Thread Content
Research on Recovery and Treatment Technologies for Wastewater from Recycled Paper Pulping Authors: Ma Xiaou, Li Shaoquan Abstract: The characteristics of wastewater from recycled paper pulping are discussed, and the treatment technologies for such wastewater are analyzed and evaluated. It is pointed out that the coagulation-sedimentation–biochemical method or chemical treatment–coagulation-sedimentation method can effectively treat waste paper pulp-making wastewater. Keywords: waste paper pulp-making wastewater, wastewater characteristics, coagulation sedimentation, biological treatment, chemical treatment. Since the recycling of waste paper for paper production enables effective use of resources and protection of the ecological environment, it is receiving increasing attention. As waste paper pulping technology continues to advance, waste paper has become an important raw material for pulp production. In 1995, foreign waste paper accounted for 30%–34% of the raw materials used in pulp production〔1〕. In recent years, the number of waste paper recycling and paper manufacturing plants in our country has also been increasing. To separate the fibers in waste paper and remove ink from them, large amounts of chemical agents are often added during the production process, and washing is used to eliminate various impurities present in the waste paper. Therefore, waste paper recycling for paper production generates large amounts of wastewater contaminated with fine fiber inks, resins, pigments, chemicals, and mechanical impurities. Compared to the process of directly using plant fibers for pulping, the pollution load of wastewater generated from recycled paper manufacturing is relatively lower; however, it still far exceeds the emission standards. If discharged without treatment, it will cause pollution and harm to the environment [2]. As the proportion of recycled paper continues to increase, waste water from paper production generated by recycled paper has become a new source of pollution that has attracted attention. In recent years, a series of treatment technologies have been developed based on the characteristics of wastewater from waste paper pulp production, laying the foundation for the effective treatment of such wastewater. 1 Characteristics of wastewater from waste paper pulp production. The process of recycling waste paper into paper can be divided into two main stages: pulping and papermaking. During processes such as dregs removal, pulp washing, and rinsing in the pulping section, a large amount of washing wastewater is generated. Depending on the source of the waste paper and the manufacturing processes used, the characteristics of the washing wastewater vary. The levels of pollutants in such wastewater are roughly as follows: CODCr 600–2400 mg/L, BOD5 125–585 mg/L, SS 650–2400 mg/L, and color intensity of 450–900 times; the wastewater appears black-gray in color. The amount of wash wastewater is 100–200 t per ton of paper [3–5] ； As in conventional papermaking processes, the papermaking stage in waste paper recycling also generates \"white water\" containing fibers, fillers, and chemicals. Air flotation is commonly used to treat this wastewater, to recover fibers and fillers, and to enable the recycled \"white water\" to be reused. In our country, the “white water” treatment technology has become mature. It can be seen that the wastewater from waste paper recycling for paper production mainly comes from the washing wastewater in the pulping process. This wastewater not only has a high SS content and high color intensity, but also contains large amounts of COD substances with complex compositions. These COD substances consist of organic materials such as soluble slurries, chemical additives, and insoluble fibers. Among the soluble COD components, they are mainly composed of low-molecular-weight components with a molecular weight below 1000 (such as solubles in waste pulp) and high-molecular-weight components with a molecular weight of over 100,000 (such as chemicals and resins); components with an intermediate molecular weight are very rare [6]. Due to the large differences in molecular weights among the COD components in waste paper pulp-making wastewater, a single treatment method can only remove a portion of these COD substances, making it difficult to achieve satisfactory results; therefore, integrated treatment technologies must be employed. The coagulation-precipitation method can remove COD components with higher molecular weights from wastewater, while methods such as biochemistry and adsorption can remove those with lower molecular weights. The wastewater treatment technology for waste paper pulp production that was developed in our country at an early stage is coagulation sedimentation. 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 **reduce the COD content of the wastewater. This method has been applied in some factories. Since the coagulation-precipitation method cannot remove COD components with lower molecular weights, it is difficult to achieve effective treatment of wastewater. In recent years, researchers have studied the advanced treatment of wastewater from waste paper pulp production, and have successively developed technologies such as coagulation sedimentation–biological treatment and chemical treatment–coagulation sedimentation. The following introduces these processing techniques. 2 Methods for treating wastewater from waste paper pulp production 2.1 Coagulation sedimentation method The coagulation sedimentation method features a simple process, ease of operation, high efficiency, and low investment costs. The basic principle is as follows: under the action of coagulants, suspended solids, colloids, and other substances that can be flocculated in wastewater are aggregated into “flocs” through electrochemical processes such as compression of the double electric layer on the surface of microparticles, reduction of the interfacial ζ potential, and electrostatic neutralization, as well as physicochemical processes such as bridging, trapping, and adsorption” ； The flocculated wastewater is then separated into solid and liquid phases using sedimentation equipment; the \"flocs\" sink to the bottom of the sedimentation equipment and form sludge, while the liquid that flows out at the top is clear water with lower color and turbidity. Practice has shown that when coagulation-precipitation is used to treat waste paper pulp-making wastewater, the SS removal rate can reach 85%–98%, the color removal rate can exceed 90%, and the CODCr removal rate can reach 60%–80%. Due to the good quality of the treated clean water, it can be reused for pulp washing and papermaking ； The resulting slurry can be reused as pulp for boxboard interlayer paper [7]. The process flow for treating waste paper and papermaking wastewater using coagulation sedimentation is shown in Figure 1. Figure 1 Process flow of treating waste paper pulp-making wastewater by coagulation sedimentation. The effectiveness of treating waste paper pulp-making wastewater using the coagulation sedimentation method depends on the quality of the coagulation process. First, an appropriate coagulant should be selected. Currently, inorganic and organic polymer coagulants are mainly used, such as polyferric sulfate, polyaluminum chloride, complexes of binary or ternary metal salts, polyacrylamide and its derivatives, etc. One or both of these can be selected depending on the characteristics of the wastewater. The author’s research shows that the rational combination of inorganic and organic polymer coagulants can effectively improve the flocculation quality of wastewater, resulting in large and dense \"floc\" with good treatment effects. Other factors affecting the coagulation process, such as the dosage of the coagulant, the method of adding it, and the coagulation reaction time, can be determined through experiments. If both the treated waste pulp and clean water can be reused, the coagulation sedimentation method also offers good economic benefits. Under normal operating conditions, the cost of coagulant required to treat 1 ton of wastewater is 0.1–0.2 yuan, with the total treatment cost being around 0.5 yuan. However, since reclaimed water and pulp generate revenue while reducing expenses such as wastewater disposal fees and resource usage costs, it offers significant economic benefits. Since the coagulation-precipitation method can only remove a portion of the COD, the COD level in the treated wastewater fails to meet the discharge requirements. This is the reason why the coagulation-sedimentation method has significant limitations in practical applications. 2.2 Biochemical treatment method: Since high-molecular-weight COD substances have poor biodegradability, it is inappropriate to carry out biochemical treatment on waste paper and papermaking wastewater without prior treatment. The effectiveness of biochemical treatment in removing COD varies with the BOD/COD ratio of the wastewater. Currently, several indicators have been proposed to determine whether biochemical treatment is possible. For example, if BOD/COD > 0.6, biochemical treatment is possible ； If BOD/COD < 0.2, biochemical treatment cannot be carried out. According to available data, for wastewater from waste paper pulp production, biochemical treatment is suitable when BOD/COD ranges from 0.4 to 0.7. Experiments show that after coagulation and sedimentation treatment, the BOD/CODCr value of waste paper pulp-making wastewater is almost always within the range of 0.4–0.7, making it suitable for biochemical treatment. At the same time, coagulation and sedimentation treatment can also remove fine inorganic particles that can reduce the activity of biochemical sludge (such as pigments, fillers, etc.). Therefore, it is necessary to carry out coagulation and sedimentation treatment on waste paper pulp-making wastewater before biochemical treatment (i.e., the so-called primary treatment), with biochemical treatment serving only as a process for advanced treatment of the wastewater (secondary treatment). In biochemical treatment, organic matter (BOD components) in wastewater is removed through the following mechanisms: (1) Oxidation of organic matter ； ⑵The formation and growth of microbial cytoplasm in activated sludge, that is, the proliferation of activated sludge ； ⑶Cytoplasmic oxidation of activated sludge microorganisms, that is, the self-decomposition of activated sludge. Therefore, in biochemical treatment, the activity of the activated sludge that has been cultivated and acclimated, as well as its oxygen supply capacity, are important factors determining its treatment efficiency. The activated sludge used to treat papermaking wastewater from waste paper should be cultivated and acclimated; only activated sludge grown under suitable conditions possesses good activity and sedimentation properties. Studies have shown that when untreated and uncultured sludge is used to treat coagulation-precipitation effluent, the BOD removal rate is only 28.2%, while the COD removal rate is 68.9% ； When this wastewater is treated with sludge that has been cultivated and acclimated, the BOD removal rate can reach 76.1%, while the COD removal rate reaches 81.2% [8]. There are various methods for supplying oxygen in biochemical treatment; for example, the activated sludge process uses the addition of air or oxygen-enriched air to the wastewater (i.e., aeration) to provide the system with the necessary oxygen ； Biological rotating disks utilize the rotation of the disk to alternate contact with wastewater and air, allowing the microorganisms and organic substances adsorbed on the biofilm on the disk to come into full contact with air and thereby obtain the necessary oxygen. When the properties of the wastewater to be treated are constant, the oxygen supply capacity of the device can be improved by increasing the contact area between the gas and liquid phases, the contact time, and the concentration difference of oxygen between these phases. Due to the large amount of wastewater generated in paper recycling, and the high cost of using oxygen as an oxidizing agent, air ventilation is generally employed. To this end, when designing oxygen supply systems, emphasis should be placed on ensuring good air distribution and improving mixing efficiency (or increasing the gas-liquid contact time) [9]. The process flow for treating waste paper and papermaking wastewater using the activated sludge method is shown in Figure 2. Figure 2 Schematic diagram of the activated sludge process for treating waste paper pulp-making wastewater, aiming to improve the removal efficiency of COD and BOD in the wastewater so that the effluent meets the discharge standards. However, the biochemical method has problems such as large land occupancy and high capital investment costs, which limit its widespread application. Therefore, developing other simpler and more feasible processing methods is more practical for enterprises (especially small and medium-sized enterprises). 2.3 Chemical treatment methods: Certain soluble pollutants in wastewater can be converted into forms that are easier to separate from water through chemical oxidation-reduction processes, and then removed from the water using conventional treatment techniques such as coagulation and sedimentation, adsorption, etc ； Or it can be converted into harmless new substances (such as CO2) to achieve the removal of COD and BOD. The usual practice is to combine chemical treatment with coagulation and sedimentation. Chemical treatment agents (i.e., oxidants) can be added to the wastewater first for pre-oxidation, followed by coagulation and sedimentation ； It can also be added simultaneously with coagulants to achieve a synergistic effect between the coagulants and oxidants ； It can also be added at a certain treatment stage after coagulation and sedimentation to carry out advanced treatment of wastewater [10]. The oxidants used in chemical treatment methods mainly include potassium permanganate, sodium hypochlorite, chlorine dioxide, Fonton reagent, etc. For wastewater of different properties, the effects achieved using different oxidants vary significantly [11–13]. The author has conducted research on the effectiveness of potassium permanganate, sodium hypochlorite, and Fonton reagent in treating waste water from paper manufacturing. The results showed that potassium permanganate is an effective pretreatment agent, while sodium hypochlorite is a good agent for advanced treatment. The method of pretreatment with potassium permanganate for waste paper pulp-making wastewater has advantages such as a simple process, easy operation, low investment, and good treatment effects. Its process flow is exactly the same as that of the coagulation-sedimentation method; it is sufficient to add an appropriate amount of potassium permanganate solution to the adjustment tank and allow a certain reaction time in order to carry out the coagulation-sedimentation process. Experiments were conducted to treat the waste paper and papermaking wastewater from a certain factory, achieving satisfactory results, as shown in Table 1. The cost of potassium permanganate required to treat 1 ton of wastewater is approximately 0.08–0.10 yuan. Table 1 Results of Chemical Treatment for Papermaking Wastewater from Waste Paper Parameter SS/mg.L‑1, Colority/x, CODCr/mg.L‑1, CODCr Removal Rate/%, BOD5/mg.L‑1, BOD5 Removal Rate/% Original wastewater: 615, 400, 609.0, ——, 252, —— Treatment by coagulation and sedimentation: 3.52, 169.27, 2.27, 2.97, 1.1 Treatment by KMnO4 pre-oxidation: 626, 5.38, 9.32, 5.98, 9.7 Advanced treatment with NaClO: 3.84, 72.88, 8.0, —— Key equipment required for some parameters (environmental protection projects) Parameter number, Project name, Construction content/main products, Key equipment, Equipment procurement D0662, Energy-saving and environmental treatment project for electrolytic aluminum production, 50,000 tons of electrolytic aluminum, electrolytic aluminum ingots; annual production capacity of 50,000 tons of electrolytic aluminum; domestic procurement E0462, Comprehensive environmental treatment project and technical upgrade project for 20,000 tons/year of hydrogen peroxide production, 1. Gas generation and blowing air recovery system ； 2. Carbon dioxide recovery system from decarburization gas ； 3. Stepwise concentration system for dilute ammonia water ； 4. JTL—4 Precision Desulfurization System ； 5. “Shuangjia” system ； 6. Synthetic air release hydrogen production system ； 7. Methanol distillation system ； 8. Technical renovation and expansion of hydrogen peroxide production. Hydrogenation fixed-bed, oxidation tower, extraction tower, purification tower, clay bed, distillation tower, compressor, waste heat boiler – domestic procurement. Project for the utilization of fertilizer waste phosphogypsum and wastewater treatment: utilizing fertilizer waste phosphogypsum, with advanced German technology and the introduction of key equipment, to build a facility capable of producing 10 million square meters of gypsum board per year ； 7,000 tons of grouting gypsum powder and a wastewater treatment center. Equipment such as filter presses, meters, calcination furnaces, calcination and drying fans, filters, cutting knives, shaping workbenches, drying ovens, etc. Domestic procurement for I0234 wastewater comprehensive treatment project: alkali recovery and filling in gaps ； Intermediate stage water treatment ； White water recycling ； Black liquor extraction and multi-stage bleaching ； Utilities ； Paper processing workshop. Main products: Recycled alkali 4,328 t/d ； Recycle clean water and fibers. Plate evaporator, thickener, coater, white water recycler, intermediate water treatment equipment, 20-ton Golden Horn furnace, steam turbine generator. The use of sodium hypochlorite for advanced treatment of water after coagulation and sedimentation, as introduced from abroad, can also yield good results (see Table 1). However, compared with the potassium permanganate pre-oxidation method, it features a larger dosage of reagents and slightly poorer performance. Therefore, the potassium permanganate pre-oxidation method is more appropriate. As can be seen from Table 1, after being treated using chemical methods, the various parameters of the waste paper pulp-making wastewater are superior to the **discharge standards, and it also meets the requirements for water quality suitable for reuse. Since chemical treatment methods reduce the accumulation of harmful substances, it is necessary to maximize the reuse of water in practical applications in order to lower production costs and minimize water waste. 3 Conclusion At present, waste paper recycling in paper manufacturing holds a rather important position. Therefore, there is an increasing demand to address waste paper and papermaking wastewater as this new source of pollution. As can be seen from the analysis in this paper, the treatment technologies for waste paper and papermaking wastewater have become increasingly mature, which plays a positive role in accelerating the effective treatment of such wastewater. Coagulation sedimentation is effective in removing suspended solids and color, and it can also eliminate some of the dissolved COD; however, the COD level in the treated wastewater remains high, making it difficult to meet the discharge standards. Therefore, coagulation-sedimentation can only be used as a primary treatment process for waste paper pulping wastewater, or in combination with chemical pre-oxidation. Wastewater from waste paper pulp production is suitable for biochemical treatment after coagulation and sedimentation treatment. Since biochemical treatment can remove organic substances with lower molecular weights, thereby compensating for the shortcomings of coagulation and sedimentation methods, it is reasonable to use a two-stage treatment process that combines these two methods. After being treated through this combined process, the wastewater from waste paper papermaking meets all the emission standards. The advantage of this method is its good treatment effect and low operating costs ； However, the disadvantages are a long processing cycle, large floor space required for the equipment, and high initial investment. Treating papermaking wastewater from waste paper using chemical methods not only overcomes the shortcomings of biological methods but also boasts advantages such as good treatment efficiency and low treatment costs. In particular, the potassium permanganate pre-oxidation method enables effective treatment of wastewater with little need to add additional equipment, building on the coagulation-precipitation method ； At the same time, since the pre-oxidation method can reduce the accumulation of harmful substances, it helps to increase the recycling rate of reclaimed water, and it is expected to enable a closed-loop circulation of water. Therefore, the author believes that this method is quite suitable for China’s national conditions, and further research and development are necessary.