Wetlands are ecosystems with known potential for particulate material retention. Many water bodies develop near shore wetlands or reed belts supported by external natural o artificial incomes from the drainage basin. The influent slows down and the particulate material sediments and is mineralized avoiding a damage to the water body. In a study done in a reed belt of Las Perdices pond, which receives municipal wastewaters from San Miguel del Monte city, it was concluded that in a surface of 2 – 3 Ha, the influent organic matter could be mineralized, the dissolved oxygen recovered and the inorganic nitrogen and soluble phosphorus concentrations reduced, under certain conditions. Knowing the involved process it is possible to design «tailored ecosystems» that can improve water quality and avoid environmental damage.