Just the phrase "waterless urinal" sounds like some kind of smelly malfunction, not something you would actually want to use or live with. Controlling unpleasant smells was actually one of the main problems in the development process for waterless urinals [source: Davis]. Sewers are full of all kinds of odors, and even gases that can be health hazards if they escape. One of the main purposes of the sealant that floats on top of the urine in waterless urinal trap cartridges is to keep any foul odors from escaping through the pipes [source: Waterless]. The sealant stays floating on top of the urine in the trap at all times. The seal that it creates also keeps the smell of urine that is sitting in the trap for long periods of time from escaping. Also, since no flush is needed to empty out the urinal, there is no chance that urine will pool in the bowl. Waterless urinals have ultra-smooth bowls designed with a steep enough slope that all liquid will flow directly down into the reservoir without pooling up [source: Waterless].