Mixed-Culture Fermentor for Simulating Methanogenic Digestors

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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- 14 C]propionate; this value indicated that CH 4 produced from propionate accounted for 14.8% of the CH 4 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 (H 2 , CO 2 , and acetate) equalled those measured in the digestor, propionate degradation was inhibited. When the H 2 turnover rate was lowered by decreasing addition of H 2 -generating substrates or by allowing a portion of the H 2 degradation to occur in an isolated compartment, propionate degradation in the fermentor resumed. The possibility is discussed that in digestors, much of the H 2 is produced and degraded within microenvironments associated with particles. Thus, the gross turnover rate of H 2 measured in digestors is an average, and specific microenvironments within the digestor may have different rates of turnover.