[China Packaging Network News] As the wave of green environmental protection has become more and more popular, people who are suffering from the pain of pollution in the paper industry have begun to seriously exercise the right to citizen supervision. More and more sewerage paper mills have been reported and stopped by local residents. Wastewater treatment in the paper industry can no longer be done.
Here, Xiao Bian popularizes some paper industry wastewater sources, characteristics and processing knowledge.
Sources and Characteristics of Papermaking Wastewater
The paper industry is one of the six major industrial pollution sources in the world and accounts for about 10% of China's total industrial wastewater. Papermaking wastewater is mainly high-concentration organic wastewater, and contains lignin, residual alkali, sulfides, chlorides and other pollutants. Its characteristics are large amount of waste water, high concentration of COD, high content of suspended fibers in waste water, divalent sulfur, high chroma, and odor odor of mercaptans. There are three major sources of papermaking wastewater: pulping waste (black liquor), mid-water, and paper machine white water. Papermaking wastewater will cause serious pollution to the surrounding water bodies and damage to the ecological environment. According to statistical data in recent years, the national wastewater discharge rate of the pulp and paper industry accounts for about 10% to 12% of the total national wastewater discharge, ranking third; the COD in the discharged sewage accounts for about the national total discharge amount. 40%-45%, ranking first. The serious pollution and harm caused by the waste water from the papermaking industry have caused widespread concern. With the continuous enhancement of people's environmental awareness, it is extremely urgent to completely solve the pollution of the paper industry wastewater to the environment.
Three types of wastewater are mainly produced during the pulping (chemical process) and papermaking process: black (red) liquor, mid-stage wastewater, and white paper machine water. The black (red) liquid is mainly retort pulping wastewater. The mid-stage water includes pulp washing, screening and bleaching wastewater, and the paper machine white water is papermaking workshop wastewater. Among them, the environmental pollution of cooking wastewater is the most serious, accounting for 90% of the total paper industry pollution. The main components of black liquor are lignin, cellulose, hemicellulose, monosaccharides, organic acids and sodium hydroxide, which can be recycled Useful substances; Mid-stream wastewater pollutants are complex, contain relatively high concentrations of lignin, cellulose, and resinate, and other difficult-to-biodegrade material components, and are rich in organic chlorides that are harmful to the environment caused by the bleaching stage. Deep color and great toxicity, pH 9~11, suspended matter 1000mg/l, COD600-2500mg/1; papermaking wastewater, also known as "white water", mainly from beating, slurry purification screening and paper machine Wet department. Contaminants in wastewater mainly include suspended solids such as fibers, fillers, coatings, dissolved wood components, added wet strength agents, and preservatives.
The basic method of wastewater treatment in papermaking industry
Physical treatment
Adsorption adsorption method utilizes a huge specific surface area of ​​adsorbent and has certain adsorption performance. It separates the organics from papermaking wastewater. The commonly used adsorption methods include clay adsorption, fly ash adsorption, activated carbon adsorption and hydrolysis adsorption. . Activated carbon is widely used as an adsorbent in wastewater treatment to remove odor-causing organics. The greatest advantage of activated carbon as an adsorbent is that it can be regenerated (up to 30 or more times) without significant loss of adsorption capacity.
The flocculating polymer flocculants have good flocculation and decoloring capabilities and are easy to use and operate. They are mainly divided into three categories: synthetic inorganic polymer flocculants, organic polymer flocculants and natural organic polymer flocculants. In general, the higher the molecular weight of the flocculant, the higher the flocculating activity.
Electrodialysis technology Electrodialysis is a membrane separation operation in which the potential difference is used as a driving force and the ion exchange membrane is used for selective permeability to remove or concentrate the electrolyte from the solution. Under the action of an external DC electric field, the selective permeation of the membrane allows the cations and cations in the black liquor to migrate directionally, so that the lignin precipitates at the anode, and the NaOH is recovered at the cathode region. The production process of electrodialysis combined with the conventional alkali recovery system, Alkali and lignin can be obtained by treating the black liquor of papermaking.
Compared with other membrane electrolysis technologies, ultrasonic membrane electrolysis technology can significantly improve the recycling effect of papermaking wastewater. Although membrane electrolysis technology is a common technique in water treatment. However, if it is used to treat papermaking wastewater, membrane fouling is serious and practical purposes cannot be achieved. Ultrasonics, because of its cavitation, ensure the normal use of the membrane and the smooth electrolysis. And because it has stirring effect, and other membrane electrolysis technology, it has better practicality.
Papermaking wastewater chemical oxidation treatment
Hydrothermal Oxidation The hydrothermal oxidation technology is a very effective new type of chemical oxidation technology. It is the use of air or oxygen and other oxidants in hot water tanks to treat dissolved water in papermaking wastewater under high temperature and pressure conditions. The suspended organic matter or reduced inorganic matter is oxidized and decomposed in the hot water tank. The obvious feature of the hydrothermal oxidation technology is that the reaction is performed in a hot water tank, so the energy consumption is high.
Photocatalytic Oxidation Due to its advantages of non-toxicity, good chemical stability, and high photocatalytic activity, TiO2 has been widely used in the photocatalytic degradation of various toxic and hazardous biodegradable organics. Studies have shown that photocatalytic oxidation of TiO2 can effectively degrade phenolic organics in pulping wastewater. In addition, the photocatalytic oxidation method has a very good degradation effect on organic substances that are toxic and difficult to be biodegraded, such as dioxin in papermaking wastewater. Photocatalytic treatment of waste water is simple, small in size, and can avoid the secondary pollution caused by traditional treatment methods. It is a promising water treatment technology.
The wet oxidation oxidation method uses oxygen or air as the oxidant under high pressure (150~350°C) high pressure (5~20MPa) to oxidize dissolved or suspended organic matter or reduced inorganic matter in water to produce carbon dioxide and A treatment of water.
The presence of toxic and difficult-to-biodegrade substances in the advanced chemical oxidation process affects the treatment effect of the biological treatment process and can be treated by advanced chemical oxidation. The advanced chemical oxidation process generally refers to the chemical oxidation process in which a large amount of hydroxyl radicals participate in the reaction process. For papermaking wastewater, heterogeneous light photocatalytic oxidation can use sunlight as the reaction light source, and the cost of the oxidant is low.
Electrochemical oxidation mainly uses optical, acoustic, electromagnetic, and other non-toxic reagents to catalyze the oxidation of organic wastewater. Because of the transfer of electrons between electrodes, the composition of pollutants is destroyed. Its advantages are: it only occurs in water, and it does not need additional catalyst to avoid secondary pollution; it has strong controllability; no selectivity; mild conditions, low cost, both air flotation, flocculation, and bactericidal action; Metal ions can make both positive and negative electrodes act simultaneously, and electrochemical oxidation is most effective for "triple" organic contaminants, which are difficult to biodegrade and are extremely harmful to humans.
Papermaking wastewater biological treatment
Aerobic biological treatment Aerobic biological treatment is a method of degrading pollutants by the action of aerobic microorganisms (mainly aerobic bacteria) under aerobic conditions. The method can be divided into activated sludge and biofilm methods according to the different states of aerobic microorganisms in the treatment system. Papermaking wastewater contains a large amount of organic matter and has good biodegradability. Generally, good results can be obtained by using aerobic biological treatment of papermaking wastewater.
Anaerobic biological treatment The anaerobic biological treatment is a treatment technology that utilizes facultative anaerobic bacteria and obligate anaerobic bacteria to degrade organic pollutants under anaerobic conditions. During anaerobic biological treatment, complex organic compounds are degraded and converted into simple, stable compounds that simultaneously release energy, most of which occurs in the form of methane. The anaerobic method is suitable for lime straw cooking waste liquid, alkaline pulping waste water and the like. Commonly used anaerobic treatment devices are Anaerobic Fluidized Bed (AFB), Baffled Anaerobic Reactor (ABR), Upflow Anaerobic Sludge Blanket (UASB) and Hair Carrier Biofilm devices.
Comprehensive treatment of papermaking wastewater
Anaerobic-aerobic combined treatment anaerobic-aerobic combined treatment process can give full play to the advantages of anaerobic microorganisms to bear high concentration, high load and effective energy recovery, and can also use aerobic microorganisms to grow fast and have good water quality. advantage. The combined treatment process saves operating costs, has a small amount of excess sludge, has a modification effect on difficult-to-degrade organics, can improve the biodegradability of the wastewater, and can inhibit the growth of filamentous bacteria and prevent the sludge from swelling in an anaerobic state. It is particularly suitable for High-concentration organic wastewater treatment.
The comprehensive management technology of alkaline straw pulp wastewater mainly based on biological methods and supplemented by physical and chemical methods “comprehensive treatment technology based on biological method and physicochemical method†first adopts physical method (filtering), followed by biochemical method as the main means. Significantly reduce the organic load in the black liquor and mid-water, and only use the materialization method as an auxiliary means to realize the discharge or reuse of wastewater.
The two-phase anaerobic membrane-biochemical system adopts the system MBS combined with traditional two-phase anaerobic technology (BS) and membrane separation technology to treat the black liquor wastewater. The COD removal rate can reach 73% on average. The MBS system has higher stability. .
Physicochemical and biochemical methods Chemical precipitation, aeration, activated sludge, and anaerobic treatment can all be used to treat papermaking wastewater, and the combination of these methods is also applicable. Studies have shown that the use of SBR + physicochemical treatment of paper in the middle section of the water investment is low, the operating costs are low, the paper plant outside the discharge of stable quality standards, governance costs within the acceptable range of the manufacturer.