Innovative two-stage fermentation system converting organic solid waste to hydrogen and methane

Abstract

Hydrogen is regarded as the most promising future energy carrier because of its environmentally friendly aspect and high energy yield. Conventional physico-chemical methods for $H_2$ are energy intensive and generate green house gases; thus interest in biohydrogen has increased substantially. Among various biological ways, dark fermentation is considered to be practically applicable since its production rate is fast and could degrade organic solid waste such as food waste and sewage sludge. However, as there is a limitation of waste degradation and energy recovery in $H_2$ fermentation, second stage process such as $CH_4$ fermentation should be followed. Therefore, the main objective of this work was to develop an innovative two-stage fermentation system converting organic solid waste to $H_2$ and $CH_4$. Firstly, continuous pilot-scale $H_2$ fermenter treating food waste was operated at various C/N ratios. Although stable $H_2$ production was achieved after 45 d of start-up, $H_2$ yield was limited to 0.53 mol $H_2/mol$ hexose_{added} which is equivalent to 2.43% of total energy content in the food waste. Moreover, increase of C/N ratio decreased the $H_2$ yield, and alkaline shock treatment was not a permanent method recovering the performance. In order to overcome the limited performance of current $H_2$ fermentation method treating organic solid waste, an innovative method named ‘$H_2$ fermentation of food waste without adding external inoculum’ was suggested. This method was found to be feasible if proper pretreatments were applied by inhibiting LAB activity. Especially, the heat treatment (90\degC for 20min) was the most effective method and $H_2$ production was increased with the pretreatment heating temperature increase. Optimal substrate concentration was 30 g Carbo. COD/L and the decrease of $H_2$ yield at higher substrate concentration was due to product inhibition. Scaled-up fermenter (3 L) where heat-treated food waste was added showed a clear...