This paper reports on how the calculation of volatile solid destruction (VSd) that may provide a relatively inexpensive and quick method of evaluating digester active volume and mixing efficiency. When a digester deviates from ideal or complete mix conditions the Van Kleeck method of VSd calculation under predicts solids destruction, while calculation using the mass balance approach over estimates VSd. Using the difference between the two calculated VSd measurements it is possible to identify the true VSd of the system using an iterative calculation method. Combining the results of the iterative analysis, developed in this study, with the commercial process simulator BioWin, it was possible to estimate the loss of active volume in the digesters at San Jose’s water reclamation plant, approximately 17.5 percent. The results of this study suggest that not only a relatively rapid an inexpensive method for active volume estimation could be developed but also provides insights into the interpretation of digester process data, and suggests that operators should calculate VSd using both methods.