Review of Constructed Subsurface Flow vs. Surface Flow Wetlands
The purpose of this document is to use existing documentation to review the effectiveness of subsurface flow and surface flow constructed wetlands in treating wastewater and to demonstrate the viability of treating effluent from Savannah River Site outfalls H-02 and H-04 with a subsurface flow constructed wetland to lower copper, lead and zinc concentrations to within National Pollutant Discharge Elimination System (NPDES) Permit limits. Constructed treatment wetlands are engineered systems that have been designed and constructed to use the natural functions of wetlands for wastewater treatment. Constructed wetlands have significantly lower total lifetime costs and often lower capital costs than conventional treatment systems. The two main types of constructed wetlands are surface flow and subsurface flow. In surface flow constructed wetlands, water flows above ground. Subsurface flow constructed wetlands are designed to keep the water level below the top of the rock or gravel media, thus minimizing human and ecological exposure. Subsurface flow wetlands demonstrate higher rates of contaminant removal per unit of land than surface flow (free water surface) wetlands, therefore subsurface flow wetlands can be smaller while achieving the same level of contaminant removal. Wetlands remove metals using a variety of processes including filtration of solids, sorption onto organic matter, oxidation and hydrolysis, formation of carbonates, formation of insoluble sulfides, binding to iron and manganese oxides, reduction to immobile forms by bacterial activity, and uptake by plants and bacteria. Metal removal rates in both subsurface flow and surface flow wetlands can be high, but can vary greatly depending upon the influent concentrations and the mass loading rate. Removal rates of greater than 90 per cent for copper, lead and zinc have been demonstrated in operating surface flow and subsurface flow wetlands. The constituents that exceed NPDES limits at outfalls H-02 a nd H-04 are similar in nature and within typical concentrations found in municipal wastewaters, mine drainage and landfill leachate, all of which have been successfully treated in constructed wetlands. Contaminant-specific removal rates required to bring outfall H-02 and H-04 effluents to within NPDES limits have been achieved by various types of wetlands, including SSF wetlands. Therefore a constructed wetland would be a viable approach for treating effluent from the two outfalls. SSF wetlands generally can be smaller than SF wetlands for a particular wastewater flow, but can cost more per acre. Thus the potential range of costs for constructing a SSF system may overlap with the cost range predicted for a SF system. However, capital costs and operations and maintenance costs for both SSF and SF wetland treatment systems are lower than for conventional wastewater treatment systems. Preliminary scoping level cost estimates for a SSF and a SF system for outfalls H-02 and H-04 are presented in this document using a range of values found in the literature.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC09-96SR18500
- OSTI ID:
- 835229
- Report Number(s):
- WSRC-TR-2004-00509; TRN: US0407493
- Resource Relation:
- Other Information: PBD: 1 Sep 2004
- Country of Publication:
- United States
- Language:
- English
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