Desalination and water treatment by reverse osmosis (RO) can highly increase clean water supply in today's world. However, biofouling of polyamide (PA) RO membranes is a serious obstacle for a wider use of this technology. One of the promising ways of biofouling control is membranes surface modification with zwitterionic polymers. A number of published works showed that zwitterionic coatings can improve the resistance of PA membranes to the initial bacteria adhesion, however no long-term experiments with real treated water effluents were conducted. In this work a commercial PA RO membrane was surface-modified with a zwitterionic polymer and its resistance to biofouling was tested in both short-term bacteria adhesion experiments and longer filtration tests conducted using real treated wastewater effluents, spiked with a small level of nutrients. The obtained results showed that, initially, there was a clear improvement in the biofouling resistance of the modified membranes and their permeation flux remained stable, in contrast to the non-modified counterpart. However, eventually, the permeabilities of the two membranes declined to a similar degree. The results indicate that antifouling coatings might not promise a better membrane performance in long term. The analysis of the biofilms grown on the pristine and the modified membranes suggested that adaptation capabilities of biofilms overcame the favorable changes in surface properties of the membrane achieved by the modification. The presented results emphasized the importance of long-term filtration experiments as an ultimate test for assessing biofouling resistance of the modified desalination membranes. (C) 2016 Elsevier B.V. All rights reserved.