TREATMENT WETLANDS - CHAPTER 23 pptx

Because wetland systems are constructed using local labor and local materials, it is not possible to offer precise universal cost estimates that will apply to all treatment sys-tems.. To

HSSF wetlands with a median design flow of 9.5 m3/d, and

566 VF wetlands with a median design flow of only 2.1 m3/d

(Wallace and Knight, 2006) System areas from the WERF

database show 330 FWS wetlands with a median area of 1.6

ha (16,000 m2), 710 horizontal subsurface flow (HSSF)

wet-lands with a median area of 140 m2, and 544 vertical flow

(VF) wetlands with a median area of 44 m2 Therefore, SSF

wetlands do not enjoy the economy of scale experienced by

FWS wetlands There is now enough information to

deter-mine approximate capital cost functions that represent these

scale effects

Because wetland systems are constructed using local

labor and local materials, it is not possible to offer precise

universal cost estimates that will apply to all treatment

sys-tems Generally, the basic components of a wetland treatment

system—earthwork, gravel (in the case of SSF wetlands),

lin-ers, and plants—are produced in regional markets that are

distance sensitive For instance, the installed cost per cubic

meter of gravel is highly dependent on the distance between

the source of supply (a local gravel pit) and the site of

wet-land construction Labor costs are also highly variable To

assess the feasibility of a wetland treatment system, local

cost figures should be used to compare the capital and

oper-ating costs of a wetland system against that of other treatment

technologies

Within the United States, Construction Cost Indices

(CCI) are published by the Engineering News Record (ENR)

These cost indices track inflationary changes within the

con-struction industry over time The ENR CCI started at 100 in

the year 1913 and has increased to 7,856 in March 2007 Cost

indices are available as a national average, but are also

pub-lished for 20 major metropolitan areas in the United States

For the purposes of this chapter, costs are based on the 2006

United States national average ENR CCI of 7,751 As a side

note, construction in high-cost metropolitan areas is almost

double that of low-cost metropolitan areas

In general, capital costs of treatment wetlands are

comparable to alternative technologies for accomplishing

the same task However, the costs of operating a treatment

passive systems Energy costs are typically close to zero for gravity-driven FWS wetlands, and are generally low for all types of treatment wetlands (see Table 1.1) Water quality monitoring is often a principal part of O&M costs However, operation and maintenance (O&M) costs can become appre-ciable if it is attempted to maintain specific vegetation types, thus encountering “weeding” costs

23.1 CAPITAL COSTS

Although it is not possible to offer universal cost guidelines, every system shares a similar set of construction compo-nents Therefore, it is possible to estimate the cost of each component within a regional market The basic direct cost components of a wetland treatment system include:

LandSite investigation and system designEarthwork

LinersMediaPlantsWater control structures and pipingSite work (site preparation, fencing, access roads, etc.)

Human use facilitiesThese costs include material, labor, overhead, and profit, and represent the contractors installed cost Additionally, there are indirect costs associated with permitting, engineering, financing, mobilization, and construction management In general, these costs are all incurred prior to system start-up Detailed estimates are usually made after final sizing and sit-ing More precise economic estimating is possible after final design drawings have been prepared

and other design features The cost of labor and materials

within a particular regional market plays a large role in the

cost of a wetland treatment system Cost differentials are

even greater when comparing across worldwide geographic

locations and their accompanying economies The effect

of regional market cost factors is illustrated in Figure 23.1,

which demonstrates treatment wetland capital cost

distri-butions for various locations HSSF wetland systems in

Poland (Kowalik and Obarska-Pempkowiak, 1998) exhibit

lower capital costs than those in Nicaragua (Platzer et al.,

2002) or the Czech Republic (Vymazal, 1996) Severn Trent

tertiary systems in the United Kingdom (Green and Upton,

1994) are more expensive than those in the Czech

Repub-lic; HSSF systems in the United States have a distribution

of capital costs that spans the range from the inexpensive

Czech systems to those that are more expensive than the

U.K systems

Some of the regional differences have to do with design

sizing criteria Different criteria are used in different

coun-tries, and also at different times in the same country Others

have to do with structural specifications For instance, a

num-ber of U.K systems are sited in basins lined with brick or

concrete, and some have stainless steel level and flow

con-trol elements Clearly, such systems will have greater capital

costs than those sited in earthen basins with plastic piping

and simplified flow control elements

There are also economies of scale, which will be

addressed in a subsequent section However, it is useful to

first examine the various components of capital costs in more

detail

Cost ($1000 USD/ha)

FIGURE 23.1 Capital costs distribution for treatment wetlands All are HSSF systems except FWS (United States) Costs adjusted to 2006

using CCI  7,751 and the 2006 exchange rate Data for Poland from Kowalik and Obarska-Pempkowiak (1998) In Constructed Wetlands

for Wastewater Treatment in Europe Vymazal et al (Eds.), Backhuys Publishers, Leiden, The Netherlands, pp 217–225 Data for the Czech Republic from Vymazal (1996) Ecological Engineering 7: 1–14 Data for Severn Trent from Green and Upton (1994) Water Environment Research 66(3): 188–192 Data for Central America from Platzer et al (2002) Investigations and Experiences with Subsurface Flow Con- structed Wetlands in Nicaragua, Central America Mbwette (Ed.) Proceedings of the 8th International Conference on Wetland Systems

for Water Pollution Control, 16–19 September 2002, Comprint International Limited: University of Dar Es Salaam, Tanzania, pp 350–365 Data for the United States: Various.

Land

Land costs are highly site specific Information on land ability and land costs is generally obtained with the assis-tance of real estate professionals who are familiar with local market conditions Wetlands are more land intensive than many other wastewater treatment processes If land needs to

avail-be purchased for the project, this can avail-be a significant cost component In the United States, treatment wetland land purchase prices have ranged from $3,000/ha in remote loca-tions with low population density and low agricultural utility

to over $100,000/ha in urbanizing agricultural landscapes Land acquisition in or near urban areas is sometimes viewed

as preservation of green space, and valued for ancillary benefits

Land costs can be a significant fraction of the total tal cost For example, Wossink and Hunt (2003) identify three categories of land cost for urban stormwater wetlands

capi-in North Carolcapi-ina: zero cost because the project is required for community green space, $125,000/ha for vacant land that may be used for residential development, and $550,000/ha for land that may be used for commercial development

As will be discussed, land costs occupy a special role in the present worth analysis of a wetland system

Site Evaluation

The design and construction of a wetland requires that site characteristics be well understood—including soils,

cells, and the need for long or short distribution and

collec-tion canals, which for large systems can influence the project

cost The potential need to level cell bottoms for purposes

of evening the hydraulic flow distribution can only be

evalu-ated with detailed site topography The shallow water depths

in wetlands, especially large FWS systems, creates a need

for accurate as-built topography as an aid to understanding

water depth and movement The costs for such surveys are

typically $50/ha–$500/ha, depending on scale and grid size

requirements For instance, the topographic survey for the

Incline Village, Nevada, treatment wetland cost $370/ha for

the 175-ha wetland, adjusted to 2006 However, the survey

work for cell 4 of STA2 of the Everglades phosphorus control

project required only $45/ha

Geotechnical Investigations

Many small-scale wetland projects are designed in

conjunc-tion with soil infiltraconjunc-tion systems In these situaconjunc-tions, it is

common to use shallow soil borings or backhoe pits to

deter-mine local soil characteristics Costs for these initial site

investigations vary with the size of the project In the U.S.,

the cost for a site investigation can range from a few hundred

dollars (for a single-home system) up to several thousand

dol-lars (for a system serving a small community)

Larger-scale projects, including those that discharge to

surface waters, also require soil investigations Even though

such large projects rarely require liners, there is a need to

assess the potential for seepage from the project, and hence

the need for seepage collection canals It is critical to

deter-mine if site soils are adequate for the berms and levees, and

if so how much compaction may be required, and how much

allowance for subsidence

Soils that may be used for rooting media in FWS systems,

as well as the gravel substrate for SSF systems, may need to

be assessed for contaminants of concern, including nutrients

A modest amount of chemical testing may be required to

identify any potential problems, or to form the basis for

fore-casting the sorption life expectancy for contaminants

Hydrogeological Investigations

The location of the groundwater table and direction of

groundwater movement can be a critical factor in wetland

ated cost, will be determined through site-specific logical investigations

hydrogeo-In some cases, there may be a concern for regional use

of the groundwater as a potable water supply The treatment wetland might be viewed as a possible source of contamina-tion if partially treated water entered into the unprotected drinking water aquifer, and moved to the wells that withdraw potable water This would usually occasion the need for a hydrogeological study to ascertain the depths and directions

of regional groundwater flow, and the consequences of even small leaks from the treatment wetland to the water quality of the aquifer This was the case at the Columbia, Missouri, proj-ect The hydrogeological study included calibration and mod-eling to address this issue, at a cost of $750/ha for the 36-ha

wetland, adjusted to 2006 (Brunner et al., 1992).

Earthwork

The construction of a wetland treatment system requires excavation and grading of the site to produce level basins that are enclosed by earthen berms For small systems (generally less than 0.05 ha) backhoes or similar types of excavation equipment are commonly used Larger basins are generally constructed using bulldozers or construction scrapers Area-specific earthwork costs are the product of two components The first component is the cost to move earth, which is a volumetric (per cubic meter) cost This cost component is

a function of equipment costs, labor costs, and the source

of earth supply (on-site or imported) The second nent is the amount of earth that must be moved to grade the site This areal requirement (m3of earth per m2of wetland area) is a function of the project site conditions Earthwork costs are lowest on flat sites that require minimal grading and have suitable soils on-site (low areal grading require-ment and volumetric cost) Sloping a site requires terracing, which increases earthwork costs (due to the large areal grad-ing requirement) If the fill needs to be imported, earthwork costs will increase (due to the high volumetric cost) Earth-work construction techniques are essentially the same for FWS and HSSF wetlands, and hence volumetric earthwork costs for the two types should be comparable for similar-sized wetlands

compo-Clearing and Grubbing

If the undeveloped site has undesirable vegetation,

build-ings, or other existing features that are incompatible with

the wetland, these will need to be removed as part of the

earthwork process Brush and trees were removed from the

sites at Ouray, Colorado; Sorrento, Louisiana; and West

Jack-son County, Mississippi, at an average cost of $9,800/ha On

larger projects, roads and ditches may require degrading, and

buildings may require removal

FWS Wetlands

In a study of two municipal FWS wetland systems (West

Jackson County, Mississippi and Gustine, California) the

U.S Environmental Protection Agency (EPA) estimated a

volumetric cost of $10.80/m3 when adjusted to 2006 USD

(U.S EPA, 2000a) The 20.2-ha wetland system in West

Jackson County, Mississippi, had an areal grading

require-ment of only 0.26 m3/m2, and the 9.7-ha system in Gustine,

California, had an areal grading requirement of 0.35 m3/m2

Larger treatment wetlands require far less earthmoving

on a per-hectare basis than do small systems For instance,

Cell 4 of STA2 of the Everglades phosphorus control

proj-ect required only 0.056 m3/m2 of earthmoving based on the

816-ha wetland footprint However, the construction of such

a large system does not involve scraping a thin layer from the

entire footprint Rather, the inlet spreader canal, outlet

col-lection canal, and seepage return canal are the source of the

fill material for the containment levees

Even large wetlands may require significant

earthmov-ing if built on slopearthmov-ing terrain For example, the Inman Road

treatment wetland in Clayton County, Georgia, has 22 wetted

hectares in a terraced arrangement of 22 cells The site

pre-sented slopes of 2–25%, and about 30 m of vertical variation

(Inman et al., 2001, 2003) Construction required moving

420,000 m3 of earth, or 1.9 m3/m2

Regional factors and site conditions can alter the cost of

earthmoving Cell 4 of STA2 of the Everglades phosphorus

control project moved 460,000 m3 of material, in a nearly

balanced cut and fill The breakdown of per cubic meter costs

was: $2.61 to blast rock, $2.13 to excavate, and $2.09 to build levees, for a total of $7.96/m3(adjusted to 2006 USD)

HSSF Wetlands

Earthwork costs for some HSSF wetland systems in the Minnesota–Wisconsin regional market are summarized in Table 23.1 Data from these seven HSSF wetland systems clearly show the impact of local site conditions on earthwork costs Areal grading requirements varied from 0.21 m3/m2 to 1.73 m3/m2, with a median value of 1.03 m3/m2

Volumetric earthwork costs for the systems in Table 23.1 varied between $2.17/m3 and $18.15/m3, with a median value

of $7.56/m3 (cost adjusted to 2006 USD, ENR CCI  7751) Combining areal grading requirements with volumetric earthwork costs resulted in areal earthwork costs ranging between $1.68/m2 and $13.97/m2, with a median value of

$5.06/m2 (adjusted to 2006 USD)

Liners

The decision to install a liner in a wetland system, and which type of liner to use, depends on the project goals, regulatory requirements, and feasibility Some wetlands are unlined,

either because the in situ native soils are deemed to have

suf-ficient sealing properties, or because groundwater recharge is

a function of the system Very large FWS wetlands cannot be plastic lined because it is not feasible for systems of more than

a few hectares, but clay lining has been implemented on tems up to 40 ha, such as Columbia, Missouri If the wetland must be lined, there are a variety of liner materials available The two most common liner materials used are 0.76-mm polyvinyl chloride (PVC) and 1.0-mm high-density poly-ethylene (HDPE) PVC liners are generally factory seamed, one-piece liners used on small projects less than 0.1 ha

sys-in size HDPE lsys-iners generally come sys-in rolls and are field seamed for larger projects

The total installed cost of the liner includes not only the

material cost but also the labor cost associated with field seaming, seam testing, material inspection, and leak test-ing If local soil conditions include sharp or angular rocks,

a layer of sand or other granular material may have to be

placed before the liner can be installed If the gravel used

to line the bed has sharp or angular edges, it may be

neces-sary to cover the interior of the liner with a protective layer

of geotextile fabric, since sharp rock can puncture a liner

Table 23.2 shows approximate installed costs of a variety of

liner materials

Liner costs for the FWS wetland system at Ouray,

Colo-rado, were $6.97/m2, and $13.23/m2 for the HSSF wetland

sys-tem at Ten Stones, Vermont (both adjusted to 2006 USD) (U.S

EPA, 2000a) Liner and geotextile fabric costs for 12 HSSF

wetlands in the Minnesota–Wisconsin regional market are

Media and Mulch

The major cost variation between FWS and SSF wetlands results from differences in the rooting media FWS systems typically use 30–40 cm of soil, which may be entirely avail-able on the site, whereas SSF wetlands use 50–80 cm of gravel or other similar media, which must usually be pur-chased and transported to the site

Soils for FWS Systems

If in situ native soils are suitable, they are typically used as

the rooting substrate for emergent wetland plants in FWS

systems If in situ native soils are unsuitable for use as

root-ing media, soil may be imported and mixed with existroot-ing soil to create conditions adequate for rooting wetland plants For these reasons, the cost of the rooting medium is usu-ally reflected in the volumetric earthwork costs However,

it is sometimes advantageous to add organic material to the rooting soil for a FWS system, to provide sorption capac-ity immediately upon start-up (Figure 23.2) Such a blending

a Assumes $25/m 3 delivered, placed, and compacted.

Source: Data from U.S EPA (2000a) Constructed wetlands treatment of

municipal wastewaters EPA 625/R-99/010, U.S EPA Office of Research

and Development: Washington D.C.; and Interstate Technology and

Regula-tory Council (2003) Technical and RegulaRegula-tory Guidance Document for

Con-structed Treatment Wetlands http://www.itrcweb.org/WTLND-1.pdf.

TABLE 23.3 Liner and Geotextile Costs for Example HSSF Wetlands System Name

Design Flow (m 3 /d)

Wetland Area (m 2 )

Liner ($/m 2 )

Geotextile ($/m 2 )

Cambridge-Isanti School District, Minnesota 39 1,196 9.07 3.13

Note: Geotextile is a nonwoven, needle-punched polypropylene material (230 g/m2 fabric weight) used as a protective layer on top of the liner.

operation may involve approximately 10 cm of amendment

material, thus adding 0.1 m3/m2 of earthmoving to the

proj-ect, plus the cost of the composting material Municipal yard

waste compost has been successfully used at Isanti-Chisago,

Minnesota, and at Saginaw, Michigan, treatment wetlands

The use of purchased topsoil, either entirely or as an

amend-ment, is a very expensive option, because high-quality

(hor-ticultural) topsoil typically sells for up to $25/m3

Media for SSF Wetlands

A number of innovative types of media have been used in

SSF treatment wetlands Examples include recycled glass

fragments, such at Millersylvania State Park, Washington,

blast furnace slag (Mann and Bavor, 1993; Drizo et al., 1999),

and lightweight expanded clay aggregates (LECA) (Zhu et

al., 1997; Jenssen and Krogstad, 2003) The cost of LECA is

quite high; for instance, Scholz et al (2001) report $204/m3

(2006 costs), but it is available in large units (Figure 23.3)

The wide variety of SSF wetland media leads to wide tions in costs, but gravel is perhaps the most common mate-rial used in Europe and North America (Figure 23.4)

varia-In a HSSF wetland, the rooting medium comprises the material used in the main wetland bed (typically gravel) and the coarser material used in the inlet and outlet zones (typi-cally coarse rock) Costs for these materials are a function of regional market conditions as well as the distance between the source of supply and the project site For instance, the 4.0-

ha VF wetland at Connell, Washington, utilized 36,000 m3 of 2.6-mm coarse sand that was available immediately adjacent

to the bed, thus incurring only a cut-and-fill cost (Burgoon

et al., 1999) That situation is extremely rare, and most often

media will be mined, screened, washed, and transported to the wetland site at considerable cost

Media costs for 12 selected HSSF wetlands in the sota–Wisconsin regional market are summarized in Table 23.4 This region was glaciated and typically has ample sources of gravel within a reasonable distance of a project Data indicate that installed gravel media costs (used in the main portion

Minne-of the bed) range between $15.95/m3 and $70.26/m3, with a median cost of $41.87/m3 (adjusted to 2006 USD) The gravel used in these systems was 9–25 mm in size Within the Min-nesota–Wisconsin regional market, distances between the source of supply (local gravel pit) and the project site were the primary factor in determining unit costs As the depth of the bed media is established during the design process, cal-culation of areal costs is relatively straightforward All of the HSSF systems in Table 23.4 were designed with a bed depth

of 0.45 m, resulting in a median areal cost of $18.84/m2.Media costs presented in Table 23.4 are higher than those presented in the literature for other HSSF systems For three HSSF wetlands (Mesquite, Nevada; Carville, Louisiana; and Ten Stones, Vermont), U.S EPA (2000a) reported bed media costs of $14.33, $23.58, and $14.74/m2, respectively (2006 USD) These were deeper beds (0.8, 0.75, and 0.75 m, respec-tively), and the volumetric cost of the media was less

FIGURE 23.2 Organic materials may be added to the rooting soils to promote early sorption potential At the Hillsdale, Michigan, site,

soils were borrowed from an adjacent floodplain, creating a pond and island complex in the borrow site.

FIGURE 23.3 Light expanded clay aggregate (LECA) can be

pur-chased in large units in Europe.

In cold-climate regions, it may be necessary to insulate a

HSSF system This may be done with straw, or cover blankets

for small systems Mulch may be used as an insulating layer

for cold-climate wetlands, and is a common design feature

of HSSF wetlands in Canada and the northern regions of the

United States (Figure 23.5) All of the wetlands in Table 23.4

were insulated with 0.09 m of reed-sedge peat Installed costs

for this peat material (2006 USD) ranged between $23.94/m3

and $82.98/m3, with a median cost of $49.89/m3 This

con-verts to an areal cost of $4.49/m2

Coarse Stone

The berm slopes of FWS wetlands may be armored against

burrowing animals and wave erosion by the use of rip rap

Coarse rock is a common choice (see Figure 18.19), but

con-crete matting has also been used (see Figure 18.23)

HSSF wetlands commonly use a coarser material (drain rock) in the inlet and outlet portions of the bed (Figure 23.6)

In the United Kingdom, the stone inlet and outlet zones are typically 0.5 m wide, at full bed depth, and packed with 50–

200 mm stone (Cooper et al., 1996) Drain rock used in the

12 HSSF wetlands summarized in Table 23.4 was 20–75 mm

in size Installed drain rock costs ranged from $25.53/m3 to

$74.41/m3, with a median cost of $47.59/m3 (adjusted to 2005 USD) U.S EPA (2000a) reports that costs for outlet materials were $10.39/m3 (50-mm stone) and $24.24/m3 (100-mm stone) for the HSSF wetland at Ten Stones, Vermont (2006 USD)

Plants

The plant component of capital cost varies according to the method chosen for vegetation establishment It is presup-posed that the media is in place, either the soil layer in a FWS system or the gravel in an SSF system The FWS

FIGURE 23.4 Gravel media being placed at the Grand Lake, Minnesota, HSSF wetland.

TABLE 23.4

Media Costs for Selected HSSF Wetlands

System Name

Design Flow (m 3 /d)

Wetland Area (m 2 )

Gravel ($/m 3 )

Mulch ($/m 3 )

Coarse Stone ($/m 3 )

Cambridge-Isanti School District, Minnesota 39 1,196 46.22 50.30 44.87

wetland offers the largest number of options, including

natu-ral recruitment, seeding, and planting

Natural Recruitment

This option is the least costly but the least controllable,

and usually of the longest duration It is not free from cost,

because moist soil conditions must be maintained, which in

turn requires water management on the wetland Virtually

all of the Florida stormwater treatment wetlands (STAs) were

established in this way A newer strategy for these STAs is

the use of a seeded sacrificial rice cover crop (Oryza sativa),

which assists in soil stabilization and initial nutrient lization This cover crop lasts only one growing season, does not renew itself, and gives way to other wetland vegetation

immobi-Seeding

Seeding is the next least expensive method of vegetation establishment The techniques range from scattering in the wind (Figure 23.7), to back-pack broadcasting, to the use of seed drills The seed is then pressed into the soils using a roller or cultipacker, or by light raking The use of foot travel has also been used effectively, at the Isanti-Chisago site in

Minnesota (Loer et al., 1999) Volunteer labor has been used

successfully at Brighton, Ontario, and Roblin, Manitoba, where grades 9 and 10 science students participated in seed-ing (PFRA, 2002) Hydroseeding has been used successfully (U.S EPA, 2000a) but requires the addition of detergent to loosen the seed covering In the Great Lakes region of the United States, the optimum seeding time is autumn through late spring

Seeds for wetland plants may be harvested from local

sources as part of the project work, particularly for Typha,

which produces large seed heads with extremely numerous seeds, and which are easily picked in autumn Seeds are also available from wetland nurseries in the United States, for over

a hundred wetland species The cost is considerable, ranging from $125 to $1,500 per kilogram of live seeds for common wetland plants in the northern United States (Table 23.5) The seeding rate is typically on the order of 2–4 kg of live seed per hectare The purchased price of the seed is consequently

in the range of $400–$3,000 per hectare, with a median of about $1,200 per hectare The cost of installation includes the seeding itself (minor) and the fostering of germination and growth, via soil moisture/water management (major) Dry seeding has been estimated at ten man-hours per hectare, and hydroseeding at $145/ha (U.S Army Corps of Engineers, 2000) Such required activities are usually inexpensive, and are less than the cost of the seed itself

FIGURE 23.5 Peat mulch in place at the Grand Lake, Minnesota, wetland.

FIGURE 23.6 Inlet coarse rock zone at the Fish and Royer, Indiana,

HSSF wetland.

The choice to plant originates with the desire to propagate

a selected suite of plant species in a short period of time

Planting costs are a function of several different factors

These include the form of plant material (plugs or rootstock),

FIGURE 23.7 Seeding of the Brighton, Ontario, wetland (a) and the resulting growth (b) (Photos courtesy J Pries.)

TABLE 23.5

Examples of Prices for Plants and Seeds (2007 USD)

Individual Species

Emergent Wetland Mix

Stormwater Wetland Mix

the source of material (locally harvested, on-site nursery,

or commercial nursery), the method of planting (manual or mechanical, volunteer or, contractor labor), and the overall planting density (number of plants per square meter) Because

of these variables, planting costs vary widely among wetland

projects In projects that employ hand planting, plant

mate-rial costs may be a small fraction of the installed cost

Commercial nurseries sell a wide range of wetland plants

as potted “plugs” or bare root stock (Table 23.5) The median

(2007) cost of bare root propagules was $0.65 (n  14), and

for plugs, $0.93 (n  13) Transportation costs must be added

to these, leading to a delivered cost of about $1.20 per plant

It is sometimes possible to obtain plants from existing stands,

such as cattails from roadside ditches Extraction requires

mechanical digging, typically with a backhoe, to a depth of

about 30 cm, so as to gather the majority of the root mass The

plants are then separated from the mass by hand, with each

propagule containing at least one healthy shoot and at least 20

cm of associated rhizome The shoot is topped to a height of

30–40 cm, to reduce the amount of foliage that the transplant

must maintain after the trauma of transplanting On-site

nurs-eries may also be established prior to wetland construction, for

the purpose of supplying plant materials However, the

plant-ing contractor may prefer to avoid the effort needed to extract

and prepare the plants, and opt for the use of purchased plants,

as was the case at Columbia, Missouri (Brunner and Kadlec,

1993) Established treatment wetlands may serve as sources of

plant materials for new systems For example, the plants for

the Connell, Washington, system (Kadlec et al., 1997) were

extracted from the Arcata, California, project, and

success-fully established in the new wetland Marsh establishment

on the Texas coast required 11.3–29.3 man-hours per 1,000

plants to hand-dig, separate, and transplant various propagule

types of 11 marsh species (Dodd and Webb [1975], as

refer-enced by U.S Army Corps of Engineers [2000])

The total cost of plants depends on planting density,

which ranges over about 0.25–4.0 plants per square meter

(spacing of 0.5–2.0 m) If the plants are on 1-m centers, there

are 10,000 per hectare This value is used in Table 23.5 to

establish the per-hectare cost of nursery-purchased plant

plugs, which ranges from $8,000 to $31,000, with a median

of $9,250/ha

The cost of putting the plants into the ground can be extremely variable In some instances, innovation has been employed to quickly and efficiently plant a wetland with mechanical equipment (Figure 23.8) However, most small wetlands are planted by hand, with rates of up to 150 plants per person per hour for experienced planting crews Allowing for lesser efficiency of 50 plants per person per hour, thus requir-ing 200 man-hours per hectare at 10,000 plants per hectare This matches estimates provided by the U.S ACE publication (U.S Army Corps of Engineers, 2000) For labor at $20/h, insertion costs would be about $2,000/ha at 10,000 plants per hectare, or $0.40 per plant When combined with the per-plant purchase price, the cost would be $1.60 per plant installed.U.S EPA (2000a) suggests a planting cost for FWS wet-lands of $0.83 per plant installed (adjusted to 2006 USD) This may be a reasonable estimate for locally harvested plants, but is probably too low for nursery-purchased plants.Table 23.6 summarizes planting costs for HSSF wet- land systems in the Minnesota–Wisconsin, U.S regional market All of these systems were hand-planted with root-

stock (primarily Scirpus atrovirens, Scirpus fluviatilis, and

Scirpus cyperinus) purchased from commercial wetland

plant nurseries in Wisconsin All 12 systems were planted

at a density of 2.7 plants/per square meter This represents

an aggressive planting approach that uses well-established rootstock and is designed to achieve complete plant cover-age in about two growing seasons (which is approximately

18 weeks per year in this climatic region) The median of

$4.58 per plant installed, in Table 23.6, is much higher than the estimate mentioned previously The difference may

be attributable not only to labor costs and other regional market factors but also to the unfamiliarity of the contrac-tors with planting as a construction activity On an areal basis, Table 23.6 shows a median of $12.32/m2 In another adjacent climatic region, for the Minoa, New York, HSSF wetland, planting costs were $9.45/m2

For small to medium size wetlands (less than 10 ha), planting costs are less than 10% of the total capital cost This

FIGURE 23.8 Homemade planter used for the Hillsdale, Michigan, treatment wetland (a) The 1.4 ha of basins were quickly planted (b)

See Figure 19.6 for a view of the wetland one year later.

fraction applies to all regions, such as Spain (Nogueira et al.,

2006) For very large FWS wetlands, planting is too

expen-sive, and natural recruitment is relied on

Structures

Hydraulic control of wetlands is generally provided through

distribution piping and water level control structures Inlet

distribution, outlet collection, and water level control are

required in all systems

Many FWS systems operated with parallel paths, thus a

splitter structure is needed to apportion the flows among flow

paths Depending on the size of the system and the

complex-ity of the structure, costs will vary, but may be about $5,000–

$10,000 for a 1-ha wetland (Wallace and Knight, 2006) In

FWS wetlands, inlet distribution is normally accomplished

using an inlet deep zone across the width of the wetland, also

called an inlet spreader canal The cost for this is part of the

cut and fill for the overall project These deep zones may be

filled with coarse rock to discourage rodents, thus incurring

the cost of the rock

Sometimes a perforated pipe is placed at the bottom of the

deep zone, to aid in uniform introduction of the water The

cost of 10-cm diameter perforated PVC is roughly $20/m

In warm climates, distribution piping may be above ground

and above water (see Figure 18.23) Gated aluminum

irriga-tion piping costs on the order of $30/m for 30 cm diameter,

with lesser costs for smaller diameters The 30-cm size can

convey up to 15,000 m3/d without excessive headloss, and

smaller diameters may be used for smaller flow rates

Small HSSF wetlands may receive nearly raw waste-

water, which will necessitate some form of inlet trash

screen-ing In the United Kingdom, this may be a Copasac™

cham-ber with a filter bag; in other countries, a cleanable bar

screen is used If the water is delivered from a septic tank,

or other settlement chamber, then this screening device may

be unnecessary

In small- to mid-sized HSSF wetlands, flow is generally distributed across the width of the bed in the inlet zone using perforated pipes or infiltration chambers Perforated PVC pipe (100-mm diameter) is commonly used as a low-cost dis-tribution method for small systems The drawback of using perforated pipe is that the influent organic loading is concen-trated at the perforations Infiltration chambers (Figure 23.9) have a louvered sidewall which provides a greater number

aCosts in 2006 USD.

FIGURE 23.9 Placement of inlet distribution chamber at Lake

Elmo, Minnesota (From Wallace and Knight (2006) Small-scale constructed wetland treatment systems: Feasibility, design cri- teria, and O&M requirements Final Report, Project 01-CTS-5,

Water Environment Research Foundation (WERF): Alexandria, Virginia Reprinted with permission.)

of openings and a larger cross-sectional area, both of which

are desirable for evenly distributing the influent organic load

Generally, the same device (perforated pipe or infiltration

chamber) is used at the outlet end of the bed for effluent

col-lection Installed costs for perforated pipe and infiltration

chamber approaches in the northern United States are about

$20/m and $40/m, respectively (Wallace and Knight, 2006)

The water level within a wetland cell is regulated by a

downstream water level control structure A typical outlet

structure for small FWS wetlands is shown in Figure 18.27,

and an outlet for a moderately large system in Figure 18.28

The costs for typical low-flow level control structures are

given in Figure 23.10 A simple flexible pipe outlet level

con-trol structure for a HSSF wetland is shown in Figure 23.11

Some small systems use premanufactured “pond valves” that

utilize stacking plates (with a 100 mm inlet and outlet pipe) to

set the water level Costs for this approach are below $1,000

for flows under 500 m3/d (Wallace and Knight, 2006) Larger

projects may use precast or cast-in-place concrete structures

with a weir device (stop log, fixed plate, or adjustable weir

gate) The ability to adjust the water level is an important control element, and providing operational control over the water depth in the wetland is well worth the incremental cost over fixed-level structures

Large inlet and outlet structures for FWS systems can

be fairly complicated, with many cost components (see ure 18.22) These may involve remote sensing of water lev-els, sent to an operations center by telemetry, and remote control of opening and closing Therefore, there is a need for electrical power, and the associated control equipment The structure requires appropriate bedding, which usually means imported gravel The cost breakdown for an example

Fig-is given in Table 23.7 Despite the apparent complexity and cost, such structures are more economical when measured in terms of cost per unit flow rate The structure of Table 23.7 cost approximately $0.42 per m3/d, whereas the AgriDrain™ units cost about $0.71 per m3/d

Capacity (m 3 /d)

FIGURE 23.10 Example pricing of simple in-line stoplog structures The height of these contained units is variable; data are shown for

1.83 and 3.05 total structure height The capacity is for a height over the weir of 25 cm See Figure 18.26 (Data from Agri Drain tion, Adair, Iowa Date: March 30, 2007, CCI  7,856 http://www.agridrain.com/watercontrolproductsinline.asp )

Corpora-FIGURE 23.11 Outlet level control structure at Pribraz, Czech

Republic Flexible pipes may be raised or lowered to adjust water

level There are two cells controlled by this device.

TABLE 23.7 Capital Cost of One Inlet Structure for STA2 Cell 4

Piping 40 m of 1.68-m diameter @ $983/m 39,323

Excavate, bed, install 917 m 3 gravel 9,095