The Evolution of Constructed Wetlands from Small-Scale Effluent Polishing Solutions to an Essential Water Treatment Technology

The desire to protect aquatic ecosystems and drinking water supplies from the adverse effects of nutrients, trace organic contaminants, and other constituents in municipal wastewater has led to a need for additional treatment, particularly in watersheds with insufficient dilution. Constructed wetlands have emerged as viable alternatives to advanced wastewater treatment processes for effluent polishing due to their low cost and ancillary benefits. Starting in the late 1980s, experiences gained from several decades of operating wetlands in small communities increased confidence that these nature-based treatment systems could be effective and reliable. As a result, investments were made in larger constructed wetlands, with surface areas greater than a million square meters (1 Mm²) and flows often exceeding 1 m³ s^-1, on effluent-dominated rivers and as part of potable water reuse projects. More recently, new wetland designs have improved the constructed wetland system performance by taking advantage of sunlight-mediated processes in the water column and microbial processes on subsurface porous media. Research that provides additional insight into contaminant removal mechanisms, demonstrates long-term viability, and further improves treatment performance could expand the application of constructed wetlands to other difficult-to-solve water quality challenges, including the treatment of municipal water reuse concentrate and the mitigation of nonpoint source pollution.