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A sag curve

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When an oxygen demand chemical is discharged into a river it affects the concentration of dissolved oxygen in a manner which varies with distance from the point of discharge. The variation of the concentration of dissolved oxygen is shown in the following graph, commonly referred to as a 'Sag curve'.

Note how the concentration of dissolved oxygen

• drops in the decomposition zone,
• remains low and approximately constant in the septic zone, and
• increases in the recovery zone to its orginal level.

In the decomposition zone conditions are ideal for the rapid growth of plants, algae and other organisms. This rapid growth quickly depletes the level of dissolved oxygen preventing continued aerobic respiration.

The level of dissolved oxygen in the septic zone is too low to support organisms that live by aerobic respiration. In this region anaerobic organisms (especially bacteria) flourish and produce noxious waste products such as methane (CH₄) and hydrogen sulphide (H₂S).