Purifying wastewater by reverse osmosis is long done. At chemical plants, it reduces the volume of the dirty effluents. In California during the last drought, it provided usable water from the sewage. What smaller units are conceivable?
Washing machines could save sweetwater. Between two uses, treat 0.1m3 in one or several days. Add tanks, accept to squander water when the machine serves more frequently. In 24h it's 1.2cm3/s. Recovering 80% of the water takes some 60 bar, at 70% efficiency 10W only.
Dishwashers are very similar.
Laundries, hotels, restaurants, collective houses would rather have a collective reverse osmosis unit. Reusing water only at washing machines or dishwashers is conceptually more acceptable.
0.2€/kWh electricity cost 0.5€/m3 saved water, less during nighttime. In rich and rainy Germany, water costs some 4€/m3 in house quantity.
To clean floors, professionals have trolleys with brooms, floorclothes, surfactants, water... Purifying the used water would waste less time renewing it. 5L in 15min need 50W, hence a motor and battery. Economics may tell "only where water is scarce".
Manned or robotic cars wash the floor at supermarkets, undergrounds... They already have electricity and collect the used water. 1L/min needs 150W if done in real time. This saves idle time and water cost. It may reduce the car's size.
The size, procurement cost and maintenance cost of reverse osmosis units must still be checked for these uses.
Marc Schaefer, aka Enthalpy