Drain water is produced during hydroponic cultivation in greenhouses. Drain water is the left over feed water that was not absorbed by the crop. Often it is not allowed that this water is released into the surface water. Therefore, it needs to be purified and reused or disposed through the sewer. Drain water contains all sorts of substances and microorganism that consume oxygen. This can cause anaerobic conditions and a rotting process may occur. Installing an Oxydent aeration system is therefore advisable.