The aim of this study was to compare the removal of ammonia-nitrogen (NH4+-N) in a bacterial suspension system (BSS) and a bacterial immobilization system (BIS) using the iron-reducing bacterium Klebsiella sp. FC61. The experimental results showed that the pellets in the BIS could adsorb part of the iron ions in the solution. Although the adsorption of Fe3+ was stronger than that of Fe2+, single-layer physical adsorption was observed in both cases. Furthermore, the self-inhibition effect of the bacteria from the suspended state to the immobilized state was effectively improved in the BIS, with a maximum NH4+-N removal rate (r(max)) of 0.7241 mg/L/h for the BIS and 0.3732 mg/L/h for the BSS. Moreover, strain FC61 could effectively remove the NH4+-N in both the BIS and BSS, while maintaining a lower concentration of iron ions, nitrate, nitrite, and the quantity of microorganism in the solution, The results of response surface methodology showed that the predicted maximum removal ratio of NH4+-N in the BIS was 70.31% (actual value = 72.15%), which was higher than that of the BSS at 59.59% (actual value = 61.37%) at the same time point of 48 h. Analysis of bacterial distribution and growth data suggested that removal of NH4+-N by strain FC61 would be more favorable in the BIS than in the BSS. (C) 2016 Elsevier B.V. All rights reserved.