Case Study on Wastewater Treatment in a Paper Mill

       The Problem

　　Wastewater from paper mills primarily originates from the beating, washing, and papermaking sections, with additional contributions from pulping and bleaching processes. This wastewater contains high levels of suspended solids, recalcitrant organic compounds, chemical oxygen demand (COD), and biochemical oxygen demand (BOD). In some paper mills, the effluent also includes complex pollutants such as ink, adhesives, and fillers. The wastewater is characterized by high suspended solids (SS) concentration, a low BOD₅/COD ratio, poor biodegradability, and significant fluctuations in water quality.

　　Client Requirements

　　The wastewater discharge complies with the "Discharge Standard of Water Pollutants for Pulp and Paper Industry."

　　Maximize wastewater reuse to reduce water consumption and effluent discharge, thereby enhancing both economic and environmental benefits for the enterprise.

　　The reclaimed water must meet the stringent requirements of papermaking processes regarding parameters such as hardness, conductivity, and pH.

　　Treatment challenges

　　It is challenging to select an appropriate pretreatment process; a suitable method must be chosen to remove suspended solids and enhance the biodegradability of the wastewater.

　　Biological treatment is challenging due to the poor biodegradability of the wastewater. An appropriate biological process must be selected and its operating parameters optimized, while maintaining system stability to prevent sludge bulking or loss of microbial activity.

　　If an anaerobic-aerobic combined treatment process is adopted, the control and management of the anaerobic stage are particularly challenging, requiring stable operation of anaerobic microorganisms and prevention of gas leakage.

　　When using a combined treatment of coagulation-sedimentation and decolorizing agents, it is challenging to determine the optimal dosages of coagulants and decolorizing agents and to control the pH during the treatment process.

　　The diverse sources of waste paper lead to significant fluctuations in wastewater quality, placing high demands on the adaptability of the treatment process.

　　Treatment effectiveness

　　Through comprehensive treatment, the wastewater discharged from paper mills can meet the "Discharge Standards for Water Pollutants from Pulp and Paper Industry." Part of the treated wastewater, after further advanced treatment, can be reused as process water in papermaking, achieving wastewater recycling. This significantly improves environmental quality and enhances both the economic and environmental benefits for the enterprise.

　　Treatment processes and water treatment chemicals

　　Pretreatment

　　Bar screens and sedimentation tanks: Initially intercept large suspended solids and remove a portion of suspended solids and some dissolved organic matter through gravity settling, without the need for chemical additives.

　　Dissolved air flotation (DAF) unit: Removes fine suspended solids and ink particles through flotation. Coagulants (e.g., polyaluminum chloride, polyferric sulfate) and flocculants (e.g., polyacrylamide) may be added to promote particle aggregation and enhance flotation efficiency.

　　Biological treatment

　　Hydrolysis and acidification: A hydrolysis-acidification tank is used to convert recalcitrant large-molecule organic compounds into readily biodegradable small-molecule organics, thereby improving biodegradability. Typically, no specific chemicals are required, but appropriate environmental conditions—such as pH and temperature—must be maintained.

　　Sequencing Batch Reactor (SBR): Simultaneously removes organic matter and ammonia nitrogen through five cyclic phases—filling, reaction, settling, decanting, and idle—to achieve efficient biological treatment. Acid or alkali additives may be required during operation to control pH.

　　Anaerobic–aerobic combined treatment process: In the anaerobic stage, anaerobic bacteria convert large-molecule organic compounds into smaller molecules; in the subsequent aerobic stage, aerobic bacteria further degrade the organic matter. Nutrients may need to be added during the anaerobic phase to support microbial growth.

　　Advanced treatment

　　Sand filtration: Further removes suspended solids (SS) and some dissolved organic matter, without the need for chemical additives.

　　Ultrafiltration (UF) and Reverse Osmosis (RO) systems: Further remove dissolved organic matter and impurities to enable wastewater reuse. Scale inhibitors, biocides, and other chemicals may be added during operation to protect the membrane modules.

　　Coagulation, sedimentation, and decolorization: Coagulants (e.g., polyaluminum chloride, polyferric sulfate) and decolorizing agents are added to aggregate and precipitate colorants and suspended solids in the wastewater, thereby removing color and suspended matter. Acid or alkali pH adjusters may also be required to control the pH during the process.

　　Disinfection: Ultraviolet (UV) disinfection or chemical disinfectants (e.g., sodium hypochlorite) are used to ensure that the effluent meets microbiological standards.

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