Open Hydro-Infrastructure: A Zero-Waste RO and Dedicated Drinking-Water Line, Solved in Twelve Puzzles

An open-source reference design for a zero-waste reverse osmosis (RO) system with a dedicated drinking-water line for homes and buildings. Using only standard, commercially available parts, it sends no water to drain: it uses the pressure drop across the building's pressure reducing valve (PRV) to return RO reject water (concentrate) to household use for washing and flushing. The design is presented the way it was developed, as twelve problems solved in sequence, each giving the problem, the physics, the solution, and what it led to. Pressure settings, tank sizes and component choices are left to manufacturers and engineers so the design can evolve freely. Limits: contaminants, including short-chain PFAS, are redirected to wash water, not destroyed. The system is not certified to NSF/ANSI 58 or reviewed against ASHRAE 188. The dead-leg flushing approach (Puzzles 9 to 12) is under study and has not been tested as a Legionella control. Released under CC BY 4.0 so anyone may build, adapt, manufacture and sell it, anywhere, with credit.