The performance of a microwave applicator loaded with a lossy dielectric sludge material is affected by several factors. In this study, the effect of the geometry, properties, and position of the sludge material load inside the oven on the microwave heating efficiency and electric field distribution were investigated numerically and experimentally. Electric field distribution was simulated for different load geometries, dielectric values, and positions of the sample in different locations inside the oven. The temperature distributions on the sludge samples were experimentally measured and found compatible with the simulation results, which show that the electric field distribution and efficiency of the system are greatly affected. In addition, we found the optimal location to improve the performance of the microwave applicator.