Moving-Bed Bioreactors (MBBRs) are commonly used for treatment of combined effluent treatment (domestic and industrial effluent combination). While MBBRs usually give significant reduction in pollutants, the carrier element fraction also plays an important role. If the carrier element concentration becomes too high, the micro-organisms get detached from the surface and get removed out of the system. This lowers the pollutant removal efficiency. Also, if the carrier element concentration is too high, more aeration is required to keep them in suspended form and increases operation cost of the plant. Hence, optimization of carrier element concentration is crucial for efficient effluent treatment. The main objective of this study was to understand correlation of carrier element fraction to the COD and phosphate removal. Based on the results obtained for the effluent selected for study, 60% carrier element fraction was found to be optimum in reduction of COD & phosphate.