Yes, the ground acts as the filter.<p>There&#x27;s a similar setup in Zurich, ground water is directly drinkable (iirc they oxygenate it a bit for test), while the lake water goes through sand filters (it is generally drinkable, but much better to remove particles).<p>Something weird I learned while visiting is that they put extra chalk on purpose to prevent pipes from deteriorating in old buildings (they experimented with lower amounts and had some troubles with leakage).