Constructed Wetlands • Artificial wastewater treatment systems consisting of shallow ponds or channels which have been planted with aquatic plants to treat wastewater. Rely upon natural processes: • biological • physical • chemical
Constructed Wetlands • Engineered structures to • control direction of flow • regulate water level • regulate detention time • Structure includes • impervious clay or synthetic liner • may contain inert porous media (rock, gravel, sand or synthetic media)
Overview and Basic Treatment • Often referred to as • “Microbial Rock Filters” • “Microbial Rock Plant Filters” • “Gravel Bed Filters”
Types of Constructed Wetlands • Free Water Surface (FWS) • Vegetated Submerged Bed (VSB) • Vertical Flow (VF) • Due to health risks subsurface flow filters are used for residential systems.
Free Water Surface Wetland • FWS wetlands have exposed water bodies similar to natural marshes.
Free Water Surface Wetland FWS Wetland operating near Pensacola, Florida
Typical applications are for polishing effluent from a lagoon, activated sludge, or other secondary treatment process Free Water Surface Wetland
Vegetated Submerged Bed • VSB wetlands employ a gravel bed planted with wetland vegetation. The water is kept below the surface of the gravel.
Vegetated Submerged Bed VSB Wetland operating near Lindstrom, Minnesota
VSB Wetlands • Most commonly used for onsite wastewater treatment for single-family homes
Vertical Flow Wetland Gravel Distribution pipes Sand Discharge pipes Impermeable liner
Vertical Flow Wetlands • VF wetlands have much higher oxygen transfer rates, allowing for nitrification. • 2 design types: • Recirculating (more common in US) • Single-pass (more common in Europe)
Constructed Wetland • What is it? • Largely anaerobic reactor unless air is artificially injected • Usually contained in a lined cell • Cell is filled with 20-24 inches of solid media, usually gravel • Appropriate wetland vegetation is planted • Liquid surface is 3 or more inches below the surface • Typical minimum cell length for single family home - 15 yards
VSB Wetland • How does it work? • Wastewater moves horizontally through the gravel • Physical processes of filtration and sedimentation are predominant removal mechanisms • Plants are primarily for aesthetic purposes NSFC
VSB Wetland • Flow governed by Darcy’s Law: Q = ksAcS Q = average flow through wetland ks = hydraulic conductivity Ac = cross-sectional area of bed S = slope of hydraulic gradeline
VSB Wetland Processes • Biomat growth and accumulation of sediment dramatically reduce the hydraulic conductivity • Recommended design values: • Initial 30% of VSB: design K value = 1% of clean bed Ks 99% reduction in flow capacity!! • Final 70% of VSB: design K value = 10% of clean bed Ks 90% reduction in flow capacity
Overview and Basic Treatment • Microbes attach themselves to the stone media. • The microbes digest organic matter measured in TSS and BOD. • Either soft tissue or hard tissue (woody) plants can be used in wetlands. • Hard tissue plants provide a better path for oxygen to get into the media, while soft tissue plants provide different floral options.
Role of Plants in VSB Wetlands • Oxygen transfer from plants is minimal (about 0.02 g/m2d) • Plant roots generally do not penetrate to bottom of gravel bed • Plant roots support symbiotic bacteria and fungi, resulting in a more diverse microbial environment • Net effect of plants on treatment is minimal
Oxygen Transfer in VSB Wetlands • Oxygen transfer through atmospheric diffusion and plant-mediated transport is minimal • BOD removal is mainly through physical-chemical processes • Insufficient oxygen transfer for nitrification • Alternative VSB designs (aeration, tidal flow) address some of these limitations
Nitrogen Cycling in VSB Wetlands • Insufficient oxygen transfer for nitrification • Reducing conditions suitable for denitrification • Significant nitrogen reduction only occurs when the influent nitrogen has already been converted to nitrate • Plant Harvesting • Removes less than 10% of applied nitrogen • Not a cost-effective nutrient management option
Phosphorus Removal in VSB Wetlands • Adsorption onto medium is only a short-term removal mechanism with standard media • Expanded shale and clay aggregates with very high phosphorus sorption capacities have been used to increase phosphorus retention in VSBs • Sacrificial bed media • Plant Harvesting • Removes less than 5% of applied phosphorus • Not cost-effective
Reasons for use to reduce organic strength and solids (meets secondary standard) aesthetic features passive operation & maintenance needs Constructed Wetland
Also called “Vegetated Subsurface Beds” 2002 USEPA onsite manual groups this with anaerobic upflow filters (high specific-surface area) Constructed Wetland
Bottom should slope – maximum of 1% Sufficient cross-section area must exist to move liquid through it Constructed Wetland
Constructed Wetland • A drainfield may be used for dispersal
A second-unlined cell may be used for dispersal, if soils permit Constructed Wetland
Typically, have hydraulic retention times of 2 to 3 days Constructed Wetlands
usually have berms around them should have means to vary the liquid depth all media should be clean Constructed Wetlands
Monitoring and maintenance critical Manage the wetland as a rock garden May need to remove dried vegetation and plant new Must keep pores in media open Constructed Wetland
Constructed Wetlands • In cold climates, the liquid depth may be lowered • In cold climates, vegetation may appear to be dead on the surface • Expected treatment efficiencies: BOD5 - < 30 mg/l TSS - <30mg/l Fecal coliform - 99 – 99.9 % reduction Nitrogen - Insignificant
Contaminant Removal • Free Water Surface (FWS) • BOD 60-80% • Total Suspended Solids 50-90% • Fecal Coliform 90-99% • Nitrogen Not significant* • Phosphorous Not significant* • Vegetated Submerged Bed (VSB) • BOD 60-80% • Total Suspended Solids 50-60% • Fecal Coliform 90-99% • Nitrogen Not significant* • Phosphorous Not significant* *A properly designed FWS wetland with fortuitous conditions of sunlight, temperature, wind and wastewater strength may remove significant amounts of nitrogen and from 2 to 3 logs of coliforms.