Straw stalk and wood pulp are widely used as raw materials in China's paper industry. Pulping and papermaking generally consists of pulping, washing, bleaching and papermaking processes. Pulping in paper industry includes alkali pulping, chemical mechanical pulping and mechanical pulping. According to different pulping methods, different raw materials, different pulping yield, different kinds of papermaking and whether or not chemicals are recovered, the occurrence and discharge of pollutants are very different, but basically contain a large number of suspended substances, BOD, COD and some toxic substances. Usually, physical and biological methods are still used to treat the wastewater.

Sources of papermaking wastewater

(1) Waste liquor from cooking wood pulp (or straw pulp), also known as black liquor.

(2) Wastewater discharged from beaters and refiners is called beating wastewater.

(3) Paper mill wastewater, which can be used directly, is called white water.

Characteristics of Papermaking Wastewater

Paper mill wastewater is a kind of industrial wastewater which is difficult to treat because of its complex composition and poor biodegradability.

(1) Suspended solids: including settling suspended solids and non-settling suspended solids, mainly fibers and fine fibers (i.e., broken fibre fragments and miscellaneous cells).

(2) Biodegradable organic compounds: including low molecular weight hemicellulose, methanol, acetic acid, formic acid, carbohydrates, etc.

(3) Refractory biodegradable organic matter: mainly from lignin and macromolecular carbohydrates contained in the raw materials of fibers.

(4) Toxic substances: rosin acid and unsaturated fatty acid in black liquor.

(5) Acids and alkali poisons: the pH value of alkali pulping wastewater is 9-10, and that of acid pulping wastewater is 1.2-2.0.

(6) Chromaticity: Residual lignin in pulping wastewater is highly colored.