A stock suspension of Ag-NPs (Sigma Aldrich, Missouri, USA) was prepared by suspending 0.5 g Ag-NPs per L of 0.25mMsodium citrate solution. The particle size of the Ag-NPs suspension was determined as 118.5 1.2 nm by Zetasizer (Malvern Instruments, UK). Ag-NPs were introduced into the OMBR from Day 64 by adding the stock suspension to the influent wastewater to a final concentration of about 0.30 mg/L.