Removal Mechanisms in Constructed Wetlands CE 421 Presented by

1
Removal Mechanisms in Constructed WetlandsCE
421 Presented by
Removal Mechanisms
Suspended Solids
Organic Matter
Overview
Nitrogen
Phosphorus
Stephen Norton December 04, 2007
Case Study
Pathogens
Metals
2
Removal Mechanisms
Suspended Solids
Organic Matter
Overview
Nitrogen
Phosphorus
Case Study
Pathogens
Metals
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
Case Study
Pathogens
Metals
6
Suspended Solids

• FWS Wetland
• Flocculation/Sedimentation
• Influenced by particle size, shape, specific
  gravity, and fluid media
• Discrete settling found by Newtons Law and
  Stokes 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
Case Study
Pathogens
Metals
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
Case Study
Pathogens
Metals
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
Case Study
Pathogens
Metals
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
Case Study
Pathogens
Metals
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
Case Study
Pathogens
Metals
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
Case Study
Pathogens
Metals
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 ammonianitrogen 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
Case Study
Pathogens
Metals
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

• 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?