The invention relates to a method for treating wastewater by means of photosynthetically active algae that form a sediment in the resting state, comprising the following steps: a) supplying wastewater to photosynthetically active algae that form a sediment in the resting state, b) conveying the wastewater supplied to the algae against gravity from a lower level to a higher level while simultaneously exposing the wastewater mixed with the algae to light, c) introducing the wastewater supplied to the algae from the higher level into the upper region of a sedimentation tank, d) allowing the algae to sediment in the sedimentation tank, and e) removing the wastewater freed of the algae by sedimentation as treated wastewater.