Hydrocyclones for the separation of impurities in pretreated biowaste

The aim of the mechanical pretreatment in case of anaerobic digestion of biowaste is to produce a substrate without impurities. To facilitate a failure free operation of the anaerobic digestion process even small impurities like stones or sand should be separated. As a result of an insufficient pretreatment or impurities separation, plant malfunctions, increased equipment wear or pipe clogging are reported. Apart from grit chambers or pulper systems, a hydrocyclone is a cost-efficient and space-saving option to remove impurities. The aim of this work was to investigate the efficiency of hydrocyclones for the separation of impurities. Two hydrocyclones at two different plants were investigated regarding their capability to separate the small inert impurities from pretreated source separated biowaste. In plant A, the hydrocyclone is part of the digester system. In plant B, the hydrocyclone is part of the biowaste pretreatment line (after milling and sieving the biowaste) before digestion. Separation rates of inert impurities such as stones, glass and sand were determined as well as the composition of the concentrated solids separated by the hydrocyclone. Due to the heterogeneity of the biowaste the impurity separation rates showed variations, therefore the following mean results were obtained in average: the investigated hydrocyclones of plant B, part of the biowaste treatment, separated more than 80% of the inert impurities in the waste stream before anaerobic digestion. These impurities had a size range of 0.5-4mm. The hydrocyclone integrated in the digester system of plant A showed separation rates up to 80% only in the size range of 2-4mm.