Papermaking wastewater reuse method

Papermaking wastewaterLand treatment of wastewater is a simple method, but its effectiveness varies under different conditions. For ammonium sulfite pulping and papermaking wastewater, in areas with suitable geographical conditions, adopting a land treatment system for wastewater is an economical and effective management approach. Yu Xiuling et al. used soil column experiments to simulate a land treatment system to remove COD from industrial wastewater of the Rushan Paper Mill. The experimental results showed that at temperatures of 10-28°C, with a water application rate of 20,000 mL/d and a hydraulic load of 0.25 m/day, the COD removal rate was around 80%. Based on these findings, an industrial wastewater treatment project for the Rushan Paper Mill was designed. Wastewater irrigation is another method that requires caution. Liu Jinsheng et al. found that irrigating rice fields with papermaking wastewater from two pulping methods significantly improved the physical and chemical properties of the soil. Diluting ammonium sulfite papermaking wastewater with Luan River water in a certain proportion could increase rice yield by 8% to 11%. Xu Lianyuan et al. explored the use of certain physiological characteristics of organisms to reduce pollutants in wastewater from small paper mills. Specifically, they utilized the acid-producing characteristic of certain nitrifying bacteria to lower the pH of alkaline wastewater from paper mills to below 8.00, and then used the water-loving and alkali-tolerant nature of reeds to irrigate reed fields with this wastewater. Li Yazhi applied a water hyacinth-aquatic plant artificial wetland to treat recycled pulp papermaking wastewater. Under conditions of influent pH 7.12-7.49, BOD5, CODCr, and SS concentrations of 440.5 mg/L, 354.2 mg/L, and 290.7 mg/L respectively, and a hydraulic load of 0.05 m/d, the removal rates for BOD5, CODCr, and SS were 98%, 93%, and 89% respectively. The system performance was stable, the effluent water quality met discharge standards, and it could be used for agricultural irrigation. Stabilization ponds can be used to treat pulping and papermaking wastewater, or a combination of flotation tanks, anaerobic ponds, and facultative ponds can be employed. Field experiments and soil column experiments showed that the naturally formed biochemical layer in the vadose zone at the bottom of papermaking wastewater channels has the ability to degrade COD. When the thickness of the vadose zone is greater than 10 mm, the biochemical layer develops well, and the COD degradation rate can reach over 97%.

　　The experiments by Yang Chonghao et al. found that when treating papermaking wastewater with anionic ion-exchange resins, acrylic skeleton strong anion resins have better resistance to organic pollution than styrene skeleton strong anion resins, and are preferred in selection. Studies by Wang Ping et al. show that magnetic treatment of wastewater can reduce the COD index of papermaking wastewater, but the COD reduction is greatly related to the wastewater composition and magnetic field strength. Advanced chemical oxidation is an emerging modern water treatment technology with broad application prospects in treating refractory and toxic organic pollutants in water. Wu Shubin introduced the basic principles and characteristics of several typical advanced chemical oxidation processes and their research applications in treating papermaking industry wastewater. Zhu Yiren et al. treated papermaking black liquor using the liquid membrane method. Under suitable process conditions such as separation time, emulsion-to-water ratio, oil-to-water ratio, wastewater pH, and stirring speed, the COD removal rate in the membrane system composed of LMA-1-TOA-kerosene-H2SO4 can reach 98%, and the effluent pH is close to neutral. Chen Guoqing used membrane separation technology to treat papermaking wastewater and recover lignin and sugars, with recovery rates of about 60% for both.

　　2 Cleaner production and wastewater reuse

　　The application of cleaner production processes in the papermaking industry has changed the concept of papermaking wastewater treatment. It not only saves water resources, saves energy, and prevents environmental pollution, but also achieves good economic and social benefits, showing a ray of hope for the thorough treatment of papermaking wastewater in the new century. The application of biotechnology to reform the pulp and papermaking industry has excellent prospects and can be involved in all aspects, such as biological improvement of tree species, debarking of logs and storage of wood chips, biological pulping, enzymatic bleaching, production of high-quality dissolving pulp, enzymatic deinking technology, production of by-products from papermaking wastewater and waste materials, and wastewater treatment. Song Yanru et al. introduced the research on the lignin biosynthesis pathway and transgenic plants to reduce the lignin content of papermaking raw materials, and proposed suggestions for China's strategy to prevent and control pulp and papermaking pollution from the source.

　　Pollutant emissions can also be reduced through process modification. Based on China's actual situation, papermaking pulp bleaching should preferentially adopt chlorine bleaching technology, and develop TCF bleaching technology when the time is ripe, to minimize the generation of harmful substances. Wu Yuying et al. proposed a process for producing furfural and papermaking pulp by using dilute sulfuric acid hydrolysis and comprehensively utilizing straw. Through the discussion of factors affecting the hydrolysis reaction, the optimized hydrolysis process conditions were found to be: acid amount 20%, time 180min, maximum temperature 110℃, and liquid ratio 1:5. The pollution load of washing pulp wastewater and rectifying wastewater is far lower than that of conventional alkaline straw pulp mill wastewater, and lower than the secondary standard for papermaking industry wastewater discharge stipulated by GB3544—1992. Liang De found that the sulfate wood pulp bleaching wastewater load can be significantly reduced by using a deep delignification cooking method, pre-bleaching oxygen delignification, and replacing chlorine with chlorine dioxide. After complete secondary biochemical treatment, the wastewater can meet the national papermaking wastewater discharge standard. Tai Mingqing studied the wastewater treatment process for papermaking using sulfite pulping with Lygodium japonicum as raw material. Practice has proved that this process is reasonable, the management and operating costs are low, and the discharged wastewater can basically meet the papermaking industry discharge standards. Qian Xueren et al. briefly described the characteristics and applications of supercritical fluids, focusing on their applications in the forest products industry, including supercritical fluid extraction of lignin from black liquor and supercritical water oxidation of papermaking wastewater.

　　The key to papermaking wastewater treatment is in-plant management. Only by recycling a large amount of wastewater within the plant can the discharge volume be reduced, thereby fundamentally lowering the cost of off-site wastewater treatment and making it more conducive to meeting discharge standards.

　　3 Wastewater Resource Recovery and Comprehensive Treatment

Ye Xueming et al. selected and bred "Strain No. 5088" from bark using the plate separation method, transforming waste scraps from flax spinning mills into "Pi Gu Jun Si," a substitute for corn in feed. They also successfully utilized white mud from paper mills and sludge from viscose fiber plant wastewater treatment as feed additives for calcium and zinc. Gu Yugang et al. studied the process conditions and influencing factors for preparing gypsum dihydrate, a raw material for chemical building materials, using white mud from paper mills, carbide slag, and industrial waste sulfuric acid. Simultaneously, they adopted a closed-loop wastewater recycling process to prevent secondary pollution. Experiments showed that by preparing waste residue into a 10% concentration slurry and reacting it with low-concentration waste sulfuric acid, high-quality gypsum dihydrate with a content of over 90% could be manufactured. Li Yinhuan et al. investigated the use of spherical lignin sulfonated cation exchange resin prepared from papermaking black liquor as a biological carrier to treat high-concentration organic wastewater in an upflow anaerobic fluidized bed reactor. They studied the relationship between gas production and organic volumetric load, hydraulic retention time, and COD removal, and sought optimal process conditions. Fu Shaobin et al. first proposed the idea of treating papermaking wastewater for use as injection water in oilfields. They researched and designed a low-investment and low-cost treatment process, after which the treated papermaking wastewater could be used as injection water for medium and low-permeability oilfields. Liu Jingjin et al. introduced a technology for flue gas desulfurization and dust removal in small and medium-sized coal-fired boilers using alkaline industrial wastewater from printing and dyeing, papermaking, and leather industries, which are currently the largest emitters in China. They also conducted a technical and economic analysis. Tao Jinmei introduced the domestic situation regarding alkali recovery and the production of lignin and its chemical products from alkaline papermaking pulping wastewater (referred to as black liquor) as raw material, providing a reference technical route for the resource utilization and treatment of papermaking black liquor. Xue Jinjun et al. used the introduced *Eisenia fetida* No. 2 to treat the settled sludge from lime-based papermaking wastewater, and the experimental results were good.

　　Gong Fenglian et al. treated mixed wastewater samples from recycled paper papermaking and printing and dyeing wastewater using a process flow of mixed regulation, flocculation reaction, inclined tube sedimentation, contact oxidation, and secondary sedimentation. They also improved the aeration method of traditional aeration devices. The application results showed that the pH of the mixed wastewater was neutral, which was beneficial for subsequent wastewater treatment processes. Li Ping et al. and Fu Ruwen et al. introduced comprehensive treatment and utilization technologies for coal ash, SO2-containing coal gas, and alkaline wastewater from the intermediate stage of papermaking.