Do all animals excrete nitrogen compounds in their urine?
The biologically useful nitrogen in the system you describe is in the sewage. Plants need nitrogen for life but can only absorb nitrogen when it’s in a soluble form (there are a few plants can fix atmospheric nitrogen). If the sewage was processed and incorporated in soil, the nitrogen would quickly leach out and become unavailable to plants (although this element eventually cycles naturally). Holding it in sewage tanks where duckweed can access it, keeps nitrogen available to grow plant tissue – essentially speeding up the recycle rate of nitrogen. The chemistry of life that you speak of involves proteins. Proteins are made of amino acids and, in each amino acid, is an amino group with a nitrogen atom. The nitrogen is incorporated into plant proteins, fish eat it and make fish proteins, humans eat it and make human proteins – and we all excrete the extra protein that we can’t utilize. The economics that makes a system like this useful comes from the fact that we have to fertilize our
