What is a treatment wetland?

Have you ever wondered what exactly IS a treatment wetland? This is a generic term we use to describe a wetland that has been designed to treat water. These have become known by a wide range of names, such as passive treatment systems (PTS), passive water treatment systems (PWTS), constructed wetlands (CW), constructed wetland treatment systems (CWTS), not to mention the many trademarked and brand names, and so on. However, it is not the name so much that matters, but why they are used. These are distinctly different from wetlands that provide habitat for wildlife, or are intended to compensate or reclaim/restore impacted wetlands (an aspect which is addressed later in this post). Instead, a treatment wetland is designed to remove compounds from water, using natural processes to either degrade the compounds into benign forms, or sequester them into the soils rendering them less bioavailable.

It is important to remember that it is the water quality, constituents of concern, and water quality goals that matter more than the origin of the water. For example, the scientific and biogeochemical processes driving the treatment of ammonia, selenium, sulphate, arsenic, or uranium, although different from one another, remain constant regardless of the source of the water (just to name a few!).

Some of the water sources and industries that have benefited from treatment wetlands include municipalities, pulp and paper, and industrial facilities and manufacturing. However, by far the area we do most of our work in is the mining and oil and gas sectors.

A treatment wetland is constructed of specially designed soils and plants native to the site that are tolerant of the water to be treated. Perhaps most importantly, the treatment wetland harbours naturally beneficial microbes which will carry out the desired treatment processes. The plants, soils, and wetland designs will strongly influence the identity, characteristics, and productivity of the microbes, and must be designed with them in mind.

When properly designed, a treatment wetland can efficiently, robustly, and predictably treat water. Constituents that can be treated include:

  • Acidity
  • Ammonia
  • Biological oxygen demand
  • Chemical oxygen demand
  • Chlorinated compounds
  • Hydrocarbons
  • Organics
  • Metals
  • Metalloids
  • Nitrate
  • Nitrite
  • Nutrients
  • Sulphate
  • Total suspended solids
  • Emerging compounds of potential concern (e.g., naphthenic acids from oil sands, etc).

Treatment of these constituents is often accomplished through a series of steps (as a treatment train), capitalizing on biogeochemical processes that are performed by microbes. A key aspect of the sustainability of treatment wetlands is that if designed properly, metals can be mineralized into soils, minimizing plant uptake.

It is very important that treatment wetlands be distinguished from a natural or constructed (man made) habitat wetland, or the reclamation/restoration of an otherwise impacted natural wetland.

In contrast to habitat wetlands, treatment wetlands should be designed to deter wildlife. This may seem surprising at first, but if you think about it, why would you want to attract wildlife to an area with water that is still in the process of being treated?

Wildlife can be deterred from treatment wetlands by doing the opposite of what is recommended for habitat wetlands. For example, aspects that can be used to deter wildlife include: steep inclines on the shores and armouring with riprap; no open water; monocultures of plants (no diversity); and even fences. Additionally, wildlife-attracting wetlands can be built downstream or nearby to provide a preferential habitat for wildlife.

Interestingly, these design aspects that deter wildlife are also important features that positively influence the treatment capacity of the wetland.

Specifically, the steep banks allow for consistent hydraulic retention time as accretion progresses; the armouring prevents erosion; open water can often be a dead zone for treatment; and monocultures provide consistency of treatment.

There are many different types of treatment wetlands, and approaches that can be taken. We will be starting to cover these in future posts. What would you be most interested in discussing? Let us know in the comments section.

Want to learn more about treatment wetlands? Stay tuned for more posts to come, and in the meantime, check out this recent article.