Anaerobic digester foaming is a common operational problem in wastewater treatment plants. Several factors contribute to foaming. This study focused on filamentous organisms and solids retention time (SRT). PAX-18, which has been used in other facilities for foaming control in activated sludge basins, was added into one treatment train at the facility of study. These solids were digested in a bench-scale anaerobic digester with a 15 day SRT and compared to others that were operated with 10, 15, and 25 day SRTs without PAX-18 treatment. It was not possible to determine if PAX-18 had a significant effect on foaming from study results. The 25 day SRT bench-scale digester had less foaming than the ones with 10 and 15 day SRTs. Higher than average extracellular polymeric substance concentrations and lower than average calcium concentrations were associated with foaming events, but this only indicates correlation, not causality.