ecological engineering 35 ( 2 0 0 9 ) 175–183 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecoleng A comparative study of surface and subsurface flow constructed wetlands for treatment of combined sewer overflows: A greenhouse experiment Annelies M.K. Van de Moortel a,∗ , Diederik P.L. Rousseau b , Filip M.G. Tack a , Niels De Pauw c a Department of Applied Analytical and Physical Chemistry, Laboratory of Analytical Chemistry and Applied Ecochemistry, Ghent University, Coupure Links 653, 9000 Ghent, Belgium b Department of Environmental Resources, UNESCO-IHE Institute for Water Education, P.O. Box 3015, 2601 DA Delft, The Netherlands c Department of Applied Ecology and Environmental Biology, Laboratory of Environmental Toxicology and Aquatic Ecology, Ghent University, J. Plateaustraat 22, 9000 Ghent, Belgium article info Article history: Received 7 December 2007 Received in revised form 27 August 2008 Accepted 29 August 2008 Keywords: Waste water treatment Ammonium Nitrogen Phosphorus Phragmites Fractionation Treatment wetlands Stormwater k–C* model First-order removal abstract The use of surface flow (SFCWs) and subsurface flow constructed wetlands (SFCWs) for the treatment of combined sewer overflows was assessed at pilot scale. Synthetic wastewa- ter was applied in three batches with decreasing concentrations to mimic concentration profiles that are obtained in the field during overflow events. Three simulated combined sewer overflows were applied on each wetland. Composite water samples (60 in total) were taken for a period of 8 days to study the removal of total nitrogen (Ntot), NH 4 –N, NO 3 –N, total COD (CODtot) and total phosphorus. Redox potential, which was monitored at vari- ous locations along the wetlands, was more negative in the SSFCWs. In general, removal occurred faster in the SSFCWs and the final concentrations were lower. The removal of Ntot was only 36.6 ± 3.3% in the SFCWs due to nitrification-limiting conditions. The conditions in the SSFCWs, in contrast, seemed to promote Ntot removal (removal efficiency 96.7 ± 1.9%). The removal of P was hampered in both wetland types by reducing conditions. P that was initially removed was released again from the substrates later on. First-order removal rate constants were derived for the removal of both CODtot (SSFCWs: 1.1 ± 0.3 m d -1 ; SFCWs: 0.17 ± 0.06 m d -1 ) and Ntot (SSFCWs: 0.4 ± 0.1 m d -1 ; SFCWs: 1.7 ± 0.5 m d -1 ). © 2008 Elsevier B.V. All rights reserved. 1. Introduction During rain events, the capacity of combined sewer systems may be too confined to allow transport of all waste- and stormwater to a treatment plant. Combined sewer overflows (CSOs) are provided for diverting the excess water to the ∗ Corresponding author. Tel.: +32 9 264 59 95. E-mail addresses: Annelies.Vandemoortel@UGent.be (A.M.K. Van de Moortel), d.rousseau@unesco-ihe.org (D.P.L. Rousseau). nearest surface water, thereby potentially causing a sudden pollution shock. The study of devices applicable for the tempo- ral storage and treatment of stormwater and combined sewer overflows has been pointed out as one of the 10 priority areas in urban wet weather flow research (Heaney et al., 1999). The occurrence of uncontrolled or poorly controlled discharges 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.08.015