Calculating Passive Treatment of Effluent in Constructed Wetlands

October 4, 2022   |  

Natural Versus Constructed Wetlands

As opposed to natural wetlands, which are typically found at topographic depressions or in areas with high slopes and low permeability soils, or between stream drainages when land is flat and poorly drained, modern treatment wetlands are constructed systems that have been designed to emphasize specific characteristics of wetland ecosystems for improved treatment capacity.

The technology for passive effluent treatment in constructed wetlands has evolved over the last several years into new system configurations and a much broader range of treatment applications. Passive treatment is often attractive to mining proponents because they are relatively low-cost and low maintenance when compared to other treatment alternatives.

Formula for Effluent Treatment

Effluent treatment in a wetland is influenced by a variety of biological processes and biogeochemical cycles that are not always easy to predict and design for. Other factors, including metal species, metal concentration, flow volume, water temperature, and pH all factor in the applicability of wetland treatment. A simple calculation can provide you with a rule of thumb area required to meet the necessary retention time for treatment, helping to determine if a treatment wetland is a feasible alternative.

The formula is: A = [Qd (Ci – Ct)] / RA

A = required wetland area (m2)
Qd = mean daily flow-rate (m3/day)
Ci = mean daily influent contaminant concentration (mg/L)
Ct = required concentration of contaminant in final discharge (mg/L)
RA = area-adjusted contaminant removal coefficient (g/m2/day) (dependent on metal)

It is important to note that the contaminant removal coefficient varies significantly by metal; and, certain metals (e.g., zinc) are not so easily remediated. For example, a calculation for a site with an inbound flow rate of 5 m3/min and iron and zinc at equal concentrations and equal regulatory discharge limits is shown below. A wetland for zinc removal would require over 100 hectares of area, not practically feasible. The removal of iron requires significantly less area.

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