Greywater is wastewater with low levels of contamination, for example from the shower. It can be used as a source of heat and - treated on site - also as service water, for example for irrigation or flushing toilets. Urine, the yellow water, on the other hand, contains many nutrients, especially phosphorus. The world's phosphorus reserves will not last forever and its extraction leads to environmental damage in the deposits and elsewhere. So what could be more obvious than to rescue the valuable substances from the "pee" and produce fertiliser from it? And finally, black water: this is toilet flush water with faeces, which most people prefer to make disappear very quickly. In our country, it is therefore diluted with a lot of clean water and washed underground. Elsewhere, it is sunk into pits or dumped into the nearest river. Yet the faeces contain a lot of energy; our ancestors knew that when they used cow dung for heating. So why not modernise the age-old process and produce hygienic fuel pellets from the unloved faecal sludge?