Bio-Gasification Under Partially Aerated Conditions

Abstract

Anaerobic digestion (AD) under mild aerobic loading is investigated. Aeration can apparently have both positive and negative impacts on biogas generation. This study aims to recognize and describe these different aeration effects on AD. Seven series of batch experiments (BT 1- 7) were conducted under varying feed and oxygenation conditions at 35 oC. Oxygen was introduced as headspace air in four of the batch experiments and as pure oxygen in two series. Effects of extensive initial aeration of inoculum were investigated in one batch series. The methane yield of the reactors fed with starch (1.09 g COD/LReactor) was positively influenced by oxygenation in the load range up to 38 % O2 (as % of feed COD). A negative influence of oxygen was observed at the next experimental level of 79 % O2. For the glucose fed reactors under similar operating conditions, the oxygen impact was near linearly negative in the tested range of 0 - 49 % O2. These results demonstrate that digestion of soluble substrates like glucose is not enhanced by O2, while substrates requiring hydrolysis are. Thus, limited aeration can enhance biogas production from non-soluble substrates under batch conditions. According to the experiment where the effects of extensive initial aeration was investigated, lengthening of the aeration period increasingly slowed down the gas generation rate but eventually resulted in the same methane yield. The lag period before gas generation started was three times longer for the aerated inoculum reactors than un-aerated. These observations suggest that oxygen induced inhibition of the biomass activity is mostly reversible. Reduced biogas production rates with initial aeration extent suggest some permanent damage to the anaerobes. Extensively aerated inoculums generated less methane compared to the un-aerated inoculum, probably due to the loss of some biogas potential initially present in the inoculum. Oxygen assisted degradation of some recalcitrant organic matter, including biomass cells themselves, present in the inoculum is also detected in an unfed inoculum test series.