ABSTRACT
After the pretreatment, the next step in the process is to let the flow of wastewater slow way down so that anything else that will settle by gravity can do so. This process is known as primary settling. A primary settling tank can be rectangular or circular. What matters is how last the flow is allowed to go through the tank. This velocity is usually kept around 1 ft/min or 0.3 m/min. Notice that the velocity is based on minutes instead of seconds like the grit channels. A grit channel velocity of 1 ft/s would be 60 ft/min.
