Water that has been used. 1. (RO, ultrafiltration, electrodialysis) The stream of water (not product water) created as the result of processing water-the reject water or concentrate. 2. (ion exchange and filtration) The spent water used in the total backwash and/or regeneration cycle. 3. The used water and solids from a residence or a community (including used water from industrial processes) that flow to a septic system or a treatment plant. Storm water, surface water, and groundwater infiltration also may be included in the waste water that enters a waste water treatment plant. The term sewage usually refers to household wastes, but this word is being replaced by the term waste water.