Wastewater from pulp and paper mills often holds suspended solids from fibres, wood and bank, mucilage, ash and filling material. Water consumption in paper production is enormous and so is the quantity of wastewater. Production of a ton of paper needs around 200m³ water.

Cleaning the water from the sieves in a pre-treatment stage reduces the use of fresh water. It also allows the recovery of some 0.5 to 5 Kg solids per m³ of water. The treatment itself has three stages: mechanical, chemical and biological.

The mechanical stage may include sedimentation, flotation or filtration, of which sedimentation is the one used most. It removes suspended fine particles from the water. To enhance sedimentation chemical flocculators may be used, such as aluminum sulfate or polyelectrolytes. The pH must be measured and set carefully, as the chemicals may modify its value.

The speed of biological cleaning depends to a large extent on the temperature, the pH and the concentration of nitrates and oxygen. For best operation the temperature should be between 20°C and 30°C, the pH between 6.5 and 8.5. The BOD-degradation is modest in a purely oxidizing tank, while the flow velocity must be kept low and the tanks should be large. Aeration chiefly improves oxidation. Measurements of pH, dissolved oxygen and the flow are important. Activated sludge tanks are used for large quantities of wastewater with a high BOD requirement. An advantage is the short detention time of the wastewater, through the installation is costly. The pH and the oxygen content should be measured and regulated.