TREATMENT WETLANDS - CHAPTER 16 pptx

The total municipal wastewater flows from municipal sources undergoing treatment in the United States is 45 × 109 m3 per year, serving approximately 72% of the tion U.S.. But wetlands cl

The preliminary step in the design of a treatment wetland

is to acquire a fundamental understanding of the site of the

wetland Site conditions dictate the physical, chemical, and

biological environment of a wetland treatment system

Con-ditions that should be evaluated during planning of a wetland

treatment system include climate, geography, groundwater

and its chemistry, soils and geology, rainfall and runoff water

chemistry, biology, and socioeconomic factors The

impor-tance of each of these conditions may vary, but all should be

investigated to some extent Detailed studies may be needed

to determine the importance of those site conditions that

affect technical feasibility

This book primarily considers performance-based design

algorithms The first steps in the process require the assembly

of the basis of the design, which includes the following steps:

1 Determine inlet concentrations and flows

2 Determine target concentrations (regulatory limits

and allowable exceedance factors)

3 Determine allowable inflow and seepage rates

4 Determine rain, ET, and temperature ranges for

the project site

5 Select wetland type (FWS or SSF)

Often, the establishment of flows and concentrations will

require the acquisition of data on flows and concentrations,

at least to confirm estimates based on prior operations or

knowledge of the technology of the source

There are, unfortunately, numerous examples of

inappro-priate selection of the design basis for treatment wetlands The

difficulty is often not the misunderstanding of current

condi-tions, but rather incorrect assumptions about future conditions

This may involve actions outside the control of the designer

For instance the “failed” Gustine, California, system

added wetlands to existing lagoons (Walker and Walker,

1990) The source water was a combination of municipal and

milk processing wastewaters, the latter having very high

bio-chemical oxygen demand (BOD) The design presumption

was that milk wastewater would be discontinued, but this did

not occur Instead of the design influent BOD of 60 mg/L,

the wetland actually experienced BOD of approximately

600 mg/L The wetland could not meet design targets for the

unplanned tenfold-higher inlet concentrations Another

exam-ple is the “successful” wetland system treating potato

waste-water at Connell, Washington (Kadlec et al., 1997; Burgoon

et al., 1999) The system was built according to a design

based on operating data from a fixed-capacity processing

plant However, coincident with wetland start-up, the plant

implemented water conservation The loads of pollutants

remained the same, but concentrations went up considerably

as flows decreased Fortunately, in this case the wetland tem was robust enough to accommodate the change and still achieve goals These anecdotes serve notice that the basis of design must be carefully set forth, and reasonable changes anticipated in influent flows and loads

sys-16.1 PROJECT SETTING

S PACE C ONSIDERATIONS : L IMITED VERSUS U NLIMITED S PACE

Free water surface (FWS) treatment wetlands are in the gory of land-extensive technologies At the end of this chapter,

cate-it will be seen that horizontal subsurface flow (HSSF) wetlands for the same purpose are not much different in size However, the site conditions—primarily property boundaries and topog-raphy—can limit the potential size of a treatment wetland for

a particular source-water volume This is particularly true of urban stormwater wetlands, which need to be sited in built-up areas and which often utilize high-value lands Siting of wet-lands inside the boundaries of major cities, such as Orlando, Florida (Palmer and Hunt, 1989), or Toronto, Ontario (Helfield and Diamond, 2004), means that the size of the wetland is dic-tated by existing streets, highways, and buildings, not by the size needed to achieve a particular performance goal

Topography may also limit the potential size of a ment wetland The presence of steep slopes adjacent to the site can preclude construction beyond a certain limit, defined

treat-by the practicality of earth moving (Figure 16.1) When the site demands, the treatment wetland may be established in terraces, with elevation drops occurring between the succes-

sive cells of the system (Navarra, 1992; Inman et al., 2003).

Land ownership can also constrain opportunities for land construction Above and beyond questions of acquisition costs, there is the issue of the willingness of the owner to sell property Building a treatment wetland rarely falls into the category of eminent domain acquisition, although that has happened in connection with the phosphorus removal wet-lands of South Florida For large wetlands, suitable parcels are often already in agricultural use Wetlands are frequently viewed as valuable landforms across the regional landscape, but aquatic and terrestrial landforms are also valuable The construction of a treatment wetland implies the removal of other types of plant, animal, and human communities There-fore, competing uses may block the construction of a wetland

wet-on a particular plot of land

Perhaps the most serious potential constraint of ing landform is the presence of naturally occurring wetlands

preexist-on property under cpreexist-onsideratipreexist-on for a treatment wetland In

the United States, it is generally not allowed to build any

proj-ect that destroys existing wetlands But what if the projproj-ect is

a constructed wetland? That situation is obviously confusing

and unclear, and it is therefore not surprising that a variety of

rules and regulations apply in various states

Sometimes the constructed treatment wetland may be

viewed as self-mitigating; that is, it inherently compensates for

the loss of preexisting wetlands That situation has occurred

at the West Jackson County, Mississippi, constructed wetland

site It is probably most acceptable when the preexisting

wet-land is degraded, and of low regional value However, in many

other circumstances, construction in wetlands must be avoided

For instance, HSSF and vertical flow (VF) wetlands do not

offer the same type of habitat that occurs in natural wetlands,

and construction of these systems in natural wetlands is often

blocked by regulatory constraints Of course, the extreme

cir-cumstance is the use of natural wetlands for wastewater

treat-ment, which is outside the scope of this book

For these area-constrained situations, the design methods

described herein are not used to select wetland area, but rather

are used to forecast performance of the available wetland area

This predictive mode is readily accommodated in a rate

coef-ficient approach, but is very awkward, if not impossible, for

a loading design approach due to the data scatter inherent in

loading charts

S OILS AND G EOLOGY

For planning purposes, site soils in the United States can

be characterized by using USDA Soil Conservation Service

soil surveys, which are generally available for most

coun-ties within the United States Other countries often have

similar mapping resources Soil surveys typically include

maps of soil types as well as summaries of soil properties,

groundwater conditions, climatic information, and plant community information

Soils are classified by soil scientists based on a complex array of physical and chemical characteristics Soil informa-tion that might be important during project planning includes the presence of hydric soils, which occur in natural wetlands (even if formerly drained) and could be a potential regula-tory constraint for a constructed wetland site; soil texture and composition as a suitable medium for berm construction or for impeding leakage to the groundwater; depth to seasonal high groundwater; and depth to confining layers of clays or rock horizons On-site soils are typically preferred for the rooting media in FWS wetlands In some cases, the sorption potential of these rooting soils will be a design variable, such

as for metal removal

The construction of wetlands entails the excavation of the wetland basin, including any deep zones, possibly together with conveyance and seepage interception canals Therefore, the soil thickness above bedrock is an important piece of design information, because that material is movable without blasting The characteristics of the bedrock are important if such blasting is required (Figure 16.2) Construction in rock

is extremely expensive, and is to be avoided if possible

At the other extreme, on-site materials may be unsuitable for the construction of embankments, because they cannot withstand exposure to the water (Figure 16.3)

G ROUNDWATER

Infiltration of wastewater to the groundwater is important because infiltration affects the wetland water balance and could pose regulatory problems under some conditions Soil infiltration rates published in soil surveys typically overesti-mate the actual infiltration rates under sustained, saturated soil conditions and are not reliable for project planning or design Surface infiltrometer tests or well slug tests provide better estimates of the groundwater leakage that can be

FIGURE 16.1 The treatment wetlands in the Tucush valley of the

high Andes Mountains of Peru (4,100 masl) are constrained to a

fixed area by extremely steep slopes They treat the drainage

com-ing from the wasterock dump of a mine operation (Photo courtesy

Compañía Minera Antamina Reprinted with permission.)

FIGURE 16.2 Deep zones and canals for South Florida’s

storm-water treatment areas require blasting of the limestone bedrock A thin veneer (0.3–1.0 m) of peat overlies the limestone.

expected from a full-scale wetland treatment system

Meth-ods for measuring infiltration rates are described by the Soil

Conservation Service (SCS) (Hansen, 1980; U.S Bureau of

Reclamation, 1993) Field tests are the most reliable method

of estimating groundwater infiltration rates For constructed

wetlands, it may be necessary to construct pilot wetland

basins on a proposed site and then instrument inflows and

outflows to develop an accurate water balance

Wetlands can be built on leaky soils as long as regulatory

requirements can be met and adequate hydroperiods can be

maintained with the wastewater addition and net rainfall In

fact, wetlands have been designed with groundwater recharge

as a specific project goal (Ewel and Odum, 1984; Knight and

Ferda, 1989) Groundwater infiltration can be eliminated as

a project concern for constructed wetlands by using a clay or

synthetic impervious liner Although this approach may not

be necessary if the wastewater has received secondary

pre-treatment, it is recommended when wastewater is less than

secondary quality, or is known to contain contaminants of

concern for the regional groundwater and its intended uses

Percolation tests are often used as the basis of sizing

infiltration fields for septic tank effluent disposal, although

such tests are probably insufficient for ensuring adequate

performance of the field (Crites and Tchobanoglous, 1998)

The allowable hydraulic loading for the infiltration field is set

according to a published table or curve, relating the allowable

loading to the time for the water level in a test pit to drop a

specified amount (usually 2.5 cm) Data collected from

perco-lation tests are then typically related to a prescriptive hydraulic

loading that is usually much less than the observed percolation

rate The reduction in hydraulic loading is to account for the

long-term accumulation of microbial biomass and particulate

matter in the soil, which substantially reduces the infiltration

rate (Tyler and Converse, 1994) Allowable hydraulic

load-ings are usually in the range of 1–5 cm/d Additionally, there

must be a specified vertical travel distance to the groundwater

table, typically about 1 m of unsaturated soil (to allow for the

removal of pathogens) These requirements are commonly set forth in local codes and rules, and are enforced as a condition for acceptability of new on-site (septic) systems These codes are typically intended for single-home treatment systems, but are often extrapolated to larger systems due to a lack of more appropriate regulatory guidelines

The focus of on-site (septic) system codes is the posal of primary effluent into the soil matrix When water

dis-is pretreated, organic and pathogen loads are substantially reduced, and soil-based treatment is less critical for regula-tory compliance

Given this basis, constructed wetlands are frequently viewed by the on-site regulatory community as a means for justifying higher loadings or lesser unsaturated travel dis-tances in the infiltration bed, or both For example, the state

of Indiana allows reduction in the size of the absorption field associated with a subsurface-constructed wetland based on the soil loading rate (Indiana Department of Environmental Management, 1997) For soil loading rates less than or equal

to 5 cm/d but greater than 2 cm/d, the allowable reduction in field size is 50% For soil loading rates of less than 2 cm/d but greater than or equal to 1 cm/d, the allowable reduction

in the field is 33% Similar reductions in infiltration area are allowed in other states

In general, it is beneficial to understand the directions and flows of regional groundwater under the project site Dif-ferent levels of hydrogeological surveys may be performed, depending on the requirements of the specific project Con-siderable detail is necessary for groundwater remediation wetlands that intercept a plume of contamination, because those studies provide the flows and concentrations needed

to determine wetland size or performance For instance, the design of the Hillsdale, Michigan, project involved multiple monitoring wells, studied over several years, and three-dimensional computational fluid mechanics (Ecology and Environment Engineering, 2004) Modeling at a similar level was necessitated at the Columbia, Missouri, project, because

of proximity to the city’s potable water well field (Brunner and Kadlec, 1993) If the water leaving the system is trans-ported by unsaturated flow, more complex models will be required (Langergraber, 2001; Davis, 2007)

A LTITUDE

As the use of treatment wetland technology has grown across the planet, the site conditions have broadened to include a wider range of conditions, among which is the altitude of the project A few experiences have identified special issues, such

as the types of wetland plants that are adapted to high-altitude

conditions: Phragmites is not a mountain plant! (Navarra,

1992) Other concerns have yet to be explored For instance, treatment wetlands have now been built at up to 4,000 m above sea level (see Figure 16.1), at which altitude the atmosphere is approximately at half sea-level density Therefore, the partial pressure of oxygen is half that at sea-level, with potential con-sequences on the ability of the wetlands to process reactions that require dissolved oxygen, such as nitrification

FIGURE 16.3 This collection canal in the Lakeland, Florida, FWS

system was built using unstable materials from on site Despite the

attempt to reinforce the embankment with concrete matting,

ero-sion caused the discharge structure to drop into the water.

B IOLOGICAL C ONDITIONS

The addition of any type of water or wastewater will alter

biological conditions at a site Constructed wetlands

fre-quently replace upland habitats with wetland vegetation The

upland habitats that are lost might include plant

communi-ties such as grassland, forest, scrub, desert, or agriculture

The environmental values of these upland habitats should be

assessed during project planning Likewise, wastewater

dis-charge to natural wetlands can cause biological changes of

varying magnitudes (see Chapter 3) Existing plant and

ani-mal communities in natural wetlands will change depending

on the degree of changes to surface water quality and

hydrol-ogy Construction-related impacts will result in

replace-ment of part of the existing vegetation by distribution pipes,

boardwalks, and monitoring structures For most constructed

wetland projects, site-specific biological conditions do not

represent a major technical constraint

16.2 CHARACTERIZATION OF DOMESTIC

AND MUNICIPAL WASTEWATER

Wastewater quality varies widely among domestic, municipal,

industrial, agricultural, and stormwater categories Different

wastewater sources have unique mixtures of potential

pollut-ants, so that even a single wastewater source category, such as

municipal wastewater or urban runoff, may vary considerably

depending on local, site-specific circumstances However, for

some chemical constituents, the qualitative and quantitative

composition of wastewaters from different sources varies less

In general, any summary of “typical” wastewater

concentra-tions and loads must be considered cautiously

Site-specific wastewater data showing historical flows

and mass loads provide the best information for wetland

treatment system design However, because many treatment

systems are designed for new facilities or because historical

monitoring may be nonexistent or insufficient, it is useful

to know the typical concentrations of major constituents in similar wastewaters This section summarizes information from a number of sources on the typical pollutant composi-tion of wastewater applied to engineered wetlands These

“typical” concentrations and loads should only be used when site-specific information is not available

The total municipal wastewater flows from municipal sources undergoing treatment in the United States is 45 ×

109 m3 per year, serving approximately 72% of the tion (U.S EPA, 2007) In addition to industrial and munici-pal wastewaters, nonpoint source pollution contributes about two thirds of the total pollution load to U.S inland surface waters (U.S EPA, 1989) Sources of nonpoint flows include urban and suburban runoff, diffuse agricultural runoff, forestry activities, runoff from concentrated agricultural activities such as feedlots, mine drainage, and runoff from undisturbed areas However, in certain areas urban runoff

popula-or other stpopula-ormwater sources provide the greatest percentage

of uncontrolled pollutants Wetlands are often used in junction with other treatment devices, including septic tanks, lagoons, and mechanical treatment plants (Figure 16.4) In those circumstances, the water quality of interest for the wet-land design is that exiting a pretreatment step

con-The amount and timing of the water to be treated is the first and foremost item of the design basis This informa-tion should include the possible seasonality of flows and the anticipated progression of flows over the life of the design This is more important for treatment wetland design than for conventional concrete and steel treatment plants, because of the implied life cycle of the process and the nature of urban and industrial growth It is customary to plan for a 20-year life expectancy for conventional wastewater treatment plants, because mechanical equipment often wears out during this period But wetlands clearly can continue to function for far longer periods than two decades; for example, there are receiving wetlands that have been in operation for periods of

70 years (Great Meadows; Yonika et al., 1979) and 90 years

Surface Discharge

Infiltration Bed

Subsurface Discharge

Sludge Reed Bed

VF Wetland

HSSF Wetland

FWS Wetland Settling

Basin

Lagoon

Oxidation Pond

Activated Sludge

Biofilm (RBC) Source

Septic Tank

Sludge Bed

Combination Wetland

FIGURE 16.4 Simplified options for treatment trains involving treatment wetlands.

(Brillion; Spangler et al., 1976) Projecting flow estimates far

into the future is risky, so it is necessary to be explicit about

flow capacity at the time of design

Most of the pollutants that are common to many of these

wastewater sources can be effectively treated by wetland

sys-tems The normal concentration range of these pollutants is

an important consideration in evaluating wetland treatment

system options This section compares and contrasts these

wastewater sources to facilitate initial alternative evaluation

W ATER Q UANTITY

The information on water quantities and timing is assembled

into the annual and monthly water budgets for the design,

including any seasonal or event storage that may be necessary

Such water budgets are easily prepared within the framework

of a spreadsheet program on a personal computer This

infor-mation is later linked to the computation of the expected

reduc-tions in pollutant concentrareduc-tions Interestingly, the addition of

a wetland to any of the several forms of pretreatment provides

dampening of flow pulses Although it is necessary to account

for the diurnal cycles in the inflows for hydraulic purposes,

the wetland will typically “hold” several such daily pulses,

because of the extended detention time used in the wetland

S MALL D OMESTIC S YSTEMS

Most design information in engineering textbooks is based

on large-scale sewer networks that have a continuous base

flow Small-scale wastewater treatment systems often do not

have a continuous base flow On the contrary, low flows are

zero (no flow), and peak flows are many times larger than the

average flow These differences in water use patterns raise

issues that are not encountered in the design of larger sewage

treatment works

For design of single-family home treatment systems, the

accepted practice in the United States is to base the design

flow on the number of bedrooms within the home These scriptive flow determinations (typically ranging from 455 to

pre-568 L/d per bedroom) are used to provide a sufficient factor

of safety for soil infiltration of septic tank effluent They do not represent actual water use Prescriptive flow determina-tions are commonly interpreted as representing the maximum expected occupancy of the home (two occupants per bedroom) and a corresponding peak flow rate As a result, peaking fac-tor determinations and infiltration/inflow allowances are typi-cally not necessary when using prescriptive flows based on

a bedroom count Special provisions may apply in some cumstances (Minnesota Pollution Control Agency, 1999).Flow projections may be based on population for small communities A prescriptive criterion of 379 L/d per person

cir-is commonly used in North America (Great Lakes UMRB, 1997) This per-person flow guideline is intended to repre-sent an average dry weather flow from domestic wastewater sources plus a “normal” amount of infiltration for gravity sewers built with modern construction techniques If the only available information is the number of homes, an aver-age number of people per household may be used to approxi-mate the total population The average household size in the United States is 2.7 people (American Housing Survey, 2003), although this varies by geographic location An appropriate peaking factor must be applied to determine peak flows

P ATTERNS OF S MALL F LOWS

Wastewater flow from individual residences is delivered to a small-scale treatment system via a series of discrete pulses triggered by flush toilets, washing machines, dishwashers, etc Low flows in small systems will be zero (no flow) Most water use occurs in the morning, evening, and at mealtimes In the United States, water use from single-family homes has been idealized for design purposes, as indicated in Figure 16.5 As more and more homes are added to the system, flow pulses overlap If there are enough homes in the collection network, flow pulses overlap to form a continuous base flow, and flow peaks start to attenuate

10%

15%

5%

0 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 24:00

Time of Day

FIGURE 16.5 Idealized water use pattern for an individual home (Adapted from NSF International (2000) Residential wastewater

treat-ment systems NSF/ANSI 4–2000, NSF International: Ann Arbor, Michigan Reprinted with permission.)

For single-family homes, the ratio of the peak flow to the

average flow (peaking factor) can be five or higher Larger

treatment systems will experience lower peaking factors due

to overlapping flow pulses and the presence of a continuous

base flow In the United States, Recommended Standards for

Wastewater Facilities (Great Lakes UMRB, 1997) suggest a

formula based on population to determine the ratio of the

peak hourly flow to the average daily flow (Equation 16.1)

For small populations (less than 100 people), this

relation-ship results in a peaking factor of approximately 4.5

Q Q

P P

peak hourly average day

The pattern of peak flow events can be altered dramatically

if wastewater is collected and pumped into the treatment

system, as might be the case for septic tank pretreatment

Sources of inflow and infiltration from homes (sump pumps,

footing drains, roof leaders, and furnace drains) can easily

produce much higher flows unless they are identified during

the design process with a home plumbing survey program

and subsequently separated from the wastewater collection

system

A CTUAL W ATER U SE

Water use studies in the United States estimate an average

daily water use of 189 to 265 L/d per person for homes built

before 1994; implementation of standards for water-efficient

appliances since then has reduced water use in newer homes

to approximately 161 to 227 L/d per person (U.S EPA,

2002c) Water use is strongly influenced by cultural practices

and varies widely from country to country Across Europe,

typical flow rates in small communities (less than 500 people)

range from 80 to 120 L/d per person (IWA Specialist Group

on Use of Macrophytes in Water Pollution Control, 2000) In

Germany, water use rates are much lower than in the United

States (Gesellschaft zur Förderung der Abwassertechnik d.V

(GFA), 1998), at 100–150 L/d per person In urban areas of

developing countries, water use is approximately 60 L/d per

person (Nhapi et al., 2003).

Lagoons

There are many variants on the concept of aquatic units for

wastewater treatment, ranging from single-pond units (Water

Environment Federation, 2001) to complex arrays of multiple

units (Craggs, 2005) Often, other treatment process units are

added to complement the pond itself (Middlebrooks et al.,

2005) The combination of a pond followed by a wetland has

been explored at a number of locations (Horne, 1995;

Stein-mann et al., 2003; Tanner and Sukias, 2003; Kadlec, 2003d;

Polprasert et al., 2005; Wang et al., 2005; Kadlec, 2005e)

Because the wetland is often an add-on, the flow of the water exiting the pond is often known from performance data.The prescription for lagoon operation may be continuous discharge, typical of warm climates, or episodic discharge, typical of cold climates Lagoon systems often discharge to surface waters, for which the goal is to minimize water quality impacts Maximum dilution occurs at high flow of the recipi-ent, which in turn occurs during freshets, i.e., the spring thaw and the autumn wet season Therefore, lagoon discharges are traditionally scheduled for those times of maximum dilu-tion When a wetland is added to the system, there are more options for scheduling the discharge For instance, the system may be designed for discharges that avoid ammonia toxicity

in the recipient (Kadlec and Pries, 2004) Winter storage may

be contemplated, provided capacity is present or designed Therefore, the designer has an added degree of freedom: the total annual volume may be managed to optimize treatment, perhaps at the expense of more pond volume This design feature is discussed in more detail subsequently

Mechanical Plants

Pretreatment systems, such as activated sludge plants, are small-retention devices, which do not typically have much capacity to dampen the incoming flow pulses Hour-to-hour, day-to-day, and month-to-month flow variations are likely to

be passed through the pretreatment system, and thus affect what is entering a follow-on treatment wetland These pulses will then be partially evened out by an add-on wetland Flows, whether municipal or industrial, are often seasonal in character It is necessary to anticipate those patterns, because the wetland must function appropriately under these variable hydraulic conditions Monthly flow estimates will be required for most point-source projects

I NFILTRATION AND I NFLOW

Infiltration is defined as groundwater that seeps into a water collection system It invariably introduces additional flow into the collection network Infiltration is strongly influenced by groundwater elevation, workmanship of sewer construction, quality of construction materials, and fraction

waste-of the overall collection network that relies on gravity flow Typical sources of infiltration include poorly installed service laterals, leaking joints on sewer pipes, cracked sewer pipes, and leaking manholes Exfiltration (movement of water out of the collection system) can also occur Portions of collection systems that are pumped (such as pressure sewers) have posi-tive internal pressures and are often pressure-tested during construction As a result, pressure sewer collection systems have a much lower potential for infiltration

Inflow is defined as extraneous water that is directly charged to the wastewater collection system In combined sewer systems, stormwater is a major source of inflow It is driven by rainfall intensity and amount of impervious sur-face present within the catchment area In newer collection networks, stormwater is almost always excluded In these

dis-situations, major sources of inflow are generally limited to

roof leaders, sump pumps, and foundation drains Because

most inflow sources are driven by rainfall, these tend to be

high-flow, short-duration events These “surge” events can

have major impacts on treatment systems The combined

effect of infiltration and inflow depend on a number of

fac-tors, including the integrity of the sewer system, size of the

collection pipes, the presence of high groundwater, and other

factors Typical allowances for combined infiltration/inflow

range from 0.09 to 0.9 m3/d/cm/km (Metcalf and Eddy,

1998)

W ATER Q UALITY

The concentrations of the pollutants in the water to be treated

are critical to the sizing process, and to the prediction of the

wetland performance in the face of unknown future

varia-tions A clear definition of the incoming water quality,

includ-ing the anticipated temporal distribution of concentrations,

is essential There are often seasonal fluctuations for point

sources, as well as diurnal fluctuations Incoming patterns

of chemical composition propagate through the wetland and

undergo modification, resulting in a spectrum of output

com-positions Some of this output variability may be predicted

by the design models, namely, those variations that represent

responses to moderately slow input changes (those which

occur on monthly or less) Faster events involve ecosystem

processes that are not included in the design models available

at the present time, and therefore will give the appearance of

generating stochastic variations

In domestic and municipal wastewater collection

sys-tems, the following components contribute to sewage flow:

Human excreta (feces and urine)

Wastewater generated by personal use, including

washing, laundry, food preparation, etc

(graywa-ter), and water used as the carrier media for human

bodily wastes (blackwater)

Water that inadvertently leaks into the collection

system (infiltration and inflow)

Wastewater from commercial or industrial sources

to consider how the community is utilizing water

Small Domestic Systems

There is often no composition data to be used for the design

of treatment systems for small systems It is necessary to resort to estimating methods that consider water use and population in the source community Untreated human urine and fecal material can introduce a variety of pollutants into the environment Typical per-person generation rates are summarized in Table 16.1 (Del Porto and Steinfeld, 2000) Graywater includes spent water from bathtubs, showers, washbasins, washing machines, laundry tubs, kitchen sinks, and dishwashers In developed countries, graywater accounts for 50 to 82% of household water use and represents about half of the organic waste solids produced in the home When conventional flush toilets are used in a waterborne sewer sys-tem, graywater is often combined with blackwater Relative contributions of pollutants by source (for a combined sewer system) are summarized in Table 16.2 (U.S EPA, 2002c) Typical constituent concentrations for residential septic tank systems are given Table 16.3

Lagoons

Another source of treatment wetland influents arises from pond treatment as the initial component of the treatment train One or more facultative, anaerobic or aerated ponds

or lagoons may be used (Shilton, 2005) Because the land is often an add-on, the quality of the water exiting the pond is often known from performance data If the entire system is constructed at the same time, the lagoon elements should be designed according to the currently accepted

wet-TABLE 16.1 Typical Per-Person Waste Generation Rates

Moisture content (g per capita·day) 95% 70% — Organic carbon (g per capita·day) 8.5 22 30

Source: From Del Porto and Steinfeld (2000) The Composting Toilet System Book Center

for Ecological Pollution Prevention, Concord, Massachusetts Reprinted with permission.

methods (Shilton, 2005) Lagoon systems are typically

designed for reduction of BOD and total suspended solids

(TSS), and occasionally ammonia Data and older models

exist for pond pathogen reduction, but have not been recently

updated and synthesized (Davies-Colley, 2005) Phosphorus

data for lagoon systems are not voluminous, because it is not

frequently regulated in lagoon discharges Some

approxima-tions of the effluent characteristics of several types of lagoons

are shown in Table 16.4

Mechanical Plants

Table 16.5 summarizes the typical quality of

medium-strength, raw, municipal wastewater in the United States and

provides a range of values for commonly observed

constitu-ents Municipal wastewater is composed of a variable array

of components characterized by the presence of

biodegrad-able organic matter (paper, feces, and food), particulate and

dissolved solids, and nutrients Many municipal wastewaters

also receive some component of industrial waste These flows

and residential sources may add trace metals and pesticides

to typical municipal wastewater

Table 16.5 also provides a range of estimated ment efficiencies for conventional primary and secondary treatment processes, and summarizes the typical quality of secondarily treated municipal wastewaters These removal efficiencies vary widely depending on the types of treatment processes However, it is generally observed that at least 70%

treat-of the BOD and TSS are removed from municipal wastewater during primary and secondary treatment Treatment require-ments have generally increased over the past decades, and many treatment plants now include at least partial nitrifica-tion, perhaps denitrification, and phosphorus removal These blur the terminology, because they range from “advanced secondary” to “tertiary” and beyond The follow-on treat-ment wetland may therefore be termed “tertiary” or, as might

be supposed, “quaternary.”

This summary can be used as a rough estimate of the ent water quality to be applied to a wetland system designed for primary, secondary, or advanced wastewater treatment

influ-TABLE 16.3

Typical Wastewater Component Concentrations Entering and Leaving a Residential Septic Tank

Parameter

Raw Waste Central Estimate Range

Septic Tank Effluent

Source: Data from Metcalf and Eddy Inc (1991) Wastewater Engineering, Treatment, Disposal, and Reuse Tchobanoglous and

Burton (Eds.), Third Edition, McGraw-Hill, New York; Crites and Tchobanoglous (1998) Small and Decentralized Wastewater

Management Systems McGraw-Hill, New York.

TABLE 16.2

Typical Per-Person Combined Sewage Generation Rates

Parameter (Mean Values)

BOD 5 (g per capita·day)

Suspended Solids (g per capita·day)

Nitrogen (g per capita·day)

Phosphorus (g per capita·day)

Source: Adapted from U.S EPA (2002c) Onsite wastewater treatment systems manual EPA 625/R-00/008 U.S EPA Office of Research

and Development: Washington, D.C.

TABLE 16.4

Typical Composition of Lagoon Discharge Water and Percent Removals at Various Levels of Treatment

Parameter Primary Anaerobic Secondary Aerobic Facultative Aerated Facultative Aerated Partial Mix

Source: Shilton (2005) In Pond Treatment Technology Shilton (Ed.), IWA Publishing, London; Metcalf and Eddy Inc (1991) Wastewater Engineering, ment, Disposal, and Reuse Tchobanoglous and Burton (Eds.), Third Edition, McGraw-Hill, New York; Crites and Tchobanoglous (1998) Small and Decentral-

Treat-ized Wastewater Management Systems McGraw-Hill, New York; Crites et al (2006) Natural Wastewater Treatment Systems Meyer (Ed.), CRC Press, Boca

Raton, Florida; U.S EPA (1983a) Design manual: Municipal wastewater stabilization ponds EPA 625/1-83/015, U.S EPA Office of Water: Cincinnati, Ohio; U.S EPA (1983c) Wastewater stabilization ponds: Nitrogen removal U.S EPA Office of Water: Washington, D.C.; Rich (1999) High Performance Aerated

Lagoons American Academy of Environmental Engineers, Annapolis, Maryland.

TABLE 16.5

Typical Composition of Municipal Wastewater and Percent Removals at Various Levels of Treatment

Constituent

Raw Wastewater (mg/L) Percent Removal Secondary Effluent (mg/L)

Nichols (1985) In Ecological Considerations in Wetlands Treatment of Municipal Wastewaters Godfrey (Ed.), Van Nostrand Reinhold Company, New York,

pp 351–391; Krishnan and Smith (1987) In Aquatic Plants for Water Treatment and Resource Recovery Reddy and Smith (Eds.), Magnolia Publishing, Orlando, Florida, pp 855–878; Williams (1982) In Water Reuse Middlebrooks (Ed.), Ann Arbor Science, Ann Arbor, Michigan, pp 87–136.

16.3 CHARACTERIZATION OF

OTHER WASTEWATERS

I NDUSTRIAL W ASTEWATERS

Although industrial wastewater quality varies among

indus-tries, it has a fairly consistent intrasystem effluent quality

Table 16.6 summarizes the typical quality of raw wastewater

from a number of industries that have used wetlands treatment

technology Raw industrial wastewater usually receives some

level of pretreatment before discharge to a wetlands treatment

system If total concentrations of BOD, suspended solids, and

ammonia nitrogen in untreated industrial wastewater are in the concentration range of hundreds to thousands milligrams per liter, it is generally not acceptable for wetlands discharge without additional pretreatment

L ANDFILL L EACHATES

Treatment and disposal of liquid leachates is one of the most difficult problems associated with the use of sanitary land-fills for disposal of solid waste Leachates are produced when rainfall and percolated groundwater combine with inorganic and organic degraded waste In unlined landfills, leachates

TABLE 16.6

Typical Pollutant Concentrations in a Variety of Untreated Industrial Wastewaters

Constituent Units

Pulp and Paper a

Landfill Leachate b

Coal Mine Drainage d

Petroleum Refinery e Electroplating f Breweries g

Note: ND  not detected.

aFrom Jorgensen (1979) Studies in Environmental Science 5 Elsevier, New York.

bFrom Staubitz et al (1989) Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural

Hammer (Ed.), Lewis Publishers, Chelsea, Michigan, pp 735–742; Lema et al (1988) Water, Air, and Soil Pollution 40:

223–250; Bolton and Evans (1991) Water, Air, and Soil Pollution 60: 43–53.

cFrom Wildeman and Laudon (1989) In Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and

Agricul-tural Hammer (Ed.), Lewis Publishers, Chelsea, Michigan.

dFrom Girts and Kleinmann (1986) National Symposium on Mining, Hydrology, Sedimentology, and Reclamation University

of Kentucky Press, Louisville, Kentucky, pp 165–171.

eFrom Adams et al (1981) Development of Design and Operational Criteria for Wastewater Treatment Enviro Press,

Nash-ville, Tennessee; ANL (1990) Environmental consequences of, and control processes for, energy technologies Argonne

National Laboratory (ANL) and Noyes Data Corporation: Park Ridge, New Jersey.

fFrom OECD (1983) Emission Control Costs in the Metal Plating Industry Organization for Economic Cooperation and

Development (OECD).

gFrom Cooper (1978) The textile industry Environmental control and energy conservation Noyes Data Corporation Park

Ridge, New Jersey; Wildeman et al (1993a) Wetland Design for Mining Operations Bitech Publishers, Vancouver, British

Columbia.

frequently discharge to groundwater or appear as surficial

drainage around the base of the landfill In modern lined

landfills, leachates are collected from the lined cells and

routed to treatment units The use of constructed wetlands to

treat these landfill leachates is a well-developed technology,

currently undergoing rapid expansion in application This

application is discussed further in Chapter 25

The highly variable nature of solid waste, differences in

age and decomposition, and the diversity of chemical and

biological reactions that take place in landfills result in a

wide range of chemical quality of leachates (McBean and

Rovers, 1999) Reviews of “average” landfill concentrations

of COD, volatile acids, and nitrogenous compounds show

increases during the first few years of operation and then

decline over ten or more years Table 16.6 provides typical

ranges encountered in landfill leachates Flows are generally

low, but vary depending on management and minimization

of percolation from rainfall Clearly, the expected volume

and chemical quality of a landfill leachate is highly

site-spe-cific, may change over time, and must be estimated on a

case-by-case basis for wetland treatment system design A detailed

discussion of the leachate quality that may reach a treatment

wetland is found in McBean and Rovers (1999)

P ULP AND P APER W ASTEWATER

The pulp and paper industry converts wood products

includ-ing pines, spruce, poplar, beech, birch, and aspen, as well as

recycled paper, into liquefied cellulose pulp and paper Raw

wood and wood chips are converted to pulp (cellulose fibers)

by mechanical grinding (ground wood) or through

chemi-cal degradation and leaching (sulfite and Kraft processes)

At an increasing number of pulp and paper mills, this pulp

is bleached to delignify and decolorize the cellulose fibers

before paper manufacture About 29–34 m3 of raw

wastewa-ter is produced for each metric ton of pulp and paper

pro-duced (Britt, 1970) Total wastewater flow for the U.S pulp

and paper industry is about 20 × 106 m3/d (Greyson, 1990)

Table 16.6 summarizes the typical composition of this

waste-water, although different manufacturing processes result in

different wastewater qualities This flow is equivalent to a

raw organic matter (BOD5) load of about 15 × 106 metric tons

per day (Greyson, 1990)

Raw wastewater from pulp and paper mills typically

receives primary treatment through settling, either in ponds

or in primary clarifiers When required to meet discharge

limitations, secondary treatment at most pulp and paper

mills includes biological conversion of BOD5 and additional

solids settling in aerated lagoons or in conventional

acti-vated sludge treatment systems To meet reduced effluent

limitations, some pulp and paper mills are being required to

provide treatment beyond the secondary level The goals of

additional treatment depend on site conditions, such as the

quality of the effluent after secondary treatment and water

quality permit limits in the receiving water One goal may

be to further reduce BOD5, TSS, nitrogen, phosphorus, color,

chlorinated organics (such as adsorbable organic halides or

dioxin), and whole effluent toxicity Constructed and natural wetland treatment systems have been used at a number of pulp and paper mills to provide this advanced secondary or

tertiary treatment (Knight et al., 1994; Knight, 2004).

M INE D RAINAGE

During and following mining operations, runoff and leachate from tailings and from abandoned tunnels and shafts dissolve trace metals, contaminating nearby surface waters Leach-ates from coal and metal mines contain residual trace met-als, notably iron, manganese, and aluminum from coal mines (Table 16.6) Seeps from abandoned mines typically received

no treatment in the past, but there is an increasing emphasis

on corrective measures (Younger et al., 2002) Constructed

wetlands are used as a technology to reduce metal

concentra-tions in mining wastewater (Younger et al., 2002; PIRAMID

Consortium, 2003a, 2003b) As discussed in Chapter 11, als are precipitated and sequestered in sediments, and taken

met-up by plants in wetlands Wetland design for metals removal is sometimes limited by the need to avoid toxic concentrations in tissues that could subsequently accumulate in the food chain Information necessary to evaluate the ability of wetlands to provide treatment of these waste products is summarized

in Chapter 11 and in the references listed previously Both removal rate specification and rate constants have been used

in design, but the available level of detailed design data port is not as large as for municipal wastewater treatment.During coal mining, iron pyrite and other metal- bearing minerals are exposed to percolating water, which leads to the release of acidic leachates to surface water These drainages typically have low pH and elevated concentrations

sup-of dissolved iron, sulfate, calcium, and magnesium In tion, the drainages have variable and somewhat elevated concentrations of aluminum, copper, manganese, nickel, and zinc (Table 16.6) Many of the streams and impoundments in the Appalachian coal mining region of the United States are affected by acid mine drainage Conventional treatment of leachates at these sites includes surface grading and recon-touring to reduce or divert flows and chemical buffering and precipitation with mechanical treatment plants to improve water quality Because these processes have relatively high capital and lifetime costs, there has been considerable inter-est in developing more cost-effective alternatives Beginning

addi-in the early 1980s, research focused on the potential of bic wetlands for precipitation of ferric sulfate to neutralize

aero-pH and reduce dissolved ferrous iron concentrations structed wetlands are now used at many sites in the United States and Europe to increase the pH and reduce concentra-tions of iron and manganese at coal mine sites (Kleinman

Con-and Hedin, 1989; Younger et al., 2002; PIRAMID

Consor-tium, 2003a, 2003b) of coal mine drainage

P ETROLEUM I NDUSTRY W ASTEWATER

Because of the diverse processes at refineries and ated transportation facilities, and the storage of flammable

associ-liquids, land area requirements are large and include many

kilometers of piping and hundreds of tanks and storage areas

Wastewater is generated by manufacturing processes,

cool-ing tower blowdown, water and sludge drainage from tanks,

and stormwater drainage and runoff (UNEP, 1987) Typical

wastewater pollutants at petroleum refineries include BOD5,

COD, oil and grease, TSS, NH4-N, phenolics, H2S, trace

organics, and heavy metals Concentrations of many of these

pollutants are reduced through source control and

prelimi-nary treatments such as sour water stripping, oxidation and

neutralization of spent caustics, and cooling tower blowdown

treatment Table 16.6 lists some examples of pollutant

con-centrations remaining in refinery wastewater

Raw wastewater from petroleum refineries typically

receives additional treatment including gravity separation of

oils and greases, primary clarification, dissolved air flotation,

and secondary treatment, including oxidation ponds, aerated

lagoons, activated sludge, trickling filters, and activated

car-bon The API separator process typically removes 60 to 99%

of the oil and grease, and smaller proportions of other

pollut-ants Primary treatment removes 20 to 70% of the BOD5 and

TSS and 10 to 60% of the COD Secondary treatment will

reduce 40 to 99% of the BOD5, 30 to 95% of the COD, 40 to

90% of the TOC, 20 to 85% of the TSS, 60 to 99% of the oil

and grease, 60 to 99% of the phenol, 9 to 99% of the NH4-N,

and 70 to 100% of the sulfide (ANL, 1990)

As described in Chapter 13, constructed wetlands are

pro-viding advanced secondary and tertiary treatment of process

water and stormwater at a large number of refineries (Knight

et al., 1999; API, 1999) Constructed wetlands typically will

reduce remaining concentrations of BOD5, COD, TSS, NH4-N, oils and grease, phenols, and metals to advanced treatment levels

A NIMAL I NDUSTRY W ASTEWATERS

Animal industry wastewater contains high BOD5, COD, TSS, and nutrients and is qualitatively similar to municipal waste-water Mass loadings from animal feed lots and other con-centrated agricultural activities require intensive treatment systems to provide environmental protection Traditional treatment methods such as anaerobic lagoons and spray irri-gation are not always adequate to provide high-quality water for off-site discharge Constructed wetlands are being used

in a growing number of cases to receive pretreated dairy and

swine wastes (NADB database, 1998; Knight et al., 2000)

These wetland treatment systems must be designed with sonable organic loadings to prevent plant mortality, odors, and poor treatment efficiencies Treatment wetlands are a compatible component of on-farm, total waste management Their land intensiveness is not a serious limitation in most instances Farmers typically have the equipment and skills necessary to build their own wetlands and operate them suc-cessfully Table 16.7 summarizes the composition of wastes from animal operations, both entering and leaving treatment wetlands

rea-S TORMWATER R UNOFF

Concentrations of most parameters in stormwater are time dependent Stormwater concentrations and loads are cyclic

TABLE 16.7

Average Wetland Influent and Effluent Concentrations of Selected Animal Facilities (mg/L)

Source: Data from NADB database (1998) North American Treatment Wetland Database (NADB), Version 2.0 Compiled by

CH2M Hill Gainesville, Florida; and Knight et al (2000) Ecological Engineering 15(1–2): 41–55.

with periods of dry fall and deposition, then 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 Chapter 14 provides a more complete

description of the expected flows and concentrations for such

event-driven systems

Table 16.8 provides typical mean concentrations for

con-stituents The averages are flow-weighted to provide realistic

estimates of the total constituent load that escapes during

multiple storm events Instantaneous concentrations will

be considerably higher than these averages Pollutant

con-centrations and loads generally range from low levels, from

undeveloped and park lands to low-density residential and

commercial, to agricultural, to higher-density residential and

commercial, and finally to high-density commercial, trial, and agricultural land uses Mean concentrations per event for BOD5 vary from 1.45 mg/L for undeveloped lands

indus-to 20 mg/L for high-density urban areas TSS concentrations vary from 11 mg/L for undeveloped areas to 150 mg/L for high-density urban areas

The mass loading rates provided in Table 16.8 represent normalized pollutant loads that are somewhat independent

of local rainfall amounts Because pollutant loads per area per time are relatively constant between similar land use areas, variable local rainfall washes these loads off the land

in a few large events 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

Load (kg/ha·yr)

Concentration (mg/L)

Load (kg/ha·yr)

Concentration (mg/L)

Load (kg/ha·yr)

Concentration (mg/L)

Load (kg/ha·yr)

Source: Dames and Moore (1990) Lakeland Comprehensive Stormwater Management and Lake Pollution Study, Volume I Report to the City of

Lakeland, Florida (May 1990); U.S EPA (1983b) Design principles for wetland treatment systems EPA 600/2 83/026, Hammer and Kadlec (Eds.), National Technical Information Service; Marsalek and Schroeter (1989) Water Pollution Research Journal of Canada 23: 360–378; Bastian (1986)

Potential Impacts on Receiving Water Urbonas and Roesner (Eds.) Proceedings of the ASCE Engineering Foundation Conference: Urban Runoff

Quality—Impact and Quality Enhancement Technology, 23–27 June 1986 American Society of Civil Engineers: Henniker, New Hampshire, pp

157–160; Lager et al (1977) Urban stormwater management and technology: Update and user’s guide EPA 600/8–77/014, U.S EPA Office of

Research and Development, Municipal Environmental Research Laboratory: Cincinnati, Ohio; Marsalek (1990) Water Science and Technology 22: 23–30; Driscoll (1986) Lognormality of Point and Non-Point Source Pollutant Concentrations Urbonas and Roesner (Eds.), Proceedings of the

ASCE Engineering Foundation Conference: Urban Runoff Quality—Impact and Quality Enhancement Technology, 23–27 June 1986 American

Society of Civil Engineers: Henniker, New Hampshire, pp 438–458; Shelley and Gaboury (1986) Estimation of Pollution from Highway Runoff—

Initial Results Urbonas and Roesner (Eds.) Proceedings of the ASCE Engineering Foundation Conference: Urban Runoff Quality—Impact and

Quality Enhancement Technology, 23–27 June 1986 American Society of Civil Engineers: Henniker, New Hampshire, pp 459–473; Novotny

(1992) Water Environment Technology January: 40–43.

it comes from leaf and wood litter, rubber, and road

sur-face material (Novotny, 1992) The high metal content of

highway solids comes from vehicle emissions Novotny

(1992) reported that the average total nitrogen 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)

Urban and residential runoff is being treated with wetland

detention basins (Kehoe, 1993) and constructed wetlands

(Carleton et al., 2001).

Wastewater treatment and disposal are regulated by an

ever-increasing number of federal, state (provincial), and local

laws, rules, ordinances, and standards In some cases, the

most challenging part of implementing a wetland treatment

project is complying with regulations through the permitting

process A detailed knowledge of the pertinent regulations

is essential to evaluate the feasibility of a wetland treatment

project, and to design it properly An up-to-date, detailed

survey of federal, state, and local ordinances should be

con-ducted to determine those that might be relevant to specific

projects

Treated water may be destined for one of three primary

receivers: surface water, groundwater, or irrigation (reuse)

There are often stringent specifications of quality that must

be met to allow discharges to these recipients, and they are

quite different The intent of specifications for surface water

discharges is the preservation or improvement of the

des-ignated uses of those waters No matter what the receiving

ecosystem or post-wetland treatment element, proper design

requires a clear statement of the required water quality

leav-ing the treatment wetland

R ECEIVING W ATER S TANDARDS

In the United States, the Clean Water Act created the National

Pollutant Discharge Elimination System (NPDES)

permit-ting program An NPDES permit is required for nearly all

point discharges of water or wastewater into waters of the

United States, including municipal and industrial

wastewa-ter NPDES permits specify allowable flows and chemical

quality of discharges into waters of the United States based

on established water quality standards for those receiving

waters The Clean Water Act guides water quality standards,

which are promulgated individually by the states Water

quality standards vary among water bodies within a state

and among states, depending on specific receiving water

resources Many wetland treatment systems discharge to

surface waters, and therefore must meet the conditions of a

discharge permit The conditions of the permit dictate the

required performance of the wetland, and therefore govern

its sizing

Traditionally, permits have been developed to control

both flows and loads of pollutants There are typically annual

averages and monthly and weekly maxima, perhaps adjusted

seasonally The relation between averages and maximum

allowable concentrations may have been determined from other technologies, and may be inappropriate for a wetland system Similar procedures are in place for other countries.Discharge in some circumstances is directed to down-stream wetlands, which are often a combination of surface and subsurface waters These wetlands are often federally regulated waters, and subject to appropriate regulations However, the upstream treatment wetland is a treatment system, typically regulated according to a different set of rules The design goals for the treatment wetland therefore become the water quality and quantity desired for the man-agement of the downstream, jurisdictional wetland These are likely to be stricter than for discharge to a large river, for example (due to lower dilution factors in the natural wetland environment)

G ROUNDWATER D ISCHARGES

Groundwater discharges are regulated via either the codes for single-home on-site (septic) drainfields or the rules for infiltration of treated wastewaters from municipal treatment plants The septic drainfield codes are not based explicitly

on water quality, and typically use a prescriptive approach based on a presumed reduction in pathogen counts Larger systems typically target specific water quality parameters, such as nitrate, and hydrogeologic modeling is often required

to determine the fate and transport of these parameters in the subsurface environment

Groundwater discharges of treated water are feasible in

a number of circumstances The problems of avoidance of eutrophication of surface receiving waters are replaced by problems of ensuring proper quality for the aquifer to be recharged If the groundwater beneath the wetland, or a follow-

on infiltration bed, is a drinking water source, then tion must be paid to nitrate, pathogens, and metals, as well

atten-as to trace organic chemicals However, many aquifers are not, and will not be, used for potable water supply Wetlands, therefore, have a role in pretreatment for conventional rapid infiltration basins (RIBs) and in posttreatment for nitrate removal from underdrained RIBs

The primary concepts of regulation of groundwater charges concern nitrates, pathogens, and perhaps salts The limit of 10 mg/L nitrate-nitrogen as a drinking water stan-dard in the United States results in specifications of nitrate,

dis-or by implication, total nitrogen in such discharges The need

to regulate pathogens if any drinking water use is present is obvious Salt content is of concern if it is a perceived threat

to potable water supplies Groundwater discharges may be

a preferred alternative because of the phosphorus-binding potential of many soils Land application has a long tradition

as a means of wastewater disposal But it is often plagued

by a surplus of nitrogen, which escapes crop utilization and nitrifies during transport to the groundwater Wetlands have the potential to strip excess wastewater nitrogen before land application In this application, wetland design targets the requirements of the subsequent treatment process (i.e., land application)

The design of the treatment wetland interfaces with the

design of the infiltration system Several sources discuss the

design of rapid infiltration systems (Crites and

Tchobano-glous, 1998; Water Environment Federation, 2001; Crites

et al., 2006).

I NTERFACING TO R EUSE

In many parts of the globe, water is in short supply As

treatment technologies improve, it has become possible to

consider the treated water as a resource, with a variety of

potential beneficial uses The irrigation of agricultural crops

is the leading consumer of treated water Crops include trees,

pastures, and fodder crops in North America, but food crops

are irrigated with treated water in other parts of the world

The key consideration is the potential for passing of

patho-gens to consumers of the crop Treatment wetlands have been

used to help condition the water for these applications (Crites

and Tchobanoglous, 1998) Landscape irrigation is another

reuse candidate, with applications for ornamentals and golf

courses Again, treatment wetlands have been employed as

conditioners in this application (Wallace and Kadlec, 2005)

The reuse water quality standards to be met vary from

state to state in the United States U.S EPA has guidelines

for various categories of reuse (U.S EPA, 2004) State

regu-lations or guidelines are in place in virtually all states, and

typically apply to several categories: (a) unrestricted urban

use, (b) restricted urban use, (c) nonfood crops, and (d) food

crops Some of these regulations contain extremely low

requirements for solids and pathogens For instance, the

state of California requirement is for turbidity less than 2

NTU, and total coliforms less than 2.2 MPN/100 mL, for

the highest reuse category (which incidentally includes use

for flushing toilets) (California Code of Regulations, 2001)

Other states have less stringent requirements; for instance,

Colorado allows unrestricted irrigation of water with a

BOD less than 20 mg/L, fecal coliform bacteria less than

25 MPN/100 mL, and TSS less than 40 mg/L (Colorado

Department of Health Water Quality Control Division,

2005) States that are driven primarily by liability concerns

tend to have very stringent reuse limits, whereas states driven

by water scarcity often have less strict limits to reduce the

economic barriers to water reuse

In water-scarce areas such as Mediterranean countries,

water scarcity may be a driving factor for reuse, and lower

levels of treatment may be acceptable (e.g., control of

para-sites but not bacterial or viral contaminants) In these cases,

alternate treatment guidelines will apply (Korkusuz, 2005)

Clearly, there is a lower limit to the treatment

achiev-able in constructed wetlands, because natural processes

cre-ate background concentrations that may be in excess of local

regulatory requirements for water reuse

E XCURSION C ONTAINMENT AND S AFETY F ACTORS

All treatment technologies possess a spectrum of effluent

concentrations, which is predictable only in the probabilistic

sense Therefore, in addition to the mean effluent tration (which may vary in a deterministic way with tem-perature and loading), there is an associated bandwidth of concentration Regulations may constrain both the mean and the maximum of the band, via specification of a limit on the maximum daily, weekly, or monthly value, together with a limit on the average annual value In design, care must be taken to accommodate the most restrictive of multiple aver-aging tests given by the regulation

concen-The average system performance will depend on either the season of the year or the water temperature, or both These are deterministic variations; that is, they may be pre-dicted from the seasonality of the removal rate coefficient or, more directly, from information on observed trends in treat-ment wetland outlet concentrations Theta factors and trend properties (given in Part I) allow the designer to forecast sea-sonal deterministic trends There remains the variability not predicted by such seasonality (see Chapter 9, Figure 9.48, for example)

Probabilistic effects are important in the utilization of design models for predicting removal performance of treat-ment wetlands Regulatory requirements often employ a standard other than a long-term average There may be a maximum monthly concentration not to be exceeded, or a specified concentration not be exceeded for more than a cer-tain percentage of samples To illustrate, consider the repre-sentation of Figure 16.6, showing the hypothetical reduction curve for a typical wetland The information in Part I is unequivocal; as wetland size increases, there is a downward trend in pollutant concentration

However, there is also a scatter in the individual ments that make up the trend During any specified part of the year, the concentration of a pollutant follows a decreasing curve, with wetland size (detention time), and has an associ-ated bandwidth of scatter in expected values (Figure 16.6) In this hypothetical example, that scatter is shown as a uniform distribution about the trend line, with a bandwidth propor-tional to the trend value It is supposed that the regulatory limit is a concentration of 30 mg/L, as a maximum allow-able The trend model tells us what size wetland is needed

measure-to meet 30 mg/L as a long-term average, which is a tion time of 6.1 days However, the scatter is such that half the time the measured values will be higher, up to 42 mg/L The exceedance frequency is expected to be 50% because the design is based on the mean performance As this level of excursions is likely to be quite unacceptable from a regula-tory standpoint, it would be necessary to increase the size of the wetland

deten-At a detention time of 9.1 days (almost a 50% increase

in wetland size), a large majority of excursions are contained (95% this hypothetical example) below the regulatory limit, and the system would experience exceedances only 5% of the time In this case, the 50% increase in size is needed for excursion containment of an expected, quantified scatter At the size that contains excursions (9.1 days’ detention), the trend value is 21 mg/L, or 70% of the limit value for the

design This fraction is called the coefficient of reliability

(COR) Crites and Tchobanoglous (1998) present a method

for its estimation from the coefficient of variation (variance/

mean) of one or more datasets, adapted from activated sludge

technology In this book, an exceedance multiplier is used,

which is just the reciprocal of the COR:

Values of such multipliers were determined for many

pollut-ants for many wetlands, and the average values are tabulated

in Part I for various exceedance frequencies The relationship

between monthly trend average effluent concentration and the

90th percentile monthly concentration for typically regulated

constituents is given in Table 16.9, which is extracted from the various results in Part I

Exit concentrations fluctuate with an amplitude of about 1.75 times the mean for the 90th percentile, meaning that this percentile is about 75% higher than the mean (Fecal coliform bacteria are a separate case; the multiplier for FC

in the example in Figure 16.6, it could be decided to contain excursions below 20 mg/L instead of 30 mg/L That leads

to a yet larger area requirement, corresponding to 13.1 days’ detention in this example

TABLE 16.9 Trend Multipliers Required to Contain the 90th Percentile

of Excursions around Trend Means for Various Pollutants

Note: Other percentiles can be found in the pollutant chapters of Part I.

FIGURE 16.6 A hypothetical example of design for excursion containment The trend represents P  3, C*  5 mg/L, and k  30 m/yr

The scatter is a uniform distribution with a o50% bandwidth On average, a goal of 30 mg/L can be met with 6.1 days’ detention To avoid exceedances at the 95% level (one time in 20), the detention time should be 9.1 days More wetland area (more detention time) may be added

as a safety factor; i.e., 13.1 days should contain outlet concentrations to less than 20 mg/L most of the time.

20

0

40 60 80 100 120 140

HRT (days)

Scatter Trend 95th Percentile

Trend design (6.1d)

Excursion containment (9.1d)

Safety factor (13.1d)

30 mg/L

Historically, Kadlec and Knight (1996) determined

mul-tipliers corresponding to the 100th percentile of monthly

means from the NADB These were relative to the long-term

mean value for a particular wetland, and therefore seasonal

variations, whether temperature driven or not, were included

in the multiplier In this book, an annual trend is computed as

the basis of the multiplier, thus excluding seasonal

phenom-ena from this measure of random scatter Further, wetland

data may sometimes contain sufficient intrasystem

variabil-ity to place the 100th percentile above the median inlet

con-centration For wetlands with very low inlet concentrations,

it may not be possible to design a wetland to totally avoid

the possibility of monthly exceedances Consequently,

multi-pliers for various frequencies of occurrence are tabulated in

Part I, and are used in Part II for design purposes

Curiously, the use of a COR has been ascribed as an

attri-bute of areal first-order models but not an attriattri-bute of

volu-metric first-order models (Water Environment Federation,

2001; Crites et al., 2006) Incredibly, the absence of a COR

in a volumetric model has been described as an advantage,

because then it has “no limiting impact on the mathematical

results of design models” (Crites et al., 2006) Conversely,

Crites et al (2006) portray the use of a COR to adjust the

design goal as a disadvantage of the areal model, because it

“may result in excessive wetland sizes to achieve low

con-centrations.” Of course, the use of a COR has nothing to do

with how one determines the trend values, as is apparent

from its use with the volumetric model (Crites and

Tchob-anoglous, 1998) And there is no doubt that the larger wetland

sizes needed to contain excursions are required to achieve an

acceptable level of regulatory compliance

O THER D ESIGN P ARAMETERS

Some of the specifications of regulatory permits or licenses are

outside the commonly encountered groups of rational design

parameters, which include BOD, TSS, nitrogen compounds,

phosphorus pathogens, metals, organics, and temperature

These do not have loading charts, nor do they have k-values.

pH

There is often a specified range of allowable pH for

dis-charges to surface waters, typically 6.0–9.0 Most treatment

wetland applications are not likely to exceed such ranges, as

detailed in Chapter 5 Exceptions are the acid mine drainage

wetlands, in which the design goal includes raising the pH of

the incoming water Treatment wetlands are not particularly

effective at neutralizing strong acid, whereas they are quite

good at creating circumneutral pH for more benign influents,

such as food wastewaters Other exceptions include industrial

processes, for which a neutralization step is included as part

of pretreatment

Toxicity

The U.S Clean Water Act prohibits the discharge of toxic

sub-stances to waters of the United States For this reason, whole

effluent toxicity (WET) monitoring is included in the NPDES permits for many municipal treatment plants WET tests are the standardized procedure to detect levels of acute and chronic toxicity in municipal and industrial effluents Individ-ual pollutants that contribute to toxicity may be monitored in some instances, such as for nitrite and ammonia, but biomoni-toring is required to assess the overall net potential for acute and chronic toxicity to receiving water biota or surrogates.The short-term chronic toxicity tests that were eventu-ally developed by the U.S EPA are a relatively inexpensive method of assessing WET (U.S EPA, 1994) Freshwater chronic toxicity tests utilize three organisms:

The water flea (Ceriodaphnia dubia) The fathead minnow (Pimephales promelus) The green alga (Selenastrum capricornutum)

Test methods require a seven-day (96 hours for the algal test) testing period, with renewals of the testing solution using the tested effluent three times during that seven-day period For the water flea, the test encompasses three repro-ductive cycles, involving three broods of young (neonates) The adult water fleas are typically fed three times per day during the testing period Acute toxicity is assessed via mor-tality Chronic toxicity is assessed via the number of young produced per female Tests may be conducted using just con-trol water and 100% effluent, or one or more diluted effluent concentrations

Fathead minnow testing utilizes larval fish, tested over

a seven-day growth period with test water renewals Acute toxicity is assessed through observed mortality of the fish Chronic toxicity is assessed by measurement of the final dry weight of the tiny fish at the end of the test period Laboratory controls are utilized and one or more effluent concentrations are tested to assess the lethal and sublethal effects of the efflu-ent on the fish Some states have specific protocols that vary from the federal WET guidelines, because of the use of other vertebrate and invertebrate species Two alternate species are

rainbow trout (Salmo gairdneri) and a different water flea, Daphnia magna Fathead minnows may be replaced with indigenous fish species such as the bluegill (Lepomis macro- chirus) or the bannerfin shiner (Cyprinella leedsi).

T OXICITY R EDUCTION IN FWS W ETLANDS

A number of studies have shown that constructed wetlands

can be effective in reducing toxicity (Knight et al., 1997;

U.S EPA, 1999; Wetland Solutions, Inc., 2003) U.S EPA- sponsored synoptic studies at six constructed treatment wet-lands in the United States (McAllister, 1992; McAllister, 1993a; McAllister, 1993b), including standardized toxicity tests The Collins, Mississippi, wetland had significant acute and chronic toxicity at the wetland inflow, probably due

to high unionized ammonia concentrations, but acute and chronic toxicity were absent in the wetland outflow West Jackson County, Mississippi, had slight acute and chronic toxicity to the water flea at the wetland inflow but no toxicity

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