ENGINEERED WETLANDS

Modern constructed wetlands (CWs) for wastewater treatment usually consist of a number of individual rectangular and/or irregularly-shaped basins (cells) connected in series and surrounded by berms of earth, clay, rock, concrete or other materials. Water in them usually flows in two or more parallel trains. These passive treatment systems often include a variety of ancillaries (e.g., ditching, lagoons, cascades, land treatment fields). Three types of cells may be used in a CW system: pond, free water surface (FWS), and sub-surface flow (SSF) cells. Free Water Surface wetland cells are artificial marsh ecosystems in which water flows on the surface through emergent wetland vegetation. In them, the submerged portions of wetland plants as well as soil and detritus act as substrates for microbial biofilms, and these and physical filtration are responsible for much of the pollution removal. With Sub-Surface Flow constructed wetland cells, water flows just under the surface of porous materials (substrates) consisting of beds of gravel, sand or rock. Pollutant removal is via biofilms in the interstices of the substrate beds, and plant root systems growing in them. SSF wetland cells may be horizontally fed (HSSF cells) or the wastewater may move vertically in the substrates (VSSF cells). SSF wetlands can be used where the wastewater being treated is noxious or odorous; where a higher degree of freeze protection is desired; where the attraction of wildlife (especially waterfowl) may be undesirable (e.g., at airports); where ample, economic supplies of substrate material are readily available; and/or where operation in an engineered wetland mode is desired.

Engineered wetlands (1) are special, advanced, semi-passive kinds of constructed wetlands in which operating conditions are more actively monitored, manipulated and controlled in such a manner as to allow contaminant removals to be optimized. At the same time, cold weather operability is improved, as is the ability to deal with otherwise adverse conditions and recalcitrant wastewaters such as landfill leachates and mine drainage. Constructed wetland systems may be “engineered” in many ways as shown in the box. With engineered wetlands (EWs), competing reactions that are carried out in the same cells of ordinary CWs (e.g., aerobic nitrification and anaerobic denitrification) can be carried out in separate EW cells. (In the case of the nitrogen reactions, virtually stoichiometric conversion of ammonia to nitrate, and nitrate to nitrogen gas can be achieved much more efficiently in a much smaller EW system.) (Higgins, 2000 a, b, c)

With EWs, many kinds of biological and chemical process systems (e.g., aerobic and anaerobic bioreactors, limestone drains) can be “expressed” as cells of the system. The box shows some of the kinds of cells that can be used in an EW. Engineered wetlands can be used to bridge the gap between active treatment and eventual, fully passive treatment in the ordinary constructed wetlands that they can evolve into over time if influent contaminant concentrations decline naturally, ending the need for the more aggressive treatment methods.

1 Although the names “engineered wetland” and “constructed wetland” are sometimes used interchangeably, it is appropriate to define the former more narrowly by limiting the engineered designation to apply only to the more advanced, semi-passive systems. All engineered wetlands are constructed wetlands, but not all constructed wetlands are engineered ones.