The present disclosure describes a modular humidification-dehumidification (HDH) apparatus and system for concentrating a solution including a plurality of internal modules coupled to each other. The plurality of internal modules includes a humidification module and a dehumidification module in gas flow communication with the humidification module. The humidification module includes humidification media facilitating evaporation of liquid from the solution to gas as the solution passes through the humidification media thereby producing a concentrated solution and a humidified gas. The dehumidification module includes a condensing heat exchanger for condensing vapour from the humidified gas.