An environment rich in oxygen allowing aerobic organisms to flourish is required. Decomposition occurs in the treatment plant, not in the field such as with just primary treatment. Aerobic treatment by Nayadic plants can restore failed fields, in fact. Can be done by various means, physiochemical and biological. Large quantities of carbon such as peat soils can also remove nutrients.
Tertiary treatment using plants completes the ecological cycle.
The minerals are taken up by other plants so the cycle is complete. We have acted in an ecologically sustainable manner. I hear of all the problems with domestic wastewater. Not without treatment first. It was once thought simply putting it into the ground was sufficient.