How can I add a washing machine drain line to an existing laundry sink drain? The sink has a trap under the drain which leads straight down through the floor where it connects to the main drain line leading out of the house to the septic system. Currently the washing machine drains directly into the laundry sink. I also have a drain hose coming through the floor to the sink that is connected to a sump pump below a utility sink in the basement directly below the laundry sink. Can I add a second drain line next to the washing machine drain line?