Читать книгу Automation of Water Resource Recovery Facilities - Water Environment Federation - Страница 250

Rotating scum troughs are turned to allow the scum to enter the trough. The main control is frequency and timing of the rotation. The trough needs to rotate at the time the scum rake or flight is approaching the trough to maximize the amount of scum collected. In addition, it is important to have enough water along with scum to help the scum travel along the trough and to the pit so that the scum does not accumulate in the trough. At the same time, too much water could overload the scum pit and downstream scum removal and/or treatment processes.

Typically, a discrete position indicator for the skim arm (i.e., circular clarifier mechanisms) or flight (i.e., rectangular clarifier mechanisms) is provided to indicate when the position of the flight is optimal to lower the trough. The controller will count these events and lower the trough when the count reaches the setpoint. The count setpoint can be lower (perhaps 1) when there is a large scum accumulation and set higher when there is less accumulation. There is also a “down-time” setpoint. When the trough is at the low position for a time equal to the down-time setpoint, it is raised again.