The present invention discloses an up-flow anaerobic sludge blanket fixed film (UASB-FF) bioreactor (10) consists of up-flow anaerobic sludge blanket (UASB) with fixed film reactor in a single reactor and settling tank for recycling low suspended solid (SS). The fixed film reactor acts as buffer zone that against complete biomass washout alongside the provision of the internal sludge recirculation. The microbes enriched sludge and the retaining of the majority of dispersed and flocculated sludge in the lower part of the reactor by the fixed bed portion allows biomass recirculation that improved the interaction among the bacterial consortium which contributes to the prompt development of bio-granules, thus, significantly accelerated the start-up process. In a specific embodiment, the UASB-FF bioreactor (10) comprising: a reactor (12); an effluent feeding system (11); an isothermal operation system (13); a recycling and drainage system (14); and a biogas collection and measurement system (15).