Wetlands have been called the “Earth’s kidneys,” and for at least sixty years constructed (or artificial) wetlands have been used to treat a variety of waste waters including urban run-off, municipal, industrial, agricultural and acid mine drainage.

Not to be confused with created or restored wetlands which have the primary function of providing wildlife habitat, constructed wetlands use natural processes involving wetland vegetation, soils and their associated microbial assemblages to improve water quality. Constructed wetlands are engineered structures that control the flow direction, liquid retention time and water level in a highly reliable, low-cost, simple, and aesthetically pleasing way. Operationally, constructed wetlands are the simplest of systems, but biologically they are a sophisticated and efficient means for controlling nutrients, pathogens, suspended solids, and some heavy metals. There is little doubt that they represent a superior approach to protecting groundwater resources when compared to septic drainage fields, and that they are particularly appropriate in rocky areas like those found in the Gulf Islands where significant pressure exists to maintain and preserve water resources.

The two general types of constructed wetlands used are (1) subsurface flow, and (2) free water surface systems. A typical constructed wetland consists of a series of vertical and horizontal flow basins (often less than one metre in depth), also called cells, that are lined with a watertight membrane. The cells are filled with gravel and other substrate materials (e.g., sand) to support the root structure of aqueous vegetation like phragmite grasses, rushes, and cattails. These plants translocate oxygen down to the roots to a colony of beneficial bacteria and other microorganisms. Nutrients like nitrogen are absorbed and these microbes convert organic nitrogen into useable inorganic forms (e.g., NO3−, NH4+). Constructed wetlands are highly effective at reducing the biochemical oxygen demand (BOD) of waste water, and have even been used in highly challenging environments.

Subsurface flow constructed wetlands are normally designed to require no electricity, and are relatively easy to monitor for performance and environmental impacts. Variables like the following are often monitored: temperature of discharge, oxygen level, pH level, total suspended solids, oil/grease content, ammonia level, chloride level, BOD, average discharge per day, maximum high for the month, and total monthly discharge.

Several environmental benefits are associated with the use of constructed wetlands including the ability to handle fluctuating water levels, they fit harmoniously in the landscape, offer new wildlife habitat, and provide various public use benefits including education. Constructed wetlands can be designed intentionally for ground water recharge, and even provide ancillary benefits. For example, a study on the harvesting of cattails to produce bioethanol as a fuel suggests that between 4000-9680 liters of bioethanol per hectare can be produced from plants harvested seasonally from constructed wetlands, thus adding more value to the land while offsetting greenhouse gas emissions through substitution.

While relatively land-intensive in terms of overall footprint, these systems offer an effective means of handling waste water. With proper planning, design, construction and operation, many of the potential adverse environmental impacts can be reduced and controlled including concerns about altering the natural hydrology of the area, the introduction of potentially invasive species, and disruptions of natural plant and animal communities. In short, constructed wetlands represent a significant attempt to stay ahead of emerging environmental regulations while providing strong environmental and community benefits.

Dr. Michael D. Mehta lives on Gabriola Island but works in Winnipeg. He is a Professor of Environmental Studies at the University of Winnipeg, and was founding Principal of Richardson College for the Environment. His website is www.policynut.com