Removal Mechanisms in Constructed Wetlands CE 421 Presented by Suspended Solids Organic Matter Overview Nitrogen Phosphorus Stephen Norton December 04, 2007 Pathogens Metals Case Study

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Overview Types of Wetlands Free Water Surface Wetland (FWS) Shallow Water Flowing Over Plant Matter Floating Plants Known as Macrophytes Vegetated Submerged Bed Wetland (VSB) Water Flows Underneath Surface Media Plant Roots Grow in Course Media Various Types of Constructed Wetlands (Vymazal, 2006)

Overview Removal Processes Physical Sedimentation and Plant Trap Sediment Biological Phytodegredation – uptake through roots Rhizodegredation – secretion of contaminants Phytovolitization – transpiring of contaminants Bacteria – soil bacteria metabolize organics Chemical Adsorption – transfer of ions to soil particles Precipitation – converting metals to insoluble forms Photo oxidation – uses sunlight to breakdown and oxidize compounds Volitization – breaks down compounds and expels as gas Mechanisms in present in a FWS Wetland (EPA, 1999)

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Suspended Solids FWS Wetland Flocculation/Sedimentation Influenced by particle size, shape, specific gravity, and fluid media Discrete settling found by Newton’s Law and Stoke’s Law Flocculent settling found experimentally Filtration Does not play large role since plant stems are far apart Interception Plays important role where biofilm absorbs colloidal and soluble matter Typical suspended solids concentration of 3 mg/L

Suspended Solids VSB Wetland Highly effective due to low velocity and large surface area of media Sedimentation Straining Adsorption onto gravel and plant media Rock media of less than 5cm to stop clogging while maintaining performance 60-75% percent of solids removal happens in first 1/3 of wetland United Kingdom - Primary Treatment Five different types of gravel media analyzed over two years Average of 82% removal less than 5 mg/L

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Organic Matter Overview Aerobic microorganisms Aerated surface waters Consume oxygen to breakdown organics Provides energy and biomass Anaerobic microorganisms Anaerobic soils Breakdown organics and produce methane Store organic carbon in plant biomass

Organic Matter FWS Wetland Physical Sorption and Volitization Biofilms on plants VOC removal rate of 80-96% Biological Aerobic Oxygen serves as terminal electron acceptor Most efficient Anoxic Nitrates, sulfates, and carbonates serve as terminal electron acceptor Less efficient than aerobic Anaerobic Organics serve as terminal electron acceptor Least efficient of three processes Bacteria Actinomycetes and fungi most important role Macrophytes Organic matter transformations in a FWS Wetland (EPA, 1999)

Organic Matter VSB Wetland Functions as fixed film bioreactor Hydrolysis Produces soluble organic matter which adheres to plant Biological Aerobic/Facultative Predominant metabolic mechanism Anaerobic Methanogenisis Sulfate reduction Gentrification Decomposition rather low due to oxygen concentration less than .1 mg/L

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Nitrogen Important issues High nitrates cause blue baby syndrome High nitrogen causes eutrofication Plant uptake Use nitrates and ammonium as nutrients Stored as organic nitrogen Microorganisms Inorganic nitrogen broken down mostly by denitrification Nitrogen usually pretty high

Nitrogen Ammonia Volitization If pH greater than 9.3 ammonia can be lost to gas forms Ammonification Organic nitrogen converted to ammonia Catabolism of amino acids by aerobic, anaerobic, and obligate anaerobic Nitrate – Ammonification First anoxic process after oxygen is depleted Reduction of nitrate to molecular nitrogen or ammonia Fixation Converting nitrogen gas to organic nitrogen Aerobic or Anaerobic by bacteria and blue-green algae More important in natural wetlands due to already nitrogen rich environment

Nitrogen Plant uptake Converts inorganic nitrogen to organic nitrogen Ammonia or nitrate used as energy or cell growth Ammonia Adsorption Ionized ammonia adsorbed by inorganic sediment Organic Nitrogen Burial Nitrogen incorporated into soil of wetland ANAMMOX Anaerobic ammonia oxidation Nitrite used as terminal electron acceptor being oxidized to ammonium

Nitrogen Nitrification Aerobic bacteria oxidize ammonia to nitrite Soil bacteria include Nitrosospira, Nitrosovibrio, Nitrosolobus, Nitrosococcus, and Nitrosomonas Bacteria oxidize nitrite to nitrate Soil bacteria include Nitrobacter Denitrification Nitrate is converted to nitrogen gas Anaerobic and anoxic conditions breakdown organics as energy source Bacillus, Micrococus, and Pseudomonas are important denitrifying organisms in soils Pseudomonas, Aeromonas, and Virbio are important in aquatic environments Nitrogen transformations in a FWS Wetland (EPA, 1999)

Nitrogen Directly reduces nitrogen Ammonia volatilization Denitrification Plant uptake Ammonia adsorption Organic nitrogen burial ANAMMOX Nitrification is limiting step in nitrogen removal Denitrification is primary mechanism for nitrogen removal Removal efficiencies vary between 40 and 50%

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Phosphorus Causes eutrofication Removal lower since no metabolic pathway to remove Phosphorus present in organic and inorganic forms Phosphorus transformations in a FWS Wetland (EPA, 1999)

Phosphorus Major removal done by uptake of plant roots Plants store phosphorus Storage usually greater below ground Phosphorus released when plant dies Soil adsorption and precipitation Soluble inorganic phosphorus stored by soil particles Bacteria uptake of phosphorus is quick Drawbacks Plants and soils reach storage capacity Bacteria are unable to store large amounts

Phosphorus VSB Wetland Adsorption of phosphorus through soil media FWS Wetland Uptake from free floating macrophytes Macrophytes can be replaced to increase removal Removal efficiencies vary between 40 and 60% Unable to meet primary removal standards

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Pathogens Removal accomplished by sedimentation Reports show good removal 57% total coliforms 62% fecal coliforms 98% giardia 87% cryptosporidium Bacteria accumulate on sediment floor Can be disrupted by human activities Filtering through root structure

Pathogens Mohammad Karim study of pathogen removal by sedimentation Results Fecal coliforms and colifages removed more by root structure Multispecies wetland 73% removal of giardia 58% removal of cryptosproridium Duckweed wetland 98% removal of giardia 89% removal of cryptosproridium Constructed wetlands offer promise for removing pathogens

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Metals Removal mechanisms Plant uptake Soil adsorption Precipitation Removal depends on types of plants and types of metals Duckweed can store large amounts of copper, cadmium, and selenium Cadmium, copper, nickel, lead, and zinc form insoluble compounds with sulfides Chemisorption Chromium, copper, lead, and zinc form chemical complexes with organic material Chromium and copper can chemically bind to clays and settle out

Metals (A) Small scale wetland, (B) large scale wetland (Maine et al., 2006) M.A. Maine et all, study of metal uptake in small and large wetland 80% Eichhornia crassipes (water hyacinth) 14% Typha domingensis (cattail) 4% Panicum elephantipes (elephant panicgrass) 81%, 66%, 82% removal of Cr, Ni, Cu in small wetland 86%, 67%, 95% removal of Cr, Ni, Cu in large wetland Cr, Ni, Zn found in macrophytes in large wetland Cr, Ni, Zn found in sediment in smaller wetland

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Case Study Bilal Tuncsiper tested three types of wetlands in Turkey Horizontal-subsurface flow (H-SSF) Surface flow (SF) Free water surface flow (FWS)) The three different types of constructed wetlands used in study (Tuncsiper, 2007)

Case Study Results 49 – 52% removal of ammonia–nitrogen for all three 58% removal of nitrates on SF wetland 60% removal of phosphorus in H-SSF wetland Did not meet drinking water or irrigation standards 94% removal of fecal coliforms for all three Conclusions Constructed wetlands can be used as secondary treatment of primary treated wastewater

Removal Mechanisms Suspended Solids Organic Matter Overview Nitrogen Phosphorus Pathogens Metals Case Study

Summary Suspended Solids Removed by flocculation/sedimentation and filtration/interception Organic Matter Removed by physical (sorption and volitization) and biological (aerobic, anaerobic, and anoxic environments) Nitrogen 30-50% removal mostly by nitrification and denitrification Phosphorus 40-60% removal by plant uptake, adsorption/precipitation, and storage in microorganisms

Summary Questions? Pathogens High percentage of removal of fecal coliforms, giardia, and cryptosporidium by sedimentation Metals Selecting proper plants can yield high removal by plant uptake, soil adsorption, and precipitation Constructed wetlands Good secondary treatment systems for treating domestic wastewater Aesthetically pleasing Use of simple technologies to remove contaminants Questions?