What a Waste
When Holly was in the final stages of concluding that moving to the surface was the only way her relationship with Artemis could continue, she finally brought up something that had bothered her since before they met: sewage.
Artemis laughed hollowly. "There are compelling reasons we prefer to treat rather than recycle: cholera, typhoid and dysentery are among them. Human faeces spread disease and we don't have magic to protect us; we cannot simply put them on our crops. Our treatment methods could be less wasteful, perhaps, but I doubt we'll abandon use of the water closet any time soon."
Because I just re-read book one, and got rather annoyed at Holly for judging us for not having 'fixed' our first decent large-scale solution to what was, and still is in some parts, a huge public health problem to her satisfaction. It's all very well and good that fairies can use night soil on their cabbage patches, doesn't mean it's safe for us.