Circular and/or bioeconomy are common terms used by the politicians in Finland and the EU. Among others, here in Finland the newly elected government has set goals for bioeconomy related projects, so called spearhead projects, which should yield us new innovations in the field. The terms give to the general public really unclear picture, but let us go a step closer to real applications.
An example of circular economy is the circulation of nutrients from waste water side streams in agriculture. Waste water treatment produces both solid and liquid side streams. The formed sludge, i.e. the biologically treated organic matter, is a potential nutrient source for re-use. The side stream cannot be directly circulated by spreading it on the fields, used for crop production, due to mainly health reasons exhibited by the present pathogens.
As stated, the solid stream contains considerable proportion of nutrients, which could be used as fertilizers in the fields. This recirculation is possible by first destroying the pathogens through pyrolyzing the solid material to biochar. Note that the feasibility of the processing is also highly dependent on the water content of the solid material. In addition to the biochar, also bio-oil and gas are produced in the pyrolysis. The gas has a relatively high energy value, which is typically burned. Bio-oil could be used as a liquid fuel, but due to the high water content and chemical instability of the bio-oil, its usage has been modest. Naturally, the overall economics of this type of processing are against it. On the other hand, the bio-oil contains also a number of valuable components like levoglucosan and carboxylic acids. Separation and recovering these components to pure products could result into considerably better economics. It is, however, completely different question, which should be the correct separation sequence for this to work. This is a question, which I and my fellow researchers should answer.
Text: Jani Kangas