Ozone-coagulation precipitation technology for advanced treatment of printing and dyeing wastewater
Printing and dyeing wastewater is difficult to treat because of its high content of organic pollutants, high alkalinity, deep color and complex composition. The advanced treatment methods of printing and dyeing wastewater mainly include membrane treatment, biological treatment, coagulation precipitation and ozone oxidation. A single advanced treatment process has been unable to remove new pollutants in printing and dyeing wastewater. This paper takes the effluent of a dyeing and dyeing wastewater treatment system of a printing and dyeing enterprise as the research object, and uses ozone-coagulation precipitation process to further treat printing and dyeing wastewater. The coagulation effect of ozone-coagulation precipitation process on printing and dyeing wastewater was studied systematically from the aspects of coagulant type, coagulant dosage and ozone concentration.
1. Test part
1.1 Main Materials
There are four types of coagulants, namely ferric chloride (FeCl3), aluminum sulfate (Al2(SO4)3), polyaluminum chloride, and ferrous sulfate (FeSO₄). All of them are analytically pure reagents and are formulated in an 8% (mass fraction) solution. The coagulant aid is polyacrylamide (PAM), which is a pure reagent for analysis and prepared into 0.3% (mass fraction) solution.
1.2 Ozone-coagulation test
1.2.1 Optimization test of coagulant
At room temperature, 500mL wastewater water sample was taken into beaker to study the effect of adding different mass concentrations (50, 100, 150, 200, 300, 400mg/L) of ferric chloride, aluminum sulfate, polyaluminum chloride, ferrous sulfate on the coagulation effect. ZR4-6 type coagulation test mixer is adopted. The coagulation mixing procedure is as follows: the first step is rapid stirring, the stirring speed is 250r/min, and the stirring time is 1min; The second step is medium speed stirring, stirring speed is 150r/min, stirring time is 2min; The third step is slow stirring. After the medium stirring, add the coagulant PAM and stir at a slow speed. The stirring speed is 55r/min, the stirring time is 15min, and the precipitation is standing for 25min. The supernatant 2cm below the liquid level was taken by syringe to determine the water quality index.
1.2.2 Optimum dosage test of coagulant
At room temperature, 500mL wastewater water sample was taken in beaker to study the strengthening effect of different dosage of ferric chloride (25, 50, 100, 125, 150, 180mg/L) on coagulation and precipitation. The coagulation test is the same as above.
1.3 Basic water quality indexes
Using glass electrode method (determination of pH of wastewater; Turbidimeter method was used to measure turbidity. The colorimetry was determined by platinum - cobalt standard colorimetry. COD in water samples was determined by rapid digestion spectrophotometry.
2. Results and discussion
2.1 Influence of different coagulants on coagulation effect
The influence of different coagulants on coagulation effect is mainly analyzed from three aspects: COD, turbidity and chroma, and the results are shown in Figure 1.
It can be seen from FIG. 1 (a) that the addition of four coagulants can effectively reduce the effluent COD content. When the dosage of coagulants exceeds 100mg/L, the coagulant effect of ferric chloride and aluminum sulfate on effluent COD gradually decreases, but the coagulant effect of ferrous sulfate and polyaluminum chloride on effluent COD shows a certain trend of increasing.
It can be seen from Figure 1 (b) that the addition of four coagulants can effectively remove the turbidity of wastewater. When the dosage of coagulant exceeds 60mg/L, the turbidity removal effect of aluminum sulfate, ferric chloride and polyaluminum chloride reaches the best, with the removal rate as high as 95%. However, with the increase of coagulant dosage, ferrous sulfate shows the phenomenon of backmixing, and the turbidity can reach twice that of raw water.
It can be seen from Figure 1 (c) that the addition of four coagulants can effectively remove the chroma of wastewater. The decolorization rates of the three coagulants, aluminum sulfate, ferric chloride and polyaluminum chloride, all increase with the increase of the dosage of coagulants, and can basically reach 22%. However, with the increase of the dosage of coagulants, the decolorization rates of ferrous sulfate vary greatly, and no regularity can be found.
2.2 Influence of dosage of coagulant on coagulation effect
The following analyzes the influence of ferric chloride dosage on coagulation effect mainly from three aspects: COD, turbidity and chroma, and the results are shown in Figure 2.
As can be seen from FIG. 2 (a), with the increase of ferric chloride dosage, effluent COD first decreases and then fluctuates in a small amplitude. When the dosage of ferric chloride is 100mg/L, the effluent COD is small, about 104mg/L, and the COD removal effect is the best.
As can be seen from FIG. 2 (b), when the dosage of ferric chloride is 0-25 mg/L, the turbidity removal effect becomes better and better with the increase of ferric chloride dosage, and the optimal removal rate reaches about 95%. When the dosage of ferric chloride is 25 ~ 105mg/L, the turbidity removal rate is stable at about 95%. When the dosage of ferric chloride is 105 ~ 180mg/L, with the increase of ferric chloride dosage, the turbidity removal effect becomes worse and worse, and then backmixing phenomenon occurs, and the turbidity can reach 3 ~ 4 times of the raw water.
As can be seen from Figure 2 (c), with the increase of ferric chloride dosage, the decolorization rate showed a trend of first increasing and then decreasing. When the dosage of ferric chloride was 0 ~ 100mg/L, the decolorization rate increased from 0% to 44%. When the dosage of ferric chloride is 100 ~ 180mg/L, the decolorization rate decreases from 44% to 30%.
2.3 Influence of ozone concentration on coagulation effect of ferric chloride
The following mainly analyzes the influence of ozone dosage on coagulation effect from three aspects: COD, turbidity and chroma. It can be seen from the test data that effluent COD shows a gradual decreasing trend when the ozone concentration increases from 10mg/L to 30mg/L. When the ozone concentration is 30mg/L, effluent COD is 82mg/L. With the ozone concentration increasing from 10mg/L to 30mg/L, the effluent turbidity showed a decreasing trend. When the ozone concentration was 30mg/L, the turbidity removal rate reached 95%. With the ozone concentration increasing from 10mg/L to 30mg/L, the decolorization rate of effluent showed an increasing trend. When the ozone concentration was 30mg/L, the decolorization rate of effluent reached 78%.
2.4 Mechanism analysis
In coagulant ferric chloride, Fe3+ can be hydrolyzed to produce positively charged hydrolysate, which has the function of compression double electric layer and adsorption bridge, and can effectively treat printing and dyeing wastewater. Compared with Fe2+ and Al3+, Fe3+ hydrolysis reaction conditions are lower, the reaction rate is faster, and the treatment effect of dyeing wastewater is better. When the dosage of ferric chloride is 100mg/L, the pH is about 3.6, and studies have shown that when Fe3+ is used as a coagulant, the optimal pH is 3.5 ~ 4.0. Too low and too high pH will affect the hydrolysis effect of Fe3+, and thus reduce the coagulation effect. In addition, to increase the ozone oxidation process, mainly because ozone has a micro-flocculation effect, which can effectively condense the particles in the water into floc, thereby increasing the subsequent coagulation efficiency and improving the coagulation effect.
3. Conclusion
Ozone-coagulation precipitation process can effectively remove the turbidity, chroma and organic matter in the wastewater of printing and dyeing wastewater. For the ozone-coagulation precipitation process, the coagulant is ferric chloride, the dosage is 100mg/L, the ozone concentration is 30mg/L, the coagulation effect is the best, the effluent COD is 82mg/L, the turbidity removal rate is 95%, and the decolorization rate is 78%.
