Degradation of oleic acid in anaerobic filters: the effect of inoculum acclimatization and biomass recirculation.

The degradation of oleic acid in anaerobic filters was studied and the effect of an acclimated inoculum and biomass recirculation was evaluated. Three anaerobic filters (R1, R2, and R3) were operated in parallel. The anaerobic filters R1 and R2 were inoculated with nonacclimated biomass, whereas the anaerobic filter R3 was inoculated with acclimated biomass. In the anaerobic filters R2 and R3, biomass settling and recirculation were applied. The use of an acclimated inoculum and biomass recirculation (R3) was beneficial in terms of removal efficiency, which was 4 to 8% higher than in the anaerobic filters R1 and R2 when oleate was the sole carbon source fed to the reactors at an applied organic load of 12.5 kg of chemical oxygen demand (COD)/m3 x d, even with an oleate to calcium and magnesium ion molar concentration ratio of 6.8. Biomass recirculation significantly reduced the biomass washout and the toxic effect on the acetogenic and methanogenic populations. The use of an acclimated inoculum was beneficial in terms of methane yield, which was 50% greater than that observed for the reactors inoculated with nonacclimated inoculum for the highest applied organic loading rate (12.5 kg COD/m3 x d). At the end of the operation, the biomass was encapsulated by a whitish matter, which was well detected by microscopic examination. When this sludge was incubated in batch vials at 37 degrees C where no substrate was added, methane production from the adsorbed organic matter was evidenced, attaining a maximum value (at standard temperature and pressure) of 39.7 mL/g volatile solids x d for the biomass taken from R1. With stirring (150 r/min), the methane production rate was 13.8 times higher than under static conditions. When oleate was added to this sludge, methane production was delayed, suggesting that adsorbed matter can be an intermediate of oleate degradation such as stearic, palmitic, myristic, or other saturated acids.