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Removal Mechanisms in Constructed Wetlands CE 421 Presented by

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1 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

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

3 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)

4 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)

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

6 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

7 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

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

9 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

10 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)

11 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

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

13 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

14 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

15 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

16 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)

17 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%

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

19 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)

20 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

21 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

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

23 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

24 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

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

26 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

27 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

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

29 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)

30 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

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

32 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

33 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?

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