Recirculation of Process Water in a Wet Fermentation of Organic Fraction of Municipal Solid Waste

Document Type: Original Article


Institute of Waste Management and Circular Economy, Technische Universität


The mechanical biological treatment plant of Freienhufen is used to stabilize residual waste. Since the rural
districts Elbe - Elster and Oberspreewald - Lausitz match their waste management with federal law, organic
fraction of municipal solid waste (OFMSW) will be collected separately in future. Hence the anaerobic
digestion process has to be converted. The accomplishment has to refer to the existing operating regime to
reduce investment costs. This contains a wet fermentation. In order to facilitate the conversion of the
operating process, suitable particle sizes and volumetric loads have to be examined. In addition, the liquid
phase of the digestate shall be recirculated maximal to save both fresh water and waste water disposal costs.
The one year lasting investigations were performed in lab-scale with a various number of reactors. Before
feeding the bio-waste was pre-treated. In order to do that, the bio-waste was milled to particle sizes of 2, 4, 8,
and 10 mm. In addition, the digestate was dewatered to gain process water. While using the process water
fresh water was substituted in varying proportions. The feeding of the reactors was adjusted to the standards
of the operating plant. For that reason, the dry matter content in the reactor was adjusted at 10.5 %.
Depending on the delivered raw material, this restriction led both to unsteady water requirements and
volumetric loading. As a result of investigations an optimal particle size as well as optimal proportion of
recirculated process water were defined. For that reason, comprehensive analyses were conducted weekly to
characterize the delivered raw material as well as the solid and liquid phase of the digestate in order to
determine critical moments due to recirculation of process water. In conclusion, liquid and solid phase of the
digestate should be evaluated with regards to application as fertilizer.