One of the X10 pumps used for the fill pump for the iV runs both directions. The drain pump will do the initial drain. The fill pump fills it and then it is stirred for a few seconds and then the fill pump drains it. It then fills it a small amount. Then the drain pump run long enough to make sure it is completly drained. The fill pump then dispenses 100ml into the iV for the alkalinity test. I have both the drain and fill pump running back into the sump but since it is done this way you could have the drain pump dump to waste but you would have to add some fresh saltwater to replace the amount that is getting dumped to waste. I test 4 times a day on a 42 gallon tank and have no issue with the initial drain going back into the sump. So it is kind of nice that you can do that for some instances.