Removal Mechanisms in Constructed Wetlands

CE 421 Presented by

Stephen Norton

December 04, 2007

Suspended Solids Organic

Matter

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

Removal Mechanisms

Removal Mechanisms

Suspended Solids Organic

Matter

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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)

•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%

Nitrogen

Removal Mechanisms

Suspended Solids Organic

Matter

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

•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

(A) Small scale wetland, (B) large scale wetland (Maine et al., 2006)

Removal Mechanisms

Suspended Solids Organic

Matter

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

•Bilal Tuncsiper tested three types of wetlands in Turkey

•Horizontal-subsurface flow (H-SSF)• Surface flow (SF)•Free water surface flow (FWS))

Case Study

The three different types of constructed wetlands used in study (Tuncsiper, 2007)

•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

Case Study

Removal Mechanisms

Suspended Solids Organic

Matter

Nitrogen Phosphorus

Case Study

Overview

Pathogens

Metals

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

•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 Questio

ns?