Water Resources Management 18: 541–565, 2004. C  2004 Kluwer Academic Publishers. Printed in the Netherlands. 541 Design Methodology of Free Water Surface Constructed Wetlands MARIA A. ECONOMOPOULOU and VASSILIOS A. TSIHRINTZIS ∗ Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, Democritus University of Thrace, Xanthi 67100, Greece ( ∗ author for correspondence, e-mail: tsihrin@otenet.gr; tsihrin@env.duth.gr) (Received: 7 January 2003; in final form: 1 April 2004) Abstract. Simple criteria, guidelines and models are established for free water surface (FWS) con- structed wetland selection and preliminary sizing. The analysis employs models for FWS constructed wetland design, considering simultaneously the removal requirements and the hydraulics of the system. On the basis of these models, a step-by-step methodology is developed outlining the de- sign procedure for new and performance evaluation for existing FWS constructed wetland systems. This methodology is combined with simple equations predicting the maximum wetland capacity in summer, so as to assist designers in sizing installations in tourist areas with increased summer populations. Furthermore, this methodology is further simplified, based on sensitivity analysis of the unit area requirements for wastewaters of various strengths, and various design conditions and performance criteria. In addition, comparison of the unit area requirements of FWS constructed wet- land systems, subsurface flow (SF) constructed wetland systems and stabilization pond systems for wastewaters of various strengths and design conditions, provides designers with general guidelines concerning the preliminary selection between alternative natural treatment systems in areas where the use of natural systems is favored because of their low-cost, simple operation and high removal performance. Key words: design methodology, free water surface constructed wetlands, seasonal performance, sensitivity analysis, stabilization ponds, subsurface flow constructed wetlands, unit area requirement, wastewater treatment 1. Introduction Natural treatment systems are characterized by low-maintenance, simple and re- liable operation and high removal efficiencies. These systems are highly favored in small to medium communities, where the resources and the skilled personnel required for the operation of conventional systems are often limited (USEPA, 1988; Sartoris et al., 2000). Furthermore, they are considered a favorable treatment al- ternative for the production of effluents that can be reused for irrigation, limiting the fresh water consumption and the possibility for pollution of receiving waters (WHO, 1989). Therefore, emphasis is placed on the development of practical de- sign and analysis procedures that promote the proper evaluation and use of natural systems (e.g., Economopoulou and Tsihrintzis, 2003).