This work analysed the hydraulic behaviour and treatment efficiency of an upflow baffled septic tank (UBST) through tracer tests and mathematical modelling using the axial dispersion and the tank‐in‐series (TIS) models. The tracer tests were performed under different HRTs (12, 18 and 24 h) and configurations (UBST, UBST with sludge and UBST with sludge and zeolite filter). UBST followed a non‐ideal flow, and configuration modifications, rather than HRT, altered its hydraulic behaviour. Mathematical modelling indicated that the TIS model calculated with mean squared error (φ) described adequately UBST hydraulic behaviour (R2 = 0.9833). Additionally, UBST is recommended to operate under 24 h HRT to reach satisfactory hydraulic and pollutant removal efficiencies. At this HRT, total COD and ammonium removals were 75.1% and 49.3%, respectively, which were better than those obtained without the zeolite (71.2% and 1.8%, respectively). However, it quickly saturated, so it is necessary to deepen the research on this topic.