Do you know if your drains have pee traps? If they don't it would be the cause of the constant smell as there is nothing to stop the stale, stagnant water coming back up the pipe and causing the smell. If you do have pee traps, then the water can still smell a little if there is some sitting in the bottom of the s-bend. I always use a mix of boiling water and eucalyptus oil and pour down our bathroom floor drain when it's a bit smelly and it lasts for a while. You could also try vinegar. If the smell is caused by a buildup or blockage of scum, fat/food etc (in kitchen drains) then you may need to hit it with some caustic soda to erode it away.