Wastewater Treatment in the Paper Industry
The paper industry uses very high volumes of process water. Correspondingly, there are also high requirements for wastewater treatment and energy-saving wastewater technology. No matter which product is made or which process is used in a factory, water plays a key role. Because of the costly nature of water processing, paper manufacturers around the world have in recent years worked hard to reduce their water footprint. Today, the production of paper, cardboard and pulp in Germany still requires an average of almost eleven litres of water per kilogram of product, and some countries use even more.
Heat Recovery from Process Water in a Paper Factory
Water is needed to make paper pulp out of fibres such as cellulose, wood or recovered paper. If recovered paper is used, the print colours are often removed using a suitable de-inking process. The paper pulp is then treated with a range of additives such as fillers, colourants or auxiliary materials and finally processed into a paper web in the paper machine. Most of the water is removed at this point using meshes and heated cylinders. This wastewater must undergo extensive purification before it can be returned to the river from which it was taken.
The paper industry is one of the most energy-intensive industry sectors in the world. Heated wastewater from the process chain is problematic not only for treatment processes and the environment. Even in technologically advanced Europe, a factory still loses around 300 kilowatts of energy per ton, according to wastewater data from the Papiertechnische Stiftung (PTS) in Munich, Germany. The use of heat exchangers can significantly improve this energy balance. A heat exchanger designed for wastewater loaded with solids makes it possible to recover heat energy from the wastewater and return it to the process water.
Wastewater in the Paper Industry
Wastewater from paper factories includes substances such as de-inking chemicals, bleach, process chemicals and additives, in addition to leftover fibres. It is characterised by a high chemical oxygen demand (COD) and is predominantly treated first mechanically, followed by full biological treatment in either a factory-operated or municipal wastewater treatment plant.
To save on costs, some plants already handle water in a closed loop. However, problems such as salting, lime sedimentation, foam formation and odour release must all be additionally dealt with. If the water is to be reused in production, then the quality standard of purification must be high.