Water isn't necessarily wasted. In the case of RO, all we are doing is temporarily using the 'water' part of the mix and sending the dissolved salts to the wastewater system where it will eventually be recombined with that 'water' in the form of the consumed beer.
Reusing RO wastewater is an iffy proposition. Undoubtedly, that water will have significant mineralization and it could be mineralized to the point that its injurious to the recipient. For example, if the water supply is ion-exchange softened prior to RO treatment, the resulting wastewater could have sodium concentration that could kill plants that are watered with it. Water can also be objectionable if the raw water has modest levels of iron or manganese. That water could stain clothing if used for washing.