In the papermaking process, including pulping, bleaching, and papermaking, high-concentration sewage, namely blackwater and whitewater, is generated. Fiber separation occurs during pulping, while dilution, pressing, and drying of the pulp during bleaching and papermaking processes exclude pollutants containing a large amount of fiber, inorganic salts, and pigments, resulting in highly concentrated sewage, namely blackwater. Significant amounts of middle-stage water, containing high concentrations of lignin, cellulose, and resin acids, which are difficult to biodegrade, are produced during the washing and bleaching processes. Additionally, whitewater, discharged from the paper machine, contains large amounts of fiber, filler, and adhesive sewage.

Characteristics of sewage:

sewage from paper mills typically contains a large amount of fiber, inorganic salts, and pigments in blackwater, while whitewater contains high concentrations of lignin, cellulose, and resin acids, which are difficult to degrade. The main pollutants include high COD and SS (suspended solids), and the color is severe.

Treatment Methods:

First, the sewage undergoes pretreatment to remove sand and large particles of suspended matter for reuse. The remaining sewage then passes through a grid to intercept short fibers for recycling. The sewage enters the regulating tank after passing through the grid, where the water quantity and quality are mainly adjusted. Mechanical stirring is applied at the front end of the regulating tank, and PAC and PAM are added for coagulation. The sewage, after coagulation, is pumped by a lifting pump to the high-level sedimentation tank. After sedimentation, the supernatant flows into the shallow pond for further treatment through shallow pond air flotation. It is possible that the effluent from air flotation still does not meet the standards for COD and BOD, so a biological filter tank is set up after air flotation to ensure that the sewage meets the standards before discharge.