The main objective of this research is to examine the modification of a well-known immobilization technique of biomass to enhance the aerobic biodegradation of phenol at high initial concentrations. This work focused on testing the effect of entrapping different low-cost mineral additives (clay and activated carbon) at the immobilization matrix, while considering the diffusive internal mass-transfer limitations. Aerobic biodegradation of phenol was performed using isolated microorganism cells from a compost pile of agricultural waste. The influence of a different initial concentration of phenol (400-2000 mg/L) on the rate of biodegradation was tested in systems that were based on free and immobilized cells. The results show that immobilized cells in modified immobilizers tolerated and completely degraded phenol at initial concentrations of 2000 mg/L and higher. A bead size of 4 mm (diameter) and a composition of alginate with clay and activated carbon was determined to be the optimal and most-effective bead size and composition for the immobilized biomass to degrade phenol at an initial concentration of similar to 2000 mg/L. The bead size significantly affected the biodegradation rate of phenol, where the highest biodegradation rate was obtained for the 1-mm beads and the lowest value was obtained for the 6-mm beads. Immobilized cells in 1-mm beads of alginate with clay and activated carbon could not tolerate an initial phenol concentration of 2000 mg/L at the first run. However, increasing the initial phenol concentration gradually enables the cells to utilize phenol at initial concentration of 2600 mg/L in the third run, using the same beads.