The hydraulic characteristic of an up-flow anaerobic sludge blanket fixed film (UASB-FF) were studied by changing two important hydraulic factors that can impact significantly on the hydraulic regime of the UASB-FF bioreactor: the Up-flow velocity (Vup) and biogas production rate (Qg). The analysis of the reactor hydraulic performance was performed by studying hydraulic residence time distributions (RTD) obtained from tracer (Rhodamine B) experiments. The region of exploration for the process

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... s taken as the area enclosed by Vup (0.5 and 3.0 m/h) and Qg (14.87 and 7.96 l/d). Three dependent parameters viz. deviation from ideal retention time (∆τ), dead volume percentage and Morrill dispersion index (MDI) were computed as response. The maximum ∆τ and dead volume percentage were 33.58 min and 26 % at Vup of 0.5 m/h and Qg of 14.87 l/d, respectively. While, the minimum responses (4.15 min and 19.3 %) were obtained at Vup of 3.0 m/h and Qg of 7.96 l/d, respectively. The values of MDI computed at the minimum and maximum Vup and Qg are identified as 11.33 and 10, respectively, showing that the hydraulic regime in UASB-FF bioreactor is a semi-complete mixing.