In the past few months, I have had a couple questions about the viability of constructed wetlands to treat septic tank effluent in cold climates. A little review is necessary before I answer the question. Our part of the wastewater industry uses constructed wetlands as an additional pre-treatment option. One way to think about this is as a media filter (just like a media, sand or peat filter). In the treatment train, it would follow the septic tank and be ahead of the final soil dispersal and treatment area.

There are three general types of constructed wetlands: open water, hydroponic and subsurface flow. In cold climates, the first two are subject to freezing in winter and reduced treatment rates during cool seasons, so subsurface flow is the type recommended for cold climates. In a subsurface flow wetland, the effluent moves through a media, usually pea rock, in which the wetland plants grow. Since flow is below the surface of the rock, they are less subject to freezing. And by controlling water levels in the media during the winter, a layer of ice can be formed near the surface to provide additional insulation.

Treatment occurs through a variety of physical, chemical and biological processes. Treatment is also impacted by the way the constructed wetland is managed in terms of how and when effluent is introduced to the system, as well as how the plants in the wetland are managed. As effluent moves through the wetland, solids are removed by filtration in the vegetation and media. Organic solids are broken down by bacteria and other organisms, reducing BOD.

NITROGEN REMOVAL

Nutrients of concern such as phosphorus and nitrogen are reduced. Phosphorus removal is primarily by adsorption to the media and plant roots. There is some plant uptake but most of the removal occurs in the pea gravel. Some studies have shown removal is improved by using iron-rich gravel materials instead of the typical pea gravel. 

Nitrogen removal is accomplished through nitrification and then corresponding denitrification. Oxygen is introduced through the plant roots from the atmosphere. This converts from the ammonium to nitrate (nitrification) form of nitrogen. Then, since the pea gravel or other rock media is saturated (without oxygen), nitrogen is released to the atmosphere (denitrification). This combination is effective for the removal of bacteria and phosphorus as well if retention time in the system is long enough. So as a part of management it is important that the wetland not be hydraulically overloaded.

In terms of treatment, the best performance occurs in cold climates such as in Minnesota, Wisconsin and Michigan during the active plant-growing season, basically May through September. Some research we conducted in Minnesota in the early 2000s showed very good reduction in BOD, about 90 to 95%, TSS 90% and fecal coliform bacteria about 99% in the summer with 80% BOD and TSS, and 96 to 99% fecal coliforms in the winter.

For nitrogen and phosphorus, treatment efficiencies were 65-80% in the summer and 25-30% in the winter. These are very good results, especially since the pretreated effluent is delivered for final treatment and dispersal in soil. It demonstrates for these constituents of concern that constructed wetlands are a viable option for use in cold climate areas with proper management and maintenance.

More recent research has been reported on the effectiveness of constructed wetlands to treat other constituents of concern, such as pharmaceuticals. Public wastewater treatment plants have had problems removing these compounds before discharging into surface waters. An example of these compounds is anti-inflammatory drug ibuprofen. When discharged to surface waters, these compounds have been shown to have negative effects on fish and other aquatic organisms. While there are methods for wastewater treatment plants to reduce levels discharged to the environment, they are expensive.

LANDSCAPING REQUIRED

Those same compounds are released into our systems, so they are a concern to us too. Research so far has shown that wetlands are effective in reducing the compounds. How the wetland is managed may have a large impact on how effective the system is for treatment. A management scenario where effluent is introduced as a batch flow instead of the typical continuous flow system is one example. This presents other management problems for the system, such as maintaining good plant growth with the flooding/drying sequence, not to mention the storage problems with periodic flows. Nevertheless, constructed wetlands are an effective treatment solution in cold climates.

In terms of management and maintenance issues, some specific necessary activities are unique to wetland systems. Effluent levels need to be monitored and controlled. If levels are not maintained, the system can dry out or freeze, killing the plants. Excess flows can reduce the treatment capabilities and cause odor problems. 

Vegetation must be maintained with dead vegetation removed periodically so the wetland does not release tied-up phosphorus. In addition, animal pests and insects must be controlled. Berms and dikes must be inspected periodically, grass mowed and any damage from burrowing animals repaired. Trees must be removed from the dikes as well as within the wetland itself. Their roots can break through the membrane liner in the system, causing leaks and system failure.

The answer to whether constructed wetlands are viable for use in cold climates is yes. But as with other systems, effective management will determine whether it meets treatment requirements.