Dry matter and sugar content as parameters to assess the quality of sugar beet varieties for anaerobic digestion

Abstract

Sugar beet is considered as biogas substrate because of its high yield. However, varieties differ in quality, in particular sugar content, which might affect biogas formation. The study aimed at analysing the impact of different beet qualities on biogas formation. Furthermore, parameters describing beet quality for anaerobic digestion should be found. From 2009 to 2011 field trials with several sugar beet varieties and a fodder beet variety were conducted with different N application rates at sites near Göttingen and Regensburg to get a broad range of beet qualities. The dry matter composition of beets, leaves, winter beet, bolters and maize was analysed. Discontinuous batch trials with fresh beet material were conducted to determine biogas formation. Sugar beet varieties did not differ in their dry matter composition and thus in biogas formation, whereas differences occurred between sugar beet and other substrates. Sugar beet was characterised by a high content of organic dry matter (98% oDM). The degradation time was mainly affected by the crude fibre content of the material. Therefore 90% of the biogas from sugar beet was formed within 3.5 days, whereas from winter beet, bolters and maize it took more than 10 days. The calculation of the specific biogas yield of different sugar beet qualities with two formulae resulted in lower values than determined in batch trials. A formula was derived to assess sugar beet quality for anaerobic digestion based on the sugar content, as increasing sugar contents lead to increasing specific biogas yields per kilogramme fresh matter.