Advanced Treatment Of Leather Effluent Biochemical Effluent Fenton Reagent

Study on advanced treatment of biochemical effluent from leather wastewater by Fenton reagent

Wang Chengjun, Huang Ruimin, Qing Haibo, Gao wu long, Zhou Yuanyuan

(School of environmental science and engineering, South China University of Technology, Guangzhou 510006)

Abstract: the biochemical effluent from a leather factory wastewater treatment plant, which is mainly made from raw cattle skin, is taken as the research object. The treatment effect and influencing factors of Fenton reagent on the wastewater are studied.

The optimum conditions for the degradation of the effluent from such leather wastewater were determined as follows: pH value 5, H2O2 dosage 600 mg/L, Fe2+ dosage 500 mg/L, reaction time 50 min.

Under these conditions, when the mass concentration of influent COD is 333 mg/L and the chroma is 90 times, the removal rates of COD and chromaticity reach 73.3% and 98% respectively, and the mass concentration of wastewater COD decreases to 89 mg/L, and the chroma drops below 5 times, reaching the first grade standard of leather waste water in the comprehensive wastewater discharge standard (GB8978-1996).

Key words: leather wastewater; advanced treatment; Fenton reagent

Chinese map classification number: X794.035 document identification code: A article number:! 1009-2455 (2008) 02-0049-03

Tannery wastewater has high pollution load and complex composition of wastewater. It not only has high pollution of odor, color, suspended solids, ammonia nitrogen and oxygen consumption, but also contains toxic substances such as heavy metal ions and sulfides, [1].

The treatment process of tannery wastewater often adopts the method of combining biochemical phase with first physicochemical process. However, most engineering practices have proved that there are still more organic pollutants in the effluent of the effluent, which is still more than 200 mg/L (COD), and can only reach the two grade standard of the wastewater comprehensive discharge standard (GB8978-1996) leather wastewater.

When the raw skin is used as the main processing object, the effluent treatment is more difficult and the water quality is worse.

Because of the high toxic substances such as sulfide and chloride in wastewater, it has a great influence on anaerobic treatment. Therefore, tannery wastewater is less treated by anaerobic biochemical method, and aerobic biochemical method is used to exchange the relatively stable operation of the system with longer aerobic residence time, but it is difficult to remove some refractory organic matter in wastewater.

Chemical oxidation has proved to be very effective for the treatment of wastewater containing refractory organic matter [2-3].

Fenton reagent is a commonly used chemical oxidant. Compared with other chemical oxidants, Fenton has the advantages of simple operation, rapid reaction, no need for complex equipment, no toxicity to biochemical treatment and friendly environment. It has been gradually applied to [2-7], such as printing and dyeing, pesticide, leachate, printed circuit board and other industrial wastewater treatment, but it is rarely reported in leather wastewater.

In this study, Fenton reagent oxidation method was used to treat the effluent from a tannery wastewater in order to further degrade the refractory organic matter in the wastewater and achieve the first-order discharge standard of leather wastewater.

1 materials and methods

1.1 wastewater quality

The experimental waste water is taken from the effluent of the two sink of a tannery wastewater treatment plant in Guangdong.

The effluent is light brown, and COD is 200~400 mg/L, pH 8~9 and chroma 80~100 times.

As the sulphur content of the effluent has reached the standard, no further tests have been carried out in this test.

1.2 test method

100 mL of water sample was poured into 250 mL cone bottles, and pH value was adjusted after adding FeSO4 and H2O2.

Shake 2 min to promote its reaction.

After the reaction was completed, the pH value was adjusted by NaOH solution at 10, so as to terminate the reaction and precipitate most Fe2+ and Fe3+ so as not to interfere with the determination of COD.

Then, the unreacted H2O2 was driven by micro heating. After cooling, the supernatant was used to determine the COD value.

The oxidation degradation efficiency of Fenton reagent for wastewater was calculated.

2 results and discussion

2.1 orthogonal test

The main influencing factors of Fenton reagent include Fe2+ concentration, H2O2 dosage, initial pH value and reaction time.

In this experiment, 4 factors and 3 levels were determined. L9 (34) orthogonal table was used to determine the best operation parameters by orthogonal test. The orthogonal test is shown in Table 1.

The wastewater (COD) is 333 mg/L and the chroma is 90 times.

From the results of orthogonal experiment and range analysis, it can be seen that the initial pH value is the main influencing factor, followed by the H2O2 dosage, and the reaction time has the least influence.

The initial operating conditions determined by this method are:

The pH value was 3, the dosage of H2O2 and Fe2+ was 600, 400 mg/ L, and the reaction time was 60 min.

In order to further determine the optimal operating conditions, a single factor test is also needed.

2.2 single factor impact test

The influence of 2.2.1 pH value

Because the form of Fe2+ is governed by the pH value of the solution, Fenton reagent can only react under acidic conditions. In neutral and alkaline environments, Fe2+ can not catalyze H2O2 and produce OH.

To determine the best pH value of the reaction, the dosage of H2O2 and Fe2+ fixed at 600, 400 mg/L and 60 min were first determined. The effect of different pH values on COD and chroma removal rate was determined, as shown in Figure 1.

As can be seen from Fig. 1, when the pH value is greater than 5, the removal rate of COD and chromaticity increases rapidly with the decrease of pH value. When pH value is 3~5, the pH value has little effect on the removal rate of COD, and basically maintains at 70%. When pH value is 4, the removal rate is as high as 74.2%, but it is only 0.6% higher than pH value 5. When pH value is 3, the removal rate decreases slightly. This is because if the pH value is too low, it will affect the re reduction of the Fe3+ to the catalyst, so that the catalyst consumed will not be replenish in time, thus affecting the generation of "."

The chromaticity has dropped to 10 times when the pH value is less than 5.

Taking into account the cost of adding medicine, the pH value of 5 is determined as the best reaction condition.

The influence of 2.2.2 H2O2 dosage

Fig. 2 shows the influence of H2O2 dosage on the COD and chroma removal rate of the solution pH value of 5, Fe2+ dosage of 400 mg/L and reaction 60 min.

It can be seen from Fig. 2 that when the H2O2 dosage is less than 600 mg/L, the removal rate of COD and chroma increases rapidly with the increase of H2O2 dosage, and then the removal rate increases slowly. The COD removal rate basically stays between 72% and 74%, and the chroma removal rate is as high as 95%.

This is due to the low dosage of H2O2, and with the increase of H2O2 dosage, the amount of OH is increased and can react with organic pollutants rapidly. When the dosage of H2O2 is too high, the generated OH will be relatively high. However, the reaction between free radicals and organic compounds in waste water needs a certain time. However, too much OH has not yet been able to react with organic matter to react with Fe2+, which has consumed Fe2+ and H2O2, thus reducing the effective concentration of the reagent.

The influence of 2.2.3 Fe2+ dosage

Fig. 3 shows the influence of Fe2+ dosage on COD and chroma removal rate when the pH value of solution is 5, the dosage of H2O2 is 600 mg/L, and the reaction time is 60 min.

Fe2+ is a necessary condition to catalyze the production of free radicals. Only when Fe2+ exists, can H2O2 decompose and produce free radicals.

When the amount of Fe2+ is low, the yield and production rate of OH are very limited, and the degradation process is inhibited. When the Fe2+ dosage is relatively high, it will reduce H2O2 and oxidize itself to Fe3+, which will increase the chroma of the effluent while consuming the reagent. The introduction of a large amount of Fe2+ will also increase the sludge production after the callback pH value.

As can be seen from Fig. 3, when the Fe2+ dosage is less than 500 mg/L, the removal rate of COD and chroma increases with the increase of Fe2+ dosage. When the dosage of Fe2+ is 500 mg/L, the COD removal rate reaches the maximum, then the COD removal rate decreases slightly, but the stability is more than 70%. The removal rate of chromaticity increases steadily. This is because the interference of Fe2+ and Fe3+ to chromaticity is avoided by adding alkali in the experiment.

The influence of reaction time on the removal rate of COD and chroma is shown in Figure 4 of 2.2.4 reaction time. It can be seen that Fenton reagent reaction is rapid. In the first 30 min, the COD and chroma removal rate increase rapidly with the increase of reaction time, and then increase slowly, and the COD and chroma removal rates tend to be stable at 50 min.

Therefore, the best reaction time is 50 min.

3 conclusion

Through orthogonal test, it is confirmed that the Fenton reagent is the most important factor affecting the degradation efficiency of two grade effluent of leather waste water. The pH value is followed by H2O2 dosage, Fe2+ dosage and reaction time.

The optimum operating conditions were determined by single factor test: pH value was 5, H2O2 dosage was 600 mg/L, Fe2+ dosage was 500 mg/L, reaction time was 50 min, under these conditions, the COD removal rate of wastewater was 73.3%, and the effluent water (COD) was reduced to 89 mg/L, which reached the first class standard of leather wastewater of "comprehensive discharge standard for sewage" (GB8978-1996).

After treatment, the chroma of wastewater is less than 5 times, and the treatment effect is very obvious. It proves that using Fenton reagent to treat the wastewater is very effective.

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