TREATMENT WETLANDS - CHAPTER 14 pptx

540 Treatment WetlandsFor modeling purposes, the time series of water flows entering the treatment wetland inlet hydrograph is sometimes needed.. Stormwater concentrations and loads are

The treatment of stormwaters of various origins is of

grow-ing concern as attempts to rectify point-source pollution

reach maturity Runoff from urban, agricultural, and

indus-trial sources comprises a sizeable fraction of the total

pollu-tion load to receiving waters in many locapollu-tions Often, these

sources are relatively dilute compared to primary or

second-ary domestic sewage but, nevertheless, they may negatively

impact receiving waters

Urban stormwater wetlands were first surveyed by

Strecker et al (1992), who documented the performance of

25 natural and constructed wetlands treating runoff The

implementation of the technology and the knowledge base

continued to build, resulting in a compilation of data from

76 wetlands worldwide by Wong et al (1999), and from 49

wetlands in the United States by Carleton et al (2001) In

North America, these are all FWS systems, and that is the

predominant wetland type elsewhere as well A typical

con-figuration consists of a sedimentation basin as a forebay,

fol-lowed by some combination of marshes and deeper pools

Design guidelines have now been promulgated by a number

of sources (for example: Schueler, 1992; Breen and Lawrence,

1998; Wong et al., 1999; LEC, 2000; Center for Watershed

Protection, 2001)

Agricultural stormwater occurs as runoff from crops and

pastures Early work on constructed wetlands for row crop

runoff control was centered in the state of Maine (Higgins et

al., 1993) (Wengrzynek and Terrell, 1990; U.S Department of

Agriculture, 1991) Runoff from sugarcane is being treated in

the huge FWS wetlands (16,000 ha) called stormwater

treat-ment areas (STAs) of South Florida (Goforth, 2001) However,

there has also been significant application of constructed

wet-lands at a more modest scale for vegetable farming (Rushton

and Bahk, 2001) Nonetheless, effective control of runoff

may require consideration of the entire watershed (Crumpton,

2000) More recently, attention has been on controlling

pas-ture runoff (Tanner et al., 2003).

Industrial stormwaters have received less attention than

urban and agricultural sources These typically will

con-tain contaminants specific to the industry in question For

instance, rain runoff from a fertilizer plant site will

poten-tially contain high levels of nutrients, whereas rain

run-off from a petroleum facility will carry hydrocarbons For

instance, in South Africa, a number of reed bed wetlands

were established to treat waters generated from truck

wash-ing operations at oil industry depots (Wood, 1993) An HSSF

wetland treated runoff from a 0.8-ha vehicle yard in Surprise,

Arizona, with 54–92% removal of oil and grease (Wass and

of wetland removal mechanisms in different ways than their continuous-flow counterparts

14.1 SOURCE CHARACTERIZATION

The amount of water to be expected from a given shed is variable and is keyed to rainfall in the contribut-ing basin The land uses and soils in the contributing basin are an important modifier of the runoff and infiltration The antecedent dryness in the basin is also a contributing factor Detailed methods of estimating runoff are avail-able, for instance, the SCS (U.S Soil Conservation Service) method (McCuen, 1982), which has been summarized by Novotny (1995) There are also numerous computer models that account for very detailed features of the contributing watershed and produce both the quality and quantity of the runoff to be treated The focus here is the treatment wetland, and it will be presumed that the incoming flows will be char-acterized to the appropriate level of detail

water-For purposes of rough estimation, the rational formula

may be used to estimate peak flow, QP (ASCE, 2006):

QP CR• •I Ab (14.1)

where

A C

b R

watershed area, mrunoff coefficient,

2dimensionlesspeak runoff flow, m /h

aP

3

Q I

 vverage rainfall intensity, m/h

The values of CR are a function of land use and storm sity, as well as the climatological region An example of the

inten-range of CR for an arid region is shown in Table 14.1 To illustrate the regional effect, consider that agricultural land

in Arizona is rated at CR 0.1–0.2 for high return-frequency events, whereas the value for agricultural land in South

Florida is CR y 0.5 Water is used consumptively in arid regions, whereas the high water table and rainfall in a wet region leads to more runoff Modifications to this simplest formula have been presented on a site-specific basis (see, for example, Brezonik and Stadelmann, 2002; ASCE, 2006)

540 Treatment Wetlands

For modeling purposes, the time series of water flows

entering the treatment wetland inlet hydrograph is sometimes

needed This time series is driven by the pattern of rainfall

associated with a particular event (represented by a

hyeto-graph), together with the collection and transfer

character-istics of the basin In some instances, such transfer is solely

by gravity, but in some cases pumps may be used Usually,

the inlet hydrograph contains both a rising and falling limb,

although the rising limb is normally quite steep (for an

exam-ple, see Figure 14.1)

In some instances, the events are separated by interevent

periods of no inflow to the treatment wetland These periods

are important because the wetland will act as a batch reactor

during much of these no-inflow durations A typical sequence

is (1) wetland filling with no outflow, (2) flow through with

both inflow and outflow, (3) draining with no inflow, and (4)

finally, a batch-holding mode with neither inflow nor outflow

The rates and duration of these inflows and outflows are in

part controlled by structures The volume of water in the

wet-land is also in part controlled by structures, but

evapotrans-piration may be an important component during interevent

periods The durations of no-flow periods are very much a

function of the climatological region in which the system is

located For instance, in Florida, during the rainy season, the

most frequent periods are measured in hours (4–20 hours, Wanielista and Yousef, 1991) In contrast, the dry season

of central north island New Zealand has periods of months with no rain, and interevent periods can be several months

(Tanner et al., 2005b).

Many stormwater wetlands are fed by pumps These may range in size from small sump pumps for small local-ized urban systems to the huge pumps that send water to the Florida stormwater treatment areas (a.k.a STAs, or treat-ment wetlands) Those Florida inflow and outflow pumps are among the largest in the world, up to three stories tall, with capacities up to 10,000,000 m3/d (Figure 14.2) A key feature

of pumped systems is intermittent feed to the treatment land, usually at fixed but incremental rates, corresponding to the number of pumps that are operated, for periods of time dictated by conditions in the contributing watershed

wet-There is typically a hydraulic limitation to the event size that may be treated in a given wetland It is generally not feasible to size a wetland to treat the 100-year return fre-quency storm because of cost and footprint considerations and because of the need for maintenance water for the (large) wetland under normal conditions Consequently, part of the design decision process is the determination of the maximum design storm to be treated Even if the system is sized to treat

TABLE 14.1 Runoff Coefficient (C) for Use in the Rational Method in Maricopa County, Arizona

Return Period

Streets and Roads

Mountain terrain (slopes  10%) 0.6–0.8 0.66–0.88 0.72–0.95 0.75–0.95

Source: Adapted from the Drainage Design Manual for Maricopa County, Volume 1.

Event-Driven Wetlands 541

a modest-sized storm flow, there remains the possibility that

dry conditions might persist in some years to the point that

the integrity of the wetland ecosystem is jeopardized If such

detrimental dryout conditions are expected, then a source

of irrigation water for ecosystem maintenance should be

identified

Concentrations of most parameters in stormwater are time

dependent, as are the flows Stormwater concentrations and

loads are episodic due to periods of dryfall and deposition,

followed by the first flush of runoff after rain, followed by

exponential decreases in runoff constituent concentrations as

storages rinse from the landscape, and finally, dry conditions and deposition until the next storm event The time series

of concentrations in the inflow to the wetland is called the

chemograph An example chemograph for an agricultural

runoff wetland, targeting nitrogen reduction, is shown in Figure 14.3

In some watersheds, the chemograph is not synchronized with the hydrograph, but instead provides higher concentra-tions early in the inflow event This phenomenon is termed

first-flush behavior, referring to the surge of pollutants

con-tained in the first water to leave the contributing basin For instance, Wanielista and Yousef (1991) report that, in Flor-ida, the first 25 mm of runoff from urban systems typically carries 90% of the pollution However, for the large agricultural

0.0 0.5 1.0 1.5 2.0 2.5 3.0

FIGURE 14.1 Time series of flows entering a treatment wetland from an improved pasture in Toenepi, New Zealand Several rain events

occurred during this winter wet season, as indicated by the repeated spikes in inflow The runoff coefficients were 0.23–0.30 and were the

result of tile drains (Adapted from Tanner et al (2005b) Agriculture, Ecosystems and Environment 105(1–2): 145–162.)

FIGURE 14.2 The outflow pump station from STA1E (Stormwater Treatment Area 1 East) of the Everglades Protection Project The

capac-ity of this pump station is 9,740,000 m 3 /d, and it serves to drain a 2,700-ha FWS wetland.

542 Treatment Wetlands

watershed of South Florida, there is no such first-flush effect

Despite the site-specific nature of the chemograph, there is

no reliable database documenting dynamic time series of

concentrations in any given watershed Of necessity, average

concentrations of some sort must be used, of which the

flow-weighted concentration is most useful This may be

evalu-ated on a long-term basis or as an event mean concentration

for the inlet water:

and where integration is over the period of one event The

numerator is the mass of pollutant entering or leaving during

the event As described in Chapter 6, the performance of the

wetland may be described in terms of the loads applied to

and emanating from the system

Tables 14.2 and 14.3 provide long-term mean

concen-trations for constituents in urban stormwater The averages

are flow-weighted to provide realistic estimates of the total

constituent load that escapes during multiple storm events

Instantaneous concentrations may rise considerably higher

than these averages Pollutant concentrations and loads

gen-erally range from low levels from undeveloped and park

lands to low-density residential and commercial, to

agri-cultural, to higher-density residential and commercial, and

finally to high-density commercial, industrial, and

agricul-tural land uses Mean concentrations per event for BOD5

vary from below detection for undeveloped lands to 20 mg/L

for high-density urban areas Total suspended solids trations vary from about 10 mg/L for undeveloped areas up

concen-to 150 mg/L for high-density urban areas Typical tions for other stormwater pollutants are also summarized in Tables 14.2 and 14.3

concentra-The mass loading rates represent normalized ant loads that are somewhat independent of local rainfall amounts Because pollutant loads per area per time are rela-tively constant between similar land use areas, variable local rainfall washes these loads off the land in a few large events

pollut-or over many smaller events Urban pollutant loads increase with the imperviousness of the watershed Although 20 to 40% of the material on street surfaces is organic, it does not biodegrade easily because it comes from leaf and wood litter, rubber, and road-surface material (Novotny, 1992) The high metal content of highway solids comes from vehicle emis-sion Novotny (1992) reported that the average total nitro-gen load from urban lands is 5 kg/ha·yr (1 to 38.5 kg/ha·yr), and the total phosphorus load averages 1 kg/ha·yr (0.5 to 6.25 kg/ha·yr)

Constructed wetlands are being increasingly used to treat runoff from intensive animal operations (CH2M Hill

and Payne Engineering, 1997; Tanner et al., 2003) and from

crops (U.S Department of Agriculture, 1991; Crumpton, 2000) Concentrations and loads from agricultural land uses vary considerably Flows and loads are typically highest from areas with high animal densities or high fertilization rates Runoff pollutant concentrations from animal feedlots can

be extremely high unless runoff is collected and treated Pollutants from feedlot runoff typically include high levels

of organic and inorganic solids and associated nutrients Nutrient concentrations and loads from row crops and pas-tures depend on fertilization practices and type of soil.There exist many computer models for the amounts of contaminants that may be expected in runoff from different

0 5 10 15 20 25 30 35 40 45

FIGURE 14.3 The time series of nitrate nitrogen arriving at an agricultural runoff treatment wetland in McDowell County, North Carolina

The rain event started at time zero and drove runoff that entered the wetland by stream flow (Adapted from Kao and Wu (2001) Water ence and Technology 45(3): 169–174.)

Sci-Event-Driven Wetlands 543

landscapes Novotny (1995) summarized the characteristics

of six models for urban watersheds, and seven for

agricul-tural watersheds, and the number has grown since that time

A large part of the design of event-driven treatment wetlands

is the determination of the flows and loads to be treated

TABLE 14.2

Pollutant Concentrations for Source Areas for Stormwaters

Constituent

TSS a (mg/L)

TP b (mg/L)

TN c (mg/L)

(1,000 #/mL)

Cu a (Kg/L)

Pb a (Kg/L)

Zn a (Kg/L)

Source: Data from Center for Watershed Protection (2001) New York State Stormwater Management Design Manual Report by the Center for Watershed

Protection (CWP) for the New York State Department of Environmental Conservation, Albany, New York.

aData from Claytor and Schueler (1996) Design of Stormwater Filtering Systems Center for Watershed Protection: Ellicott City, Maryland.

b Average of data from Steuer et al (1997) Sources of contamination in an urban basin in Marquette, Michigan, and an analysis of concentration, loads, and data quality Water Resources Investigation Report 97–4242, U.S Geological Survey; Bannerman et al (1993) Water Science and Technology 28(35): 241–259; Waschbusch (2000) Sources of phosphorus in stormwater and street dirt from two urban residential basins in Madison, Wisconsin, 1994–1995 Proceedings of the

National Conference on Tools for Urban Water Resource Management and Protection; U.S Environmental Protection Agency: Washington, D.C, pp 15–55.

cData from Steuer et al (1997) Sources of contamination in an urban basin in Marquette, Michigan, and an analysis of concentration, loads, and data quality.

Water Resources Investigation Report 97–4242, U.S Geological Survey.

TABLE 14.3

Typical Concentration Data for Pollutants in Urban Stormwater

Source: Data from Center for Watershed Protection (2001) New York state stormwater management design manual Report by

the Center for Watershed Protection (CWP) for the New York State Department of Environmental Conservation, Albany, New

York; pooled NURP/USGS Smullen and Cave (1998) Updating the U.S nationwide urban runoff quality database 3rd

Interna-tional Conference on Diffuse Pollution, 31 August–4 September 1998; and Schueler (1999) Watershed Protection

Techniques 3(1): 551–596.

Event-driven wetlands are dynamic in all respects, and the principal underlying hydraulics exhibit variable water depths and flows The behavior is strongly conditioned by the nature

544 Treatment Wetlands

of inflow and outflow structures that may be designed to

improve detention and treatment (Somes and Wong, 1997)

The most general situation may have several complicating

factors, but here a simple and common case is explored for

purposes of illustration It will be presumed that the wetland

is relatively small and consequently behaves as a level-pool

system, with no gradients in stage Stormwater wetlands

experience event flows and concentrations, followed by

peri-ods of batch operation It is then necessary to account for the

dynamics of water storage within the wetland

The dynamic, level-pool water mass balance for the

wet-land for unsteady state inflows and meteorology is

The stage is often controlled by an outlet structure, here

sup-posed to be a rectangular weir Thus the outflow-stage

rela-tion would be given by the Francis formula (French, 1985):

It is noted that the wetted area is not constant but changes with

stage according to the bathymetry of the wetland, represented

by the stage–area–volume relationship (see Chapter 2)

Fur-ther, the wetland catches rainfall over an area typically greater

than the wetted area but loses water to ET from just the wetted

area Infiltration further contributes to water losses and may

be a significant fraction of the output from the wetland, as in

the case of the Hidden River wetland in Tampa, Florida (Carr

and Rushton, 1995)

An example of this analysis is illustrated in Figure 14.4

For illustration, a wetland of 2,500 m2 is subjected to a

5-cm rain event and the associated runoff from the

contrib-uting basin The presumed conditions and wetland design

are shown in Table 14.4 The wetland does not have a level

bottom; rather, there is a presumed quadratic

stage–vol-ume relation, which implies that the wetted area increases

linearly with stage The hypothetical sequence of events is

The wetland loses some water to ET (1.0 cm/d),

and some to infiltration (1.0 cm/d)

For the first half day (t 0–0.5 days), the wetland fills due to direct collection of rain

At t 0.65 days, the wetland has filled to the top of the outflow weir, and outflow commences

For the next half day (t 0.5–1.0 day), the wetland fills because of direct rain and incoming runoff

For the next day (t 1.0–2.0 days), the wetland fills because of incoming runoff

During t 2.0–3.1 days, there is no inflow, and the

wetland is draining over the outflow weir At t3.1 days, the level has decreased to the top of the weir, and outflow stops

After time t 3.1 days, the wetland loses water to

ET and infiltration, and the level continues a slow

Stage area: A  aH, a  2,500 m 2 /m

Weir coefficient  31,623 (m 3 /d)/(m 1.5 )

ET rate  1.00 cm/d Infiltration rate  1.00 cm/d Rain catchment  3,000 m 2

Rainfall

Rain rate  5.00 cm/d Basin catch  6,000 m 3 Inflow start  0.50 d

Inflow rate  2,000 m 3 /d Runoff  3,000 m 3 Storm/wetland volume ratio  2.40

Event-Driven Wetlands 545

Because of the assumed bathymetry, the wetted area first

grows and then shrinks through this course of events

This simple model has been found to fit event wetland

data quite well (Kadlec, 1994; Hey et al., 1994; Kadlec,

2001a) However, there are several different inlet and outlet

structures that may be used, and the level-pool model must

be modified accordingly for other situations

Event-driven wetlands operate with periods of flow through

and periods of water-holding or batch processing Any water

that does not escape the wetland during a particular event will

be held until the next event, and possibly longer Therefore,

it is subject to the water quality improvement functions of

the wetland for not only the event duration but also the

interevent period Conversely, water that enters and leaves

the wetland during the event is subject to treatment only

dur-ing the (possibly brief) period of detention durdur-ing the event

Therefore, at the first level of consideration, it would seem desirable to contain the entire volume of water associated with an event within the wetland This requirement is often referred back to the contributing watershed in terms of the rain amount and runoff coefficient that generated the storm volume The wetland may be sized to nominally contain the runoff volume associated with a rain event of specified amount or return frequency

One limit on performance is the expulsion of water that resides in the wetland at the time of event initiation This water is at a background concentration if the events are widely spaced In this example, the time for background to be reached was on the order of two or three days The length of time for the wetland water surcharge to dissipate, and thus for stage to reach the weir elevation, was also of the order of two or three days If the events are more closely spaced, then the resident antecedent water will not be at background but rather will exhibit a concentration representing only partial treatment

of the previous event A second limit on performance is the

0 500 1,000 1,500 2,000 2,500 3,000

(a)

FIGURE 14.4 Response of a hypothetical stormwater wetland to a one-day steady rain Runoff into the wetland begins after half a day

The wetland fills, and outflow persists for just over three day Infiltration and ET deplete the water after the event See Table 14.4 for parameters.

0.7 0.8 0.9 1.0 1.1

Time (days)

Wetland Water Stage (m) Rain

Inflow Outflow

(b)

546 Treatment Wetlandsultimate removal that would be associated with a continuous

water input at the event average flow rate This is likely to

be a relatively low percentage reduction because of the high

flow in the event

A complicating factor in analysis is that wetlands are not

hydraulically simple and do not operate on a basis of

plug-flow displacement Therefore, the fraction of incoming water

that remains in the system for a particular event is not

deter-mined merely from displacement Depending on the

configu-ration of the wetland, some of the very first water that enters

the wetland may find its way to the exit far in advance of

nominal retention time during the event This is partly due to

preferential flow paths, and partly due to the positioning of

water-level control structures

If it is assumed that the wetland behaves hydraulically

as several mixed tanks in series, it is reasonable to use the

known detention time distribution to compute how much of

the incoming water is still in the wetland after a period of

flow of known magnitude Figure 14.5 shows the results as a

function of the nominal number of wetland volumes of water

that have passed through the system If the flow were plug

flow, then the retention is 100% until the nominal detention

time is reached, after which the new event water begins to

exit For less ideal flow conditions, the amount of the new

event water that is held is less

The presumption of Figure 14.5 is that all of the wetland

water is involved in flow That can be far from true because of

wetland design factors (see Appendix B) For instance, if there

is a point inlet to the system, and a point outlet aligned directly

opposite, then the incoming pulse of water can travel directly

to the outlet, never reaching zones to the side of the most direct

flow path That renders the corner areas essentially out of the

flow path and reduces the volumetric efficiency quite

mark-edly (Agunwamba, 2006) Walker (1998) investigated this

sit-uation using computational fluid mechanics (simulations) of

unvegetated FWS basins He determined that aspect ratio

(L:W) was the major determinant of such shortcircuiting and calculated a retention-displacement chart (Figure 14.6).For individual, separated inflow events, the concept of retained volume provides a strong correlating factor for per-formance If the event is small, it is wholly contained and held until the next displacing flow Treatment proceeds in the batch mode for the interevent duration If the event is large, then it is processed by flow through at the detention time of the event flow Because such flows are typically large, the wetland will have a short detention time, which results in decreased treatment performance Therefore, mass reduc-tions decrease (exponentially) as the number of nominal dis-placements caused by the event increases

The storage and treatment potential for individual events may ultimately be linked to the sequence of events that are likely to occur over a long period of record The three required probability distributions are for the event duration, the event intensity, and the interevent duration (Wong and Somes, 1995) To further complicate matters, when the wet-land basin is “full,” water will be diverted away (bypassed) Thus, a wetland that drains quickly, i.e., one with short deten-tion time, will be in a position to detain more water upon refilling than a wetland that drains slowly Wong and Geiger (1997) examined the runoff patterns for Melbourne, Austra-lia, and concluded that the hydrologic effectiveness (the per-centage of runoff that is exposed to treatment) was inversely proportional to the wetland detention time

These factors all point to the existence of a set of mance determinants that go beyond those for continuous-flow treatment wetland systems Batch interevent rate coefficients and background concentrations are important The inter-nal flow patterns are also important as determinants of the concentration and timing of the flushed antecedent water The dynamics of water flow and storage of the wetland are important because they determine the volumetric constraints

perfor-on performance

0.0 0.2 0.4 0.6 0.8 1.0

Size of Event (Wetland Volumes)

Event-Driven Wetlands 547

Ultimately, the computed description of performance will

either be expressed as long-term mass removal or by dynamic

modeling that produces time series of outflow and effluent

concentrations in response to the inlet time series Most of

the stormwater wetland literature reports mass removals over

some period of record However, there are now

first-genera-tion dynamic models for some applicafirst-genera-tions For instance,

there is a flow and phosphorus model for application in south

Florida called the Dynamic Model for Stormwater Treatment

Areas (DMSTA) (Walker and Kadlec, 2005)

14.2 TECHNOLOGY STATUS

Data reporting falls into two general categories: event

time-series analyses, and summaries of removal efficiencies Two

concepts are needed to organize these bodies of information:

event mean concentrations and mass balancing Because the

concentration pulses and flow pulses are often out of sync,

the event mean concentration (EMC) is used:

V

 33

where

C concentration in a water parcel, g/m = m3 gg/L

water volume in the parcel, m3

In effect, the EMC is the mass average concentration over

the course of an event and may be calculated for both inlet

and outlet flows of various types for the wetland If only the

directed inflow and outflow are considered, the EMC

The outflow from the wetland corresponding to a given

inflow event may total less than the inflow due to infiltration

and evapotranspiration The mass load reduction for a given pollutant is

land-of urban runland-off control is focused on highway runland-off (Shutes

et al., 2001; Bulc and Sajn Slak, 2003; Pontier et al., 2003)

However, the greater number of systems are used to treat rainwater runoff from more generalized urban areas Here the focus is upon gravity-driven constructed systems, as opposed

to pumped or natural systems Such systems are often quite small, serving small catchments (Figure 14.7)

In this section, attention is on gravity-fed, constructed systems treating urban stormwater Pumped systems have

been included in other summaries, such as Carleton et al (2001) and Wong et al (1999), but many of these do not

reflect the random and episodic nature of wetlands ing rain-driven events and are omitted here, as are natural wetland systems A sampling of TSS performance is given in Table 14.5, in which it is clear that there is a wide spectrum

receiv-of performances for all common constituents, with median removals far from 100%

Removal percentages are insufficient for design poses, and hence it is necessary to find relationships between system characteristics and performance measures Carleton

pur-et al (2001) concluded that long-term pollutant removals

could be described in terms of the same kinds of first-order, steady-flow design equations currently employed for waste-water treatment wetlands They present rate coefficients that

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Size of Event (Wetland Volumes)

Plug Flow L:W = 8 L:W = 4 L:W = 2 L:W = 1 L:W = 0.5

FIGURE 14.6 Fraction of an inflow event contained in a wetland as a function of the event volume and the aspect ratio (L:W) (From Walker

(1998) Ecological Engineering 10(3): 247–262 Reprinted with permission.)

548 Treatment Wetlands

are intended to be used in a procedure such as that described

by Wong and Geiger (1997) and Wong et al (2006) As

dis-cussed in Chapter 6, that method has inherent drawbacks,

and calibrations for event-driven systems provide only

cen-tral tendencies Additionally, rate coefficients derived from

single events may not be adequate for a time series of

meteo-rologically driven flows

Data sets analyzed by Strecker et al (1992) and extended

by Carleton et al (2001) include about half natural wetland systems that receive stormwater Strecker et al (1992) in fact

found that natural wetlands performed somewhat better than constructed wetlands, but the use of natural wetlands is not encouraged because of real and perceived negative impacts (U.S EPA, 1993e) The median areal rate coefficients reported

FIGURE 14.7 (A color version of this figure follows page 550 ) An urban stormwater wetland in the city of Blue Mountains, Australia.

TABLE 14.5

Suspended Solids Reduction in Constructed Urban Runoff Treatment Wetlands

Name Location Reference WWAR Area Ratio

(%)

HLR (cm/d)

Reduction (%)

Note: All are FWS except for Slovenia, which is HSSF.

Event-Driven Wetlands 549

by Carleton et al (2001) for FWS constructed gravity-

fed systems (N  9) are

Total Phosphorus: 8.3 m/yr

Ammonia: 5.0 m/yr

Nitrate: 6.7 m/yr

These are very low compared to the k-values reported for

continuous-flow systems in the preceding chapters

Individ-ual events are afforded much better treatment

Carleton et al (2001) also suggest that

wetland-to-water-shed area ratio (WWAR) may be used as a predictor of

per-formance Their regression equations are developed for small

data sets, including natural wetland systems, and have low

correlation coefficients (R2y 0.15–0.45) Within the range 0 

WWAR  0.1, the data scatter was particularly large

Regres-sions of the data in Table 14.5 are unsatisfactory; in general,

the fractions remaining increase slightly with WWAR

Duncan (1998; as referenced and reported by Wong et al.,

1999) suggested the use of regression equations for purposes

of sizing Relations are based on analysis of subsets of 76

stormwater wetland systems, presumably including pumped

systems, inclusive of those in the United States and

comple-mented by systems in Australia The proposed empirically

derived relations are

hydraulic loa

 dding rate, m/yr

The range of loading rate data for this regression is

approxi-mately 2  q  200 m/yr, and the percents remaining ranged

from 10 to 100%; about 30 systems were involved in each

regression

The acquisition of knowledge about wetland treatment of

agricultural runoff has often been spurred by regional water

quality problems as well as by the slow growth of experiences

on small-scale systems Nutrients from agriculture have been

implicated in eutrophication of marine environments

(nitro-gen) and freshwater (phosphorus) environments

Agricul-tural runoff is episodic by nature, and there is an extensive

literature on predicting the amounts of water and particulate

matter that leaves the fields However, fertilization practices

dictate the availability of nutrients to be washed off the fields,

and hence the patterns of loading are different from those in

urban or industrial settings Non-point source (NPS)

pollu-tion from agriculture may occur when nutrients are applied at

1991; Tanner et al., 2003), to small-order streams (Stone

et al., 2003), to large regional landscape units of thousands

of hectares (Reilly et al., 2000) Some of these receive

pumped water at relatively constant rates, and these have been included under the discussion of continuous flow (but possibly variable flow) wetlands Those that receive water as

a result of meteorological events are considered here

Target: Nitrogen

Nitrogen compounds are among the principal pollutants of concern in fresh and marine waters because of their role in eutrophication, their effect on the oxygen content of receiv-ing waters, and their potential toxicity to aquatic invertebrate and vertebrate species For example, the nitrogen content

of the streams and rivers of the midwestern United States

is of particular importance at this point in history because

of hypoxia in the Gulf of Mexico, together with the ciated ecological and economic consequences (Diaz and Solow, 1999) Both point and nonpoint sources contribute to the nitrogen content of waters within the Mississippi River drainage basin About 60% of the waterborne total nitrogen

asso-is in the form of nitrate (Goolsby and Battaglin, 2000).The source of the nitrogen is about two thirds from agri-culture and one third from other sources, including urban runoff, atmospheric deposition, and point sources The result

is a surface-water nitrate concentration, in the upper portions

of the basin, of about 4 mgN/L

Federal and state officials have agreed on an action plan that includes a component intended to promote restoration and enhancement of natural systems for nitrogen retention and denitrification (U.S EPA, 2001a) The premier natural system that has the capability to effectively remove nitrate from surface water is the free water surface wetland, based simply on its ability to place contaminated water in intimate contact with the biogeochemical cycle that removes nitrogen More than half of the presettlement wetlands in the upper Mississippi River basin have been lost to drainage (Dahl, 1990) It is therefore synergistic to restore wetlands that are positioned to effectively function for nitrate reduction Con-siderable attention has therefore been devoted to constructed and restored wetlands for nitrogen control in the midwest-

ern United States (Kovacic et al., 2000; Hey, 2002; Mitsch

et al., 2005; Hey et al., 2005; Kadlec, 2005b) Similarly, the

Chesapeake Bay in the eastern United States is impacted by

agricultural runoff from farms in Maryland (Whigham et al., 1999; Jordan et al., 1999) Studies on seven constructed

550 Treatment Wetlands(restored) wetlands demonstrated that nutrients could be

removed from field runoff Wetland systems have also been

implemented for runoff control from other row crops, such as

potatoes (Wengrzynek and Terrell, 1990; U.S Department of

Agriculture, 1991; Higgins et al., 1993).

The problem of excessive nitrogen runoff has also been

identified as detrimental to the Baltic Sea The role of

wet-lands in southern Sweden has been thoroughly explored

Data were acquired on eight constructed wetlands and used

to extrapolate the effects of 40 wetlands of total area 92 ha

on the runoff from a 22,400-ha catchment (Arheimer and

Wittgren, 1994; 2002) In Finland, retention performance of

constructed wetlands has been studied at four agricultural

study sites (Puustinen and Koskiaho, 2003) In Norway,

field studies at five sites were conducted over several years,

focusing on nutrients and sediments (Braskerud et al., 2000;

Braskerud, 2001b; 2002a; 2002b; 2003)

Rural settings often contain pastures, which form a

source of nutrients and TSS to the drainage waters FWS

wetlands are a convenient method of intercepting some of

the nutrient loads, and have been implemented in Australia

(Raisin et al., 1997) and New Zealand (Tanner et al., 2003;

Tanner et al., 2005b) Typical on-farm systems are not large

(Figure 14.8)

Target: Phosphorus

Although phosphorus has also been studied in the various

small-scale studies discussed previously, much of the research

on agricultural phosphorus removal in treatment wetlands has

originated from the Everglades protection projects of South

Florida The receiving Everglades ecosystem is conditioned

by very low total phosphorus, in the range of 6–12 µg/L

Even modest amounts of field runoff phosphorus pose a threat

to that ecosystem (Davis, 1994) Tests were performed on

dozens of mesocosm tanks, thirty 0.2-ha test cells, and four

2-ha field scale wetlands In parallel, a 1,500-ha, five-cell

wetland, the Everglades Nutrient Removal Project (ENRP) was operated as a research device These FWS systems vari-ously contained emergent vegetation, submerged plants, or algae, and testing was conducted over the period 1994–2005

A summary of results is given by Walker and Kadlec (2005) Details of ENRP results were presented in a special issue of

Ecological Engineering (Vol 27, Issue 4, 2006) Although

these studies were directed toward the design of pumped event-driven systems, the trials were in general conducted

at conditions of steady flow Plans to study event operation were abandoned because the full-scale wetlands were placed

in operation and became the study platforms

There are presently six Stormwater Treatment Areas (STAs) in operation, aggregating over 16,500 ha and treat-ing an annual average flow of 4.4 r 106 m3/d (annual average HLR  2.7 cm/d) These FWS wetlands have been in opera-tion for periods of two to eight years and receive episodic deliveries of pumped inflows, occasioned by the flat topog-raphy of South Florida An example of the flows and total phosphorus concentrations over the life of STA6 is shown

inFigure 14.9 Summaries of performance may be found in annual reports (e.g., SFWMD, 2006)

The stormwater runoff flows and quality from “industrial” facilities cannot be generalized because those facilities are quite different in the chemicals that find their way into run-off A broad definition of industrial is taken here, excluding urban and agricultural runoff This category includes inor-ganic sources of nitrogen, such as nitrates and ammonia from fertilizer manufacturing plants (TCI, 2005), and urea from

airport de-icing (Thoren et al., 2004).

The Simplot engineered FWS wetland (TCI, 2005) was designed to intercept, retain, and treat previously uncontrolled, meteorologically driven, nitrogen-contaminated groundwater forced to the surface in wet weather conditions and otherwise

FIGURE 14.8 A constructed FWS wetland system treating tile drainage from a pasture in Toenepi, New Zealand Wetlands are incised to

capture tile drainage by gravity.

COLOR FiguRe 1.13  Single-home HSSF wetland in Comfort Lake, Minnesota.

COLOR FiguRe 3.33  Trees growing in the Vermontville, Michigan, constructed treatment wetland after 15 years of operation.

COLOR FiguRe 7.10  Venting groundwater at this Wellsville, New York, site contains iron, which oxidizes upon contact with air.

COLOR FiguRe 11.6  This HSSF wetland outlet structure at Tamarack, Minnesota, has become coated with elemental sulfur.

COLOR FiguRe 22.7  Wetland EW3 at the Des Plaines River site near Wadsworth, Illinois. The fringe zone is vegetated by cattails and 

bulrushes, whereas the interior has submerged aquatic vegetation and floating-leaved plants. Open water shows as dark areas on this false  color infrared photo.

COLOR FiguRe 14.7  An urban stormwater wetland in the city of Blue Mountains, Australia.

COLOR FiguRe B.8 

Progress of Rhodamine WT dye tracer through a FWS wetland, cell 4 of the ENRP in Florida. The dye was intro-duced along the upper boundary, and flow proceeds from the top to the bottom. Note the short-circuit along the left-hand side, which is  partially redistributed by the central cross canal. (Photo courtesy of T. DeBusk.)

COLOR FiguRe B.22 

Rhodamine dye dispersing through an inlet deep zone, with a spikerush band providing resistance to shortcircuit-ing. (From CH2M Hill (2003a) PSTA Research and Demonstration Project, Phases 1, 2, and 3 Summary Report, Prepared for the South 

Florida Water Management District.)

Event-Driven Wetlands 551

contributing nutrients to the Assiniboine River A 4-cell, 22-ha

constructed wetland was implemented in Autumn 2003, near

Brandon, Manitoba Approximately 100,000 m3 of nutrient-

rich water entered the wetland in 2004, over the

April-through-November operating period, corresponding to an annualized

HLR of 0.19 cm/d (0.28 cm/d instantaneous) Flows varied by

a factor of ten, depending upon rainfall Inlet concentrations

were 400 mg/L total nitrogen (TN) and 165 mg/L ammonia

nitrogen, and were reduced by 39% for total nitrogen and

82% for ammonia The system was zero discharge in 2004,

losing 460 mm of water This corresponds to design net ET of

500 mm in a typical eight-month operating period

The 18-ha FWS Kalmar Dämme wetland was built near

Kalmar Airport in southeast Sweden (Thoren et al., 2004)

Urea was used as the runway de-icing agent, and annually

contributed about 41 metric tons of total nitrogen to receiving

waters during 1998–2001 Airport runoff joined agricultural

runoff from a 48-km2 catchment The wetland removed 17%

of the TN load from all sources, but removal of urea was mated to be 38%, based on source allocation Winter remov-als were much lower than summer

esti-These two examples serve to illustrate the applicability

of constructed wetlands to controlling nitrogen from sources other than municipal and animal wastewaters, or urban and agricultural runoff Both represent high nitrogen loadings, which were treated to a good degree

0 200,000 400,000 600,000 800,000 1,000,000 1,200,000 1,400,000

May 1999

May 2000

May 2001

May 2002

May 2003

May 2004

May 2005

May 2006

3 /d)

Inflow Outflow

(a)

FIGURE 14.9 Flows (a) and TP concentrations (b) entering and leaving STA6 in Florida The average inflow rate was 161,000 m3 /d; the average outflow was 103,000 m 3 /d The arithmetic mean inlet TP concentration was 58 µg/L, and 25.2 µg/L at the outlet.

0 50 100 150 200 250 300

May 1999

May 2000

May 2001

May 2002

May 2003

May 2004

May 2005

May 2006

Inlet Outlet

(b)

552 Treatment Wetlands

(Behrends et al., 2001) However, if the holding time is long,

then the mode would be termed sequential batch

opera-tion The water may be simply held without flow during

the batch time, or it may be continuously recycled (Sikora

et al., 1995a) Therefore, like the interevent period of

storm-driven wetlands, batch operation is an option for treatment

wetlands The operational advantage is that water may be

held until a specific water quality objective is met The

disad-vantage is that some form of storage is necessary if the batch

mode is to be used to treat continuous flows

Although it is often assumed that time in a batch system

is equivalent to transit time in a continuous-flow system, this

is a perilous assumption for wetlands There are several

rea-sons why these are not equivalent

Hydraulics Continuous-flow systems display a

distribution of detention times, with different

ele-ments of water spending different lengths of time in

the flowing wetland In a batch system that is filled

and drained very quickly, every element of water

spends exactly the same time in the ecosystem

The result is that the batch system in effect reflects

a circumstance strongly akin to plug flow

Con-sequently, only the distribution of reaction rate

coefficients attributable to mixtures will affect the

apparent rate of disappearance of a lumped

pollut-ant category (see Chapter 2)

Sorption In continuous-flow systems, the substrate

is exposed to a relatively constant water phase

con-taminant concentration, which causes the sorption

capacity to be equilibrated and plays no further

role In a batch system, the sorbed pollutants may

be used up by microbial and vegetative processes,

along with the water-phase materials Therefore,

at the end of a batch, which is also the beginning

of the next, the sorption sites may be empty and

capable of immediate removal of pollutants to

sorption storage This phenomenon was reported

by Sikora et al (1995a).

Vegetation The environment for vegetation is

different in the batch mode, with diffusion and

transpiration flows providing the transfer of

dis-solved substances, such as oxygen and nutrients

In the flow mode, advective processes are present

and may dominate Stein et al (2003) conducted

microcosm studies on batch-loaded planted gravel

bed systems and compared them to

accompany-ing flow through systems There were differences

attributable to plants, including more pronounced

seasonal effects and larger differences among

plant species in the batch mode The result was

superior nitrogen and phosphorus removal in the

batch mode in all seasons and from all conditions

studied

Microbes Microbial populations respond to

their environment and evolve accordingly over a

period of time In a flow through system, there are

in a batch system undergo a transient in their ulation and relative abundance, i.e., they are in a growth phase

pop-Despite these differences in site characteristics, there is typically a near-exponential decline with time of pollutant concentrations in batch wetlands, often with a residual con-

centration This is seen at the microcosm level (e.g., Gale

et al., 1993; Van Oostrom and Russell, 1994), the mesocosm

level (e.g., Burgoon, 1993), and at field scale (Lakhsman,

1981; Sikora et al., 1995a; Kadlec, 2001a) Declines in taminant concentration are well described by the k-C* model

con-using the exponential decay because of the lack of detention time distribution (DTD) effects (Figure 14.10) However, the rate coefficients in batch systems may differ considerably from those determined under flow through conditions For instance, Burgoon (1993) found a factor of two difference, with higher rate coefficients for CBOD for batch operation Kadlec (2001a) also found higher values for phosphorus

k-values for batch operation.

Experiences on full-scale batch systems are limited Studies were conducted on three batch systems at Humboldt, Saskatchewan, in which lagoon waters were treated in wet-lands utilizing a fill-hold-drain strategy (Lakhsman, 1982)

In general, BOD was reduced from 90 mg/L to below 20 mg/L in four days, total Kjeldahl nitrogen (TKN) from 20

to below 3 mg/L in 20 days, and phosphorus from 12 to below 3 mg/L in 20 days Given an operating depth of 0.3 m,

0.0 0.2 0.4 0.6 0.8 1.0

FIGURE 14.10 Decline of pollutant concentrations in batch

sys-tems Ammonia data is from FWS microcosms of Gale et al (1993)

Phosphorus data is from an FWS field-scale wetland of Kadlec (2001a) CBOD data is from SSF mesocosms of Burgoon (1993) All

fit lines are for the k-C* model using an exponential decay.