Study on the treatment of waste paper pulp-making wastewater by pre-oxidation coagulation method

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Study on the Treatment of Waste Paper Pulpmaking Wastewater by Pre-oxidation Coagulation Method Authors: Kang Siqi, Ma Xiaou, Yin Gengming Abstract: The pre-oxidation coagulation method was used to treat waste paper pulpmaking wastewater. Studies have shown that using KMnO4 as an oxidant under optimal treatment conditions, the removal rate of CODCr in wastewater can reach 89%–94%, while the removal rate of BOD5 reaches 90%. Keywords: pre-oxidation; KMnO4; waste paper pulp-making wastewater. Introduction: With the increasing scarcity of papermaking resources and the growing environmental awareness of people, the recycling of waste paper has received greater attention, and waste paper has become one of the main raw materials for paper production. Although the waste paper papermaking process does not produce \"black liquor,\" a large amount of wastewater is generated, containing pollutants such as fine fibers, ink, resins, pigments, chemicals, and mechanical impurities. If such wastewater is discharged directly without treatment, it will cause pollution and harm to the environment. Wastewater from paper production using waste paper mainly comes from the washing wastewater in the pulping process. Depending on the source of the waste paper and the manufacturing processes used, the characteristics of the wastewater resulting from washing vary. The pollutant levels in the wastewater from the paper mill under study in this article are approximately as follows: CODCr of 500–1000 mg/L, BOD5 of 200–450 mg/L, SS of 500–800 mg/L, color intensity of 250–600 units, pH value of 6.5–7.3, with a black-gray appearance. The amount of wash wastewater is 150–200 m3/t. The wastewater treatment technology currently used in our country for papermaking from waste paper is coagulation sedimentation (or air flotation). With the application of new organic and inorganic polymer flocculants, the coagulation-precipitation method can not only effectively remove SS and color from wastewater but also eliminate most of the COD. The author has previously studied the aforementioned wastewater using the flocculation precipitation method and obtained the optimal treatment conditions. After treatment, the wastewater contains almost no suspended solids; the color removal rate is over 98%, the CODCr removal rate ranges from 70% to 75%, and the BOD5 removal rate ranges from 60% to 70%. Since the treated water has become clear and transparent, the idea of reusing this treated water for pulp washing was proposed, in order to reduce the total amount of wastewater discharged and save production water [1]. To avoid the impact that high COD levels in recycled water and the accumulation of COD may have on the pulping process, and to improve the quality of the wastewater discharged, it is necessary to explore more effective treatment technologies for waste paper pulp-making wastewater. In the field of wastewater treatment, chemical oxidation methods are receiving increasing attention [2–4]. The author investigated the treatment process for waste paper pulp-making wastewater using the KMnO4 pre-oxidation method. 1 Test 1.1 Water sample The test water sample was obtained from a paper mill. The factory primarily uses imported waste paper (pamphlets, books, newspapers, etc.) as raw material to produce toilet paper, cardboard, and kraft paper. Its papermaking wastewater (i.e., “white water”) is treated using air flotation and then recycled, while the other wastewaters are combined and discharged into the drainage ditch. The water samples for this test were taken from the discharge ditch. 1.2 Main Reagents and Instruments: Instruments include a variable-speed stirrer, a pH meter of type pH-25, a spectrophotometer of type 722, an chemical oxygen demand analyzer of type HH-5, etc. Chemicals: potassium permanganate, sodium hypochlorite, hydrogen peroxide, ferrous sulfate, polyacrylamide, JA composite coagulant, etc. 1.3 Wastewater treatment process: Chemical pre-oxidation? coagulation method is employed. 500 mL of wastewater sample was taken into a beaker, and a chemical oxidant was added under stirring. After reacting for the specified time, it was treated according to the optimal conditions of the coagulation-precipitation method. Subsequently, it was slowly poured into a 1000 mL volumetric flask; after allowing it to stand for 10 minutes, the clear wastewater was collected for analysis of relevant parameters. 2 Results and Discussion In previous studies on the coagulation-precipitation method, satisfactory results were achieved in removing suspended solids and color from wastewater; therefore, the aim of this experiment was primarily to improve the COD removal rate. Condition tests were conducted using ?KMnO4, ?NaClO, and Feton reagent (H2O2+Fe2+); the treatment effect of the KMnO4 pre-oxidation method was the best. Therefore, optimization studies were carried out on the process conditions for KMnO4 pre-oxidation. To eliminate the influence of other factors, KMnO4 blank tests (with identical other treatment conditions) were conducted in each group of experiments, and the CODCr removal rate (II) was used as the criterion for evaluating each test. The expression for the CODCr removal rate (II) is as follows: CODCr removal rate (II) = (CODCr(1) – CODCr(II)) / CODCr(1). Here, CODCr(1) represents the value of CODCr remaining in the wastewater after treatment, as measured in the KMnO4 blank test ； CODCr(II) -- The residual CODCr value in the wastewater after treatment, when KMnO4 is added. 2.1 Effect of KMnO4 dosage on CODCr removal rate The dosage of KMnO4 has a significant impact on the CODCr removal rate, as is clearly shown in the relationship curve between KMnO4 dosage and CODCr removal rate (II) presented in Figure 1. Under the condition of an oxidation reaction time of 30 minutes, when the dosage of KMnO4 is below 10 mg/L, the CODCr removal rate (Ⅱ) increases gradually as the dosage of KMnO4 increases ； When the dosage of KMnO4 exceeds 10 mg/L, the CODCr removal rate (Ⅱ) increases rapidly as the dosage of KMnO4 rises, reaching its peak at a dosage of 13 mg/L ； As the amount of KMnO4 added continued to increase, the CODCr removal rate (Ⅱ) decreased slowly. It can be seen that there is an optimal amount of KMnO4 to be added, at which the oxidation and coagulation-promoting effects are most pronounced. The optimal dosage of KMnO4 obtained from this experiment was 13 mg/L, resulting in a total CODCr removal rate of 87.0%. Figure 1 Relationship between KMnO4 dosage and CODCr removal rate (II) (oxidation time: 30 min ； pH: 7.01 ； Wastewater CODCr: 644 mg/L). It should be noted that the total CODCr removal rate obtained from the KMnO4 blank test was still about 5% higher than that achieved by the flocculation sedimentation method. This is because, just like in the test with no oxidant added, there is a certain stirring time prior to flocculation and sedimentation, which allows some of the CODCr substances in the wastewater to come into contact with air and be oxidized, or to be oxidized by the dissolved oxygen in the water, thereby turning into substances that can be flocculated. 2.2 Effect of oxidation reaction time on CODCr removal rate The reaction time of KMnO4 is also a major factor affecting the CODCr removal rate. When the dosage of KMnO4 was 13 mg/L, by varying the reaction time, the relationship curve between the CODCr removal rate (Ⅱ) and the reaction time is shown in Figure 2. As can be seen from Figure 2, at an oxidation time of 25 minutes, the CODCr removal rate (Ⅱ) reaches a peak value of 22.5% (with the overall CODCr removal rate reaching 91.1%) ； Thereafter, as the oxidation time increased, the CODCr removal rate (Ⅱ) declined sharply instead. This indicates that there is an optimal reaction time between KMnO4 and the oxygen-consuming substances in the wastewater; at this time, the state of the system is most favorable for the flocculation of CODCr. Figure 2 Relationship between KMnO4 reaction time and CODCr removal rate (II) (KMnO4 dosage: 13 mg/L ； pH: 6.66 ； Wastewater CODCr: 617 mg/L) Figure 3 Relationship between pH and CODCr removal rate (II) (KMnO4 dosage: 13 mg/L ； Reaction time: 25min ； Wastewater CODCr: 979 mg/L). The pH value affects the oxidation capacity of KMnO4 in the system as well as the redox process, and it also influences the flocculation efficiency of flocculants; therefore, the pH value has a significant impact on the CODCr removal rate (II). Tests show (as shown in Figure 3) that the CODCr removal rate (II) remains at a high level when the pH is between 4 and 7, reaching its maximum at a pH of 6: the CODCr removal rate (II) was 29.2%, while the total CODCr removal rate reached 94.0% ； At pH < 3 and pH > 8, the CODCr removal rate (II) is negative. It was found in the experiments that when pH < 4, as the pH decreases, the flocculation effect significantly deteriorates. It can be inferred that at pH < 4, the main reason for the decrease in the CODCr removal rate (Ⅱ) is the deterioration of the flocculation effect ； While at pH > 8, the flocculation process of the system proceeds normally. The optimal process conditions for the KMnO4 pre-oxidation process, as determined through the above experiments, are: a KMnO4 dosage of 13 mg/L, an oxidation reaction time of 25 minutes, and a pH value of 6. Comparative tests showed (see Table 1 for details) that the pre-oxidation with KMnO4 followed by coagulation method is significantly superior to the flocculation-sedimentation method: the CODCr removal rate increased from 72.2% to 89.3%, an improvement of 17.1% ； The BOD?5 removal rate increased from 71.1% to 89.7%, an increase of 18.6% ； All parameters of the treated wastewater meet the **specified first-class discharge standards; the CODCr level has been significantly reduced, allowing it to satisfy the requirements for reuse. Table 1 Comparison of wastewater treatment results under optimal conditions
Parameter: Suspended solids (mg/L), Turbidity (NTU), Chroma (times), pH value, CODCr (mg/l), BOD5 (mg/l), CODCr removal (%), BOD5 removal (%)
Raw wastewater: 61, 51, 83, 400, 6.79, 60, 9.02, 52.0 – –
Flocculation and sedimentation method: 3.52, 26.35, 169.27, 2.97, 2.27, 1.1
Pre-oxidation-coagulation method: 6.00, None, 6.00, 65.32, 5.98, 9.38, 9.73
Conclusion: The pre-oxidation-coagulation method using KMnO4 as an oxidant can effectively increase the removal rate of COD in waste paper pulp wastewater. Under the optimal process conditions, the total removal rate of CODCr reaches 89%–94%, and all parameters meet the first-class discharge standards. Due to the simple process of the pre-oxidation-coagulation method, and since only oxidation equipment needs to be added to the coagulation-sedimentation process (an adjustment tank can also be used as an oxidation tank), it is easy to implement in factories. Especially for factories that already have coagulation and sedimentation treatment facilities, only a small amount of additional investment is required to achieve effective wastewater treatment. At the same time, the pre-oxidation-coagulation method reduces the accumulation of harmful substances, making it possible to recycle the clarified wastewater.