Hydraulic retention time (HRT) is an estimate of how long it takes a particle to travel through a specific tank or volume. HRT is calculated by dividing the tank volume by the flow through it, and is usually reported in hours. Many of the processes we use at WWPs are sized/designed to achieve a minimum HRT. For example, the HRT within the aeration tank must be long enough for the important reactions to take place.  In most cases, HRT is not something we need to actively monitor on a regular basis.  But, it can be helpful in troubleshooting a process issue.  In those cases we can calculate HRT based on operating conditions and compare them to design values.

Have you used HRT to troubleshoot an issue? What did it tell you?