When would a treatment plant typically switch from the lead unit to the lag unit?

Switching from the lead unit to the lag unit in a treatment plant is typically done when the lead unit requires maintenance. This transition is crucial for ensuring that the treatment process continues without interruptions. The lead unit generally handles the bulk of the load, and if it malfunctions or requires servicing, the lag unit is activated to take over the workload. This practice helps maintain operational efficiency and compliance with regulatory standards by preventing potential treatment lapses.

The other scenarios, such as heavy rainfall or drops in water quality, might necessitate adjustments in operation but not specifically a switch to the lag unit unless accompanied by a maintenance issue with the lead unit. Regular scheduled intervals for switching units can occur in some systems, but this is not the primary reason for such a change; it is more about operational readiness to ensure reliability in the treatment process. Therefore, maintaining functionality and addressing maintenance needs directly relate to the decision to switch units.