Five carrier media with different shapes (spherical and cylindrical), sizes, voidage and protected surface areas (112-610 m(2)/m(3)) were studied in a pilot scale moving bed biofilm reactor (MBBR). This study aimed at assessing start-up duration using biofilm formation rates. Results indicated that the spherical media required shorter periods to achieve stable biofilm formation rates associated with chemical oxygen demand (COD) (15-17 days), compared to cylindrical high surface area media (23-24 days). Protected surface area presented weaker correlations with the biofilm formation rate for COD (R-2 = 0.83) and ammonia removal (R-2 = 0.76). However, good correlations were observed with a combination of the media physical factors: dimensionality (Di), voidage (Voi), and hydraulic efficiency (HE) strongly correlated with biofilm formation rates for heterotrophic (R-2 = 0.95) and nitrifying bacteria (R-2 = 0.92). This study proposes that the media physical properties can contribute to shortening start-up, contributing to improved removal rates and fast commissioning of MBBRs.