What is oxygen used for?

What is an oxygen-sag curve? How is it used in relation to BOD?

• BOD-Biological Oxygen Demand

• Answer:

  Environmental Engineering Dictionary A graph of the measured concentrations of Dissolved Oxygen in water samples collected (1) upstream from a significant Point Source (PS) of readily degradable organic material (pollution), (2) from the area of the discharge, and (3) from some distance downstream from the discharge, plotted by sample location. The amount of dissolved oxygen is typically high upstream, diminishes at and just downstream from the discharge location (causing a sag in the line graph) and returns to the upstream levels at some distance downstream from the source of pollution or discharge. USAGE The discharge of a sewage effluent in a stream produces a biochemical oxygen demand (BOD) which decays exponentially in time and space. This oxygen demand causes an oxygen deficit, or oxygen shortage. The greater the oxygen deficit, the greater the rate of natural oxygen replenishment from the atmosphere into the stream. These two concurrent processes of oxygen consumption and oxygen replenishment produce an oxygen sag curve, i.e., a curve which sags initially due to the increased oxygen demand and recovers asymptotically downstream due to the increased rate of oxygen replenishment. The derivation of the DO sag equation is the objective of this article.