The flow at the treatment plant will fluctuate with the changes in water usage by its domestic customers. At night water usage is low and so is the flow at the treatment plant. In the morning, usually between 6:00-8:00am, water usage increases and so does the flow to the plant. But it takes several hours for the wastewater to make its way through the collection system. So that peak flow usually hits the treatment plant between 9:00-10:30am.
This peak flow can be as much as two and a half times the daily average flow. The flow will spike again between 6:00-8:00pm, which correspond with the evening peak water usage. Treatment process adjustments must be made to compensate for the high and low flows that will affect the hydraulic loading on the plant. A higher flow rate will result in decreased detention time in treatment processes and can adversely affect treatment by increasing the surface loading rate in the clarifiers.
Some systems are designed with equalization basins in the headworks. The purpose of flow equalization is to retain water during higher flow periods. The stored water can be treated later in the day when the plant flow drops. This reduces the hydraulic “bump” from sudden flow surges that can cause a loss of solids from the system.