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Novel approaches to estimating the active or effective volume for anaerobic sludge digesters and assessing digester efficiency (DE) and volatile solids reductions (VSR) are presented in this paper. The active volume and the DE metrics are used in determining to what degree the current
DE and VSR may improve by increasing the digester's active volume. VSR for a given period is typically estimated using solids mass balances or the Van Kleeck equation (USEPA, 2003); in this study, a novel dynamic simulation is used for its estimation. Active volume is estimated as
the completely-mixed anaerobic sludge digestion modeling volume that adequately simulates the digested volatile solids concentrations reported in the facility's operational records for the period. DE is calculated as the average mass of degradable volatile solids degraded in the digester
divided by the average influent mass of degradable volatile solids during the period. DE is a better indicator of digester's performance than the VSR because it clearly indicates the progress of the volatile solids digestion in the digester. The maximum possible VSR is not fixed because
it depends on the non-degradable volatile solids content of the sludge; however, the maximum degradable volatile solids removal efficiency is always 100 percent. DE determination, though simple, takes 40 to 50 days because the non-degradable volatile solids concentration enters into its calculation. Volatile
solids digestion is modeled as first order with respect to degradable volatile solids. Two anaerobic sludge digestion-modeling configurations are used in this study: (1) batch and (2) non-steady state, completely-mixed, continuous flow. The batch configuration is used to measure the anaerobic
sludge digestion facility-specific degradable volatile solids digestion rate (kad) and the non-degradable fraction of the influent volatile solids. The continuous-flow configuration is used to dynamically estimate the active volume, the DE, and the VSR. For a constant volume
digester system, the active digestion volume and the nondegradable fraction of the influent volatile solids were used as continuous-flow model fitting parameters. The model outcomes proved that the model predictions during validation are acceptable. The values that provided the best fit for
this evaluation were 0.66 for the influent non-degradable volatile solids fraction and 0.84 for the active volume fraction. The 32-day average DE was estimated as 58 percent and the average VSR as 0.20.
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