It depends on what is used to clean the sewage, but I think theres a couple ways to make fertilizer from it. Depending on the animal or type of sewage, if you use duckweed or algae to clean the water you can take that and apply it to fields. You will also have the sludge from the sewage that can also be applied directly to fields. Pig farmers will usually take some of the sludge from their lagoons and apply it directly to their fields, and they will also spray a watery-sewage on the fields. I don't like this method since its proven to travel by air and also can contaminate water tables at high levels
But if the water is high in metals and other toxins you can only do so much with the excess from this process