The objective of this work is to investigate the effects of surface area loading rates (SALRs) and hydraulic retention times (HRTs) in moving bed bioreactor (MBBR) systems on the morphology and thickness of the attached biofilm along with subsequent effects on particle size distribution and the settling characteristics of the biologically produced solids. The morphology of biofilm attached to the MBBR carriers changed from a porous biofilm to a biofilm with a more filamentous structure throughout the study at various operating conditions without observable correlation with SALR and HRT. Although, biofilm morphology did not demonstrate an effect on the biologically produced solids observed in this study, the thinnest biofilms resulted in the highest concentration of solids in the effluent. Furthermore, the particle size distribution analysis demonstrated that both higher SALRs and longer HRTs resulted in a shift towards larger-sized particles. Increases in SALR and HRT, independent of each other, also showed increases in effluent solid concentration and lower settleability of the solids.