As noted in a previous column, I have had questions about design, installation and maintenance of constructed wetland systems. The kind of system I am most familiar with is the subsurface flow constructed wetland. In this system, septic tank effluent is delivered to the head of the wetland into normal drainfield rock material. The best treatment results are obtained when flow is equalized to the wetland cell (bed). Effluent is delivered in timed dosing through a pressure distribution network in the rock.

         Both the wetland vegetation and the pea gravel substrate are involved in the treatment processes. In cold climates, treatment varies by the season, with best treatment during the growing season and reduced treatment in the winter. BOD removal averages 90-95% in the summer versus 80% in the winter. Fecal coliform bacteria are reduced by 96-99% in the winter and greater than 99% in summer. If the purpose of the wetland is to remove nutrients in lakeshore or other sensitive areas, nitrogen and phosphorus removal are 25-30% in the winter and 65 to 80% in the summer. Constructed wetlands can provide good treatment before discharge to a soil treatment unit (trenches, mounds, at-grades) or if permitted to an unlined wetland cell.

         Obtaining these levels of treatment requires that all parts of the system be properly operated and maintained. The septic tank and the pump tank, pump and timer all need to be periodically (every year at a minimum) checked to make sure they are functioning properly. While this is the same for any type of system with these components, it is more important in wetland systems. The plants need a consistent flow of water to survive, flourish and provide the highest and most consistent levels of treatment.

         Similarly, if effluent from the wetland is delivered to a series of pressure-dosed sewage treatment trenches, mound or at-grade, the pump tank and pump need to be checked. The final treatment area inspection ports should be opened and any liquid levels should be observed and reported. Beyond these typical maintenance activities, there are several unique issues or situations to address in wetland systems.

REGULATE FLOW

         There will be a need to periodically regulate the control structure at the outlet. First, wetland vegetation needs water to survive, so flow out of the wetland may need to be reduced during periods of high evapotranspiration by the plants. Flow should also be monitored going into the wetland for the same reason; to maintain adequate water or to identify the presence of excessive flow that may hydraulically overwhelm the system. Excessive flows could occur due to changes in household water use, leaky fixtures, heavy rains or rapid snowmelt.

         An additional reason to regulate the flow is to prevent freezing during the winter. This is done by lowering the water level in the pea gravel substrate after initial freeze-up in the fall. This creates an insulating pocket of air above the water level in the wetland cell. The air pocket combined with adding mulch or cover to the surface prevents the system from totally freezing.

         Vegetation in the wetland must be managed to provide the highest levels of treatment. In my area, it takes about three years for wetland vegetation to become fully established in the cell to provide maximum treatment. The bed must be weeded to get rid of any unwanted invasive species or noxious weeds, small trees and shrubs to ensure wetland plants survive and thrive. Without weeding, the proper wetland plants will not become well established. Roots from trees and shrubs can damage the liner, causing leaks.

         Dead vegetation at the surface should be removed when it gets thicker than two inches in depth. Maintaining nitrogen and phosphorus removal by the wetland requires that vegetation be harvested every five years, removed from the site and either land-applied or disposed of in a landfill.

FIGHT INVASIVES

         Just as in more conventional treatment systems, toxic chemicals can harm the wetland plants. The homeowner needs to understand that excessive use of cleaning products may damage their system. Where the wetland is serving a restaurant or bar, excessive solids can plug the media. Another type of pretreatment may be necessary.

         Take precautions to control wetland cell access to keep out small children or pets. This may entail some type of fencing that may need periodic maintenance. Also, any unwanted burrowing animals will need to be controlled. These animals may find the wetland cell a desirable place to set up housekeeping. This can cause damage to the vegetation and the liner. The vegetation should be inspected annually for other pests such as insects that can damage the vegetation and appropriate control actions initiated.

         Around the perimeter of the wetland cell, berms or dikes should be inspected and any erosion or damage repaired. Grass should be mowed periodically to discourage burrowing rodents. Trees or shrubs that start to grow around the perimeter should be removed.

         As with any other type of onsite system, constructed wetlands will provide treatment indefinitely into the future if they are properly operated and maintained.