Comparison of free water and horizontal subsurface treatment wetlands

The two most prevalent types of treatment wetland, especially during the early history of the technology, are free water surface (FWS) and horizontal subsurface flow (HSSF) wetlands. The several factors involved in the choice of which alternative to choose include size, cost, operability, together with health and nuisance issues and ancillary benefits. Contaminant removal performance differs by constituent, with the advantage to FWS for moderate to high biochemical oxygen demand (BOD), TSS, ammonia, total nitrogen and phosphorus. HSSF are more effective for tertiary BOD levels, nitrate and pathogens. Superpositions of the loading data show that the respective data clouds overlap virtually entirely for HSSF and FWS wetlands. There is little or no performance difference when they are compared on this areal basis. In general, there is little or no advantage of HSSF for space saving. In cold climates, HSSF systems are less cold sensitive, and easier to insulate for winter operation. The use of winter storage enables FWS to be used in freezing conditions, but the cost makes that option comparable to the more expensive HSSF. In general, economics do not favor the choice of HSSF wetlands. Factors other than reduction performance are also important in the selection process. Other principal reasons for selecting the HSSF option over the FWS option are prevention of human health contact problems, mosquito control and minimization of wildlife interactions.