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.