Sentence:  Since we didn't have a septic tank, we made sure there was an empty bucket underneath the sink. That's where we had a straight <WSD>drainpipe</WSD>, and after the dishes were washed, we would take the bucket from underneath the sink and throw waatever was in there outside. 
Meaning of the word marked with <WSD> tags:  A pipe that carries wastewater from a bathtub, shower, sink, etc.