A system for detecting an overflow from a storage tank holding a liquid and/or slurry. An inlet to an overflow box communicates with the tank at an overflow point and receives any overflow from the tank. A main drain pipe communicates with the overflow box and leads to a drain. An overflow discharge pipe having a smaller internal diameter than that of the main drain pipe also communicates with the overflow box, but is located closer to the inlet than the main drain pipe. The overflow discharge pipe provides a first drainage of any overflow from the tank to the main drain pipe, and is provided with a sensor for detecting and/or measuring a flow rate of any overflow through the overflow discharge pipe. A controller provides a signal to activate an alarm in order to alert an operator that an overflow condition exists. Alternatively, the signal could be used to control a flow rate of liquid or slurry into the tank by providing a signal to a flow rate controller to reduce the flow rate of liquid or semi-liquid into the tank whenever any overflow is detected within the overflow discharge pipe by the sensor.