The COD of all MBR reactor feeds, retentates, and permeates was determined on a weekly basis using a HACH DR2800 spectrophotometer (Hach, Loveland, CO, USA). Digestion of the samples was carried out in either HACH LCK 314 (15–150 mg L −1 ) or LCK 514 COD (100–2000 mg L −1 ) cuvette test vials (Hach-Lange, Manchester, UK) depending on the concentration to be measured.