That is a low-tech solution for a low-tech world. There are studies that even with the current level of water treatment, some rivers contain 90% (or so) female

That is a low-tech solution for a low-tech world. There are studies that even with the current level of water treatment, some rivers contain 90% (or so) female fish, due to the remaining pharmaceuticals in the water. European cities are now starting level 4 water treatment with activated charcoal and ozon to improve the water further but even then, some substances will remain in the water. So maybe we can use the untreated water to grow plants for energy generation but I would be very skeptical of any food production.