Mixed-culture fermentor for simulating methanogenic digestors.

Propionate degradation in an anaerobic digestor degrading animal waste (10-day retention time, 5.75 g liter day volatile solids loading rate, 40 degrees C) was 0.304 mM h, measured with [2-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. 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.