The importance of hydrogen in landfill fermentations

Forty-two samples taken from two landfills were monitored for CH4 production and apparent steady-state H-2 concentration. The rates of methanogenesis in these samples ranged from below the detection limit to 1,900 mu mol kg (dry weight)(-1) day(-1), and the median steady-state hydrogen concentration was 1.4 mu M in one landfill and 5.2 mu M in the other. To further investigate the relationship between hydrogen concentration and methanogenesis, a subset of seven landfill samples was selected pn basis of their rates of CH4 production, H-2 concentrations, sample pHs, and moisture contents. Samples with H-2 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 H-2 concentrations of > 100 nM had relatively high VFA levels (133 to 389 mmol of VFA kg [dry weight](-1)). Samples with high H-2 and VFA contents had relatively low pH values (less than or equal to 6.3). However, methanogenic and syntrophic bacteria were present in ail 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, H-2 is a critical intermediate that may be used to monitor the status of Landfill fermentations.