Sewage contaminated river ecosystems, such as the Zenne river in Brussels, are amongst the most extreme examples of ecosystems disturbed by human activity. When these systems receive urban waste from cities, they are submitted to high loads of various organic and inorganic pollutants mostly carried by suspended sediments which offer abundant surface area for adsorption and desorption. The impact of these loads on the biogeochemical functioning of the river will depend on various factors like the number of inhabitants, the river discharge (or water volume of the receiving river), the suspended solid deposition rate, the sewage treatment capacity of the city, the type of wastewater treatment, etc.
 
The consequences for both the river and the riparian population are numerous: hypoxic and anoxic waters prevent the presence of fishes and other oxygen dependant organisms, turbidity prevents any algal development, bad smells cover the riparian regions, and risks for the riparian population health increase. The river becomes dead, dangerous and looses any attractiveness for recreational activities. Until the year 2000, an extreme example of such ecosystems was the Zenne River downstream Brussels.

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