Electrochemical production of sodium hypochlorite from saline waste streams: A scalable 3D reactor approach for circular water treatment

The practical implementation of electrochemical sodium hypochlorite production remains limited by insufficient validation under realistic saline matrices and by scale-up constraints related to charge transfer, hydrodynamics and electrode utilization. This study addresses these gaps by developing and evaluating a three-dimensional electrochemical reactor for in situ NaOCl production from synthetic and real saline waste-derived streams. The work was structured as a sequential scale-up study including laboratory optimization, matrix validation and pilot-scale testing. At laboratory scale, current density and salinity strongly influenced NaOCl generation. The best-performing condition among those evaluated was obtained at 35.5 g·L⁻¹ NaCl and 215.15 A·m⁻², reaching 19.66 g·L⁻¹ NaOCl and 79% production efficiency. Matrix validation showed that real saline streams can support electrochlorination, with WWTP effluent producing up to 31% more NaOCl than synthetic brine at equivalent nominal salinity. Pilot-scale tests revealed that direct extrapolation from laboratory conditions was limited by insufficient charge density, short residence time and suboptimal hydrodynamics. Process intensification through reduced working volume, optimized recirculation flow, increased current density and electrode cleaning restored reactor performance, achieving 28.79 g·L⁻¹ NaOCl, 19.90 g·L⁻¹ active chlorine and 95.68% production efficiency. These results demonstrate that 3D electrochemical reactors can provide a scalable route for decentralized NaOCl production from saline waste streams, provided that hydraulic conditions, charge density and electrode maintenance are properly controlled.