wastewater were approximately 70%–80% and 90%, respectively, with a resin yield of 0.036–0.040. (2) The degradation curves of CODCr and volatile phenols in wastewater are essentially similar; when the inlet CODCr level is between 1400–2000 mg/L, the period before HRT reaches 10–12 hours corresponds to a rapid degradation phase, followed by a slow degradation phase. (3) Load shocks of CODCr and volatile phenols in the influent can cause temporary suppression of the biochemical system, which can be avoided by using a gradual load increase method. (4) For biochemical tests, when the influent CODCr level is between 1400–3000 mg/L and the concentration of volatile phenols is below 700 mg/L, with an HRT of ≥12 hours, the removal rates for CODCr and volatile phenols are over 95% and 98% respectively ； For the two-stage method, HRT1 of 12–14 hours and HRT2 of 14–18 hours are more appropriate, resulting in an effluent CODCr of less than 100 mg/L and volatile phenols at around 1 mg/L. (5) Phenol-formaldehyde polycondensation—a two-stage biochemical method—can effectively treat high-concentration wastewater generated in the production of phenol-formaldehyde resins.