Study on the treatment of waste papermaking wastewater by pre-oxidation coagulation

2.2 Effect of Oxidation Time on CODCr Removal Rate

The KMnO4 reaction time is also a major factor influencing CODCr removal rate. When the dosage of KMnO4 was 13 mg/L, the reaction time was changed to obtain the relationship between the CODCr removal rate (II) and the reaction time as shown in FIG. 2 . From Figure 2, we can see that at the oxidation time of 25min, CODCr removal rate (II) peaks, as high as 22.5% (CODCr total removal rate of 91.1%); thereafter, with the increase of oxidation time, CODCr removal rate (II) is rather sharp decline. this means

The optimal time for the interaction between KMnO4 and the oxygen-consuming substances in the wastewater is present. At this time, the corresponding state of the system is most favorable to the flocculation of CODCr. 

Fig. 2 Relationship between KMnO4 reaction time and CODCr removal rate (II) (KMnO4 dosage: 13 mg/L; pH: 6.66; wastewater CODCr: 617 mg/L)

2.3 Effect of pH on CODCr removal rate

Fig. 3 Relationship between pH and CODCr removal rate (II) (KMnO4 dosage: 13 mg/L; reaction time: 25 min; wastewater CODCr: 979 mg/L)

The pH value of KMnO4 affects the oxidation ability and oxidation-reduction history of KMnO4 in the system, and also affects the flocculation effect of the flocculant. Therefore, the pH value will have a great influence on the CODCr removal rate (II). The test showed (as shown in Figure 3) that the CODCr removal rate (II) maintained a high level at a pH of 4 to 7 and reached a maximum at pH=6: the CODCr removal rate (II) reached 29.2%, CODCr The total removal rate reached 94.0%; at pH<3 and pH>8, the CODCr removal rate (II) was negative. In the experiment, it was found that when pH<4, the flocculation effect deteriorated significantly as the pH decreased. It is concluded that the main reason for the decrease of CODCr removal rate (II) at pH<4 is due to the deterioration of the flocculation effect; and at pH>8, the flocculation process of the system is normal.

The optimum process conditions for KMnO4 pre-oxidation obtained by the above test are: KMnO4 dosage 13 mg/L, oxidation reaction time 25 min, pH value=6. Comparison tests showed (see Table 1 for results) that KMnO4 pre-oxidation-coagulation method was significantly better than flocculation method: CODCr removal rate increased from 72.2% to 89.3%, increased by 17.1%; BOD5 removal rate was 71.1% Increased to 89.7%, an increase of 18.6%; after treatment of wastewater indicators are better than the national first-class emission standards, CODCr content has been greatly reduced, to meet the requirements of reuse water.

Comparison of wastewater treatment results under the optimal conditions in Table 1

Project suspended matter
(mg/L) turbidity
(degree) color
(times) pH CODCr
(mg/l)BOD5
(mg/l) CODCr
Removal (%)BOD5
Removal (%) of raw wastewater 6151834006.79609.0252.0--flocculation sedimentation method 3.5226.35169.272.972.271.1 pre-oxidation-coagulation method 6.00 None 6.065.325.989.389.7


3 Conclusion

The preoxidation-coagulation method using KMnO4 as oxidant can effectively increase the removal rate of COD in waste paper papermaking wastewater. Under the optimal process conditions, the total removal rate of CODCr reached 89% to 94%, and all the indicators reached the first-class emission standards.

 As the preoxidation-coagulation process is simple, only the oxidation reaction equipment (or the conditioning tank can also be used as the oxidation tank) on the basis of the coagulation precipitation method can be easily implemented in the factory. In particular, plants that already have a coagulation and sedimentation plant can achieve effective treatment of waste water with only a small investment. At the same time, pre-oxidation-coagulation method reduces the accumulation of harmful substances, making it possible to recycle the clarified wastewater. 

(Author / Kang Siqi, Ma Xiaoou, Yin Gengming)