Ah yes, forgot about that. We don't have to pay for the sewer/sepic tank (well, it's included in the communal tax - we don't have to pay for it separately I mean), and water we indeed have to pay, but that is about the only thing that is still relatively cheap these days. On the other hand, about 80% to 90% of the water used in the house (toilets, shower, bath, washing machine, ...) uses rain water, collected from the roof via the gutters into a special tank. Only the water that is actually used for consumption is tap water. (read: which is used to prepare food with). But yes, you are right: we still also need the water company's services... if only very little.

I've also looked into filtering my own waste water via special tanks etc, but that was too big an investment which I would never win back in my lifetime. Unless the price of a litre of water becomes at least 10 times as expensive as it is now, and even then...