Biological wastewater treatment
Biological wastewater treatment is a group of methods to free water from biological and chemical pollution.
The active littoral zone
The littoral zone of a lake is the shallow part, where water plants and a great variety of small animals and microbes are living. It is biological active, that is, it has a cleaning effect upon the water. Even pharmaceuticals are digested. However, the cleaning capacity is limited.
Such a lake can be an important part of the waste water treatment in a Martian settlement. Lakes and ponds on Mars would require enclosure in a pressure vessel to avoid freezing and evaporating.