Abstract: Modular living green wall systems are provided that supply water-cooled heat rejection for building cooling, power generation, industrial, chemical, and other processes that rely on heat rejection to the ambient environment for their efficient operation. Warm water from the process requiring heat rejection is circulated vertically through channels of porous media of the system and is cooled by evaporative and/or convective heat transfer to the ambient air that flows over and/or through the porous media across or counter to the water flow direction. Cool water leaving the system is piped to a heat exchanger of the process to provide the requisite cooling and is returned warm to the modular green wall system to complete the circulation loop. Modular living green wall systems may be assembled using plant modules and water treatment modules that are nested together to form continuous porous vertical water flow channels and a water recirculation system.