French vertical flow constructed wetlands: a need of a better understanding of the role of the deposit layer

French vertical flow constructed wetlands, treating directly raw wastewater, have become the main systems implemented for communities under 2,000 population equivalent in France. Like in sludge drying reed beds, an organic deposit layer is formed over time at the top surface of the filter. This deposit layer is a key factor in the performance of the system as it impacts hydraulic, gas transfers, filtration efficiency and water retention time. The paper discusses the role of this deposit layer on the hydraulic and biological behaviour of the system. It presents results from different studies to highlight the positive role of the layer but, as well, the difficulties in modelling this organic layer. As hydraulic, oxygen transfers, and biological activity are interlinked and impacted by the deposit layer, it seems essential to focus on its role (and its quantification) to find new developments of vertical flow constructed wetlands fed with raw wastewater.

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Influence of surface layer on hydrology and biology of gravel bed vertical flow constructed wetlands

Water Science & Technology ◽

10.2166/wst.2005.0295 ◽

2005 ◽

Vol 51 (9) ◽

pp. 91-97 ◽

Cited By ~ 36

Author(s):

F. Chazarenc ◽

G. Merlin

Keyword(s):

Constructed Wetlands ◽

Matter Content ◽

Vertical Flow ◽

Treatment Efficiency ◽

Biological Profile ◽

Biological Behaviour ◽

System A ◽

Vertical Flow Constructed Wetlands ◽

One Year ◽

Raw Wastewater

In France, gravel vertical flow constructed wetlands (gVFCWs) were adapted to treat raw wastewater, which led to important accumulations of matter in filters (organic and mineral). To prevent clogging, large gravel sizes were employed (∅ 2–6 mm). The aim of this paper was to present the influences of matter accumulation on the hydraulic and biological behaviour of the system. A one-year survey of accumulated matter content and potential respiration activities was completed in three gVFCWs (operating for 3, 4 and 8 years). Cores were sampled into filters. Results showed a vertical stratification of accumulated matter and respiration rates. Dry accumulated matter quantities ranged from 20 kg m−2 (3 and 4 years operating) to 80 kg m−2 in the oldest plant (8 years). Potential respiration was larger in the oldest plant (75 g O2 m−2 h−1) than in the most recent one (15 g O2 m−2 h−1). Accumulated matter seemed to play a role both on the water retention (enhancing initial percolation time by 5 times) and biological profile (enhancing microfauna's diversity). Contrary to what is generally proposed in the literature, accumulated matter in French gVFCWs seemed to provide better treatment efficiency without leading to surface clogging compared to systems using sand.

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Constructed wetlands for the treatment of raw wastewater: the French experience

Water Practice & Technology ◽

10.2166/wpt.2014.048 ◽

2014 ◽

Vol 9 (3) ◽

pp. 430-439 ◽

Cited By ~ 4

Author(s):

S. Troesch ◽

F. Salma ◽

D. Esser

Keyword(s):

Organic Matter ◽

Constructed Wetlands ◽

Suspended Solids ◽

Oxygen Demand ◽

Vertical Flow ◽

Small Communities ◽

Vertical Flow Constructed Wetlands ◽

French Experience ◽

Raw Wastewater

Vertical flow constructed wetlands for small communities (<5,000 population equivalent) have been successfully developed in France since the 1990s (currently about 3'000 plants are in operation). This paper summarizes the results and efficiencies of 70 plants designed and built by Epur Nature or SINT. The results show clearly that the design performs well for organic matter removal and nitrification and makes sludge management easy. Therefore if well designed, such systems can achieve an outlet quality of BOD5 < 20 mg/L, chemical oxygen demand <90 mg/L, suspended solids <30 mg/L and TKN < 15 mg/L. In addition, some new configurations involving a French vertical first stage fed with raw sewage, patented by Epur Nature, are presented as an aid to reduce the global footprint.

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Performance of the first stage of the French system of vertical flow constructed wetlands with only two units in parallel: influence of pulse time and instantaneous hydraulic loading rate

Water Science & Technology ◽

10.2166/wst.2018.355 ◽

2018 ◽

Vol 78 (4) ◽

pp. 848-859 ◽

Cited By ~ 3

Author(s):

Jorge A. García Zumalacarregui ◽

Marcos von Sperling

Keyword(s):

Constructed Wetlands ◽

Loading Rate ◽

Oxygen Demand ◽

Feeding Time ◽

Vertical Flow ◽

Hydraulic Loading Rate ◽

Hydraulic Loading ◽

Vertical Flow Constructed Wetlands ◽

Removal Efficiencies ◽

Raw Wastewater

Abstract The technology of vertical flow constructed wetlands – French system for treating raw wastewater depends on several hydraulic factors, one of them being the duration of the pulse feeding and the resulting instantaneous hydraulic loading rate. This paper analyses two scenarios in the same system, the first of a faster feeding by pump and the second of a slower feeding by siphon, both with instantaneous hydraulic loading rate values lower than the literature recommendations. The system treated raw wastewater from a population equivalent of 100 p.e. in Brazil, and was comprised by only the first stage and two units in parallel. The shorter duration of feeding time and higher instantaneous hydraulic loading rate were associated with significantly higher chemical oxygen demand and total Kjeldahl nitrogen removal efficiencies, but with no significant differences in terms of biochemical oxygen demand (BOD) and suspended solids (SS). Oxygen concentrations and redox potential in the effluent were evaluated, together with the effluent flow rate profiles. The removal efficiencies were associated with the accumulation of solids in the upper part of the filter resulting from seven years of operation and to the operating hydraulic conditions, which are important elements in the performance of the system.

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

Water Science & Technology ◽

10.2166/wst.2005.0277 ◽

2005 ◽

Vol 51 (9) ◽

pp. 11-21 ◽

Cited By ~ 161

Author(s):

P. Molle ◽

A. Liénard ◽

C. Boutin ◽

G. Merlin ◽

A. Iwema

Keyword(s):

Constructed Wetlands ◽

Pollutant Removal ◽

Vertical Flow ◽

Low Level ◽

Vertical Flow Constructed Wetlands ◽

And Performance ◽

Raw Wastewater

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.

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