The results of Ag-NPs on the system performance suggested that nitrifiers in the OMBR are extremely susceptible to this new emerging pollutant. Since nitrification is a critical step in biological nutrient removal in modern wastewater treatment in mitigating eutrophication, it is important to curb the release of Ag-NPs to municipal wastewater. Indeed, the applications of Ag-NPs based novel draw solutes in the OMBR process (Han et al., 2013), as advocated in some studies, would be of grave concern, owing to its possible reverse transport from the DS into the feed and its subsequent detrimental effects on biological activities.