A modular aftertreatment assembly for an engine system is provided. The modular aftertreatment assembly includes a housing having a first section, a second section, and a third section. The modular aftertreatment assembly also includes a mixing tube positioned within and extending from the second section of the housing. The modular aftertreatment assembly further includes a diffusion assembly having a first diffusion and a second diffusion plate. The modular aftertreatment assembly includes a selective catalytic reduction assembly. The modular aftertreatment assembly also includes a plurality of perforations provided on an outlet face of the first bank of the plurality of catalysts and the second bank of the plurality of catalysts respectively. The modular aftertreatment assembly further includes a pair of outlets provided in association with the first and third sections of the housing respectively.