COD

Grease trap has the highest value of COD, whilst the value for tap water and C1 were low and the rest of the sites had a 0 value of COD.

Grease trap has a high value of COD because the decomposition of organic matter such as food molecules, organic chemicals from detergents and dead skin cells, as well as aerobic respiration by bacteria (BOD that contributes to COD) thriving in the water takes up a large concentration of DO. Hence this causes the amount of COD to increase in the Grease Trap.

In C2, not all unwanted nutrients in the water had been taken up by the plants in the phytoremediation system and hence, the COD value. However, as the water goes through a second round of phytoremediation in the C-shaped pond, almost all the unwanted nutrients had been taken up by the plants and bacterial activity had become negligible.