Pulp wastewater composition and characteristics

Wastewater from papermaking industry refers to waste water produced during the pulping and papermaking production process. Paper industry production is divided into two major process stages, namely pulping and papermaking. Pulping is to separate the fiber from the plant raw materials into pulp and then bleaching. In paper pulp, the pulp is diluted, formed, pressed and dried to make paper. Both processes consume large amounts of water, most of which are discharged as waste water.

Pulp and papermaking wastewater can be roughly divided into: pulping cooking liquor, washing waste water, bleaching waste water, and paper machine white water. Alkali pulp cooking waste, also known as "black liquor", is the main source of pollution for pulp mills.

The composition of pulp and papermaking wastewater is complex, and its composition depends not only on the pulp method but also on the variety of products and the types of raw materials. The suspended solids in wastewater mainly come from fibers and fillers lost in pulp and papermaking processes; BOD mainly comes from pulping and cooking processes, such as sugars, alcohols, and organic acids produced by the decomposition of cellulose. In chemical pulps, cooking takes place. The amount of BOD5 in the waste liquid accounted for more than 80%; COD and coloring substances mainly originated from the lignin and its derivatives in the pulping and cooking process.

The main four characteristics of pulp and papermaking wastewater:
1. High concentration of pollutants. Especially pulping line wastewater contains a large amount of raw material leachables and chemical additives, and its BOD5 concentration is even as high as 104 mg/L or more.
2. There are many components of hard-to-degrade organics, and their biodegradability is poor. Lignin, cellulose and other substances are difficult to be degraded by the activated sludge process. For example, the BOD5 wastewater from paper mills has been reduced to less than 10 mg/L, but the COD is still above 200 mg/L, and the color is still very high.
3, the waste water composition is complicated. In addition to raw material leachables, some contain chemicals such as sulfides, inks, and flocculants that are unfavorable to biochemical treatment.
4. Large fluctuations in waste water flow and load, accompanied by fiber and chemical spillage. This is more evident in factories with multiple production lines. Water volume and load fluctuations are very unfavorable to the stable operation of biochemical treatment systems.