Water Research 38 (2004) 1669–1678 Initial contaminant removal performance factors in horizontal flow reed beds used for treating urban wastewater Joan Garc ! ıa a, *, Paula Aguirre a , Rafael Mujeriego a , Yuming Huang b , Laura Ortiz b , Josep M. Bayona b a Environmental Engineering Division, Department of Hydraulic, Coastal and Environmental Engineering, School of Civil Engineering, Technical University of Catalonia, Jordi Girona 1-3, M" odul D-1, Barcelona 08034, Spain b Department of Environmental Chemistry, CID-CSIC, c/ Jordi Girona 18-26, Barcelona 08034, Spain Received 6 December 2002; received in revised form 21 October 2003; accepted 21 January 2004 Abstract This study evaluates the effect of hydraulic loading rate (HLR), aspect ratio, granular medium size and water depth on the removal of selected contaminants during the start up of horizontal subsurface flow reed beds (HFRBs). Experiments were carried out in a pilot-scale HFRB system comprising four pairs of lined beds of almost equal surface area (54–56m 2 each bed), with the following aspect ratios: 1:1, 1.5:1, 2:1 and 2.5:1. The size of the granular medium of each pair varied from coarse granitic gravel (D 60 ¼ 10mm, C u ¼ 1:6) to small granitic gravel (D 60 ¼ 3:5mm, C u ¼ 1:7). The beds of the pair with longest aspect ratio were made shallower (0.27 m) than the rest (0.5 m) The system was sampled weekly from May 2001 to January 2002. The results indicate that HLR and water depth are determining factors in the performance of the HFRBs. Beds with a water depth of 0.27m removed more COD (70–80%), BOD 5 (70– 85%), ammonia (40–50%) and dissolved reactive phosphorus (DRP) (10–22%) than beds with a depth of 0.5m (60– 65% for COD, 50–60% for BOD 5 , 25–30% for ammonia, and 2–10% for DRP). The higher efficiency observed shallower beds was related to their less reducing conditions (average redox potential (E) ranging from 351 to 338 mV) than beds with a depth of 0.5m (390 to 358mV). The difference in E status between two bed types seems to lead to differences in the biochemical processes. In fact, denitrification was estimated to be a significant reaction in shallower beds. r 2004 Elsevier Ltd. All rights reserved. Keywords: Constructed wetlands; Hydraulic loading rate; Aspect ratio; Water depth; Subsurface flow 1. Introduction Horizontal flow reed bed (HFRB) technology is mainly used for the treatment of urban and domestic wastewater from small rural communities. During the design of full scale HFRBs the most relevant parameters to be defined are: hydraulic loading rate (HLR), aspect ratio, granular medium size and water depth [1]. The effect of HLR on organic matter and nutrient removal in HFRBs has been assessed in several studies [2–4]), and the results indicate that lowering the HLR improves the efficiency. Most of the HLRs are in the range of 10– 100mm/d [5,6]). A popular notion in HFRB design has been that increasing the aspect ratio of the beds will produce a behaviour closer to plug flow and therefore better efficiency. However, because in the microscale mixing occurs, this notion is not completely correct [1]. Several studies with innocuous tracers have repeatedly demon- strated that an increasing aspect ratio delays the breakthrough time but the spread of the response curve is similar for lower and higher aspect ratios [7,8]). This ARTICLE IN PRESS *Corresponding author. Tel.: +34-93-4016464; fax: +34-93- 4017357. E-mail address: joan.garcia@upc.es (J. Garc ! ıa). 0043-1354/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.01.011