When the liquid level in a container with a drain needs to be kept within a range, it can be adjusted by the "liquid level control system". Specifically, an upper liquid level and a lower liquid level are set for the container. When the liquid level in the container reaches the upper level, the control device (mechanical or electronic) cuts off the power supply of the liquid supply pump, and the liquid outlet discharges the liquid for a period of time. When the liquid level in the container drops to a lower level, the control device turns on the power supply of the liquid supply pump, and the liquid level slowly rises again.

If the liquid level in the container with the liquid replenishing inlet needs to be kept within a range, the control device controls the action of the drainage pump, and vice versa.