TREATMENT WETLANDS - CHAPTER 17 docx

This excess material is TABLE 17.2 Phosphorus Budgets for a FWS Design Example Parameters Note: This example is based on the water budget shown in Table 17.1 ; sequential tank-to-tank ca

A performance-based procedure is unavoidable for FWS

wet-lands because they address too many applications with

vary-ing degrees of source strength and pretreatment In general,

the targets are also variable because of the differences in

reg-ulatory requirements among countries, states, and discharge

recipients Loading specifications, whether as pollutant

kilo-grams per wetland hectare per year or as wetland hectares

per person equivalent, have not, and, in general, cannot be

developed to deal with the spectrum of situations created by

FWS diversity of applications It is not an exaggeration to say

that each FWS wetland is unique

Design of FWS wetlands may be roughly divided into

two categories: sizing calculations and physical

specifica-tions Sizing requires characterization of the incoming water

and regional meteorology as well as the goals of wetland

treatment, as discussed in Chapter 16 Here, a

comprehen-sive sizing strategy is presented, based on the information

assembled Chapter 18 will deal with physical

consider-ations, including the number of cells, layout, depth,

bathym-etry, soils and plants, structures, and lining It is recognized

that wetlands are almost never stand-alone treatment devices,

but rather form part of a treatment train Other components

may be mechanical, such as clarifiers or filters, or more

natu-ral, such as settling basins or lagoons More than one type

of wetland may be involved at the same site—FWS, VF, and

HSSF This chapter focuses on only the FWS component of

treatment systems

Based on the performance-based sizing algorithm

intro-duced in Chapter 15, the key features described in this

chap-ter are the following:

Parameter selection and calculation

6 Select rate constants and seasonality

(Select plant community type.)

7 Select DTD (hydraulic) efficiency  P-value

(Select compartmentalization.)

8 Adjust area till goals are all met

Constraint checks and iteration

9 Set estimated growth cycle

10 Check biogeochemical cycles for consistency

11 Check chemical constraints

12 Check loading graph for risk assessment

13 Adjust for seasonality

Repeat this procedure as needed to meet design goals

The complexity of wetland behavior and, hence, of the

sizing calculation is such that a single equation for wetland

area cannot be written At this point in the evolution of the

treatment of wetland design, the sizing procedure must move out of the realm of the pocket calculator and into the world

of spreadsheet computations The single formula, black-box approach will not suffice

17.1 POLLUTANT REDUCTIONS AND PERFORMANCE COMPUTATIONS

The information collected is utilized as a basis to forecast the area needed to achieve the goals of the project The pro-cedure outlined here is for a steady-flow situation Different flows may need to be considered to deal with daily or sea-sonal peaking Different seasons may need to be considered,

so that the “bottleneck” period of the year is identified and analyzed

Many literature sources provide single equations into which anyone may insert numbers to compute a wetland area Although such an option would be very convenient,

it is not realistic except for a few highly specialized cases Accordingly, the procedure given here explores the effect of changing wetland area (or equivalently, detention time) on the forecasted effluent concentrations of contaminants It is presumed that the designer will conduct the design calcula-tions via a computer spreadsheet so that design changes may

be easily explored

Wetland hydrology is first determined as a necessary sor to area calculations Annualized calculations are considered first; the effects of season will be considered later in the chapter

QoQi •A P( ET I) (17.1)

qoqi(P ET I) (17.2)where

wetland area, mevapotranspiratio

2

A ET

infiltration rate, m/dpreci

I P

 ppitation rate, m/dhydraulic loading rate

flow rate, m /d3

628 Treatment WetlandsThe inlet hydraulic loading will be increased by rainfall

(≈0.5–1.5 m/yr) and decreased by evapotranspiration (ET) (≈

0.5–1.5 m/yr) However, there may be seasonal imbalances

These amounts are important if the wetland is to have a very low

hydraulic loading or, correspondingly, a long detention time

The ratio a i is the atmospheric augmentation

Evapotranspiration (rain) has two effects: lengthening

(shortening) of detention time and concentration (dilution) of

dissolved constituents The use of an average flow rate

com-pensates for altered detention time but not for dilution or

concentration The fractional error in a first-order model

prediction of concentration due to flow averaging is

approxi-mately equal to a, for a −0.5 Thus, if 25% of the inflow

evaporates, use of a first-order model with average flow

pre-dicts concentrations 25% lower than required by the mass

balance If rain adds 25% to the flow, use of a first-order

model predicts concentrations 25% higher

A possibly important feature of ET is that the

transpira-tion carries water into the root zone (see Part I, Chapter 4)

Therefore, if the pollutant mass balances are done on surface

water, then the transpiration component is the same as

infil-tration: it carries materials into the soil For a fully vegetated

wetland, somewhat more than half of ET is attributable to

transpiration Transpiration is typically about one half to two

A  transpiration fraction ofET, dimensionleess

The pollutant mass balances will be conducted on a

cells-in-series basis (see Figure 17.1) Results of the overall water

mass balance are apportioned to the cells according to the

chosen number of TIS For the first unit in the series:

Q1QinA P1( ET I) (17.4)where

area of tank number 1, mflow ra

1

A Q

 tte out of tank 1, m /d3

Flows are thus computed sequentially, from inlet to outlet,

for the number of tanks chosen (PTIS).

The input data requirements for water mass balances are

6 Apparent number of tanks in series (P-value)

An example is detailed in Table 17.1 This hypothetical example is configured to include small rainfall, considerable

ET, and some infiltration; in other words, a leaky arid region

system The net loss of incoming water is 41%, of which 44%

is ET and 56% is infiltration.

POLLUTANT MASS BALANCES

The TIS model is then carried forward via a sequential culation of pollutant concentrations for each “tank” in the chosen hydraulic model (Figure 17.1)

cal-A first-order areal model with rate constant k is selected with necessary wetland background concentration C* A

volumetric first-order model may also be chosen for which

k  EhkV The pollutant mass balance for the first of the land segments, designated by subscript “1” for steady-state,

wet-FIGURE 17.1 Conceptual TIS model for pollutant reduction.

In this simple version, rainfall has been assumed to have zero

pollutant concentration, but it is easy to add an atmospheric

input of the pollutant if it exists Infiltration is assumed to

occur at the outlet concentration Transpiration flow of the

contaminant has been included Combining Equation 17.4

with Equation 17.5 gives the concentration exiting the

hypo-thetical segment number one:

Note that the hydraulic loading rates in Equation 17.7 are the

individual tank loading rates, not the overall system

load-ing rates This computation is then repeated sequentially

for the remaining segments, using, in each case, the outlet

concentrations and flows from the preceding unit The

wet-land outlet concentration is that exiting the final

hypotheti-cal segment

Outgoing pollutant loads are calculated as the product of

the volumetric outflow (m3/d) and the outflow concentration

is extended to illustrate these calculations, continuing the

choice of P  3 TIS Phosphorus is chosen as the pollutant of interest, entering the wetland at 2.00 mg/L The value of C*

is chosen to be low, 0.01 mg/L The rate constant is selected

to be the median shown for phosphorus in FWS systems in Table 10.11, k  10 m/yr Because the transpiration flux is

presumed to draw phosphorus into the root zone, the

frac-tion of ET, that is, transpirafrac-tion, must be selected, and in the

example it is picked as A  0.5

The computed phosphorus concentration in the surface

outflow water is Co 0.62 mg/L, or a concentration reduction

of 69% (Table 17.2) However, the existence of ET losses and

infiltration creates a different result for mass removal: 82%

of the phosphorus entering does not leave with surface water

If infiltration water departs vertically downward without ther treatment, then 15% of the phosphorus mass removal

fur-is due to infiltration Thus, it fur-is seen that load reduction and concentration reduction are two different goals, which may lead to different designs Both these types of design sizing require mass balance computations for water and rate calcu-lations for pollutants

TABLE 17.1 Water Budgets for a FWS Design Example Parameters

Note: Sequential tank-to-tank calculations are based on Equation 17.4; this example has a

com-bined detention time of 19.2 days, whereas averaging the flows leads to 17.2 days; the required input data are shown in bold.

630 Treatment Wetlands

I NTERCONNECTED P OLLUTANTS : T HE C ASE OF N ITROGEN

Nitrogen species interconvert, thereby linking the mass

bal-ances for organic, ammonia, and oxidized nitrogen It is

some-times possible to disconnect these species, as, for instance, in

the case of wetlands that receive nitrate but little or no organic

or ammonia nitrogen However, in many cases, it is

neces-sary to account for the (internal) production of ammonia from

organic sources, i.e., from either the incoming water or the

decomposition of wetland necromass, and the internal

produc-tion of oxidized nitrogen A simple, presumed chemistry is

ORG-NlNH -N4 lNO -Nx lN2 (17.8)

In a simplified version of analysis, uptake and return from

biomass is not included The effects of the biogeochemical

cycle on nitrogen will be explored in a subsequent part of the

design process The three mass balances then become linked,

and the tank equations are:

(17.14)

Equation 17.12 is a direct analog of Equation 17.7 for an unspecified generic pollutant Equations 17.13 and 17.14 contain extra production terms from ammonification in the ammonia balance—and nitrification in the oxidized nitrogen balance The three must be solved sequentially—Equation 17.12 followed by Equations 17.13 and 17.14

D ESIGN P ARAMETERS : S OURCES OF I NFORMATION

The P-k-C* design model is the basis for this sizing analysis;

it is therefore necessary to select values of these three eters for all pollutants of concern

param-Background Concentrations

Wetland systems are dominated by plants (autotrophs), which act as primary producers of biomass However, wetlands also include communities of microbes (heterotrophs) and higher animals, which act as grazers and reduce plant biomass Most wetlands support more producers than consumers, resulting

in a net surplus of plant biomass This excess material is

TABLE 17.2 Phosphorus Budgets for a FWS Design Example Parameters

Note: This example is based on the water budget shown in Table 17.1 ; sequential tank-to-tank calculations are based

on Equation 17.6; the required input data are shown in bold; the shaded cells represent potential design targets.

typically buried as peat or exported from the wetland (Mitsch

and Gosselink, 1993) The net export results in an internal

release of particulate and dissolved biomass to the water

col-umn, which is measured as nonzero levels of biochemical

oxygen demand (BOD), total suspended solids (TSS), total

nitrogen (TN), and TP These wetland background

con-centrations are typically denoted by the term C* Enriched

wetland ecosystems (such as those treating wastewater) are

likely to produce higher background concentrations than

oli-gotrophic wetlands These elevated background

concentra-tions are largely due to increased biomass cycling resulting

from the higher levels of nutrients and organic carbon in the

wastewater Even land-locked wetland basins, which only

receive water inputs through precipitation, will have nonzero

background concentrations

Consequently, many pollutants are not reduced to zero

in treatment wetlands—including BOD, TSS, organic

nitro-gen, and phosphorus However, it is important to distinguish

between artifacts of data fitting and real wetland processes

Short-circuiting can lead to high values of data-fit C* for

heavily loaded systems, but these high C* can be dealt with

by improving the hydraulics Independent of hydraulics, the

wetland can manufacture water-phase organics and solids,

and cycle nutrients into and out of the water body The

chap-ters of Part I contain estimates of the C*-values for the

com-mon pollutants, and many literature sources provide ranges of

background concentrations (U.S EPA, 1999; IWA Specialist

Group on Use of Macrophytes in Water Pollution Control,

2000; U.S EPA, 2000a; Wallace and Knight, 2006; Crites

et al., 2006) A summary is given in Table 17.3.

Some individual exotic chemicals are foreign to

typi-cal wetland environments and are not expected to exhibit

background concentrations Examples include halogenated

hydrocarbons and pesticides

Number of Tanks to Be Used in the Model

The performance of the wetland depends on the number

of tanks (TIS) selected—very strongly if the design is to

approach wetland background concentrations or high degrees

of removal (see Chapter 6) If a high degree of removal is required, it will necessitate a very large wetland with poor hydraulics, or a smaller wetland with good hydraulics Figure 17.2 illustrates this high degree of sensitivity in the

region of low P for large reductions This chart quantifies the

fact that a tiny fraction of the water following a fast circuit will carry enough unreacted material to the outlet to make 99% reduction almost impossible to achieve Low

short-P numbers represent flow patterns that, by virtue of either

fast forward mixing or velocity profiles with high-speed ments, carry fractions of unreacted material directly to the outlet

ele-The P-value is somewhat at the discretion of the designer

More cells and greater length-to-width ratios can increase the

P-value As an illustration of the design decision to be made,

consider a hypothetical case relative to Figure 17.2 Suppose the wetland is to achieve a 90% reduction It is possible to

consider a one-cell wetland, with a presumptive P  3 that

needs 84 m2/(m3/d) Or, the designer can opt for two cells in a

series, each with a presumptive P  3 that needs 68 m2/(m3/d) The question might be resolved based on the economics, i.e., does the cost savings of 20% area reduction outweigh the added cost of the divider berm and structures? This illus-tration carries a zero background concentration, but the concepts are applicable to any pollutant, provided the reduc-tion fraction to background is used instead of percentage removal

It should be remembered that the designer can control

the N-value for the wetland (inert tracer tanks in series) but cannot entirely control the P-value Pollutants that are mix-

tures, which may undergo weathering in the wetland, act to

reduce the applicable P-value Table 6.3 provides some

guid-ance on the apparent P-values to be selected relative to tracer

N-values.

Rate Coefficients

In this initial, annualized analysis, the appropriate rate ficients are those (shown in the chapters of Part I) as the result of the fitting of annual data from existing wetlands Those are variable across wetlands and years, thus producing

coef-frequency distributions of the fitted k-values These are to be

found as follows:

Organic N Chapter 9, Table 9.11Ammonia N Chapter 9, Tables 9.17 and 9.20Total Kjeldahl

nitrogen (TKN) Chapter 9, Table 9.12Oxidized N Chapter 9, Table 9.23Total N Chapter 9, Table 9.14Total P Chapter 10, Table 10.11Fecal coliforms Chapter 12, Table 12.3Conspicuously absent from this list is the common constitu-ent TSS Some individual system data were presented in

Heavily Loaded

Note: In the case of fecal coliforms, lightly/heavily loaded refers

to animal use.

632 Treatment Wetlands

Chapter 7 and analyzed for rate coefficients However,

incom-ing TSS is often reduced rather quickly, and wetland-effluent

TSS results from a balance of generation and resuspension in

the FWS wetland The removal rate coefficients that pertain

to the inlet region of the wetland are often quite high, ≈10 m/d

(3,650 m/yr), as shown in Figure 7.8 Colloidal materials are

an exception to this generality The design recommendation

suggested here is the use of a high-rate coefficient for TSS

(perhaps 200 m/yr), unless the design-limiting bottleneck

happens to be TSS In that event, it is strongly suggested that

settling tests be conducted on the candidate wetland influent

waters

The frequency distributions of the reference tables are

generally quite broad It is up to the designer to narrow the

selection for a given application, either by choosing the

pre-ferred degree of risk (blind to modifying factors), or by

delv-ing further into the details of existdelv-ing wetland data sets, and

to narrow the selection to wetlands closest to the intended

application in terms of operating conditions Regrettably,

there is no modern, published, all-inclusive database to which

to turn The reader is cautioned that older databases such as

the NADB (Knight et al., 1993) and the 1994 Danish

data-base (in Kadlec and Knight, 1996) have been superseded

Also, old databases are uneven in the quality and quantity of

the data presented There are numerous examples of

misin-terpretation in such databases, and it is concluded that their

use as a sole source of narrowing the information field is

dan-gerous Because there are now so many treatment wetlands, it

is not feasible to provide detailed data In this book, the

com-promise is to provide analysis, references, and the generic list

of sources used herein (see Appendix A)

D ESIGN S IZING G OALS : L OAD R EDUCTION

VERSUS C ONCENTRATION R EDUCTION

A key feature of treatment wetlands is the ability to design

or manage the system for either concentration reduction or

for mass removal, but only one at the expense of the other (Trepel and Palmeri, 2002) Wetland performance, as mod-eled previously, follows the rule of mass action: the removal rate of a pollutant is greater at higher pollutant concentra-tions in the water The first-order model assumes a (nearly) direct proportionality: doubling the concentration doubles the removal rate As a result of this observed behavior, removal rates decrease as water passes through the treatment wet-land, and pollutant concentrations are reduced (Figure 17.3) However, the actual mass of the pollutant that is removed increases with increasing hydraulic loading Thus, increas-ing hydraulic loads result in more kilograms removed, but at the expense of higher effluent concentrations This trade-off between removal efficiency and load reduction is a key feature

of wetland design for nutrient control These may be

quanti-fied via the first-order model A simple version for C* 0 and no water loss is

¦¥

³µ´

condi-If the output load of a pollutant is to be held below some regulatory limit, then that has the same general effect as specifying an outlet concentration However, because the load of a pollutant in the wetland outflow is usually taken to

FIGURE 17.2 The effect of the number of tanks on the area required for different degrees of removal The calculations are for k  15 m/yr

and C* 0.

10 100 1,000 10,000 100,000

be that in the surface water discharge, any reduction in flow

through the wetland assists in providing lower output loads

as well as higher load reductions

All three of the potential design goals (concentration out,

load out, load reduction) may be sensible, depending on the

pollutant in question These are marked as shaded cells in

the example in Table 17.2 For instance, a low required outlet

concentration can be of use in preventing ammonia toxicity

in receiving waters But if there is a load allocation, as might

be the case for a discharge contributing to the total

maxi-mum daily load (TMDL) for an impaired water body, then

the mass of the contaminant is of more direct interest Lastly,

if there is a desire to maximize the benefits of a given

wet-land footprint, then the goal should be to dissipate or retain

the maximum amount of pollutant in the wetland

Because it is easy to confuse these potentially conflicting

goals, it is recommended that the designer clearly identify

and state the purpose of the design

17.2 AREA COMPUTATIONS

At this point in the development of constructed wetland

tech-nology, it would be disingenuous to provide overly simplistic

design-calculation procedures But it is also a mistake to think

that adding more factors that may play a significant role in

perfor-mance lends more accuracy or precision to design predictions

GOAL SEEKING: DETERMINATION

OF THE REQUIRED WETLAND AREA

The fundamental and straightforward technique for

deter-mining the area is to adjust that area until the specified

crite-rion is met That is easy if the calculations have been set up

on a spreadsheet; the area can then be sequentially and

man-ually changed by the user until the criterion is met, or

auto-matic searches may be invoked, such as the Solver™ routine

in Microsoft Excel™ The hypothetical phosphorus example

is continued to illustrate this process

Concentration Criterion

A common criterion for phosphorus in the United States is for monthly means to be less than 1.0 mg/L Realistically, the project owner would not want to encounter exceed-ances very often For the example, choose the compliance frequency to be 90% From Table 10.13, the multiplier to contain exceedance at that frequency is 1.94 Therefore, the design target is adjusted downward to 1.00/1.94  0.52 mg/L, which becomes the value to be achieved as the wetland area

is varied

In just three manual iterations, the starting guess of 24

ha (Co 0.62 mg/L, Table 17.2) is changed to 27.6 ha (Co0.52 mg/L) The same result may be obtained using Solver™,which provides the answer in an essentially instantaneous search from any plausible starting condition

The exceedance containment multipliers for common constituents may be found as follows:

Organic N Chapter 9, Table 9.11Ammonia N Chapter 9, Table 9.21Oxidized N Chapter 9, Table 9.25Total N Chapter 9, Table 9.16Total P Chapter 10, Table 10.13Fecal coliforms Chapter 12, Table 12.4(TKN is not in this list because it is not normally regulated.)

Maximum Load Criterion

The same philosophy of regulation might lead to an annual load limitation on the effluent from the system An outlet con-centration of 1.0 mg/L at a flow rate of 5,000 m3/d (inflow rate) implies a maximum annual load of 1,825 kgP/yr leaving the wetland (50% load reduction) A search for the wetland area leads to a value of 10.1 ha This is very much lower than that for the concentration goal for two reasons First, the exceed-ance factor is not in play because of the annual character

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Hydraulic Loading, HLR (m/yr)

0 50 100 150 200 250 300

2 · yr)

Concentration Reduction Load Removal

FIGURE 17.3 Concentration reduction and load reduction as a function of hydraulic load for a hypothetical nitrate treatment wetland

Parameters: Ci 10 mg/L, k  35 m/yr, C*  0, P  4.

634 Treatment Wetlands

of the load limit Secondly, credit is built into the calculation

for the loss of water to ET and for wetland infiltration.

M INIMUM L OAD R EDUCTION C RITERION

A slightly different regulatory philosophy requires a minimum

load reduction For instance, some wetland systems in South

Florida require a minimum of 75% phosphorus load reduction

A search for the wetland area leads to a value of 19.8 ha

M ULTIPLE C OMPOUNDS OF C ONCERN

The projected outlet concentrations of all constituents of

interest are calculated via the preceding Equation 17.7 (or

Equations 17.12–17.14 in the case of nitrogen) All computed

outgoing concentrations and loads vary with the selected

wetland area Area is adjusted until the most stringent

cri-terion is met Criteria often include outlet concentration

specifications, each with an associated allowable frequency

of exceedance But in other cases, outlet pollutant loads may

be specified (or load reduction) Loads are easily calculated

using the outlet concentrations and flows from the last unit

If the contaminants are considered singly, then a last step

remains: all performances are calculated on the controlling

(maximum required) wetland area

As an example of multiple contaminants, the previous

phosphorus illustration, along with the additional

require-ments to also reduce BOD and total nitrogen, is continued

BOD enters the wetland at 30 mg/L, and total nitrogen, at 30

mg/L The value of C* is chosen to be 2 mg/L for BOD, and

shown for BOD and TN in FWS systems in Tables 8.2 and

9.14, i.e., kBOD 33 m/yr and kTN 13 m/yr It is assumed

that exceedances must be contained at the 90th percentile

The multipliers are 1.56 for BOD5 (Table 8.6) and 1.55 for

TN (Table 9.16)

The calculations of the required areas for each of the

three contaminants are shown in Table 17.4 The largest area

is needed for the reduction of total nitrogen—40 ha for the

5,000-m3/d flow When that area is applied to the calculation

of performance for BOD5 and TP, those pollutants are reduced more than necessary (Table 17.5)

This concludes the preliminary calculation of the required wetland area It may seem that there are no further steps needed, but in fact there is no guarantee that these pre-liminary calculations fit into reasonable, known patterns of wetland behavior It is critical that the bounds of plausible biogeochemical cycles are not exceeded, that the required ancillary chemicals (oxygen, carbon) are present in ample supply, and that the forecasted results are reasonable com-pared to known performance data Further, the seasonality of the system has yet to be investigated via forecasting

17.3 CHECKING THE BIOGEOCHEMICAL CYCLES

In this phase of design, vegetation in the prospective land and its role in nutrient processing is brought into the picture The rate coefficient-based analysis has provided first estimates of the quantities of materials entering and leaving the system, but there has been no allocation of the removals

wet-to the various processes that comprise the entire ecosystem function It is not feasible to go too far in breaking down processes because the knowledge base does not support great detail Here, the processing of carbon, nitrogen, and phos-phorus are examined via the mass balances used in ecosys-tem analysis in Part I (see Chapter 9, Figure 9.14)

In the analysis that follows, two points are important First, the analysis is based on estimates of what the eco-system will be like Such estimates cannot be precise, and, consequently, the analysis is “order-of-magnitude” only The intent is to gain some insight into the relative importance of wetland processes that will likely be operating if the proj-ect is built Second, the analysis is for an annual period, and hence there remains the issue of seasonality Seasonal analy-sis proceeds more readily if we can first establish whether the wetland should be viewed as an agronomic system or a microbial system

C, N, AND P CYCLES

The pollutant removals simplistically described in the

pre-ceding section take no account of wetland functions; they are

based on purely empirical k-values and outlet concentration

data It is therefore prudent to examine the empirical

fore-casts and to ascertain whether they comply with an estimated

set of wetland processes Of particular interest are the

car-bon, nitrogen, and phosphorus cycles

The carbon cycle involves the growth, death, and

decom-position of biological materials, including animals, plants,

algae, and microbes For many wetlands, the standing crop

of these organic materials is relatively constant throughout

the year, but the proportions of living and dead may vary

considerably, as may the physical location (i.e., standing dead

or litter) Further, the speed of cycling varies with the nature

of the material and the time of the year (see Chapter 3) Fine

detritus from microbes cycles rapidly, whereas the

decompo-sition of some woody plant parts may take years However,

the important feature of the carbon cycle is the amount of

material that does not decompose, for it is this residual that

accretes in the ecosystem and forms storage for many

pollut-ants, including phosphorus This storage is relatively

perma-nent under appropriate hydroperiod conditions

An approximate assessment of the implied impacts of

the carbon cycle can be made on the basis of two

numeri-cal characteristics: the speed of the cycle in grams of dry

material per square meter per year, and the fraction of the

cycled material that does not decompose The speed of the

cycle may also be characterized by the standing crop, in

g/m2, plus a turnover rate, in number of times per year The

recycled organic fraction contains carbon that is a source of

support for denitrification and other heterotrophic processes

The burial fraction leads to sediment buildup and storage of

nitrogen, phosphorus, and other trace contaminants that

par-tition to organics Typical values for the vegetative standing

crop, turnover time, speed, and burial fraction are given in

Table 17.6

In general, the algal and microbial standing phytomass

is small compared to the vegetative phytomass However,

turnover times are also small, so the annualized rate of

uptake and burial for this component of the phytomass may

be estimated to be 50–100% of the vegetative annual rates

Simply stated, the amounts of N and P processed by the big

green biomass are not very much larger than those processed

by the nearly invisible microbes and algae This nondisparity

has been characterized as the “buckets and teacups” analogy

set forth by Richardson et al (1986).

The term nutrient poor would reflect very low nutrient

status, with TP < 20 µg/L and NH4-N 0.2 mg/L

Nutrient-moderate wetlands would have TP < 200 µg/L, with NH4

-N < 1.0 mg/L -Nutrient-rich wetlands would have TP ≈ 1.0

mg/L, with NH4-N ≈ 5.0 mg/L Very-nutrient-rich systems

would have TP  5.0 mg/L, with NH4-N  10.0 mg/L These

definitions do not correspond to the equivalents for aquatic

water bodies, where the dominant vegetation is plankton In

the treatment wetland context, bacterial and algal materials

are of comparable importance with above- and belowground macrophytic vegetation

The magnitude of the biogeochemical cycle increases with increasing nutrient availability—up to some presumptive limit enforced by availability of space and sunlight This fer-tilizer response is not well-quantified for treatment wetlands and, therefore, cannot be used directly in design However, rough estimates are of value in assessing the potential impor-tance of the biogeochemical cycle—particularly to check that the empirical design calculations do not imply unreasonable ecosystem functions

One technique for making such checks is to graphically link a presumed cycle to the mass balance calculations avail-able from the preliminary sizing step The biogeochemistry

check, thus, is implemented via linked ecosystem mass

bal-ances, one each for C, N, and P These do not replace, nor

can they substitute, the water column mass balances used in

k-rate removal calculations In an annualized mass balance

storage calculation, the following equations may be used:

wherebiomass burial, g/m dbiomass deco

2

B D

biomass growth, g/m d

2 2

The Carbon Cycle

The nomenclature for biomass processes in treatment lands is a bit confusing The growth, death, and decomposi-tion processes are referred to as part of the wetland carbon cycle, but more than carbon is involved However, most veg-etation and other wetland organisms are about 40% carbon;

wet-so, either dry biomass or carbon serves to track the amount

of the material involved Carbon itself is withdrawn from atmospheric sources as carbon dioxide for photosynthesis Likewise, it is returned to the atmosphere as methane from anaerobic mechanisms, or carbon dioxide from oxidative processes (respiration included)

The ability to estimate nutrient cycling rests upon our knowledge of the biomass pools in the wetlands and their changes Guidelines are shown in Table 17.6 According to Equations 17.17–17.19, on an annual basis, the important esti-mation quantities are

 necromass biomass times turnover per year)

Annual burial fraction (undecomposable residual

fraction)

636 Treatment Wetlands

An assumption is made that the nutrients taken up, but not

buried as accretion, are returned to the water column of the

FWS wetland For nitrogen, this is the maximum estimate, as

microbial processes in abovewater tissues can transfer

nitro-gen to the atmosphere without entering the water These

phy-tomass quantities, together with phyphy-tomass nutrient content

(percentage or mg/kg), allow checks on the empirical removal

calculations

For purposes of design, for order-of-magnitude checks

on the calculated nutrient removals, the total growth rate and

burial fraction are assumed based on the strength of the water

to be treated This provides a rough annual estimate of the

biomass cycle (Figure 17.4) Note that the magnitude of this

cycle and the nutrient contents are a function of the degree of fertilization of the wetland (see Chapter 3)

The wetland carbon cycle is also critical to observe formance as it relates to sediment oxygen demand and to the carbon supply for denitrification The implied supply con-straints of this carbon cycle are examined in the constraint check section of this chapter

per-The Phosphorus Cycle

For phosphorus, the calculated removal is represented as a

large uptake (GX G)—in major part balanced by the return

of soluble phosphorus from tissue decomposition (DX D) For

TABLE 17.6

Estimates of Wetland Nutrient Cycling in Biomass

Nutrient Poor Nutrient Moderate Nutrient Rich Very Nutrient Rich Organic matter

Note: Bacterial and algal cycling are included in these values.

Source: For first two categories, data from Davis (1994) In Everglades: The Ecosystem and Its Restoration Davis

and Ogden (Eds.), St Lucie Press, Delray Beach, Florida, pp 357–378 For the second two, data from Kadlec (1997a)

Ecological Engineering 8(2): 145–172.

FIGURE 17.4 Estimated annual biomass cycle in a FWS treatment wetland for a rich nutrient condition Note that the standing stock and

turnover refers to above- and belowground material, and to macrophytes, algae, invertebrates, and microbes.

Standing stock: 3,000 g/m2 · yr

Turnovers per year: 3.0 Turnover rate: 9,000 g/m2 · yr

Burial fraction: 0.250 Bulk density: 0.100 g/cm3

Accretion: 2.25 cm/yr

Necromass

Soil Air

a stable ecosystem past startup, the net phosphorus removal

associated with the k-rate calculation is assigned to accretion

Sorption and the building of additional biomass are no

lon-ger sinks for phosphorus (Kadlec, 1997a) Because the k-rate

calculation is independent of the biomass cycle calculation,

there is one degree of freedom, which is taken to be the

com-puted accretion fraction It is known through extensive data

from treatment wetlands that this percentage ranges from

≈500 to 5,000 mgP/dry kg, or from 0.05–0.5% dry weight

A somewhat narrower range is shown in Table 17.6 because

extremely nutrient-poor wetlands are uncommon in

treat-ment applications It is unlikely that higher removals can be

sustained via accretion The corresponding accretion rates

are 0.01–20 gP/m2·yr The values in Table 17.6 are provided

as an order-of-magnitude check point If a design is forecast

to exceed these approximate cycling, removal, and storage

values, then more detailed mass balances should be invoked

If it appears that the phosphorus removal calculated by a

k-rate exceeds the ecosystem capacity to store, then the k-value

should be adjusted downward

As a graphic illustration, the design example is

car-ried forward (Figure 17.5) Because the nutrient

concentra-tions are high (2 mg/L P and 20 mg/L TN), it is anticipated

that the nutrient-rich condition will prevail, as illustrated in

Figure 17.4 The biogeochemical cycle removes 27 gP/m2·yr

from the water column (9,000 gdw/m2·yr × 0.3% P), far more

than the loading to the wetland of 9.1 gP/m2·yr The k-rate

cal-culations indicate that 7.4 gP/m2·yr are removed, and, hence,

it is deduced that (27 − 7.4)  19.6 gP/m2·yr are returned to

the water from decomposition and leaching of the biomass

The removal to accretion is thus 7.4/27  28% of the biomass

uptake These uptakes and returns involve aboveground plant

parts (≈50%), belowground plant parts (≈15%), and microbes

and algae (≈35%) The 28% burial rate for phosphorus is

somewhat higher than the comparison value in Table 17.6 of 20% If phosphorus were the controlling substance for siz-ing, it would be advisable to adjust the rate coefficient down-ward from the value of 10 m/yr used in the design forecast But, the wetland is overdesigned for phosphorus reduction,

and, hence, it is probably not necessary to revise the k-rate

calculation

The Nitrogen Cycle

For nitrogen, a more complex situation occurs as a result of the multiple speciation of water column nitrogen The details of nitrogen mass balancing have been presented in Part I, Chap-ter 9 The water column contains organic, ammonium, and nitrate nitrogen Their interconversions are computed from

the empirical k-rates The biogeochemical cycle is linked in

a manner analogous to phosphorus, but with abstraction from both the ammonium and nitrate pools in the water, and return from both the ammonium and organic pools in the water This allocation recognizes a split of plant uptake between ammo-nium and nitrate (Martin and Reddy, 1997) and the fact that decomposition processes produce organic nitrogen The nitro-gen content in accreting sediments is known from extensive

data from treatment wetlands to range from ≈1.0–2.5% dry

weight (Table 17.6) A lower value would be associated with nutrient-poor wetlands, a higher with nutrient-rich systems

Again, because the k-rate calculations are independent of the

cycle calculation, there is one degree of freedom, which for nitrogen is taken to be an assumed percentage of the cycled nitrogen that is buried in accreting sediments A nitrogen deficit may be assumed to be supplied by fixation, a process known to occur in nitrogen-deficient wetland environments

FIGURE 17.5 Estimated annual phosphorus cycle in a FWS treatment wetland for a nutrient-rich condition Flows, concentrations, and

loadings leaving the wetland are from k-rate forecasts The cycle turnover and tissue-phosphorus concentrations are presumptive The

frac-tion of phosphorus buried as accrefrac-tion is calculated by mass balance.

Total Phosphorus

in Water

Litter Live

638 Treatment Wetlands

An illustration of the use of the nitrogen cycle and

inter-conversions to confirm the design k-rate calculations is shown

in Figure 17.6 The previous illustrative example is continued,

but for simplicity, the details of the k-rate calculations for the

various nitrogen species are not shown here As indicated in

Figure 17.6, the incoming nitrogen is presumed to be mostly

organic (9 mg/L) and ammonia (10 mg/L), comprising nearly

all of the 20 mg/L of TN The nitrogen loading to the wetland

is 91 gN/m2·yr, which is a low loading that places the system

in the category of an agronomic system (see Chapter 9) It is

therefore expected that the biogeochemical cycling of

nitro-gen will play an important role in the overall reduction, but

not to the exclusion of microbial processes

Once the size of the biomass cycle has been selected,

together with the phytomass nutrient content, mass balances

fix all the nitrogen fluxes for the specified inflows and

out-flows For this proposed design example, the cycling of

bio-mass nitrogen is very important The required nitrogen to

build the annually cycled biomass is 180 gN/m2·yr, which

is just about double the amount of nitrogen supplied in the

wastewater (Figure 17.6) However, there is no concern that

the plants will starve, because there is a correspondingly

large return flux of nitrogen from leaching and

decomposi-tion of necromass It is again necessary to keep in mind that

these uptakes and returns involve aboveground and

below-ground plant parts as well as microbes and algae

The ultimate fate of removed nitrogen in the example

is apportioned to accretion (56%), denitrification (21%), and

seepage (23%) Interestingly, this does not mean that there are

only small amounts of nitrification occurring, because about two thirds of the incoming nitrogen load winds up being nitrified (61.7 out of 91.3 gN/m2 yr, Figure 17.6) because of leakage and a small amount of biomass uptake The question then arises as to whether there is sufficient oxygen supply to support the nitrification implied by this mass balance This implied supply is evaluated in the next section

17.4 CHEMICAL SUPPLY CONSTRAINTS

Traditional chemistry assumptions indicate a requirement for carbon to support heterotrophic denitrification and oxygen to support nitrification

Table 17.7 lists some of the more important chemical and biological constraints on wetland processes The top block

of information shows that the carbon balance, and, hence, the implied biochemical oxygen demand (BOD), is due to removal of incoming BOD and to the carbon formed by the biogeochemical cycle Denitrification requires 4.0 g of chem-ical oxygen demand (COD) to reduce 1 g of nitrate nitrogen (U.S EPA, 1993b; Crites and Tchobanoglous, 1998) Decom-posing biomass is far more important than added BOD in the water in many instances However, some moderate fraction

of the biomass decomposition takes place in air, thus ing the available carbon for denitrification

reduc-The second block concerns nitrification, for which there are associated oxygen and alkalinity requirements An oxy-gen equivalent is retrieved as the result of denitrification (third block, Table 17.7), as well as some of the required

FIGURE 17.6 Estimated annual nitrogen cycle in a FWS treatment wetland for a nutrient-rich condition Flows, concentrations, and

load-ings leaving the wetland are from k-rate forecasts The cycle turnover and tissue–N concentrations are presumptive The fraction of biomass

nitrogen buried as accretion is assumed to be 25%.

Denitrification

Uptake Uptake

alkalinity for nitrification The carbon produced in the

wet-land supplies some or all that is necessary for denitrification

(fourth block, Table 17.7) The accretion process implies a

rate of sediment buildup determined from the sediment bulk

density Finally, the possibility of sulfur-driven

denitrifica-tion carries a demand for sulfide-sulfur

These constraints are critical to design because wetland

chemical processes cannot proceed without the necessary

ancillary compounds Nowhere is this more apparent than in

the need for oxygen to drive ammonia removal routes

There-fore, if inadequate supplies of ancillary chemicals are

fore-cast, rate constants and loadings must be reduced until the

constraints are met Alternatively, additional supplies may be

introduced into the wetland This may require costly

supple-ments, such as the addition of methanol to fuel denitrification

(Gersberg et al., 1984) However, sometimes rearrangements

of flows can resolve the supply problem, such as the feed-

forward of high BOD unpretreated influent for a carbon

sup-plement for denitrification (Burgoon, 2001)

OXYGEN SUPPLY

The various oxygen requirements for the example design are

calculated (estimated) in Table 17.8 Removal of BOD (0.35

gO/m2·d) and nitrification (0.73 gO/m2·d) appears to exert

only a small demand, which should easily be supplied by

atmospheric reaeration The decomposition of the generated

necromass would consume something like 10 gO/m2·d, but

that decomposition is in part due to anaerobic processes and,

in part, occurs above the water as standing dead materials

oxidize Another part of oxidation may involve root tion, which comprises plant oxygen transfer Therefore, it is difficult to attach great significance to the apparent need for large amounts of oxygen for necromass oxidation

of solid residuals is an important factor The implied rate of sediment buildup is 2.3 cm/yr, which is comparable to the rates observed in operating wetlands (Kadlec, 1997a)

I NTERSYSTEM P ERFORMANCE C HECKS

The question addressed next is, how does the proposed design compare with results from existing wetlands for which there

is operating data? The annual performance period is retained for this phase of investigation

The intersystem loading chart for phosphorus (ure 17.7) shows that the overall scatter follows an increasing trend of outlet concentrations with increasing loading How-ever, in design, the loading variable is usually restricted to

Fig-a fixed inlet concentrFig-ation, whereFig-as the hydrFig-aulic loFig-ading is variable in response to different choices of the wetland area Therefore, in any particular design, only a subset of the larger loading group is relevant For phosphorus and most common pollutants, a particular inlet concentration subset shows a more modest increasing trend than that of the whole set

A generic loading chart looks similar to the tual “cloud” indicated in Figure 17.8 The larger data group encompasses all systems for all inlet concentrations, whereas each subgroup at a specific range of inlet concentrations occupies a smaller group The left side of either data cloud represents less efficient systems, and hence these are proto-types of low risk for the design The right side represents more efficient systems, a riskier assumption Any perfor-mance calculation is represented by a single point on this plot and, therefore, may be compared to the intersystem data scatter on this chart

concep-The loading graphs that allow intersystem comparisons for common constituents may be found as follows:

Organic N Chapter 9, Figure 9.17Ammonia N Chapter 9, Figure 9.37

Oxidized N Chapter 9, Figure 9.51

Fecal coliforms Chapter 12, Figure 12.11

Supply and Demand Constraints

Nitrate-N equivalent of BOD

removed

COD (BOD) SOD equivalent of decomposing

biomass

Max oxygen required for BOD

Alkalinity produced by

denitrification