Question

if the critical point in a do sag curve is found to be 18 km downstream from the discharge point of untreated wastewater, would you expect the critical point to move upstream (toward the discharge point), downstream, or remain in the same place, if the wastewater is treated?

Answer 1

The critical point in a dissolved oxygen sag curve would move upstream toward the discharge point if the wastewater is properly treated, as this reduces the organic load and the oxygen demand, allowing for quicker recovery.

Step-by-step explanation:

If the critical point in a dissolved oxygen sag curve is found to be 18 km downstream from the discharge point of untreated wastewater, we would expect the critical point to move upstream (toward the discharge point) if the wastewater is treated. This is because the treated wastewater would likely have lower levels of organic pollutants and nutrients which demand oxygen for decomposition, hence reducing the biochemical oxygen demand (BOD) and allowing the dissolved oxygen levels to recover more quickly.

Wastewater treatment plants perform primary, secondary, and tertiary treatments to reduce pollutants in the wastewater. Effective treatment reduces the load of organic material and nutrients that enter the water body, thus lessening the impact on dissolved oxygen levels downstream. With reduced organic load, the recovery of dissolved oxygen occurs sooner, resulting in the critical point shifting towards the point of discharge.