The aim of this paper was to present the influence of hydraulic behaviour in the treatment of sewage (domestic wastewater) using panelled anaerobic baffle-cum filter reactor (PABFR). The PABFR has five compartments of equal size in which the first three compartments operate as anaerobic baffled reactor (ABR) followed by anaerobic filters (AF). The combined reactor has a great potential for its application due to its panelled structure and arrangement of baffles inside each compartment. Hydrodynamic behaviour of the reactor was determined by means of a pulse tracer test and by calculating the residence time distribution curves at different flow rates. At high flow rates [with hydraulic retention time (HRT) \4 h], the mixing pattern in ABR showed a completely mixed type with a maximum dead space of 14 % and as the flow decreased (HRT of 8 and 12 h), the ABR's mixing behaviour was intermediate between plug flow and completely mixed. In the case of AF, as the flow increased, the dispersion was intermediate between completely mixed and plug flow, and as the flow rate decreased, the reactor became completely plug flow with the dead space ranging between 2.2 and 7.4 %. On the whole, PABFR treatment performance in terms of chemical oxygen demand removal was 90, 89 and 64 % for 12-, 8-and 4-h HRT, respectively, which clearly shows the correlation between mixing and reactor process performance.Thus, the outcome of this research in general highlights the importance of hydrodynamic behaviour for a better operation of the reactor.