vtsnowedin wrote: I don't know why they don't use the secondary treated water directly for irrigation but the probably have good reasons. That way they could swap new water for slightly used water with some phosphorous in it that the plants can utilize and clean out of the water.
I can partially answer that. Secondary water that has not gone through distillation or reverse osmosis has a lot of nasty stuff in it you do not want getting into your food crops. Just as an example you have the metabolites and/or left over medications that every person taking medication flushed down the toilet or washed off in the shower. Many of those medications recombine in wildly unpredictable ways in the waste stream and are then taken up by plants where they can reenter the food either directly when you eat the plants or secondarily when the critters that ate the plants are eaten by yourself.
I should be able to change a diaper, plan an invasion, butcher a hog, design a building, write, balance accounts, build a wall, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, pitch manure, program a computer, cook, fight efficiently, die gallantly. Specialization is for insects.