CODT is defined as the COD measured after the whole sample has been shaken and a portion is taken for analysis while CODS is defined as the COD measured after the sample stands at room temperature (25 °C) for one hour and a portion is taken from the middle layer (no oil nor residue) of the sample [8].
The second study was conducted in mid May 2009. Untreated samples (influent) were collected at the inlet of the CRT at 0900, 1300 and 1700 h while treated effluent samples were collected at the outlet of the last chamber of the ST at time intervals of one hour on each study day, respectively. The results of CODT and CODS content for each sample were compared using the t test to check if there was any significant difference between them [24].