Overview

The pulp and paper industry is highly water-intensive, requiring approximately 300–500 m³ of fresh water to produce 1 ton of paper. Paper mill wastewater contains a significant amount of raw materials—about 20% of fibers are lost in the process—as well as chemicals and various impurities. Without proper treatment, this wastewater can cause severe environmental pollution. Understanding its characteristics is essential for selecting the appropriate treatment process.

Project Profile

Characteristics of Paper Mill Wastewater

  • Black liquor produced during pulping (wood or straw pulp).
  • Beater wastewater discharged from pulping and refining equipment.
  • Papermaking machine wastewater, where reusable water is known as “white water”.

Basic Treatment Methods

Wastewater from recycled paper pulping contains high levels of suspended solids (SS) and COD. COD consists of non-dissolved and dissolved components, with non-dissolved COD being the primary portion. Removing SS also removes most of the non-dissolved COD.

The BOD5 level of paper mill wastewater is relatively low, with a BOD5/COD ratio of 0.15–0.25, indicating poor biodegradability. Coagulation removes only part of the BOD5, so biological treatment is essential for effective removal.

Main Pollutants

  • Suspended solids: fibers and fines.
  • Easily biodegradable organics: hemicellulose, methanol, acetic acid, formic acid, sugars.
  • Refractory organics: lignin and high-molecular-weight carbohydrates.
  • Toxic substances: rosin acids, unsaturated fatty acids.
  • Acid/alkali pollutants: alkaline pulping pH 9–10; acidic pulping pH 1.2–2.0.
  • High color: caused by residual lignin.

Sinomeasure Instruments Used