Three water samples namely river water,untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to $BOD$ test. The samples were labelled $A, B$ and $C$; but the laboratory attendant did not note which was which. The $BOD$ values of the three samples $A, B$ and $C$ were recorded as $20\, mg/L, 8\, mg/L$ and $400\, mg/L$ respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?

(C) Biological oxygen demand $(BOD)$ is the method of determining the amount of oxygen required by micro-organisms to decompose the organic waste present in a water supply. If the quantity of organic waste is high,the number of decomposing bacteria increases,leading to a higher $BOD$ value.
Therefore,a higher $BOD$ value indicates higher pollution. Comparing the given values: $400\, mg/L$ (Sample $C$) > $20\, mg/L$ (Sample $A$) > $8\, mg/L$ (Sample $B$).
$1$. Sample $C$ $(400\, mg/L)$ is the most polluted,representing untreated sewage water.
$2$. Sample $A$ $(20\, mg/L)$ represents secondary effluent from a sewage treatment plant.
$3$. Sample $B$ $(8\, mg/L)$ is the cleanest,representing river water.

Label	Sample Type
$A$	Secondary effluent $(20\, mg/L)$
$B$	River water $(8\, mg/L)$
$C$	Untreated sewage $(400\, mg/L)$