Constructed Wetlands

Wetlands may be applied to treat most pollutants that occur in waters. Such treatment wetlands have become popular because of both a \u27green\u27 image and the low operating costs for the passive treatment technology. They have been applied extensively not only to domestic wastewater but also to stormwater, agricultural waters, industrial sources, mine water, and for water reclamation from impacted natural waters. There are two major types of wetlands, free water surface and subsurface flow. Subsurface flow wetlands may be designed as horizontal or vertical flow. In any design, the wetland should follow a settling process to prevent clogging from quickly causing problems. The removal mechanisms in wetlands are similar regardless of design: solids are removed by filtration as well as some settling; biochemical oxygen demand (BOD) is decreased as organics are consumed by microbes; and ammonia is microbially oxidized near the water surface, and the resulting nitrate can be removed in anoxic metabolism deeper in the wetland. Phosphorus is not biologically removed in wetlands, but some sediment materials may sorb phosphate. Somewhat surprisingly, plants have a minimal direct impact on pollutant removal. Plants do have a proven impact on treatment because they provide surfaces for microbial attachment, insulate and shade the water surface, and affect redox potential in the wetland sediment. Compared to other treatment technologies, the design and performance of wetlands involve much higher variability. Wetlands have a comparatively low volumetric activity and thus are impacted strongly by local conditions. The day-to-day performance of the highly complex system is thus \u27noisy\u27, and design equations cannot yet capture this variability. This does not mean performance is poor; well-sized wetlands produce water low in solids, BOD, nitrogen, and pathogens. Rather, the design of wetlands is overly simplistic, generally using zero-order areal loading values. This chapter reviews the removal mechanisms, performance, and design of wetlands. The focus is on domestic wastewater, but other applications are also addressed in terms of mechanisms and performance