The invention concerns a wastewater treatment plant and a method for it. The plant comprising at least one main reactor module (30) having a plurality of stackable filter-modules (50), each filter-module purifying wastewater independently by being individually charged batchwise with wastewater. The wastewater is discharged on each filter-module by means of a deflector (44) referenced to a filter-module (50), the deflector creating a back pressure in dependency of the gravity-flow-force of the wastewater leading to that the wastewater is spilled from the deflector (44) in an angle of discharge relative to a horizontal plane evenly onto every independent filter-module, respectively.