Influence of surface layer on hydrology and biology of gravel bed vertical flow constructed wetlands F. Chazarenc and G. Merlin Laboratoire OCIE, Universite ´ de Savoie, Campus Scientifique, F 73 376 Le Bourget du Lac, France (E-mail: florent.chazarenc@umontreal.ca; gerard.merlin@univ-savoie.fr) Abstract 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 (B 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 22 (3 and 4 years operating) to 80 kg m 22 in the oldest plant (8 years). Potential respiration was larger in the oldest plant (75 g O 2 m 22 h 21 ) than in the most recent one (15 g O 2 m 22 h 21 ). 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. Keywords Constructed wetlands; raw wastewater; vertical flow Introduction VFCWs are widely used in Europe and represent an adapted process to treat secondary domestic wastewater (Cooper, 1999). Sand as the main filtration layer with batch load feeding is the most common system design (Vymazal et al., 1998). Many recent exper- iments have been conducted to estimate the clogging time of the system (Blazejeweski and Murat-Blazejewska, 1997; Platzer and Mauch, 1997; Laber et al., 2000), to study performance due to oxygen supply (Green et al., 1998; Lahav et al., 2001), or to simplify the design and reduce surfaces (Weedon, 2003). Filter impermeabilisation after clogging is one of the most important problem occurring in VFCWs and often leads to treatment performance decrease (Langergraber et al., 2003). Managing feeding and rest periods could reduce clogging problems (Breen, 1997; Laber et al., 2000). This operation enables accumulated organic matter mineralization, mainly by micro-organisms associated with plants (Sun et al., 1998). Mineral matter then becomes an element of the filtration matrix and modifies its composition and properties. In France, the process was adapted to treat screened raw wastewater with loading rates ranging from 20 to 50 g m 22 d 21 of TSS and from 30 to 60 g m 22 d 21 of BOD 5 whereas loads of 20 to 25 g m 22 d 21 BOD 5 are gener- ally recommended (Cooper et al., 1996). The French system consists of a first vertical flow stage (usually 3 beds) filled with gravel (B 2–6 mm). The second stage is generally made of several vertical flow beds filled with sand. The capacity to achieve effective treatment of domestic sewage from small rural communities (less than 2000 Pe) was demonstrated according to French standards (Boutin et al., 1997). Using large gravel almost never leads to the complete clogging of the first stages. Furthermore, experiments show that new constructions require a maturation period of approximatively two years to reach optimal treatment performances. This maturation time seems to be linked to matter Water Science & Technology Vol 51 No 9 pp 91–97 Q 2005 IWA Publishing and the authors 91