Re-aeration in sewers

The gas–liquid mass exchange between flowing water and the atmosphere is one of the key processes in sewers. The transfer of oxygen to wastewater, which is commonly known as re-aeration, strongly influences the dissolved oxygen (DO) concentration of the wastewater. When re-aeration balances the microbial oxygen uptake, aerobic conditions will prevail in the sewer. On the other hand, the wastewater will become anaerobic when oxygen uptake is higher than the rate of re-aeration.

The rate of oxygen tansfer (r_O2 ) is presented by the following expression:

where \(k_La\) is the mass transfer coefficient (1/h), \(c_l\) is the equilibrium concentration of oxygen in water, and \(c_{b,l}\) is the liquid phase bulk dissolved oxygen concentration.

\(k_La\) in a sewer pipe depends upon a number of factors including hydraulic properties of the flow, pipe dimensions, slope and wastewater characteristics. A number of empirical relationships are available for the determination of the mass transfer coefficient.

Further information can be obtained as follows:

• Paper – Empirical modeling of air-to-water oxygen transfer in gravity sewers by Jensen (1995)
  
• Paper: Reaeration in sewers by Huisman et al. (2004)