The importance of hydrogen in landfill fermentations

Abstract

Forty-two samples taken from two landfills were monitored for CH4 production and apparent steady-state H2 concentration. The rates of methanogenesis in these samples ranged from below the detection limit to 1,900 μmol kg (dry weight)-1 day-1, and the median steady-state hydrogen concentration was 1.4 μM in one landfill and 5.2 μM in the other. To further investigate the relationship between hydrogen concentration and methanogenesis, a subset of seven landfill samples was selected on basis of their rates of CH4 production, H2 concentrations, sample pHs, and moisture contents. Samples with H2 concentrations of <20 nM had relatively small amounts of volatile fatty acids (VFAs) (undetectable to 18.6 mmol of VFA kg [dry weight]-1), while samples with H2 concentrations of >100 nM had relatively high VFA levels (133 to 389 mmol of VFA kg [dry weight]-1). Samples with high H2 and VFA contents had relatively low pH values (≤6.3). However, methanogenic and syntrophic bacteria were present in all samples, so the lack of methanogenesis in some samples was not due to a lack of suitable inocula. The low rates of methanogenesis in these samples were probably due to inhibitory effects of low pH and VFA accumulation, resulting from a thermodynamic uncoupling of fatty acid oxidation. As in other anaerobic ecosystems, H2 is a critical intermediate that may be used to monitor the status of landfill fermentations.