How to treat raw sewage with constructed wetlands: an overview of the French systems

TLDR

Vertical‑flow constructed wetlands for raw wastewater in France have been highly successful over the past two decades. A survey of more than 80 wetlands examined performance relative to loads and plant characteristics, and found that first‑stage sludge must be removed every 10–15 years to maintain function. The systems reliably meet low COD and SS targets, with nitrification being the most sensitive process; properly sized wetlands can achieve outlet levels of 60 mg L⁻¹ COD, 15 mg L⁻¹ SS, and 8 mg L⁻¹ TKN on 2–2.5 m² PE⁻¹.

Abstract

The development of vertical flow constructed wetlands treating raw wastewater in France has proved to be very successful over the last 20 years. In view of this a survey was carried out on more than 80 plants in order to study their performance and correct the design if necessary. This study shows that such systems perform well in terms of respecting the goals of both low level outlet COD and SS and nitrification. Pollutant removal performance in relation to the loads handled and the specific characteristics of the plants were investigated. Nitrification is shown to be the most sensitive process in such systems and performance in relation to sizing is discussed. Such systems, if well designed, can achieve an outlet level of 60 mg L−1 in COD, 15 mg L−1 in SS and 8 mg L−1 in TKN with an area of 2–2.5 m2.PE−1. The sludge deposit on the first stage must be removed after about 10–15 years.