Why not reuse the wastewater from the sink for flushing?
By deflected the pipe from the sink to connect it to the flush reserve, this system will reduce the quantity of water in this reserve.
This solution is very newly utilized by DIEHL Aviation (see "Grey Water Reuse" concept).
This device is very common on our daily environment but still rare on the sky.
There will be no more regular maintenance to do and no more waste bins for papers. There will be a reduction in paper consumption but also a gain in space in the toilets. In short, the use of a hand dryer is more hygienic and more ecological!