12/9/2011. Constructed Wetlands. Functions, Design Criteria

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1 Constructed Wetlands Functions, Design Criteria 1

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3 Functions of Wetlands Flood mitigation Water filtration/purification (treatment) Wildlife habitat, biodiversity Biogeochemical cycles Recreation Reasons for wetland construction Mitigation: Mitigation i i means replacement of wetlands and their functions. It is required in the Corps process if alternative siting and efforts to reduce impacts still result in wetland loss. Banking: Banking refers to the creation of artificial wetlands, storage of credits for these wetlands, and subsequent permission to destroy other wetlands with acreages less than the credits banked. Treatment: Creation of wetland specifically for purpose of treating a specific waste (e.g., agricultural runoff, storm runoff, domestic wastewater, mine drainage). 3

4 Advantages of Treatment Wetlands 1. Wetlands can be less expensive (both construction, operation & maintenance) than conventional treatment facilities; 2. Wetlands can tolerate fluctuations in flow; 3. Wetlands facilitate water reuse and recycling; 4. Wetlands provide habitat; 5. Wetlands enhance landscape aesthetics and functions; Limitations/disadvantages of wetlands as treatment systems Require large land areas; Performance may be temporally variable (temperature, floods, snowmelt); Biological components are susceptible to shock loadings (toxics) Require minimum amount of water; 4

5 Important characteristics of wetlands Position in landscape; Water source Vegetation; Depth of water table; Temporal heterogeneity - variability in water level (hydroperiod) Spatial heterogeneity - vegetation, flow paths, soil or substrate quality, Position in landscape: 1 5

6 Position in landscape: 2 UPLAND Site Selection No permits required for inputs; Preferred for treatment wetlands; Easier to control hydrology, Other considerations Lowland Permit required for inputs; Can restore degraded wetlands; Can result in net environmental benefit CWA Sec. 319 NPS control Public perceptions; Role in watershed (water quality, wildlife corridors, hydrologic); Hydrology, geomorphology, endangered species/habitat, zoning, health 6

7 Site Selection Criteria Historic wetland sites; Previously drained or degraded sites of certain wetland types (e.g., forested or riparian wetlands); Sites with reliable, adequate, and available water supply necessary to provide for proposed wetland functions and values; Sites situated within the landscape such that self-sustaining hydrology can be ensured; Sites containing majority of drained or hydrologically modified hydric soils; Sites with predominately hydric or hydrologically modified soils; Sites in close proximity to designated priority areas, such as state management areas, public lands, preserves; Sites connected to existing wetland systems and/or areas of wildlife significance; Sites with sufficient distance from incompatible land uses and/or urban or populated areas that may reduce long-term wetland functioning; Sites that contain adequate associated upland areas to serve as a wetland buffer; Sites in which construction will not adversely affect ecologically significant aquatic or upland areas, cultural sites, or habitat for threatened and endangered species; or Sites that will enhance habitat and species diversity, outdoor recreation, and scientific and research values. Environ. Law Inst. (ELI), 2002, Banking and Fees Objectives: Maximize species diversity; Maximize genetic diversity; Optimize wildlife habitat; Vegetation Strategies: Minimize weedy, nonindigenous, or invasive species; Use local, indigenous species; Optimize physical heterogeneity; Create temporal variations in flow Use multiple cells ; Plan capacity for sediment accumulation 7

8 Reasons for constructing wetlands: 1. Mitigation (replacement of destroyed wetlands); 2. Banking 2. NPS pollution control; 3. Point source pollution treatment (sewage, storm water runoff, acid mine drainage, industrial waste); 4. Wildlife habitat; 5. Wetland restoration; Flow regimes in constructed wetlands Surface flow easier but require more area, temporally & spatially variable Subsurface flow, saturated largely anaerobic Subsurface flow, unsaturated horizontal trickling filter, both aerobic and anaerobic, 8

9 Flow regimes: cell layout Surface feed, unsaturated subsurface flow 9

10 Subsurface flow Surface feed, surface flow 10

11 Performance Potential Frequent pollutants 1. Sediments - 1) settling basin design, τ min > settling time; 2) sediment accumulation rate dictates storage capacity (dredging will require CWA sec. 404 permit as well as easy access); Short circuiting of flow is problematic; avoid scouring; 2. Nitrogen two removal mechanisms: 1) burial of organic N, 2) denitrification. ifi ti To use first mechanism, maximize i plant growth and sediment accumulation (saturated soil). For denitrification, anaerobic zones without short circuiting are important; subsurface feed and flow are desirable. For feed lots, initial nitrification may require aerated flow. 11

12 Further pollutants Phosphorus primary removal mechanisms are burial of organic matter and sorption on oxic soils. Maximize organic matter preservation (anaerobic soils), sediment accumulation rate, and areas with oxic sediments; avoid short circuiting multiple cells desirable; Vectors & pathogens: 1. Mosquitoes minimize stagnant water - utilize natural predators 2. Bacteria retention time adequate to kill bacteria base on low temperature conditions, - solids filtration (subsurface flow) and settling Processes in wetlands 12

13 Empirical design criteria 13

14 Quasi-mechanistic design criteria 1. Hydrology: Q in Q out +P A - ET A A = dv/dt 1. Darcy s Law Q = KA(dh/dx) 2. BOD or bacteria removal: C T T ref = A exp k θ Av ( L w d) φ Co Q 2 m A v = specific area for bacterial growth 3 m L, w, d = physical dimensions φ = porosity Constructed Wetland or Pond? 14

15 What functions are fulfilled? Wetland or lagoon or pond? 15

16 References 1. U.S. EPA /626/1-88/022, Sept. 1988, Design Manual: Constructed wetlands and aquatic plant systems for municipal wastewater treatment 2. Moshiri, GA (ed) 1993, Constructed wetlands for water quality improvement, CRC Press, Boca Raton, FL, 632 pp. 3. Hammer, DA (ed) 1989, Constructed wetlands for wastewater treatment: Municipal, Industrial and Agricultural,, Lewis Publishers, Chelsea, MI, 831 pp. 4. Hammer, DA, 1997, Creating Freshwater Wetlands, 2 nd ed. CRC Press, Boca Raton, FL, 406 pp. 16