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Pulp:
According to manufacturing methods: it can be classified into chemical pulp (such as caustic soda method, sulfate method, sulfite method, etc.), mechanical pulp (such as stone-ground wood pulp, disk-milling mechanical pulp, hot-milling mechanical pulp, etc.) and chemical mechanical pulp.
According to raw material types: there are wood pulp (made from wood, which is the main type), non-wood pulp (such as bamboo, grass, sugarcane residue, etc.) and waste paper pulp (made from recycled waste paper).
According to processing depth: it is divided into unbleached pulp, semi-bleached pulp, bleached pulp, refined pulp, etc.
Definition: Pulp is a fibrous substance obtained by different processing methods from plant fibers, and is the basic raw material for papermaking.
Classification:
Papermaking: It is the process of turning the obtained pulp through a series of processes, such as sheeting, pressing, drying, etc., into paper or paperboard. Paper has various uses, such as writing, printing, packaging, etc.
Characteristics of the sources of pulp and paper wastewater:
Source:
Black liquor: The waste liquid from the alkaline pulping cooking process, with a dark color, is one of the main sources of wastewater in papermaking. Most paper mills in China use alkaline pulping to produce black liquor, which mainly consists of lignin, pentosan, and total alkali, etc.
Middle-stage water: The wastewater discharged during the screening, washing, bleaching, etc. of the pulping material after extracting the black liquor. The color is dark yellow.
White water: The wastewater from the papermaking section, originating from the papermaking process. It mainly contains fine fibers, fillers, coatings, dissolved wood components, as well as added adhesives, wet strength agents, preservatives, etc. Feature:
Hard-to-biodegrade organic matter: Mainly derived from the lignin and large-molecule carbohydrates contained in the fiber raw materials, such as lignin, inorganic salts, etc., it is difficult for microorganisms to decompose in the natural environment.
Easy-to-biodegrade organic matter: Includes the raw material components dissolved during the process, such as hemicellulose, resin acid, low-molecular sugar, alcohol, organic acid, and putrefactive substances, which can be easily utilized by microorganisms. However, if the concentration is too high, it will also cause harm to the environment.
Suspended matter: Mainly consists of fibers and fiber fines, broken fiber fragments, and impurity cells, etc., which will increase the turbidity of the wastewater and affect the subsequent treatment.
Toxic substances: Such as rosin acid and unsaturated fatty acids contained in the black liquor, and sulfuric acid hydrogen, methyl sulfide, methyl thioether, and various chlorinated organic compounds contained in the sewage condensate, etc., are toxic to aquatic organisms.
Acid-base substances: The alkaline pulping and bleaching processes will produce acid-base substances, causing changes in the pH value of the wastewater and possibly causing corrosion to equipment and pipelines.
Types of pollutants are numerous:
Large amount of wastewater: In the paper production process, a large amount of water is required, so the amount of wastewater generated is also large, especially the water volume of white water accounts for a large proportion.
High color: The residual lignin and other substances in the pulp wastewater make the wastewater have a high color, affecting the appearance.
Great difference in biodegradability: The biodegradability of black liquor and mid-stage water is poor, while the biodegradability of white water is relatively low, but the organic pollution load is relatively low.
Pulp and paper wastewater treatment process flow:
Preprocessing stage:
Grate discharge: Install a grate to remove large solid impurities in the wastewater, such as branches, plastic pieces, and fiber clusters, to prevent blockage of subsequent treatment equipment.
Sedimentation tank: Remove sand particles and coal residues from the wastewater, which are heavier inorganic particles, to reduce wear on equipment.
Equalization tank: Regulate the water quality and quantity of the wastewater to make the water quality and quantity stable, so as to facilitate subsequent treatment.
Black liquor treatment:
Alkali recovery method: This is an effective method for treating black liquor. Through the main processes of black liquor extraction, evaporation, combustion, and causticization, the suspended solids (SS), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) in the black liquor can be completely removed, and alkali can be recovered, generating secondary steam (energy).
Integrated treatment of mid-stage water and white water:
Anaerobic treatment: Common anaerobic treatment processes include upflow anaerobic sludge bed (UASB) and anaerobic fluidized bed, etc. Under anaerobic conditions, anaerobic bacteria decompose the large-molecule organic substances in the wastewater into small-molecule organic substances, reducing the COD of the wastewater and improving its biodegradability, while generating recoverable energy such as biogas.
Aerobic treatment: After the wastewater undergoes anaerobic treatment, it enters the aerobic treatment stage. Common aerobic treatment processes include activated sludge method and biological contact oxidation method, etc. Aerobic microorganisms further oxidize and decompose the organic substances in the wastewater, removing COD and BOD, and purifying the wastewater.
Air flotation method: For white water and some of the intermediate water, the air flotation method is a commonly used treatment method. After the wastewater is adjusted, it is mixed with the reduced-pressure dissolved air water in the air flotation tank, causing the fine fibers, fillers, etc. suspended in the water to adhere to the bubbles and float to the surface to form a sludge layer. Then the sludge is removed, which can reduce the suspended solids and some COD in the wastewater.
Biological treatment:
Deep treatment stage:
Coagulation and sedimentation: Add coagulants such as polyaluminum chloride (PAC) and polyacrylamide (PAM) to the wastewater that has undergone biochemical treatment. This causes the fine particles and colloidal substances in the wastewater to aggregate into larger particles, which then settle in the sedimentation tank. This further removes residual organic matter and suspended solids.
Filtration: Use filtration equipment such as sand filters and activated carbon filters to filter the wastewater, removing residual impurities and improving the clarity of the wastewater.
Disinfection: Through disinfection processes such as ultraviolet disinfection and ozone disinfection, kill bacteria and viruses in the wastewater, ensuring that the effluent meets the discharge standards or meets the requirements for reuse.
Case of wastewater treatment for pulp and paper production:
Case of a Large Paper Mill in Shandong:
Project Overview: This paper mill is a large enterprise in the local area, generating a large amount of wastewater daily.
Pre-treatment before treatment: The wastewater's COD, BOD, suspended solids, etc. indicators were severely exceeded, with high coloration and significant fluctuations in water quality.
Treatment Process: A combined process of "grate + regulating tank + alkali recovery + UASB + biological contact oxidation + coagulation sedimentation + filtration + disinfection" was adopted. Firstly, pre-treatment was carried out through the grate and regulating tank, then the black liquor was treated by alkali recovery. The middle-stage water and white water passed through UASB for anaerobic treatment and then entered the biological contact oxidation tank for aerobic treatment. Subsequently, they went through coagulation sedimentation, filtration, and disinfection for deep treatment.
Post-treatment water quality: The COD, BOD, suspended solids, and coloration indicators of the effluent all met the national discharge standards. Some of the treated water was reused in the production process, achieving water resource recycling and reducing the total amount of wastewater discharge.
Case of a Medium-sized Paper Mill in Jiangsu:
Project Overview: This factory mainly produces cultural paper, and before the wastewater treatment system upgrade, it faced problems such as non-compliant wastewater discharge and high treatment costs.
Pre-treatment before treatment: The wastewater contained a large amount of fibers, fillers, chemical agents, etc., with high COD and BOD concentrations and complex water quality.
Process Improvement: The original wastewater treatment system was upgraded, and an air flotation device was added to improve the removal effect of suspended solids and some organic substances; the biochemical treatment system was optimized, using more efficient bacterial strains and aeration methods to improve the biochemical treatment efficiency; in the deep treatment stage, an activated carbon adsorption device was added to further remove residual organic substances and coloration in the wastewater.
Post-treatment water quality: After the upgraded wastewater treatment system, the effluent quality was stable and met the standards, reducing environmental pollution, lowering wastewater treatment costs, and improving the economic and environmental benefits of the enterprise.
