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This paper evaluates data collected by the authors as well as other individuals and agencies at operational subsurface flow constructed wetlands. The purpose of the evaluation was to identify the influence of hydraulic loading, organic loading, hydraulic residence time (HRT) on removal performance for biochemical oxygen demand (BOD), total suspended solids (TSS), nitrogen and phosphorus. The results indicate that BOD and TSS removal can be very effective at a relatively short HRT and that BOD removal exhibits a linear relationship with organic loading. Effective nitrogen removal requires a longer HRT and appears to be limited by the low oxygen availability in these systems. Phosphorus removal is also somewhat limited and ranges from 30 to 60 percent. Design models for these parameters are available or under development.

A recent study for the U. S. EPA documented more than 150 constructed wetland systems in the United States, treating municipal and industrial wastewaters. During 1990 and 1991, visits were made to more than twenty of these sites for observations and discussions with the designers and operators. This first generation of systems demonstrates a lack of consensus on design procedures, and in many cases little regard for some of the critical factors affecting performance. This paper summarizes the results of the inventory and site visits and provides suggestions for design of the second generation of these systems. Water Environ. Res., 64, 776 (1992).

During 1990 and 1991 the U.S. Environmental Protection Agency (EPA) sponsored an effort to identify existing and planned constructed wetlands in the U.S., and to collect readily available information from operating systems. In addition to inquiries by telephone and mail, the effort included site visits to over 20 operating subsurface flow constructed wetlands. The inventory documented the presence of over 150 constructed wetland systems for wastewater treatment, including both free water surface (FWS) and subsurface flow (SF) systems. The majority of the systems identified were SF systems for treating municipal wastewater. FWS systems were separated into three groups based on the design level of effluent water quality. SF systems were separated into three groups based on the basic design approach. The inventory indicated that neither between nor within these groups was there consensus regarding basic hydraulic and engineering design criteria, system configuration, or any other aspect, such as type of vegetation, size and type of media, or pretreatment. Information on location, type of system, design approach, hydraulic and organic loading rates, costs, and other aspects is presented. Information gathered and "lessons learned" from the site visits are presented. Insufficient oxygen for nitrification appears to be a problem for both FWS and SF systems. Insufficient hydraulic design appears to be a problem for SF systems.

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