Natural and Enhanced Remediation Systems - Chapter 6 pot

©2001 CRC Press LLCCHAPTER 6 Constructed Treatment WetlandsCONTENTS 6.1 Introduction6.1.1 Beyond Municipal Wastewater 6.1.2 Looking Inside the “Black Box” 6.1.3 Potential “Attractive Nui

Suthersan, Suthan S “Constructed Treatment Wetlands”

Natural and Enhanced Remediation Systems

Edited by Suthan S SuthersanBoca Raton: CRC Press LLC, 2001

©2001 CRC Press LLC

CHAPTER 6 Constructed Treatment WetlandsCONTENTS

6.1 Introduction6.1.1 Beyond Municipal Wastewater 6.1.2 Looking Inside the “Black Box”

6.1.3 Potential “Attractive Nuisances”

6.1.4 Regulatory Uncertainty and Barriers6.2 Types of Constructed Wetlands

6.2.1 Horizontal Flow Systems6.2.2 Vertical Flow Systems6.3 Microbial and Plant Communities of a Wetland6.3.1 Bacteria and Fungi

6.3.2 Algae 6.3.3 Species of Vegetation for Treatment Wetland Systems6.3.3.1 Free-Floating Macrophyte-Based Systems6.3.3.2 Emergent Aquatic Macrophyte-Based Systems6.3.3.3 Emergent Macrophyte-Based Systems with Horizontal

Subsurface Flow 6.3.3.4 Emergent Macrophyte-Based Systems with Vertical

Subsurface Flow 6.3.3.5 Submerged Macrophyte-Based Systems6.3.3.6 Multistage Macrophyte-Based Treatment Systems6.4 Treatment-Wetland Soils

6.4.1 Cation Exchange Capacity6.4.2 Oxidation and Reduction Reactions 6.4.3 pH

6.4.4 Biological Influences on Hydric Soils6.4.5 Microbial Soil Processes

6.4.6 Treatment Wetland Soils

6.5 Contaminant Removal Mechanisms6.5.1 Volatilization

6.5.2 Partitioning and Storage6.5.3 Hydraulic Retention Time6.6 Treatment Wetlands for Groundwater Remediation6.6.1 Metals-Laden Water Treatment

6.6.1.1 A Case Study for Metals Removal 6.6.2 Removal of Toxic Organics

6.6.2.1 Biodegradation6.6.3 Removal of Inorganics6.6.4 Wetland Morphology, Hydrology, and Landscape PositionReferences

Creating or constructing a natural wetland sounds like an oxymoron, but this doesn’t mean that an “unnatural wetland” is by definition bad It doesn’t mean

we can’t mimic Mother Nature in giving natural birth to a desirable wetland Constructed rice paddies have been responsible for feeding more people than any other enterprise on earth.

6.1 INTRODUCTION

Natural wetlands are land areas that are wet during part or all of the year because

of their location in the landscape Historically, wetlands were called swamps,marshes, bogs, fens, or sloughs, depending on existing plant and water conditionsand on geographic setting Wetlands are frequently transitional between uplands(terrestrial systems) and continuously or deeply flooded (aquatic) systems They arealso found at topographic lows (depressions) or in areas with high slopes and lowpermeability soils (seepage slopes) In other cases, wetlands may be found at topo-graphic highs or between stream drainages when land is flat and poorly drained(blanket bogs) In all cases, the unifying principle is that wetlands are wet longenough to alter soil properties because of the chemical, physical, and biologicalchanges that occur during flooding, and to exclude plant species that cannot grow

in wet soils.1

The structural components of natural wetland ecosystems are shown in Figure6.1 These components are highly variable and depend on hydrology, underlyingsediment types, water quality, and climate Starting with the unaltered sediments orbedrock below the wetlands, these typical components are1

• Underlying strata — unaltered organic, mineral, or lithic strata, typically saturated with or impervious to water and below the active rooting zone of the wetland vegetation

• Hydric soils — the mineral-to-organic soil layer of the wetland, infrequently to continuously saturated with water and containing roots, rhizomes, tubers, funnels, burrows, and other active connections to the surface environment

• Detritus — the accumulation of live and dead organic material in a wetland, consisting of dead emergent plant material, dead algae, living and dead animals (primarily invertebrates), and microbes (fungi and bacteria)

• Seasonally flooded zone — the portion of wetland seasonally flooded by standing water and providing habitat for aquatic organisms including fish and other verte- brate animals, submerged and floating plant species that depend on water for buoyancy and support, living algae, and populations of microbes

• Emergent vegetation — vascular, rooted plant species containing structural ponents that emerge above the water surface, including both herbaceous and woody plant species

com-Natural wetlands have been used as convenient wastewater discharge sites for

as long as sewage has been collected (at least 100 years in some locations) Examples

of old treatment wetland sites can be found in Massachusetts, Wisconsin, Florida,and Ontario

Judging by the growing number of wetlands built for wastewater treatmentaround the world, this “natural” technology seems to have firmly established roots.After almost 30 years of use in wastewater treatment, constructed “treatmentwetlands” now number over 1000 in Europe and in North America.1 Marsh-type

“surface flow” systems are most common in North America, but “subsurface flow”wetlands, where wastewater flows beneath the surface of a gravel-rock bed, pre-dominate in Europe This inexpensive, low-maintenance technology is reportedly

in high demand in Central America, Eastern Europe, and Asia New applications,

Figure 6.1 Structural components of natural wetland ecosystems (adapted from Kadlec et

al., 1996).

Unaltered Sediment

Hydric Soils

Detritus Rhizomes

Emergent Vegetation

Cypress Kness

Subcanopy Tree

Shrub

Seasonal High Water Seasonally Flooded Zone Seasonal Low Water

Canopy Tree

Buttressed Stem

from nitrate-contaminated groundwater to effluent from high-intensity livestockoperations, are also increasing.

In the U.S., treatment-wetland technology has not yet gained universal regulatoryacceptance; projects are approved on a case-by-case basis Some states and EPAregions are eager to endorse them, but others are wary of this nontraditional method

of treating wastewater and contaminated groundwater In part, this reluctance existsbecause the technology is not yet completely understood Knowledge of how thewetland works is not far enough advanced to provide engineers with detailed pre-dictive models Because wetlands are natural systems, their performance is variable,subject to the vagaries of changing seasons and vegetative cycles These treatmentwetlands also pose a potential threat to wildlife attracted to this new habitat within

an ecosystem exposed to potentially toxic compounds

When utilized for benign, pretreated wastewaters, wetlands do not generally pose

a threat to human or wildlife health In these circumstances, there may be significantancillary benefits in terms of habitat creation and beneficial human use In thosesituations where a potentially hazardous condition exists, the extra expense of agravel media is warranted.1 Water and associated particulates, organisms, and sedi-ments are then located below ground, and thus out of reach of human and wildlifecontact Subsurface wetland waters are typically anoxic or anaerobic, which isoptimal for some processes such as sulfide precipitation or denitrification, but unsat-isfactory for other processes, such as nitrification of ammonium nitrogen

New efforts are underway, however, to place the technology onto firmer scientificand regulatory ground Long-term demonstration and monitoring field studies arecurrently probing the inner workings of wetlands and their water quality capabilities

to provide better data on how to design more effective systems Researchers aredocumenting the fate of toxic compounds in wetlands and the extent to which wildlifemay be exposed to them A recent study of U.S policy and regulatory issuessurrounding treatment wetlands has recommended that the federal governmentactively promote this technology and clear the regulatory roadblocks to enable wideruse Proponents argue that the net environmental benefits of constructed wetlands,such as restoring habitat and increasing wetland inventory, should be considered Afederal interagency work group is grappling with that recommendation, trying tobalance the benefits and shortcomings of this increasingly popular technology

6.1.1 Beyond Municipal Wastewater

Constructed wetland systems in North America have been designed nantly for large-scale treatment of municipal wastewater, ranging from 100,000 to

predomi-15 million gallons per day.1,2 The use of treatment wetlands is well established inEurope, where the technology originated with laboratory work in Germany 30 yearsago.3 Subsurface-flow systems are the norm because they provide more intensivetreatment in a smaller space than marsh-type wetlands — an important designconstraint in countries where open space is limited The European thrust has beenfor small-scale systems primarily for domestic wastewater treatment; for example,Denmark alone has 150 systems, most in small villages handling domestic waste-water The term “reed beds” is commonly used for treatment wetlands in Europe

Since the 1980s, constructed wetlands have also been built to treat other types

of wastewaters, including acid mine drainage, industrial wastewater, agricultural andstorm water runoff, and effluent from livestock operations.1,2 The petroleum industry

is using constructed wetlands to treat a variety of wastewaters from refineries andfuel storage tanks Food processing and pulp and paper industries are relative new-comers to treatment wetlands Stormwater runoff also has recently become a focus

of research in using constructed wetlands as a treatment method

While many of the early acid mine drainage treatment systems were marsh-likesurface flow systems, the most recent projects are “passive treatment systems” thatlink several different types of cells — vertical limestone drain as well as vegetatedcells — to sequentially treat particularly “nasty” wastewater with low pH and highmetals content.2 A wetland system for the treatment of runoff from coal piles atcoal-fired power plants with a pH of 2 and high levels of metals uses a series ofsuccessive alkalinity-producing systems, a rich organic layer over an anoxic lime-stone drain, to reduce the acidity in the wastewater before it flows into wetland cells.Landfill leachates are a subset of polluted waters requiring substantial levels oftreatment Leachates vary considerably, depending upon the materials accepted atthe landfill They may contain large concentrations of volatile and toxic organics,both as individual compounds and as COD, chlorinated organics, metals, and nitrog-enous compounds.2 Wetland treatment of landfill leachates has been successfullytested at several locations Cold climate systems are functioning properly in Norway,

as well as at several locations in Canada; reed beds are used to treat leachate in theUnited Kingdom, Slovenia, and Poland.4

Based on current understanding of the effectiveness of wetland treatment ofleachates, several U.S projects are in planning and design phases In addition, thereare about a half-dozen other projects in various locations, such as Mississippi,Indiana, Pennsylvania, and West Virginia Wetlands have been proposed for control

of stormwater runoff from capped landfills.1,2

Continued growth in the use of treatment wetlands is expected as a result of newregulatory initiatives on nutrient management, including the Clean Water Act’s totalmaximum daily load (TMDL) program Small- to medium-sized communities trying

to meet new TMDLs in sensitive watersheds for phosphorus or ammonia needsomething that is cost-effective, and wetlands are a good option

6.1.2 Looking Inside the “Black Box”

The rapid spread and diversification of treatment-wetland technology are runningahead of the mechanistic understanding of how they work These complex naturalsystems are still, somewhat, a “black box,” according to many in the field Forexample, the role of plants in transporting oxygen into the root zone to promotenitrification has been demonstrated in the laboratory but not convincingly in thefield, according to many researchers There is very little data to say whether that is

an important factor or whether the plants are more or less passive It is likely,according to some researchers, that the ratio of open water to vegetated areas ismore important in creating aerobic conditions in a wetland

Another issue quite often debated is how important the volume of water in awetland is to treatment performance Is it the bottom of the wetland or the volume

of water that is more important? The data coming in now are on the side of thewetland bottom:1,2 it apparently does not matter how deep the water is as long asthe soil is wet That is a surprise to civil engineers, who, for years, have designedtreatment systems based on their volume and hydraulic residence time

Numerous research efforts, both broad based and focused, are currently ating a great deal of new information on treatment-wetland function.1,2,5 The exten-sive research activities include gathering conventional water quality data; measure-ments of metals, biotoxicity, and organics; bird surveys; and macroinvertebratesampling Expanding the species pallet of plants used in treatment wetlands isanother focus of research among researchers in this field Most constructed wetlandsfor treatment have been built around herbaceous species so far, and many researchersare experimenting with a greater variety of plants to see how water quality changeswhen multispecies systems are used Many have found that pathogen removal ishigher in a multispecies system than in a single species system One of the thingsthat may be important in pathogen removal is having multiple types of wetlandcomponents, for example, a duckweed system followed by a subsurface wetland.5

gener-Looking deeper into the wetland, to the microbes in the soil and around the rootsystems of wetland plants, some researchers are studying the role that bacteria play

in trace element removal Researchers have found that bacteria in the root zone ofbulrush increase the plants’ ability to accumulate and volatilize selenium twofold.They are now working to identify which bacteria are most responsible, and will soonmove to mesocosm studies to see whether seeding the soil with those bacteriaincreases trace element removal

Some researchers are experimenting with an innovative wetland design — avertical flow system — to solve the oxygen depletion problem and boost nitrifica-tion.1,2 Effluent flows over a porous surface and percolates through a vegetated sandfilter, which is periodically allowed to dry to reintroduce oxygen to the system

6.1.3 Potential “Attractive Nuisances”

Aside from research issues surrounding the design and performance of thetreatment wetlands black box, another scientific issue looms large for the future ofthe technology: do treatment wetlands pose a threat to wildlife?1,5 This question is

an important one, since many wetland projects are designed with habitat creation

as one of their primary beneficial objectives It is easier to justify the land use for

a constructed wetland if it is also used for habitat restoration

Research is also being directed toward several critical issues Some researchersare working to find out exactly where toxic trace elements from wastewater end up

in a treatment wetland They are completing laboratory studies documenting traceelement uptake potential of various wetland plants and identifying where the ele-ments go in the plants: roots, stems, leaves, or plant litter They are also monitoringseveral active treatment wetlands to track trace elements in the ecosystem: sediment,water, air, plant tissues, and animal tissues

To address similar habitat-related issues, influent and effluent water have beenanalyzed for potential bioaccumulation and mutagenic activity from organic com-pounds.5 Toxicity tests were designed to look for physiological impacts on biotaliving in the system Work also continues on the control of an unplanned threat tohuman health: mosquitoes Fish have been introduced to the wetlands to consumemosquito larvae, but the density of the particular bulrush variety used may preventthe fish from reaching certain parts of the wetland Sections of the wetlands can bereconfigured and replanted to raise the water level and give the fish greater access.

6.1.4 Regulatory Uncertainty and Barriers

Treatment wetlands do not appeal to all wastewater engineers because they lackthe traditional “handles” of engineered pollution control systems, are not easy tocontrol, and may be hard to predict Regulators in the U.S have similar problemswith treatment wetlands because they do not fit easily into existing regulatorycategories Surface-flow treatment wetlands can be a point source discharge and aprotected environment at the same time

No national guidance on the use of treatment wetlands and no uniform acceptance

of them by states exist, according to researchers and consultants In this atmosphere

of regulatory uncertainty, questions abound Concerns have been expressed that under

a strict reading of the Clean Water Act, certain treatment wetlands could be considered

“waters of the U.S.,” and thus discharges into them could be tightly regulated.USEPA’s environmental technology initiative (ETI) treatment wetland policy andpermitting team of representatives from federal, state, and local agencies issued areport in January, 1997, that recommended “changes in regulation and/or policy thatwould facilitate, where appropriate, implementation of beneficial treatment wetlandprojects.”6,7 It also advocated that “net environmental benefits” of habitat creation,reduced use of energy and treatment chemicals, and recreational value — not justthe water quality impact of a treatment wetland project — should be considered inapproving it

The report catalogued numerous regulatory and policy issues Should tion of effluent be done at the inlet rather than the outlet of a wetland? When should

disinfec-a wetldisinfec-and be lined to protect groundwdisinfec-ater? Should tredisinfec-atment wetldisinfec-ands be disinfec-allowed

to mitigate for permitted wetland losses? Under what conditions should constructedtreatment wetlands be considered “waters of the U.S.?” The report also noted thatmore research is needed concerning the “fate and effect of potential wastewatertoxins and ecological risks in treatment wetlands.”

The federal interagency work group, including representatives from USEPAwetlands and wastewater offices, the U.S Army Corps of Engineers, the NationalOceanic and Atmospheric Administration, the Bureau of Reclamation, and the U.S.Fish and Wildlife Service, was created to take up these issues.6,7

The question of where treatment wetlands should be sited has been a particularlydifficult regulatory issue, and consensus must be reached on the need to handlewetland systems differently depending on whether their primary purpose is watertreatment or habitat restoration There is still some disagreement about the habitat

value of treatment wetlands and concerns about the negative impact they could have

on the environment

USEPA currently is not developing the type of specific guidance documents andformal agency actions recommended in the ETI study to promote the use of treatmentwetlands Nevertheless, wetlands experts are encouraged because the issues are nowbeing discussed at the national level

6.2 TYPES OF CONSTRUCTED WETLANDS

6.2.1 Horizontal Flow Systems

The purposeful construction of treatment wetland ecosystems is a relatively newtechnology Constructed wetlands for pollution control, wastewater treatment, and,recently, for contaminated groundwater treatment are divided into two basic types:free water surface (FWS) and subsurface flow (SSF) wetlands Both types consist

of a channel or a basin with some sort of barrier to prevent seepage and utilizeemergent aquatic vegetation as part of the treatment system The difference betweenFWS and SSF wetlands is that SSF uses some kind of media as a major component(Figures 6.2a and b) In an FWS treatment wetland, soil supports the roots of theemergent vegetation; water at a relatively shallow depth of 6 to 24 inches flowsthrough the system with the water surface exposed to the atmosphere Oxygen isprovided by diffusion through the water surface

An SSF treatment wetland bed contains a suitable depth (1.5 – 3.0 feet) ofpermeable media, such as coarse sand or crushed stone, through which the waterflows The media also support the root structure of the emergent vegetation Thesurface of the flowing water is beneath the surface of the top layer of the medium,determined by proper hydraulic design and appropriate flow control structures Inboth systems the polluted water undergoes physical, biological, and chemical treat-ment processes as it flows through the wetlands

The rate at which organic contaminants move through wetlands can be mined by several transport mechanisms These mechanisms often act simultaneously

deter-on the organics and may include such processes as cdeter-onvectideter-on, diffusideter-on, dispersideter-on,and zero- or first-order production or decay

Figure 6.2a Free water surface (FWS) wetland.

Inlet

Outlet Weir

Currently, constructed wetlands for municipal wastewater treatment are designedbased on the assumptions of plug-flow hydrodynamics and first-order biochemicaloxygen demand (BOD) removal kinetics The first assumption implies that dispersion

in the system is negligible and all the fluid particles have a uniform detention timetraveling through the system The plug-flow model seems to give a reasonablyaccurate estimate of the performance of SSF-constructed wetlands

However, some designers have recognized the limitation of using the plug-flowmodel for constructed wetlands design Three types of hydraulic inefficiencies mayoccur in treatment wetlands: one caused by internal islands and topographical features,

a second caused by preferential flow channels on a large-distance scale, and a thirdcaused by mixing effects, such as water delays in litter layers and transverse mixing

6.2.2 Vertical Flow Systems

Vertical flow constructed wetlands are vegetated systems in which the flow ofwater is vertical rather than horizontal as in FWS and SSF wetlands (Figure 6.3)

Figure 6.2b Subsurface ßow (SSF) wetland.

Figure 6.3 Vertical ßow constructed wetland.

Inlet

Effluent Porous Media

Polluted water is applied at time intervals over the entire surface of the wetland.The water flows through a permeable medium and is collected at the bottom Theintermittent application allows the cell to drain completely before the next applica-tion This type of operation allows for much more oxygen transfer than typical SSFsystems and thus may be a good option for treatment of wastewaters with a relativelyhigh oxygen demand This type of system has been recommended for removal ofhigh levels of ammonia through nitrification High BOD levels may cause cloggingdue to biomass buildup; mineral buildup may also cause clogging Intermittentapplication gives the advantage of greatly increasing the oxygen available for micro-bial reactions, but also greatly increases the mechanical and operational requirements

of the system over the more traditional wetland treatment processes

6.3 MICROBIAL AND PLANT COMMUNITIES OF A WETLAND

Because of the presence of ample water, wetlands are typically home to a variety

of microbial and plant species This biological diversity, from the smallest virus tothe largest tree, creates interspecies interactions resulting in greater diversity, morecomplete utilization of energy inflows, and ultimately, emergent properties of thewetland ecosystem.1,6,7 The treatment wetland system designer should not expect tomaintain a system with just a few known species The successful wetland designercreates the gross environmental conditions suitable for groups or guilds of species,seeding the wetland with diversity by planting multiple species, using soil seedbanks, and inoculating from other similar wetlands, and then using minimum externalcontrol to guide the wetland development.1 This form of ecological engineeringresults in lower initial cost, lower operation and maintenance costs, and the mostconsistent system performance

6.3.1 Bacteria and Fungi

Wetland and aquatic habitats provide suitable environmental conditions for thegrowth and reproduction of microorganisms, two important groups of which arebacteria and fungi These organisms are important in treatment wetland systemsprimarily because of their role in the assimilation, transformation, and recycling ofchemical constituents present in contaminated waters

Bacteria and fungi are typically the first organisms to colonize and begin thesequential decomposition of contaminants and wastes Also, microbes typically havefirst access to dissolved constituents in the wastewater or contaminated groundwater.Some bacteria are sessile, while others are motile by use of flagella In wetlands,most bacteria are associated with solid surfaces of plants, decaying organic matter,and soils Bacteria also play a significant role in altering the biogeochemical envi-ronment because they are responsible for processes such as nitrification, denitrifi-cation, sulfate reduction and methanogenesis, etc

Fungi represent a separate kingdom of eucaryotic organisms and include yeasts,molds, and fleshy fungi Most fungal nutrition is saprophytic, which means it is

based on the degradation of dead organic matter Fungi are abundant in wetlandenvironments and play an important role in treatment.

6.3.2 Algae

Algae are unicellular or multicellular photosynthetic bacteria and plants that lackthe variety of tissues and organs of higher plants Algae are a highly diverse assem-blage of species that can live in a wide range of aquatic and wetland habitats Majoralgae life forms typical in wetland environments are unicellular, colonial, filamen-tous, and macroscopic forms

For the most part, algae depend on light for their metabolism and growth andserve as the basis for an autochthonous food chain in wetland habitats Organiccompounds created by algae photosynthesis contain stored energy which is used formicrobial respiration or which enters the aquatic food chain and provides food to avariety of microbes Alternately, this reduced carbon may be directly deposited asdetritus to form organic peat sediments in wetlands

When light and nutrients are plentiful, algae can create massive populations andcontribute significantly to the overall food web and nutrient cycling of a treatmentwetland ecosystem When shaded by the growth of macrophytes, algae frequentlyplay a less important role in wetland energy flows and treatment (Figure 6.4).Filamentous algae mats are sometimes a dominant component of the plantbiomass in wetland systems The mats are made of a few dominant species of green

or blue-green filamentous algae in which individual filaments may include thousands

of cells Filamentous algae mats first develop below the water surface on the substrate

of wetland in areas with little emergent vegetation During the day, entrained gasbubbles (primarily pure oxygen resulting from photosynthesis) may cause the mats

to move up through the water column and float at the surface During the night, themats sink again to the wetland substrate.1

Filamentous algae that occur in wetlands as periphyton or mats may dominatethe overall productivity of the wetland, controlling DO and CO2 concentrationswithin the treatment wetland water column Wetland water column DO can fluctuatediurnally from near zero during the early morning following a night of high respi-ration to well over saturation (>15 mg/L) in high algae growth areas during a sunnyday Dissolved carbon dioxide and consequently the pH of the water varies propor-tionally to DO because of the corresponding use of CO2 by plants during photosyn-thesis and release at night during respiration As CO2 is stripped from the watercolumn by algae during the day, pH may rise by 2 to 3 pH units (a 100- to 1000-fold increase in H+ concentration) These daytime pH changes are reversible, andthe production of CO2 at night by algae respiration frequently returns the pH to theprevious day’s value by early morning (Figure 6.5.)1

6.3.3 Species of Vegetation for Treatment Wetland Systems

The term macrophyte includes vascular plants with tissues that are easily visible.Vascular plants differ from algae through their internal organization into tissuesresulting from specialized cells (Figure 6.6) The U.S Fish and Wildlife Service has

Figure 6.4 Major energy sources and ecological niches affecting the occurrences of algae

in wetlands (adapted from Kadlec and Knight, 1996).

Figure 6.5 Typical diurnal plots of DO concentration and pH in a wetland dominated by

PM 12

NOON 6

AM 12

more than 6700 plant species on their list of obligate and facultative wetland plantspecies in the U.S Obligate wetland plant species are defined as those which arefound exclusively in wetland habitats, while facultative species are those that may

be found in upland or in wetland areas.1

Wetland macrophytes are the dominant structural component of most wetlandtreatment systems The vascular macrophytes can be categorized morphologically

Figure 6.6 Growth forms of rooted wetland and aquatic vascular plants (adapted from

Kadlec and Knight, 1996).

Emergent Herbaceous

Cattail Duck Potato

a.

Emergent Woody

Buttonbush Shrub

d.

by descriptors such as woody, herbaceous, annual, perennial, emergent, floating, andsubmerged Woody species have stems or branches that do not contain chlorophyll.Since these tissues are adapted to survive for more than one year, they are typicallymore durable or woody in texture Herbaceous species have aboveground tissuesthat are leafy and filled with chlorophyll-bearing cells that typically survive onlyone growing season Woody species include shrubs that attain heights of up to six

to ten feet and trees that are generally more than ten feet in height when mature.1

The terms emergent, floating, and submerged refer to the predominant growthform of a plant species In emergent plant species, most of the aboveground part ofthe plant emerges above the waterline and into the air These emergent structuresmay be self-supporting or may be supported by other physical structures Emergentplant species are important because they provide surface area for microbial growthimportant in many of the contaminant assimilation processes in treatment wetlandsystems.1,2Floating species have leaves and stems buoyant enough to float on thewater surface Submerged species have buoyant stems and leaves that fill the nichebetween sediment surface and the top of the water column Floating and submergedspecies may appear in treatment wetlands when water depths exceed the tolerancerange for rooted, emergent species

Aquatic macrophyte-based wetlands treatment systems may be classified according

to the life form of the dominating macrophyte into 1) free-floating macrophyte-basedtreatment systems; 2) rooted emergent macrophyte-based wastewater treatment systems;3) submerged macrophyte-based wastewater treatment systems; and 4) multistage sys-tems consisting of a combination of the above-mentioned concepts and other kinds oflow-technology systems (e.g., oxidation ponds and sanitary filtration systems)

6.3.3.1 Free-Floating Macrophyte-Based Systems

Free-floating macrophytes are highly diverse in form and habit, ranging fromlarge plants with rosettes of aerial and/or floating leaves and well-developed sub-merged roots (e.g., water hyacinth, Eichhornia crassipes ) to minute surface-floating plants with few or no roots (e.g., duckweeds , Lemna, Spirodella, Wolffia sp.)(Figure 6.7a).2

Water Hyacinth-Based Systems: The water hyacinth is one of the most prolificand productive plants in the world This high productivity is exploited in wetlandtreatment facilities Two different concepts are applied in water hyacinth-based

Figure 6.7a Schematic description of a free-ßoating water hyacinth-(Eichhornia crassipes)

based treatment wetland system.

Influent

Effluent

wastewater treatment systems: 1) tertiary treatment systems (i.e nutrient removal) inwhich nitrogen and phosphorus are removed by incorporation into the water hyacinthbiomass, which is harvested frequently to sustain maximum productivity and to removeincorporated nutrients Nitrogen may also be removed as a consequence of microbialdenitrification; and 2) integrated secondary and tertiary treatment systems (i.e., BODand nutrient removal) in which degradation of organic matter and microbial transfor-mations of nitrogen (nitrification-denitrification) proceed simultaneously in the waterhyacinth ecosystem Harvesting of water hyacinth biomass is only carried out formaintenance purposes The latter system should include aerators, that is, areas with afree water surface where oxygen can be transferred to the water from the atmosphere

by diffusion and where algal oxygen production can occur The retention time in thesystems varies according to wastewater characteristics and effluent requirements, but

is generally on the order of from 5 to 15 days.2

The role of water hyacinths in the process of suspended solids removal is welldocumented Most suspended solids are removed by sedimentation and subsequentdegradation within the basins, although some sludge might accumulate on the sed-iment surface The dense cover of water hyacinths effectively reduces the effects ofwind mixing and also minimizes thermal mixing The shading provided by the plantcover restricts algal growth, and hyacinth roots impede the horizontal movement ofparticulate matter Furthermore, electrical charges associated with hyacinth roots arereported to react with opposite charges on colloidal particles such as suspendedsolids, causing them to adhere to plant roots, where they are removed from thewastewater stream and slowly digested and assimilated by the plant and microor-ganisms The efficiency of water hyacinths in removing BOD and in providing goodconditions for microbial nitrification is related to their capability of transportingoxygen from the foliage to the rhizosphere The extensive root system of the waterhyacinth provides a huge surface area for attached microorganisms, thus increasingthe potential for decomposition of organic matter.1,2

Water hyacinth-based wetland treatment systems are sufficiently developed to

be applied successfully in tropics and subtropics Water hyacinths are severelyaffected by frost; the growth rate is greatly reduced at temperatures below 10∞C.Consequently, in temperate regions, water hyacinth-based systems can only be used

in greenhouses or outdoors during summer Pennywort (Hydrocotyle umbellate), onthe other hand, has a high growth rate and a high nutrient uptake capacity evenduring relatively cold periods in subtropical areas.2 It has been suggested that waterhyacinths and pennywort can be alternately cultured, winter and summer, in order

to maintain performance at a high level year-round

Duckweed-Based Systems: Duckweeds (Lemna, Spirodella, and Wolffia sp.)have not been investigated as much as water hyacinths for use in wetlands treatment.Duckweeds, have a much wider geographic range than water hyacinths, however,

as they are able to grow at temperatures as low as 1 to 3∞C Compared to waterhyacinths, duckweeds, play a less direct role in the treatment process because theylack extensive root systems and therefore provide a smaller surface area for attachedmicrobial growth.2 The main use of duckweeds is therefore in recovering nutrientsfrom secondary treated wastewater A dense cover of duckweed on the surface ofwater inhibits both oxygen entering the water by diffusion and the photosynthetic

production of oxygen by phytoplankton because of poor light penetration The waterconsequently becomes largely anaerobic, which in turn favors denitrification Thelight absorption of duckweed cover restricts growth of phytoplankton and thereforethe production of suspended solids.

Duckweed-based systems may be plagued by problems, as high winds can pilethe duckweed into thick mats and eventually completely sweep the plants from thewater Therefore, in large systems, it is necessary to construct some kind of barrier

on the water surface to prevent this The retention time in duckweed-based wetlandtreatment systems depends on wastewater quality, effluent requirements, harvestingrate, and climate, but it varies typically from 30 days during summer to severalmonths during winter

6.3.3.2 Emergent Aquatic Macrophyte-Based Systems

Rooted emergent aquatic macrophytes are the dominant life form in wetlandsand marshes, growing within a water table ranging from 18 inches below the soilsurface to a water depth of 60 inches or more In general, they produce aerial stemsand leaves, and an extensive root and rhizome system The depth penetration of theroot system, and thereby the exploitation of sediment volume, is different for dif-ferent species Typical species of emergent aquatic macrophytes are the commonreed (Phragmites australis), cattail (Typha latifolia), and bulrush (Scirpus lacustris).2

All species are morphologically adapted to growing in a waterlogged sediment byvirtue of large internal air spaces for transportation of oxygen to the roots andrhizomes Most species of emergent aquatic macrophytes possess an extensive inter-nal lacunal system that may occupy 50 to 70% of the total plant volume Oxygen

is transported through the gas spaces to the roots and rhizomes by diffusion and/or

by convective flow of air Part of the oxygen may leak from the root system intothe surrounding rhizosphere, creating oxidized conditions in the otherwise anoxicsediment and stimulating both decomposition of organic matter and growth ofnitrifying bacteria Emergent macrophyte-based wetland treatment systems can beconstructed with different designs; see Figure 6.7b for an example These types ofsystems are also currently applied for the precipitation and removal of dissolvedheavy metals under anaerobic conditions as a sulfide or carbonate precipitate

Figure 6.7b Emergent macrophyte treatment wetland system with surface ßow (adapted from

Mohiri, 1993).

6.3.3.3 Emergent Macrophyte-Based Systems with Horizontal

Subsurface Flow

Design typically consists of a bed planted with the common reed Phragmites australis and underlain by an impermeable membrane to prevent seepage if required.The medium in the bed may be soil or gravel During the passage of wastewater orcontaminated groundwater through the rhizosphere of the reeds, organic matter isdecomposed microbiologically, nitrogen may be denitrified, and phosphorus and heavymetals may be fixed in the soil The reeds have two important functions in the process:1) to supply oxygen to the heterotrophic microorganisms in the rhizosphere, and 2) toincrease and stabilize the hydraulic conductivity of the soil The quantitative signifi-cance of the uptake of nutrients in the plant tissue is negligible, as the amount ofnutrients taken up during a growing season constitutes only a few percent of the totalcontent introduced with the wastewater Moreover, nutrients bound in the plant tissueare recycled in the system upon decay of the plant material Surface runoff is a generalproblem in soil-based treatment facilities because it prevents the wastewater fromcoming into contact with the rhizoshere Furthermore, the oxygen transport capacity

of the reeds seems to be insufficient to ensure aerobic decomposition in the rhizosphereand deliver the oxygen needed for quantitatively significant nitrification

6.3.3.4 Emergent Macrophyte-Based Systems with Vertical

Subsurface Flow

In a vertical flow system the requirements for sufficient hydraulic conductivity

in the bed medium and improved rhizosphere oxygenation can be established Adesign consisting of several beds laid out in parallel with percolation flow andintermittent loading will increase soil oxygenation several-fold compared to hori-zontal subsurface flow systems During the loading period, air is forced out of thesoil; during the drying period, atmospheric air is drawn into the porespaces of thesoil, thus increasing soil oxygenation Furthermore, diffusive oxygen transport tothe soil is enhanced during the drying period, as the diffusion of oxygen is approx-imately 10,000 times faster in air than in water This design and operational regimeprovides alternating oxidizing and reducing conditions in the substrate, therebystimulating sequential nitrification–denitrification and phosphorus adsorption (Fig-ure 6.7c) The limited information available on the treatment performance of suchsystems indicates good performance with respect to suspended solids and aerobicallybiodegradable organics, ammonia, and phosphorus

6.3.3.5 Submerged Macrophyte-Based Systems

Submerged aquatic macrophytes have their photosynthetic tissue entirely merged (Figure 6.7d) The morphology and ecology of the species vary from small,rosette-type, low-productivity species growing only in oligotrophic waters (e.g.,

sub-Isoetes lacustris and Lobelia dortmanna) to larger eloedid-type, high-productivityspecies growing in eutrophic waters (e.g., Elodea canadensis)

Submerged aquatic plants are able to assimilate nutrients from polluted waters.

However, they only grow well in oxygenated water and therefore cannot be used in

wastewater with a high content of readily biodegradable organic matter because the

microbial decomposition of the organic matter will create anoxic conditions The

prime potential use of submerged macrophyte-based wastewater treatment systems

is therefore for “polishing” secondarily treated wastewaters, although good treatment

of primary domestic effluent has been obtained in an Elodea nuttallii-based system

The presence of submerged macrophytes depletes dissolved inorganic carbon in the

water and increases the content of dissolved oxygen during the periods of high

photosynthetic activity This results in increased pH, creating optimal conditions for

volatilization of ammonia and chemical precipitation of phosphorus High oxygen

concentrations also create favorable conditions for the mineralization of organic

matter in the water The nutrients assimilated by the macrophytes are largely retained

within the rooting tissues of the plants and by the attached microflora Losses from

the foliage of plant nutrients upon senescence of the macrophyte tissues are readily

taken up by the periphytic community so that very little leaves the littoral detritus

and macrophyte-epiphyte complexes Much of the detrital matter produced in these

systems will be accumulated and retained in the sediments

The use of submerged macrophytes for wastewater treatment is still in the

experimental stage, with species like egeria (Egeria densa), elodea (Elodea

canaden-sis and Elodea nuttallii), hornwort (Ceratophyllum demersum), and hydrilla

Figure 6.7c Emergent macrophyte based treatment wetland system with vertical percolation.

Figure 6.7d Schematic description of a submerged macrophyte-based treatment wetland

system.

(Hydrilla verticillata) being the most promising Present knowledge suggests that

their prime area of application will be as a final step in multistage systems

6.3.3.6 Multistage Macrophyte-Based Treatment Systems

Different types of these macrophyte-based wastewater treatment systems may

be combined with each other or with conventional treatment technologies For

example, a multistage system could consist of 1) a mechanical clarification step for

primary treatment; 2) a floating or emergent macrophyte-based treatment system for

secondary treatment; and 3) a floating, emergent, or submerged macrophyte-based

step for tertiary treatment The types of secondary and tertiary treatment step will,

among other factors, depend on wastewater characteristics, treatment requirements,

climate, and amount of available land

6.4 TREATMENT WETLAND SOILS

Several individual component wetland processes combine to provide the observed

overall treatments Sedimentation and filtration remove solids Chemical precipitation

(abiotic or microbially induced), ion exchange, and plant uptake remove metals

Nutri-ents are utilized by plants and algae, and cycled to newly formed sedimNutri-ents

The definition of hydric soils indicates that any upland soil utilized for

construc-tion of a wetland treatment system will become a hydric soil following a short to

long period of flooding and continuous anaerobiosis.1

Hydric soils are defined as soils that, in their undrained condition, are saturated,

flooded, or ponded long enough during the growing season to develop anaerobic

conditions favoring the growth and regeneration of hydrophylic vegetation.8

Since most wetlands are constructed in former uplands, most constructed

wet-lands are initially dominated by mineral soils As constructed wetland treatment

systems mature, the percent of organic matter in the soil generally increases, and in

some systems, soils might eventually cross the arbitrary line between mineral and

organic (Figures 6.8a and b) Mineral soils are classified by particle size distributions,

color, depth, and a number of other factors The three major mineral soil classes are

clays, silts, and sands

Clays are soils with very fine particles packed closely together Because of their

very fine texture and low hydraulic conductivity, clays may function as aquitards

The existence of many natural wetlands depends on impermeable clay lenses in

sedimentary or wind-blown (loess) deposits Clays typically have the highest

adsorp-tion potential of any soils because of their high surface area to volume ratio resulting

from their small particle size distribution When water in a wetland is in contact

with underlying clays or when water percolates through the bottom of a clay-lined

wetland, the presence of clays may greatly increase treatment potential for

conser-vative ions such as phosphorus and metals

Organic soils, called peat, muck, or mucky peat may be classified by their extent

of decomposition Those soils with the least amount of decomposition (less than

one third decomposed) are called peat Fibric peats have more than two thirds of

the plant fibers still identifiable Saprists or mucks have greater than two thirds of

the original plant materials decomposed Hemists (mucky peat or peaty mucks) are

between saprists and fibrists

Due to their fibrous nature, organic soils may shrink, oxidize, and subside when

they are drained Fire may also accelerate this oxidation process, and agricultural

Figure 6.8a The types of soils present in a newly planted treatment wetland system (adapted

from Kadlec, 1996).

Figure 6.8b Types of soil layers developed after a period of maturation in a treatment wetland

system (adapted from Kadlec, 1996).

practices (drainage, cropping, harrowing, and burning) are known to result in soil

subsidence in highly organic soils such as those in the Everglades agricultural area

where subsidence rates have been estimated at about 3 cm/yr

Drying organic soils promotes oxidation and gasifies carbon, but not the mineral

nutrients associated with those soils Although the available nutrient content of a

peat or muck is often quite low, there are large amounts of nitrogen, phosphorus,

sulfur, and other mineral constituents organically bound in unavailable forms

Oxi-dation destroys these recalcitrant organics and releases the associated substances

Upon reflooding, those substances can dissolve and provide relatively high

concen-trations of nutrients and other dissolved minerals

Organic soils cannot easily be characterized by grain size because the necessary

act of drying destroys the physical-chemical structure The general range of hydraulic

conductivity for soils found in sedge, reed, and alder wetlands is 0.1 to 10 m/d,

placing these materials in the range of other mineral soils However, this is true only

for fully saturated soils; even a slight degree of unsaturation lowers the hydraulic

conductivity by two orders of magnitude, due to the extremely large capillary suction

pressure created in the micropores This means that organic soils and sediments are

virtually undrainable; they retain a very high percentage of water Organic soils are

typically dark in color, ranging from black mucks to brown peats

Soil chemical properties are primarily related to chemical reactivity of soil

particles and the surface area available for chemical reactions Chemical reactivity

is related to the surface electrical charge of the soil particles, is typically highest in

clays and organic soil particles

6.4.1 Cation Exchange Capacity

Wetland soils have a high trapping efficiency for a variety of chemical

constit-uents; they are retained within the hydrated soil matrix by forces ranging from

chemical bonding to physical dissolution within the water of hydration The

com-bined phenomena are referred to as sorption

A significant portion of chemical binding is cation exchange, which is

replace-ment of one positively charged ion, attached to the soil or sedireplace-ment, with another

positively charged ion The humics substances found in wetlands contain large

numbers of hydroxyl and carboxylic functional groups, which are hydrophilic and

serve as cation binding sites Other portions of these molecules are nonpolar and

hydrophobic in character The result is the formation of micelles, groups of humic

molecules with their nonpolar sections combined in the center and their negatively

charged polar portions exposed on the surface of the micelle Protons or other

positively charged ions may then associate with these negatively charged sites to

create electrical neutrality

Micelles are one form of ligand that can bind metal ions The cumulative process

of binding a metal ion to a ligand (L) to form a complex may be described by a

chemical equation; here, it is illustrated for the binding of a divalent metal ion (M):1