Could a desert animal, with the right symbiotic microbes, avert the need to urinate by creating ammonia and having bacteria in its blood/kidneys convert it into nitrate and then back into amino acids? Or is nitrification too slow a process to ever entirely replace urination?
If required, the animal can spare some of its own energy to allow the symbiotes to adapt for nitrification specifically. The animal lives is humanoid physiologically, and eats approximately the same amount of calories as modern people. The symbiotes' nitrification processes cannot be more efficient than is seen in reality, but they can do the processes more if energetically feasible