A semi-empirical model was developed to predict biomass-affected porosity, specific surface area and pressure drop as a function of the biomass concentration in two selected Submerged Aerated biorectors (SABRs). Under similar conditions two bench-scale SABRs (1m long and 100mm diameter) were operated to treat an industrial wastewater, the first packed with porcelinaite rocks and the other with polystyrene grains at hydraulic loading rates of ( 0.1–3.2 m/h) and with BOD5 concentration of (110- 436 mg/L) .Typical constant that can be used to estimate pressure drop for some of the most common design of SABRs were correlated. The proposed equations in porosity and specific surface area caused by biomass accumulation in SABR bed are based on macroscopic estimates of average biomass concentrations. In comparison to biofilm-based models, the macroscopic models are relatively simple to implement and are computationally more efficient.The effects of biomass accumulation and distribution on pressure losses and removal efficiency of biological load in SABRs were experimentally studied.Localized biomass accumulation in the SABR beds is the key factor increasing the pressure drop, which was caused by local bed clogging due to biomass growth. The highest pressure drops in the beds (porcelinaite rocks: 2,150 N/m3 and polystyrene grains: 1115 N/m3) occurred where there were high biomass levels. The pressure drop varied nonlinearly with the amount of accumulated biomass and the amount of oxygen consumed. Porcelinaite rocks caused greater pressure drops, on average 2 times higher than the polystyrene grains. Compaction, as a consequence of biomass growth and porcelinaite rocks degradation increased the pressure drop in the porcelinaite rocks bed.A comparison of the experimental and the predicted pressure drops showed that the model provided good estimates of biomass-affected porosity and pressure drop in the SABRs packed with spherical grains with even biomass distribution.