The usable potential of municipal biological waste as a source of bio-based products—the example of a composting plant | Das nutzbare Potenzial biologischer Siedlungsabfälle zur Erzeugung biobasierter Produkte – Beispiel Kompostwerk

From September 2013 to May 2014, a feasibility study was conducted in collaboration with the Enterprise for Municipal Services (Eigenbetrieb für kommunale Aufgaben und Dienstleistungen (EAD)), Handelshaus RUNKEL, the engineering firm INGUT, JAGER BIOTECH GmbH and the TU Darmstadt (Research Area: Waste Engineering), the goal of which was to determine whether biological waste produced by the city of Darmstadt could be used to create a fuel on the basis of non-polar carboxylic acids.In this context, the biological waste was first assessed with regard to its composition and material characteristics, which revealed that the majority of collected biowaste consisted of yard waste with higher lignin content: on average, 65 % by weight of the biowaste fell into this category.Subsequently, the different types of waste collected were separately treated at a composting plant. With the addition of a modified rotting box, solid-liquid filtering was integrated at the plant. This made it possible to separate the liquid-phase from the solid-phase waste; the former, also referred to as percolate, was then used for the manufacture of bio-based products.Treatment in the modified rotting box resulted in a percolate with a high percentage of organic substances. Though non-polar carboxylic acids could already be detected directly after percolation, a total of only 26 % by weight of the carboxylic acids present were determined to be non-polar. In order to increase the percentage, additional after-treatment steps were initiated. With the help of post-fermentation, the dissolved organic substance was further broken down into carboxylic acids, increasing their concentration by as much as threefold. An additional processing step, ethanol fermentation, was added after the post-fermentation. Thanks to the addition of ethanol in the course of this step, anaerobic microorganisms converted the polar carboxylic acids into non-polar ones. The tests with ethanol fermentation have shown that the method is especially well suited to the production of carboxylic acids with even numbers of carbon atoms, e.g. acetate, butanoic and hexanoic acid.In a refining process, the non-polar carboxylic acids were first extracted from the percolate, then transesterified to produce bio-based fuels.