This book describes a simplified approach to the modelling and process design of a fixed bed hybrid bioreactor for wastewater treatment. In this work a simplified model for hybrid bioreactor is developed to determine output parameters like exiting substrate concentration in bulk liquid, average substrate flux in the biofilm, effective and total biofilm thickness. The model is based on mass balance of both carbonaceous substrate and biomass under suspended and attached growth simultaneously along with substrate mass transport into the biofilm. The proposed model has also been validated with the results obtained from experimental study with municipal wastewater considering as a low strength wastewater with no inhibition. There is a flexibility of the proposed model making it a versatile one to find out the exiting substrate concentration both in hybrid bioreactor as well as in a completely mixed biofilm reactor (CMBR). The book caters to academics and practitioners working in the field of advanced wastewater treatment.