1. S. Tanigawa (EESI): Fact Sheet – Biogas: Converting Waste to Energy; http://www.eesi.org/papers/view/fact-sheet-biogasconverting-waste-to-energy; Download from 02.03.19.
2. Biogas. Fundamentals, Process and Operation;Tabatabaei,2018
3. Fachagentur Nachwachsende Rohstoffe e. V. (FNR): Guide to Biogas – From production to use; https://mediathek.fnr.de/media/downloadable/files/samples/g/u/guide_biogas_engl_2012.pdf; 5. Edition, Gülzow, 2010. p. 21–22.
4. K. Peltier: The Spruce: Acetic Acid: Definition, Cleaning Uses, Safety, & More; Last Update: 11.09.18; https://www.thespruce.com/acetic-acid-definition-cleaning-uses-1707015; Download from 02.03.19.
5. Dow Chemicals: Product safety assessment: Propionic acid; Last Update 03.02.15; http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_096d/0901b8038096db9e.pdf?filepath =productsafety/pdfs/noreg/233-00419.pdf&fromPage=GetDoc; Download from 02.03.19.