Abstract
Propionate degradation in an anaerobic digestor degrading animal waste (10-day retention time, 5.75 g liter−1 day−1 volatile solids loading rate, 40°C) was 0.304 mM h−1, measured with [2-14C]propionate; this value indicated that CH4 produced from propionate accounted for 14.8% of the CH4 produced in the digestor (34.5%, including acetate produced from propionate). The mean propionate concentration was 0.67 mM, giving a propionate turnover rate of 0.46 h−1. A continuous-, mixed-culture fermentor was developed to mimic the digestor. When degradation rates of methanogenic precursors (H2, CO2, and acetate) equalled those measured in the digestor, propionate degradation was inhibited. When the H2 turnover rate was lowered by decreasing addition of H2-generating substrates or by allowing a portion of the H2 degradation to occur in an isolated compartment, propionate degradation in the fermentor resumed. The possibility is discussed that in digestors, much of the H2 is produced and degraded within microenvironments associated with particles. Thus, the gross turnover rate of H2 measured in digestors is an average, and specific microenvironments within the digestor may have different rates of turnover.
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