It’s a marsh that you put your sewage in, and it cleans the water.

The Water Purification Marsh at Aröd was the first construction of its kind in Sweden, i.e. an artificial wetland, shaped in a manner to retain the nutrients phosphorus and nitrogen from agriculture and household on terra firma by aid of a biological multitude of plant and animal organisms,

And another source:

http://www.vattenmiljo.se/pres_eng.htm

One problem with this concept is that it requires lots of land. A reed bed might be a viable sewage treatment alternative for a small family dwelling.

Of course the easiest is a composting toilet.