Design of Equalization vessels

Equalization vessels often have sloping sides to keep the head more constant as volume changes.

A common method for combining aeration and mixing is to have a mixer at the surface to splash the liquid into the air. Because the level is changing, this mixer must be mounted on floats. The aeration in the equalization vessel will reduce the BOD of the wastewater by 10 to 20 %.

A graphical method is used to specify the volume of an equalization vessel. A plot of flow rate versus time gets a horizontal line for the average flow rate. All flows above this line are integrated to get the fluctuating volume for the vessel. This is shown in the next sketch.

The vessel volume should equal the average flow multiplied by the time period, and then add on the fluctuating volume. Note that when the flow variation is small, the fluctuating volume is small so that the required vessel size is close to that based on average flow. When the variation is great, the equalization vessel is quite a bit larger than that based on the average flow.

while on sabbatical leave, ESB, Porto, Portugal July 1996