I suspect that using reverse osmosis to treat grey water would take a lot less energy than treating sea water for the simple fact that sea water has 35 g/L of salt and grey water does not.

Absolutely. The pressure required to force water through the filter in reverse osmosis increases as the salinity of the inlet water increases.

If you look at this article, you will see that if the salinity doubles in the inlet water, the pressure required to overcome this also doubles. This means that the energy required per unit of filtered water increases dramatically as the salinity of the inlet water increases.

All reverse osmosis systems release the inlet water as its salinity increases and replace it with water that is less saline - despite the loss of energy of doing so. Efforts are made to recover some of this pressure energy and to pass it on to the inlet water and some suppliers claim 90% recovery of this energy. It is little like trying to recover some of the waste heat of steam turbines and to use it to warm up the inlet water before it is heated in the furnace.