A break tank prevents the possibility of contaminated dish washing water back flowing into the fresh water supply. The break tank is a tank into which the fresh water from the mains supply is supplied into first, and then pumped into your dishwaher. If there is a backflow from your dishwahser, any dirty water would go to the break tank which has an air gap at the top to accommodate additional water flow. this means that dirty water is never able to enter the mains fresh water supply.