Constructed Wetlands Systems: Alternative Design …

1 Constructed Wetland Systems: Constructed Wetland Systems: Design ApproachesDesign ApproachesScott D. Wallace, PresidentNorth American Wetland Engineering Curriculum Development forDecentralized Wastewater ManagementNDWRCDP DisclaimerNDWRCDP DisclaimerThis work was supported by the National Decentralized Water This work was supported by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) with Resources Capacity Development Project (NDWRCDP) with funding provided by the Environmental Protection Agency funding provided by the Environmental Protection Agency through a Cooperative Agreement (EPA No.)

2 CR827881through a Cooperative Agreement (EPA No. CR827881--0101--0) 0) with Washington University in St. Louis. These materials have with Washington University in St. Louis. These materials have not been reviewed by the Environmental Protection not been reviewed by the Environmental Protection Agency. These materials have been reviewed by Agency. These materials have been reviewed by representatives of the NDWRCDP. The contents representatives of the NDWRCDP. The contents of these materials do not necessarily reflect the views and of these materials do not necessarily reflect the views and policies of the NDWRCDP, Washington University, or the policies of the NDWRCDP, Washington University, or the Environmental Protection Agency, nor does the mention of trade Environmental Protection Agency.

3 Nor does the mention of trade names or commercial products constitute their endorsement or names or commercial products constitute their endorsement or recommendation for for DisclaimerCIDWT/University DisclaimerThese materials are the collective effort of individuals from These materials are the collective effort of individuals from academic, regulatory, and private sectors of the academic, regulatory, and private sectors of the onsite/decentralized wastewater industry. These materials have onsite/decentralized wastewater industry. These materials have been peerbeen peer--reviewed and represent the current state of reviewed and represent the current state of knowledge/science in this field.

4 They were developed through a knowledge/science in this field. They were developed through a series of writing and review meetings with the goal of formulatiseries of writing and review meetings with the goal of formulating ng a consensus on the materials presented. These materials do not a consensus on the materials presented. These materials do not necessarily reflect the views and policies of University of necessarily reflect the views and policies of University of Arkansas, and/or the Consortium of Institutes for Decentralized Arkansas, and/or the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT).

5 The mention of trade names or Wastewater Treatment (CIDWT). The mention of trade names or commercial products does not constitute an endorsement or commercial products does not constitute an endorsement or recommendation for use from these individuals or entities, nor recommendation for use from these individuals or entities, nor does it constitute criticism for similar ones not it constitute criticism for similar ones not , 2005. Constructed Wetlands : Wallace, 2005. Constructed Wetlands : Design Approaches Design Approaches --PowerPoint Presentation. PowerPoint Presentation.

6 Inin( Gross and Deal, eds.) University ( Gross and Deal, eds.) University Curriculum Development for Decentralized Curriculum Development for Decentralized Wastewater Management. National Wastewater Management. National Decentralized Water Resources Capacity Decentralized Water Resources Capacity Development Project. University of Arkansas, Development Project. University of Arkansas, Fayetteville, , of Constructed WetlandsTypes of Constructed Wetlands Free Water Surface (FWS)Free Water Surface (FWS) Vegetated Submerged Bed (VSB)Vegetated Submerged Bed (VSB) Vertical Flow (VF)Vertical Flow (VF)Free Water Surface WetlandFree Water Surface Wetland FWS Wetlands have FWS Wetlands have exposed water bodies exposed water bodies similar to natural similar to natural Water Surface WetlandFree Water Surface WetlandFWS Wetland operating near Pensacola, FloridaPhoto courtesy S.

7 WallaceFree Water Surface WetlandFree Water Surface Wetland Typical applications are for polishing effluent from Typical applications are for polishing effluent from a lagoon, activated sludge, or other secondary a lagoon, activated sludge, or other secondary treatment processtreatment processVegetated Submerged BedVegetated Submerged Bed VSB Wetlands employ a gravel bed planted with VSB Wetlands employ a gravel bed planted with wetland vegetation. The water is kept below the wetland vegetation. The water is kept below the surface of the of the Submerged BedVegetated Submerged BedVSB Wetland operating near Lindstrom, MinnesotaPhoto courtesy North American Wetland EngineeringVSB WetlandsVSB Wetlands Most commonly used for onsite wastewater Most commonly used for onsite wastewater treatment for singletreatment for single--family homesfamily homesVertical Flow WetlandsVertical Flow Wetlands VF Wetlands have much higher oxygen transfer VF Wetlands have much higher oxygen transfer rates, allowing for , allowing for nitrification.

8 2 Design types:2 Design types:zzRecirculating (more common in US)Recirculating (more common in US)zzSingleSingle--pass (more common in Europe)pass (more common in Europe)Recirculating VF wetland schematic courtesy Reactor Dynamics Wetland ProcessesFWS Wetland Processes Flow governed by Manning s EquationFlow governed by Manning s Equation21321()()wvdsn=v = liquid flow velocity, ft/sn = Manning s coefficient s/ft1/3dw= depth of water in wetland, fts = hydraulic gradient or slope of the water surface ft/ftCrites & Tchobanoglous, 2002 FWS Wetland ProcessesFWS Wetland Processes Manning s coefficient a function of the Manning s coefficient a function of the density of the vegetation and flow depthdensity of the vegetation and flow depth2wand=a = resistance factor, 16sft < very dense moderately dense vegetation> for sparse vegetationWater depth, dwResistance Factor, aCrites & Tchobanoglous.

9 20021 FWS Wetland ProcessesFWS Wetland Processes Evapotranspiration in FWS is generally Evapotranspiration in FWS is generally estimated as 80% of pan evaporationestimated as 80% of pan evaporationKadlec& Knight, 1996 Role of Plants in FWS WetlandsRole of Plants in FWS sedimentation by reducing water column mixing and surface area in the water column to increase biofilm biomass and pollutant uptake. the removal of particles from the water column by increasing biofilm and plant surfaces available for particle shade from the plant canopy over the water column to reduce algae and preserving duckweed fronds which greatly limit reaerationand light penetration into the water cause flocculation of smaller colloidal particles into larger, settleable Knight Mertz, 2000.

10 USEPA, 2000 Oxygen Transfer in FWS Oxygen Transfer in FWS WetlandsWetlandsOxygen is supplied to the FWS wetland through three passive (algae) oxygen transferFWS Wetlands can also be mechanically aerated to increase of Algae photosynthesis on dissolved oxygen levelsMara, 1976 Suspended Solids removal in FWS Suspended Solids removal in FWS WetlandsWetlands Sedimentation (discrete particle settling)Sedimentation (discrete particle settling) Aggregation (flocculation)Aggregation (flocculation) InterceptionInterception PredationPredationRemoval mechanisms predominate in emergent removal mechanisms predominate in emergent vegetation zones.