Effect of ammonia on methane production pathways and reaction rates in acetate-fed biogas processes.

In order to understand the correlation between ammonia and methanogenesis metabolism, methane production pathways and their specific rates were studied at total ammonium nitrogen (TAN) concentrations of 0.14-9 g/L in three methanogenic sludges fed with acetate, at both mesophilic and thermophilic conditions. Results showed that high levels of TAN had significant inhibition on methanogenesis; this could, however, be recovered via syntrophic acetate oxidation (SAO) coupled with Hydrogenotrophic Methanogenesis (HM) performed by acetate oxidizing syntrophs or through Acetoclastic Methanogenesis (AM) catalyzed by Methanosarcinaceae, after a long lag phase >50 d. Free ammonia (NH3) was the active component for this inhibition, of which 200 mg/L is suggested as the threshold for the pathway shift from AM to SAO-HM. Methane production rate via SAO-HM at TAN of 7-9 g/L was about 5-9-fold lower than that of AM at TAN of 0.14 g/L, which was also lower than the rate of AM pathway recovered at TAN of 7 g/L in the incubations with a French mesophilic inoculum. Thermophilic condition favored the establishment of the SAO-catalyzing microbial community, as indicated by the higher reaction rate and shorter lag phase. The operational strategy is thus suggested to be adjusted when NH3 exceeds 200 mg/L.

Influence of the geomembrane on time-lapse ERT measurements for leachate injection monitoring.

Leachate recirculation is a key process in the operation of municipal waste landfills as bioreactors. To quantify the water content and to evaluate the leachate injection system, in situ methods are required to obtain spatially distributed information, usually electrical resistivity tomography (ERT). However, this method can present false variations in the observations due to several parameters. This study investigates the impact of the geomembrane on ERT measurements. Indeed, the geomembrane tends to be ignored in the inversion process in most previously conducted studies. The presence of the geomembrane can change the boundary conditions of the inversion models, which have classically infinite boundary conditions. Using a numerical modelling approach, the authors demonstrate that a minimum distance is required between the electrode line and the geomembrane to satisfy the good conditions of use of the classical inversion tools. This distance is a function of the electrode line length (i.e. of the unit electrode spacing) used, the array type and the orientation of the electrode line. Moreover, this study shows that if this criterion on the minimum distance is not satisfied, it is possible to significantly improve the inversion process by introducing the complex geometry and the geomembrane location into the inversion tools. These results are finally validated on a field data set gathered on a small municipal solid waste landfill cell where this minimum distance criterion cannot be satisfied.