Need of adjustment of methanogenic activities from solids digester sludge: Modelling dilution effects on micro-organisms biomass concentration.

Anaerobic digestion is a widespread technology used for organic-based solid waste management. Specific methanogenic activity tests are simple and cost-effective tools for sludge characterization and system diagnosis. However, in solid digesters, substrate and organic inert material dilute micro-organisms, enlarging the activity tests and distorting experimental results. To correct this situation, correction factors, defined as the ratio of micro-organisms concentration to total volatile solids content, are considered. Due to the impossibility to have a quick measurement of the mass of micro-organisms in the digester content, correction factors were evaluated based on the simulation of a simple model. To verify the importance of the correction, hydrogenotrophic and acetoclastic methanogenic activities were studied, involving a stage for sludge acclimatization and subsequent processing in a continuous digester. This situation was compared with a wastewater up-flow anaerobic reactor where no dilution effects are presented. A correcting factor of 0.79 was obtained for the acclimatization period, whereas correcting factors of 0.25-0.30 were estimated for the two periods of the digester. Tendencies shown for raw activities differed from those observed after using correcting factors to adjust activity values for periods 1 and 2 of the digester; also, the gap between the up-flow anaerobic reactor (without solids dilution effects) and the digester activities was reduced from sixfold to double, evidencing the relevance of this correction tool. Additionally, correcting factors also enabled a reasonable calculation of the inoculum size during the design of the activity tests.

Hydrogenotrophic activity: A tool to evaluate the kinetics of methanogens.

Anaerobic-digestion-based technology is key to achieving sustainable water management and resource recovery. It is essential to understand the material flux and kinetics involved in methanogenesis to optimize the organic matter removal and methane production. In this sense, specific methanogenic activity is a cost-effective tool to characterize the biological activity of anaerobic biosludge, to monitor the performance of reactors, and study the kinetics of acetate and H2 conversion to methane. Established protocols are applied for the acetoclastic activity test. However, hydrogenotrophic activity assay remains less widespread and is not standardized. In this work, the assay design for hydrogenotrophic activity is discussed and full calculation is presented, based on the kinetics for the H2/CO2 conversion to methane. An equation to calculate the inoculum size is proposed, suitable for a wide variety of types of biosludge: from a wastewater treatment plant to solid digesters, from a high-rate reactor to lagoons. The applied zero-order model fitted adequately to data for pilot-scale and full-scale anaerobic reactors: the p-values from the ANOVA F-test were below 1E-03; standard deviations for triplicate experiments were between 3 and 12%, coherent with the values found in the literature. Microbial growth during the test was negligible, below 1.2% of the biomass dosed in the vial. As a complement, acetoclastic activity was determined for each sample. The use of both acetoclastic and hydrogenotrophic activity is relevant for the study of the methanogenesis and gives a better characterization of the performance of the biosludge in anaerobic reactors rather than only using the specific acetoclastic methanogenic activity.