The potential of a moving bed biofilm reactor (MBBR) in full scale has been investigated as an alternative for upgrading of the Kish Island Markazi wastewater treatment plant. In this regard, the activated sludge process (ASP) upgraded to an MBBR process and different operating parameters results in this change compared with ASP. Results show that with increasing the average flow influent from 1,049 ± 88 to 1,944 ± 275 m3 d−1 and reducing the aeration tank volume from 300 to 150 m3, organic loading rate (OLR), mixed liquor suspended solids (MLSS), sludge retention time (SRT), sludge volume index (SVI), hydraulic loading rate (HLR) and hydraulic retention time (HRT) were changed to 0.32 ± 0.04–1.8 ± 0.36 kg COD/m3d, 2,641.19 ± 284.99–7,354.2 ± 778.35 mg L−1, 5.28 ± 0.64–22.1 ± 1.53 d, 135 ± 37.3–29.2 ± 3.81, 23.14 ± 1.94–43.37 ± 5.04 m d−1 and 2.76 ± 0.22–1.48 ± 0.18 h, respectively. Effluent concentrations under this operation condition were well below the discharge limits for irrigation water. Therefore, the MBBR process is a good alternative for upgrading wastewater plants especially when there is inadequate space or modifications are needed that will require large investment.