Subsurface Constructed Wetlands

Page Contents:

• Advantages and Disadvantages
• Considerations
  • Siting
  • Permitting
  • Factors that Control the Level of Nitrogen Reduction
• Treatment Details
• Cost per Pound of Nitrogen Removed

Constructed wetlands are artificial wetlands that are designed and built to simulate nitrification and denitrification processes that occur in natural wetlands to treat wastewater effluent (USEPA 2000). There are two general types of constructed wetlands: surface and subsurface flow (USEPA 2000). Subsurface flow wetlands are generally applied for wastewater treatment so that it is not visible or available to wildlife, and include a lined ditch or bed filled with gravel and soil to support plant growth (USEPA 2000). Constructed wetlands are not currently approved for wastewater treatment in Suffolk County but are being evaluated by SBU CCWT at various locations including Sylvester Manor, Shelter Island, New York (SBU CCWT Constructed Wetlands for Wastewater Treatment Fact Sheet).

Advantages and Disadvantages

Considerations

Siting

Wetlands are typically land intensive but subsurface flow wetlands require less space than surface flow wetlands because they are designed to have longer residence times and more intense biological treatment. However, in colder climates systems size should be doubled to provide adequate treatment (USEPA 2000). Treatment wetlands should be constructed upland and outside floodplains, and not on steep topography, to avoid damage to aquatic resources and natural wetlands, and site suitability factors such as soil type should be considered (USEPA 2000).

Permitting

The following permits are required for installation of constructed wetlands:

• SCDHS
• New York State Department of Environmental Conservation (NYSDEC) and/or Town and Village setbacks in close proximity to surface waters or wetlands
• Town and Village Building Department

Factors that Control the Level of Nitrogen Reduction

Nitrogen removal rates are dependent upon the level of treatment; passive, horizontal flow systems have been shown to provide less removal than unsaturated vertical flow systems, and intensified systems with horizontal and vertical Nitrogen removal rates are dependent upon the level of treatment; passive, horizontal flow systems have been shown to provide less removal than unsaturated vertical flow systems, and intensified systems with horizontal and vertical flow, with aeration and reciprocating fill and drain demonstrate the highest level of removal (Nivala et al. 2019). Plants have little influence on vertical flow and intensified systems but can improve removal efficiency in horizontal flow systems (Nivala et al. 2019; USEPA 2000).  CCWT has experimented with a recirculating with a gravel layer coupled with a woodchip and/or mulch layer to promote coupled nitrification-denitrification. It has also considered a recirculation step which would redirect a portion of effluent back to the gravel layer instead of to final deposition. Both of these modifications would maximize the nitrogen removal by these systems.

Treatment Details

The influent source for the NRB is septic tank effluent, with a concentration of 65 mg/L (CDM Smith 2020). The low and high range removal rates of 40% to 17% are based on the removal efficiencies of the horizontal and unsaturated vertical flow systems determined by Nivala et al. (2019) and a flow rate of 15,000 gpd.  Incorporating woodchips and recirculation into wetlands systems would increase these removal percentages.

Constructed Wetland Nitrogen Effluent Concentrations, Nitrogen Reduction, and Annual Removal Rates

Cost Per Pound Nitrogen Removal

The capital cost (design, engineering, and construction), O&M, and replacement costs for a subsurface constructed wetland are baThe capital cost (design, engineering, and construction), O&M, and replacement costs for a subsurface constructed wetland are based on costs estimated by the Cape Cod Commission (2020) scaled to a 15,000 gpd system. Costs assume existing septic tanks are left in place. The total cost includes capital costs annualized with a 5% discount rate and the annual average O&M cost with a 2% inflation rate applied over 20 years.

Constructed Wetland Capital, O&M, Total Annualized Costs, and Costs per Pound of Nitrogen Removed