Solids mass balancing is an effective tool to optimize plant operation and assess future capacity needs in wastewater treatment plants. Two case studies are presented that illustrate applications of mass balance modeling. A computer spreadsheet model was developed and applied in mass balance evaluations at the two treatment facilities. At the Chambers Creek Wastewater Treatment Plant, a mass balance evaluation was conducted as part of the plant re-rating and optimization study. The evaluation identified a number of gaps and discrepancies in the plant data, including significant solids and BOD removal across the fine screens and under-representation of the primary effluent concentrations due to the sampling configuration. The various data discrepancies were accounted for in the mass balance model before applying the model in the plant capacity evaluation. At the South Treatment Plant, a mass balance analysis was conducted to estimate future solids production rates and to identify any potential problems with the precision of solids data, data collection and sample analysis. In addition to identifying the sources of some data discrepancies, the analysis provided an estimate of the unit solids production rate that was about one-third lower than a previous value estimated based on limited mass balance calculations. This unit solids production rate was subsequently used to size the expansion of the solids treatment facility.