Contamination of the wash water during plastics recycling
When the plastics are washed, various substances dissolve into the process water, which can lead to considerable contamination. The composition of the wastewater depends heavily on the input material (post-consumer packaging vs. clean production residues, etc.). Typically, the wash water from used plastic packaging contains organic residues such as fats, oils and proteins, food residues as well as sugar, acids or other ingredients from packaging residues. These often significantly increase the biological and chemical oxygen demand (BOD5/COD) – BOD5 values of up to 12,000 mg/l and COD of up to 20,000 mg/l are not uncommon. The water also contains plastic abrasion, i.e. tiny plastic particles that detach from the flakes during shredding and washing – microplastics.
Washing chemicals and surfactants also contribute to the pollution. Detergents (alkalis, soaps, degreasers) are added to the wash water to dissolve grease and dirt. These chemicals remain in the water and must be neutralised or removed later. The printing inks removed from films and bottle labels during washing also end up in the wastewater. Printing inks contain pigments and binding agents that cause fine colour particles and dissolved organic substances in the water.