Effect of aeration by micro - bubbles on methane production in anaerobic digestion

Abstract

This study aims to look at the effect of introducing controlled amounts of oxygen on biogas quantity and quality during anaerobic digestion. The anaerobic digestion process is studied, specifically, it’s biochemical processes, to understand the effect of addition of oxygen. Anaerobic sludge is aerated to mimic DAF (Dissolved Air Flotation) conditions, where tap water is pressurized (at 3 and 5 bar) and then depressurized in contact with the anaerobic sludge in a column reactor. During the course of this process, air micro-bubbles which were dissolved under high pressure are released due to contact with atmospheric conditions. To estimate and compare the methane production of the originally collected non-aerated sludge (anaerobic sludge not aerated in the column reactor, therefore, considered as 0 bar) and aerated sludge (anaerobic sludge subjected to high-pressure micro-air bubbles in the column reactor at 3 and 5 bar) a BMP test is conducted. Methane production was found to be lower in the aerated sludge with the BMP value for the 0 bar sludge being 296.17 +/- 45.15 NLCH4kg 1 and the value for 5 bar aerated sludge being 252.26 +/- 16.8 NLCH4 kg 1. Biogas composition of the aerated sludge was also examined with a Gas Chromatography (GC) machine and the percentage of methane, carbon dioxide and oxygen were measured for the 3 and 5 bar aerated sludge. For the 5 bar
aerated sludge, the overall percentages are averaged at 30%, 70%, and 1% respectively and for the 3 bar aerated sludge the average values are 20%, 80%, and 1% respectively. Furthermore, particle size distribution (PSD) analysis was done to compare variations for particle sizes between the aerated (5 bar) and non-aerated (0 bar) sludge. Very low variation was observed between these samples with the average size of the aerated samples being marginally smaller than the non-aerated sludge, indicating poor separation efficiency for the separation method adopted.