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WETLANDS

Introduction

Wetlands are areas that are periodically inundated with a frequency and depth sufficient to promote the growth of specific vegetation adapted to life in saturated soil conditions.

Types of Wetland Systems

The two basic types of wastewater-to-wetland treatment systems:

Natural

a. Marshes wetlands containing emergent, nonwoody macrophytes b. Swamps wetlands dominated by woody species

Constructed Constructed wetland is an engineered system that has been designed and constructed to utilize the natural processes involving wetland vegetation,

soils and their associates microbial assemblages to assist in treating wastewater (USEPA, 2000).

Classification of Constructed Wetland (CW) Types Commonly Used for Wastewater

Management

1. Free Water Surface (FWS)

Water level is above the ground surface; vegetation is rooted and emergent

above the water surface; water flow is primarily above ground; vegetation may be planted or allowed to colonize voluntarily.

wetland plants & water

soil

liner

native soil (nat. grnd)

FWS (or SF- Surface Flow) wetland consists of a shallow basin, soil or

other medium to support the roots of vegetation, and a water control

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structure that maintains a shallow depth of water (Davis, USDA-NRCS & USEPA, undated).

2. Vegetated Submerged Beds (or Subsurface Flow System)

Water level is below ground, water flow is through soil or gravel bed; root penetration is to bottom of bed; wetland plants are generally common reed,

bulrush, or cattail.

wetland plants

soil, sand or gravel

liner native soil

Wetland Functions and Values

Under appropriate circumstances, constructed wetlands can provide (Davis, USDA-

NRCS & USEPA, undated): water quality improvement flood storage and desynchronization of storm rainfall and surface runoff cycling of nutrients and other materials habitat for fish- and wildlife passive recreation, such as bird watching and photography active recreation, such as hunting education and research aesthetics and landscape enhancement.

Components of Constructed Wetlands

A CW consists of a properly-designed basin that contains water, a substrate, and, most commonly vascular plants. These components can be manipulated in constructing a

wetland. Other important components of wetlands, such as the communities of microbes and aquatic invertebrates, develop naturally (Davis, USDA-NRCS & USEPA, undated).

Water

Hydrology is the most important design factor in CW because it links all of the functions in a wetland and because it is often the primary factor in

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the success or failure of a CW. While hydrology of CW is not greatly different than that of other surface and near-surface waters, it does differ

in several important respects: small changes in hydrology can have fairly significant effects

on a wetland and its treatment effectiveness because of the large surface area of the water and its shallow

depth, a wetland system interacts strongly with the atmosphere through rainfall and evapotranspiration

the density of vegetation of a wetland strongly affects its hydrology, first, by obstructing flow paths as the water finds its

sinuous way through the network of stems, leaves, roots, and rhizomes and, second, by blocking exposure to wind and sun.

Substrates, Sediments, and Litter

Substrates used to construct wetlands include soil, gravel, rock, and organic materials such as compost. Sediments and litter then accumulate in the

wetland because of the low water velocities and high productivity typical of wetlands. The substrates, sediments, and litter are important for several

reasons: they support many of the living organisms in wetlands substrate permeability affects the movement of water through the

wetland

many chemical and biological (especially microbial) transformations take place within the substrates

substrates provide storage for many contaminants the accumulation of litter increases the amount of organic matter in the

wetland. Organic matter provides sites for material exchange and microbial attachment, and is a source of carbon, the energy source that

drives some important biological reactions in wetlands.

Vegetation Both vascular plants (the higher plants) and non-vascular plants (algae) are

important in CW. Photosynthesis by algae increases the DO content of water which in turn affects nutrient and metal reactions. Vascular plants

contribute to the treatment of wastewater and runoff in a number of ways: they stabilize substrates and limit channelized flow they slow water velocities, allowing suspended materials to

settle

they take up carbon, nutrients, and trace elements and incorporate them into plant tissues

they transfer gases between the atmosphere and the sediments leakage of oxygen from subsurface plant structures creates

oxygenated microsites within the substrate their stem and root systems provide sites for microbial

attachment they create litter when they die and decay

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Microorganisms A fundamental characteristic of wetlands is that their functions are largely

regulated by microorganisms and their metabolism (Wetzel, 1993 cited in Davis, USDA-NRCS & USEPA, undated). Microorganisms include

bacteria, yeasts, fungi, protozoa, rind algae. The microbial biomass is a major sink for organic carbon and many nutrients. Microbial activity:

transforms a great number of organic and inorganic susbtances into innocuous or insoluble substances

alters the reduction/oxidation (redox) conditions of the substrate and thus affects the processing capacity of the

wetland is involved in the recycling of nutrients.

How wetlands improve water quality

The mechanisms that are available to improve water quality are therefore numerous

and often interrelated. These mechanisms include: settling of suspended particulate matter filtration and chemical precipitation through contact of the water with the

substrate and litter

chemical transformation adsorption and ion exchange on the surfaces of plants, substrate, sediment,

and litter breakdown and transformation of pollutants by microorganisms and plants uptake and transformation of nutrients by microorganisms and plants predation and natural die-off of pathogens

Advantages of CW

Constructed Wetland Treatment Design

CWs are a cost-effective and technically feasible approach to treating wastewater and runoff for several reasons:

wetlands can be less expensive to build than other treatment options operation and maintenance expenses (energy and supplies) are low operation and maintenance require only periodic, rather than continuous, on-

site labor

wetlands are able to tolerate fluctuations in flow they facilitate water reuse and recycling

In addition: they provide habitat for many wetland organisms they can be built to fit harmoniously into the landscape they provide numerous benefits in addition to water quality improvement,

such as wildlife habitat and the aesthetic enhancement of open spaces they are an environmentally-sensitive approach that is viewed with favor by

the general public.

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Limitations of CW

There are limitations associated with the use of CWs they generally require larger land areas than do conventional wastewater

treatment systems. Wetland treatment may be economical relative to other options only where land is available and affordable.

performance may be less consistent than in conventional treatment. Wetland treatment efficiencies may vary seasonally in response to changing environmental

conditions, including rainfall and drought. While the average performance over the year may be acceptable, wetland treatment cannot be relied upon if effluent

quality must meet stringent discharge standards at all times. biological components are sensitive to toxic chemicals, such as ammonia and

pesticides flushes of pollutants or surges in water flow may temporarily reduce treatment

effectiveness they require a minimum amount of water if they are to survive. While wetlands

can tolerate temporary drawdowns, they cannot withstand complete drying.

Also, the use of CWs for wastewater treatment and stormwater control is afairly recent development. There is yet no consensus on the optimal design of wetland

systems nor is there much information on their long-term performance.

General Design of Constructed Wetlands

Design Considerations

Despite a large amount of research and published information, the optimal design of CWs for various applications has not yet been determined. In general, wetland designs attempt

to mimic natural wetlands in overall structure while fostering those wetland processes that are thought to contribute the most to the improvement of water quality. Mitsch

(1992) suggests the following guidelines for creating successful CWs: keep the design simple. Complex technological approaches often invite failure. design for minimal maintenance. design the system to use natural energies, such as gravity flow. design for the extremes of weather and climate, not the average. Storms, floods,

and droughts are to be expected and planned for, not feared.

design the wetland with the landscape, not against it. Integrate the design with the natural topography of the site.

avoid over-engineering the design with rectangular basins, rigid structures and channels, and regular morphology. Mimic natural systems.

give the system time. Wetlands do not necessarily become functional overnight and several years may elapse before performance reaches optimal levels.

Strategies that try to short-circuit the process of system development or to over-mange often fail.

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design the system for function, not form. For instance, if initial plantings fail, but the overall function of the wetland, based on initial objectives, is intact, then the

system has not failed.

Site Selection

Selecting an appropriate location can save significant costs. Site selection should consider land use and access, the availability of the land, site topography, soils, the environmental

resources of the site and adjoining land, and possible effects on any neighbors.

A site that its well suited for a constructed wetland is one that: is conveniently located to the source of the wastewater provides adequate space is gently sloping, so that water can flow through the system by gravity contains soils that can be sufficiently compacted to minimize seepage to

groundwater

is above the water table is not in a floodplain does not contain threatened or endangered species does not contain archaeological or historic sites

Physical Design Factors 1. Aspect Ratio

FWS: BOD5 and TN increases at L:W ratio greater than 4:1 to 6:1. Although,

Knight estimated an optimum L:W ratio of about 2:1

VSB: L:W = less than or greater than 1, depending on the treatment goals.

2. Compartmentalization

Constructed wetlands should be compartmentalized with several cells arranged in series or in parallel. The use of multiple cells allows redistribution of flows,

maintenance of plant communities (greater edge length to surface area), and isolation of different plant populations and any associated diseases or pathogens.

3. Multiple Input Points

A new principle in constructed wetland design is stepfeeding or the use of

alternate input points along the length of one cell. It approximates complete mix rather than plug flow. However, this design reduces the organic loading to any

given area of the wetland and may decrease the potential for nuisance vector or odor conditions.

4. Outlet Structure

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5. Dikes, Liners & Bottom Slopes

Dikes should be sloped no steeper than 2H:1V and riprapped or protected by erosion control fabric on the slopes.

Bottom slopes = 1 3 %

Liners

CWs must be sealed to avoid possible contamination of groundwater and also to prevent groundwater from infiltrating into the wetland. Where on-

site soils or clay provide an adequate seal, compaction of these materials may be sufficient to line the wetland.

On-site soils can be used if they can be compacted to permeability of

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kt = k20 x 1.06

(T-20)

Reed et al. (1987) equation

)7.0exp( 75.1 HRTktAvFCo

Ce

where Ce = effluent BOD5

Co = influent BOD5 kt = temp.-dependent first-order reaction rate constant

HRT = hydraulic residence time T = water temperature

F = fraction of BOD5 that does not settle in headworks of the system Av = specific surface area for microbial activity

= void fraction Typical values for k20 given by EPA is 0.0057/d. A maximum BOD5 loading rate of about 100 kg/ha-d has been recommended to help

prevent the occurrence of mosquito populations.

For = 0.75 & F = 0.52 and Av = 15.7 sq.m/cu.m., Reed et al. (1987) equations becomes:

Hkt

CeCoQA

)(65

)6539.0ln(ln

VSB

1. Cross Section Area

Sk

QAc

s

where Ac = cross-sectional area

Ac = depth x width ks = hydraulic conductivity of media

S = bed slope W = bed width

d = bed depth

Normal design bed depth for VSB is between 60 and 90 cm.

For VSB with medium to coarse sand, k20 = 0.806 & = 0.32.

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A guideline for minimum constructed wetlands area for removal of BOD5 is about 3 to 4

ha/1000 cu.m-d for FWS and 1.2 ha/1000 cu.m-d for VSB. Actual area should be determined using available equations.

2. VSB Surface Area

))()((

)ln(ln

dkt

CeCoQAs

where As =surface area

Ce = effluent BOD Co = influent BOD

kt = temp.-dependent 1st-order reaction rate

Q = average flowrate through the system

d = depth of submergence

= porosity of bed

Note:

Tchobanoglous indicated that typical BOD5 loading rates are in the range of 60 to 80 kg/ha-d. Other authors recommend a maximum BOD5 loading rate of about 110 to

120 kg/ha-d in both VSB and FWS.

Table 1. Summary Wetland Design Consideration

Design Consideration Constructed Natural

FWS VSB

Minimum size reqt.,

ha/1000cu.m-d

3-4 1.2-1.7 5-10

Maximum water depth, cm 50 Water level below ground

surface

50; depends on native

vegetation

Bed depth, cm Not

applicable

30-90 Not

applicable

Minimum aspect ratio 2:1 Not applicable Prefer 2:1

Min, hydraulic residence time,d 5-10 5-10 14

Max. hydraulic loading rate,

cm/d

2.5-5 6-8 1-2

Minimum pretreatment Primary;

secondary is optional

Primary Primary;

secondary; nitrification,

TP reduction

Configuration Multiple cells in parallel and

series

Multiple beds in parallel

Multiple discharge

sites

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Distribution Swale;

perforated pipe

Inlet zone

(>0.5 m wide) of large gravel

Multiple

discharge sites

Maximum Loading, kg/ha-d BOD

TN

100-110

60

80-120

60

4

3

Additional consideration Mosquito control with

mosquito fish; remove

vegetation once each 1-5

years

Allow flooding capability for

weed control

Natural hyroperiod

should be >50%; no

vegetation harvest

Table 2. Media Characteristics for SFS (USEPA, 1988)

Media Type Max 10 %

Grain Size, mm

Porosity Hydraulic

conductivity (ks),

cu.m/sq.m-d

K20

Medium sand 1 0.42 420 1.84

Coarse sand 2 0.39 480 1.35

Gravelly sand 8 0.35 500 0.86

Example for Designing FWS CW:

Given: Qd = 100 cu.m/d

BOD5 influent = 500 mg/L desired BOD5 effluent = 50 mg/L

void fraction = 0.50 HRT = 30 d

F = 0.52 Av = 15.7

k20 = 0.0057/d

Reqd: Design FWS

Solution:

1. FWS

use H = 0.50 m

Ce/Co = F exp (-0.7 kt Av1.75 HRT)

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50/500 = 0.52 exp[(-0.7)(0.0057)(17.5)1.75

(0.5)(HRT)] HRT = 5.52 d say 6 d

HRT = (A H)/Q 6 d = A(0.50m)(0.50)/100 cu.m/d

A = 2,400 sq.m

Dimensioning:

L/W = 4:1 L = 4W

A =2400 = 4W(W) W =24.5 m

L = 98 m

Calculate the required area and bed depth for a SFS where influent wastewater is from a facultative lagoon. Assume influent BOD5 to the wetlands will be 130 mg/L. The desired

effluent BOD5 is 20 mg/L. The predominant wetland plant type in surrounding marshes is cattail. Water temperatures are 6

OC in winter and 15

OC in summer. Wastewater flow is

950 cu.m/d.

Solution:

From studies in Santee, California, cattails rhizomes penetrate approximately 0.3 m into the medium. The media depth should therefore be 0.3 m.

Bed slope, say 1 %. Reed et al (1987) indicated the need to check ksS < 8.60

Choose sand as media

ks = 480 cy.m/sq.m-d k20 = 1.35/d

check actual ksS = 480(0.01) = 4.8 < 8.60 o.k.

solving for kt

winter: kt = k20(1.06)T-20

k6 = 1.35 (1.06)

6-20

k6 = 0.36/d

summer: k15 = 1.35(1.06) 15-20

k15 = 0.84/d

determine cross section area of bed

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Sk

QAc

s

Ac = (950 cu.m/d)/(480)(0.01) = 198 sq.m d=0.30 m

dimensioning width = Ac/d = 198/0.3 = 660 m w

determine surface area of bed

winter:

))()((

)ln(ln

dkt

CeCoQAs

As = [950(ln 130 ln 20)]/[0.36(0.3)(0.39) As = 42, 177 sq.m

summer

As = [950(ln 130 ln 20)]/[0.84(0.3)(0.39) As = 18, 067 sq.m

hence, winter conditions control. As = 42, 177 sq.m (~ 4.22 ha)

determine bed length

LW = As

L = 42,177/660 = 63.9 m

using say 11 cells dimensions of each cell = 60 m x 64 m

hydraulic residence time

Q

LWd

Q

VvHRT

HRT = 63.9(660)(0.39)/950 = 5.2 days