This study aims to investigate the applicability of membrane distillation for ammonia removal from its aqueous solutions. Among the different recognized membrane distillation configurations, vacuum membrane distillation is applied in the present work for ammonia removal. The effects of different operating parameters on ammonia removal from aqueous solutions of different concentrations have been investigated. Experimental results showed that high feed temperatures, low downstream pressures and high initial feed concentrations and pH levels enhance ammonia removal efficiency. The pH is found to be the most dominant factor. Temperature and concentration polarizations within feed boundary layer proved to have a significant influence on mass transport. High feed flow velocity is an important tool diminishing temperature and concentration polarizations effects. The resistance to mass transfer is found to switch from being predominantly located in the feed side at low feed temperatures and flow velocities to be nearly located within the membrane pores at 55.7 degrees C and reasonably higher feed flow velocity of 0.84 m/s. Although high feed temperatures and low downstream pressures increase remarkably the total transmembrane flux and the ammonia removal rate, the corresponding ammonia separation factors were decreased. Ammonia removal efficiencies higher than 90% with separation factors of more than 8 were achieved in this study. (c) 2007 Published by Elsevier B.V.