Every day we use approximately 30 litres per person to flush the toilet. On the same day we use twice as much, 60 litres per person for our hygiene, most often in the very same bathroom. When flushing the toilet we use water as transportation media but for sure there is no need at all to use drinkable fresh water for that matter.
The basic idea is to make a second use of the hygiene water by save enough of it, treat it, no more no less than to be able to flush the toilet safe and comfortable.
A reservoir tank is installed and dimensioned to completely remove the need to use fresh water for toilet flushing. That mean water reduction of 20% in total and 33,3% in bathroom.