TREATMENT WETLANDS - CHAPTER 21 pot

Thus, wetlands expo-such as FWS and surface-flood vertical flow VF systems cannot be used in these applications, and the regulations force the designer to use HSSF wetlands or VF wetlan

The purpose of this chapter is to provide information that

will help ensure successful outcomes for projects using

hori-zontal and vertical subsurface flow (SSF) wetlands (or

bio-solids wetlands) by discussing the internal configuration of

these systems Common construction pitfalls and start-up

errors are also discussed

Implementation of SSF wetland systems consists of the

following three stages:

1 Physical design (specification of internal components)

2 Construction of the wetland

3 Start-up (commissioning)

The sizing of the system can be done using different sizing

tools, including loading charts, scaling factors, and

first-order modeling The pros and cons of these methods have

been discussed in Chapter 20

The first implementation stage, physical design, involves

making decisions as to the number of cells, site grading,

aspect ratio, bed depth, internal piping, media size,

hydrau-lics, water level control, insulation, etc The

configura-tion of these components is then typically documented in

a set of technical drawings (engineering plans) and written

specifications

Construction of the system consists of preparing bidding

documents, retaining a contractor, and physically building

the wetland This is the most critical phase of the

imple-mentation because construction of SSF wetlands is

essen-tially a “one-way street.” Problems with site grading and

elevations become difficult and expensive to correct once

the liner (if any) is in place Problems with the liner become

next to impossible to correct once the bed media is installed

Because it is so difficult to undo construction problems, the

wetland designer is often retained in some advisory capacity

to observe and document the construction work

The final stage, commissioning, includes introducing the

flow and pollutant loading into the wetland, as well as

man-aging water levels in the wetland in a manner consistent with

vegetation establishment and regulatory compliance

The overall implementation process may seem

decep-tively simple, but it is not Determining the wetland size

is not the same as designing the system Mistakes made

in the technical drawings and specifications will lead to

serious problems down the road Mistakes made during

construction may be extremely difficult to fix, and repairs

are likely to cost many times more than the original

com-ponents Failure to understand how the wetland is

sup-posed to operate leads to start-up problems Any of these

situations is likely to lead to the intervention of regulatory

authorities

Characterization of the influent waste stream and eral site assessment issues have been previously discussed in

the necessary wetted area) have been reviewed in Chapter 20 The next stage in the implementation process is deciding how the wetland should be configured, how internal components should be placed, and how large they are Those issues are discussed in more detail here

21.1 SITING

General conditions of the potential site will have been sidered during the establishment of the basis of design There will remain a number of possibilities for the location of the system within the overall site confines These factors have been previously discussed in a general way in Chapters 16 and 18 for FWS wetlands; these also apply to SSF wetlands This chapter considers additional factors that often come into play when siting SSF wetlands These systems are typically smaller than FWS wetlands and, consequently, are often con-structed in or near existing infrastructure

con-CONSTRUCTION ACCESS

Although the as-constructed footprint area of an SSF land may be small, the designer must keep in mind that these wetland cells will be constructed with earth-moving equip-ment such as backhoe excavators The size of this equipment requires a clear perimeter area around each wetland cell; the track size of the equipment will define the minimum separa-tion distance between the wetland cell, the site boundary, and additional wetland cells Large excavators are often preferred

wet-by contractors because the long reach of these machines facilitates placement of bed media during construction An example of a construction-imposed separation distance is shown in Figure 21.1

Typical separation distances are in the range of 5–7 m around the perimeter of each wetland cell to allow access by construction equipment Designers should keep in mind that devices such as cranes (Figure 21.2) may be required to set primary treatment devices such as settling tanks, so over-head vertical clearance (from obstacles such as power lines) may be another construction access issue Excavations for tanks, pipes, wetland basins, and water level control struc-tures raise another access issue Clearly, there must be an adequate location on the site to temporarily store excavated soil With deep excavations (especially trenches), the exca-vation may have to be braced, or the sidewalls may have to

be stepped back to avoid the risk of cave-in This greatly increases the amount of site area and access needed during

the construction period This is especially important in

sandy soils that have a low angle of repose and thus require

large excavation widths

It should be noted that heavily wooded sites will require

clearing and grubbing, as discussed in Chapter 18, and remote

sites may necessitate the construction of access roads, adding

to the overall construction cost

S LOPES

SSF wetlands require the construction of level beds On

slop-ing sites, this will necessitate terracslop-ing of the areas where

wetland beds are to be constructed On steeply sloping sites,

this will require substantial cut (earth removal) on the uphill

side and substantial fill (earth placement) on the downhill

side; this additional earthwork can far exceed the volume of

the wetland basin itself Consequently, steeply sloping sites

are often not preferred construction areas, unless no other

alternative is available

It should be noted that completely flat sites may also require substantial earthworks if gravity flow through the treatment process is desired Sites with 1–3% slopes are often the easiest to work with in terms of establishing a hydraulic profile that flows by gravity

E XISTING U TILITIES

SSF wetlands are often used for homes and villages that have previously lacked wastewater treatment Because the wetland (and any associated collection system component)

is often the last utility in the ground, there is always the risk

of hitting and damaging other underground utilities, such as water pipes, gas lines, electrical power lines, or telephone wires Most utilities in North America have a “one-call” tele-phone number to dispatch an individual to locate these utili-ties Obviously, sites with known utility conflicts should be avoided, or provisions to reroute the affected utilities must be incorporated into the design process

FIGURE 21.1 Separation distance between these two vertical flow wetland cells was dictated by the need for access by construction

equip-ment at The Preserve, Minnesota The use of such a large excavator was desirable to place gravel media in the wetland cells Alternate means

of gravel placement, such as an extendable conveyor, would decrease this separation distance.

FIGURE 21.2 Installation of settling tanks at The Preserve, Minnesota Note that construction access is needed for four different

compo-nents: (1) the truck and trailer used to deliver the tanks, (2) the crane used to set the tanks, (3) the excavation itself, and (4) the area needed

to stockpile the excavated soil Thus, the total area needed for construction is far greater than the tanks themselves.

Biosolids wetlands are generally constructed to

sup-port sludge management at existing mechanical treatment

plants and are often constructed in close proximity to these

treatment works Again, knowledge of existing utilities is a

critical component of the design process

F LOODPLAINS

Siting of wetland systems in floodplains and floodways has

already been addressed in Chapter 18 It should be noted that

most flood events result in considerable sediment deposition

as flood waters recede Free water surface (FWS) wetlands

are much more tolerant of this sediment load than SSF

wet-lands, where the additional sediment could create potential

clogging issues in the wetland bed As a result, locating SSF

wetlands in floodplains requires careful consideration of the

expected frequency and severity of flooding in the project

area

Biosolids wetlands will contain organic sludge with

varying degrees of pathogens, depending on the age of the

biosolids and level of stabilization Because the potential

to export pathogenic material exists during a flood event, a

decision to locate such a facility in a floodplain would have

to be carefully evaluated

R EGULATORY I SSUES

Regulatory requirements are often a major factor in the

selec-tion of SSF technology over FWS wetlands For instance, in

the United States, approximately 25% of homes rely on

on-site (single home) septic systems (U.S Census Bureau, 1990)

There are considerable health risks associated with exposure

to raw sewage (or sewage exiting the septic tank) As a result, there is a large body of prescriptive codes that require the

treatment and disposal of the sewage without surface sure of the wastewater (U.S EPA, 2002c) Thus, wetlands

expo-such as FWS and surface-flood vertical flow (VF) systems cannot be used in these applications, and the regulations force the designer to use HSSF wetlands or VF wetlands with buried distribution piping so the water will not be exposed.Another example of regulatory requirements that dictate the wetland selection process is airports In cold climates, ice must be removed from the airplanes prior to take-off, and the associated de-icing chemicals generate stormwater run-off that is high in organic (glycol) contaminants Constructed wetlands can remove this chemical oxygen demand (COD) load However, because there is a potential for bird–aircraft strike hazard (BASH), open water bodies such as ponds and FWS wetlands are often discouraged or prohibited from being constructed near airport runways (FAA and U.S Department

of Transportation, 2004) For this reason, SSF wetlands (VF

or HSSF) are preferred for treating de-icing runoff

Siting of wetlands is also influenced by required tion distances from homes, water bodies, and other landscape features These separation distances are usually prescriptive

separa-in nature and dictated by local, state, and provsepara-incial tions In the United States, these range from approximately

regula-3 m to over regula-300 m, depending on the applicable regulations and the size of the treatment system In situations where there are limited choices for siting the wetland, these separation distances may end up not only dictating the location but also the shape of the wetland, as illustrated in Figure 21.3

Soil infiltration area

Site boundary

Dosing tank

15 m Setback

60

m S

etback

Septic tanks Sewer line

Roadwa y

House pad

Property line

15 m Setback

15 m Setback

FIGURE 21.3 The shape and location of this HSSF wetland bed in Lake Elmo, Minnesota, were dictated by regulatory setback requirements.

21.2 LAYOUT AND CONFIGURATION

Once the wetland area and location have been selected, the

designer must then make a number of decisions regarding the

layout and configuration of the wetland These include

The number of parallel flow paths

The number and type of wetlands in each flow

path

Clogging dynamics

Cell configuration (aspect ratio and bed depth)

The hydraulic profile of the wetland system

The internal configuration of the wetland cells

(flow distribution, bed depth, media size and type,

the need for insulating mulch, the water level

con-trol structures, the liner selection and type, etc.)

NUMBER OF FLOW PATHS

The decision to design the wetland system with two or more

parallel flow paths depends on several operational factors:

The need to provide operational flexibility

The need to provide treatment redundancy

The need to operate the system on a load-and-rest

basis

OPERATIONAL FLEXIBILITY

When there are multiple flow paths, one treatment train

can be taken out of service for maintenance and the flow

diverted to the other parallel wetland cells This flexibility

is a desirable attribute in a wetland treatment system, but it

comes at the added cost associated with extra berms, liner,

water level control structures, and earthwork These added

costs become especially significant for very small

treat-ment systems Let us consider the example of a single home

or 770 m2 The results of splitting this wetland into two allel beds is 40 m r 22 m, or 880 m2 In the first case, the wetland occupies 39% of the required site area; with the two parallel beds, the wetland occupies 34% of the site area, an influent splitter structure is necessary, and the outlet control structure(s) must regulate two water levels

par-Although the split-bed approach illustrated offers more operational flexibility, it comes at the cost of additional area, liner, and control structures The added costs of these com-ponents must be weighed against the alternate methods to provide operating flexibility For instance, many single-home septic tanks provide approximately three days of hydraulic retention time, so one means of taking the system out of ser-vice for maintenance or repairs would be to simply pump the septic tank This would give the operator about three days to complete the necessary work, and if additional time

is needed, the tank could be pumped again This alternate approach to flexibility is less expensive than the split-bed sys-tem, and for this reason, single-home HSSF wetlands in the United States are commonly single-bed designs

As the size of the system increases, the added cost of parallel flow paths becomes a progressively smaller compo-nent of the overall construction cost, and providing alternate means of flexibility (such as pumping of septic tanks) becomes progressively more expensive Therefore, for larger projects, two or more parallel flow paths become the preferred design approach For small community wastewater projects, systems with a design flow greater than 35 m3/d would typically have two parallel flow paths, and for larger projects, additional parallel flow paths may be warranted For instance, the HSSF wetland used to treat aircraft de-icing runoff in Edmonton, Alberta, consists of six parallel flow paths, with two cells in each flow path (Figure 21.4)

FIGURE 21.4 HSSF wetland for treatment of aircraft de-icing runoff, Edmonton, Alberta This particular system consists of six parallel

flow paths, with two treatment cells in each flow path.

TREATMENT REDUNDANCY

Many regulatory authorities require wastewater treatment

systems to be designed to treat more than the stated design

flow and load For treatment systems in the upper midwestern

United States, a commonly used redundancy requirement is

that the treatment system must be able to treat 75% of the

influent flow and load, with one treatment train (flow path)

out of service (Great Lakes UMRB, 1997) It should be

noted that this redundancy requirement was developed for

mechanical wastewater treatment plants treating raw

munici-pal wastewater and may or may not be appropriate for

treat-ment wetlands, depending on the type and intended function

of the wetland

With a redundancy requirement, two parallel flow paths

are the minimum If the designer selects two flow paths, each

wetland flow path must be designed to treat 75% of the flow

and load, and the wetland must be designed to treat 150% of

the design (2 r 75%) In certain cases, adding a third flow

path may be advantageous With three flow paths, two

treat-ment trains will be in operation when one is out of service, so

the two trains in service must each be able to treat 37.5% (one

half of 75%) of the design flow and load So, the overall

wet-land is sized to treat 112.5% of the design (3 r 37.5%) The

designer must then evaluate whether the savings in wetland

area is sufficiently justified, given the added cost of the extra

berms and control structures

LOADING AND RESTING

Some treatment wetlands require a loading and resting

regime to function properly For these types of systems, the

loading and resting sequence will determine the number of

the parallel flow paths required

Most VF wetlands are pulse-loaded at a rapid rate to

flood the surface of the bed In between pulses, water drains

from the wetland and air is drawn into the wetland bed (see

biofilms and maintains aerobic conditions within the wetland

bed Particulate organic matter is deposited on the surface

of the bed, and microbial populations are greatest in the

upper regions of the bed, where the organic loading is the

highest (Langergraber et al., 2006b) As the loading period

progresses, this leads to the development of a clogging mat

on the surface of the bed (unless very low organic loadings

are utilized) This clogging mat blocks the movement of air

into the bed, promoting anaerobic conditions and ponding of

the bed surface (Platzer and Mauch, 1997; Hyánková et al.,

2006)

Loading and resting are used as means to avoid clogging

of the bed and associated surface ponding (EC/EWPCA

Emergent Hydrophyte Treatment Systems Expert Contact

Group and Water Research Centre, 1990; Platzer and Mauch,

1997; Molle et al., 2004a) The need for resting intervals

becomes increasingly important as the organic loading on

the VF wetland is increased For instance, in France many

VF wetlands are fed with raw sewage (no primary treatment);

to operate successfully, three VF beds in parallel are needed

in the first stage (Molle et al., 2004a) One bed is fed with raw

sewage while the other two are rested (Figure 7.31) Thus, the loading and resting operating regime of these French systems dictates that the first stage of the treatment process must con-sist of at least three parallel flow paths

21.3 NUMBER AND TYPE OF WETLANDS

IN EACH FLOW PATH

Once the number of parallel flow paths has been decided, the next question facing the wetland designer is how many cells to put in each flow path There are several scenarios that would lead the designer to consider multiple cells in series:There is a need for greater treatment efficiency (and hence greater hydraulic efficiency) to increase the

number of PTIS through the use of two or more

There is a desire for staged treatment, with ferent treatment reactions occurring in different wetland cells

dif-G REATER T REATMENT E FFICIENCY

The first case is a situation where the designer wishes to increase the overall treatment efficiency of the process—or

to provide the same level of treatment in a smaller area By combining two wetland cells in series, the number of tanks

in series (NTIS) is theoretically increased The decreasing

length-to-width ratio for each physical compartment means

that NTIS is not doubled The relaxed parameter, P, accounts for both hydraulic effects (N) and effects of pollutant weath- ering; so a decision to increase P must be based on the pollut-

ant under consideration and the best professional judgment of the designer In any case, the advantage of cells in series for HSSF wetlands is not large unless the design contemplates a

close approach to C*.

As discussed in Chapter 6, compartmentalization becomes

a significant aspect of the wetland sizing process when the goal is a very high level of treatment The case of providing two wetland cells in series (instead of one cell) was previously explored for biochemical oxygen demand (BOD) removal in

Tables 20.7 and 20.8 In that particular example, there was not

a dramatic reduction in the required area because the design goal (94.4% reduction in mass load) allowed an effluent con-

centration significantly above (double) the assumed C* value.

However, as the target effluent quality gets closer

and closer to C*, the hydraulic efficiency N (and hence P)

becomes an extremely important part of the sizing process, because bypassing of untreated effluent in the TIS model has

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a large impact on the effluent quality Here, we will consider

a scenario where the wetland process must deliver an effluent

quality very close to C*, where there are very large impacts

of compartmentalization on the wetland size

Tables 21.1 and 21.2 consider the case of an aerated

subsurface flow wetland (saturated flow) to remove COD

from hydrocarbons Because of the internal mixing induced

by aeration (N 3) and pollutant weathering, the wetland

is assumed to function as 2 PTIS (aerated wetlands are

addressed in more detail in Chapter 24) The spatially

vari-able flow P-k-C* model previously introduced in Chapters 17

and 20 is utilized here The influent COD is assumed to be

120 mg/L, the regulatory compliance limit is 20 mg/L, and

C* is assumed to be 10 mg/L However, because of

variabil-ity in the wetland performance, an effluent qualvariabil-ity of 11.2

mg/L is targeted as the design objective so that the system

will meet the 20 mg/L regulatory limits on a consistent basis

(90% of the time) The lower targeted effluent concentration

is not a safety factor and has nothing to do with treatment

redundancy

Further, it is assumed that there will be significant

weath-ering of the COD mixture as it approaches C*, so the two

wetland cells in series are represented by P 3 instead of

N 6

As the target effluent concentration is so close to C*, any

bypassing of untreated water (low NTIS and therefore low

PTIS) has a very large impact on treatment performance

In Table 21.1, the required wetland area is 2,400 m2; in

Table 21.2, the required wetland area is 1,500 m2 So, in this particular example, there is a large benefit in terms of size reduction (35%) if the wetland is constructed as two cells in series, even though the “benefit” of the second cell has been reduced by pollutant weathering effects

DIVIDING WETLAND CELLS BASED ON SLOPE

On sloping sites, there will be earthwork cut-and-fill costs associated with terracing the site for wetland basins How-ever, wetland designers should be aware that dividing the wetland into multiple segments may potentially have impacts

on treatment performance (as explored in Tables 20.8,

20.9, 21.1, and 21.2) If there are large topographic changes between wetland cells, this can provide the opportunity for creative design elements, such as cascade waterfall aerators (Burka and Lawrence, 1990), if these are advantageous from

a process design standpoint

MORE THAN ONE WETLAND TYPE

Another obvious situation to divide the flow path into multiple cells is when there is a desire to use more than one wetland type along the flow path This subdivision of the treatment process into unique process elements dates back to the original work of Seidel (Seidel, 1966), which

is discussed in Chapter 15 Her work eventually led to the development of the current “hybrid” wetland systems,

TABLE 21.1

COD Reduction for One Wetland Cell, PTIS  2

Input Parameters Calculated Values

Flow rate, Q 137.1 m 3 /d Volume per tank 547.2 m 3

Precipitation, P 2.2 mm/d Area per tank 1,200 m 2

ET 0.13 mm/d Influent flow, Qi 137.1 m 3 /d

Infiltration 0.01 mm/d Effluent flow, Qo 142.0 m 3 /d

Area, A 2,400m 2 Influent mass load 16,452 g/d

Porosity, E 0.38 Effluent mass load 1,591 g/d

Bed depth 1.2 m Mass percentage reduction 90.3%

C* 10 mg/L HLR based on Qavg 0.058 m/d

k 356 m/yr HLR based on PTIS 0.059 m/d

Compliance C 20 mg/L HRT based on Qavg 7.84 d

Variability factor 1.78 HRT based on PTIS 7.70 d

Target C 11.2 mg/L Liner required 3,294 m 2

which typically employ a combination of

vertical–hori-zontal flow wetland cells (Urbanc-Bercic and Bulc, 1994;

Cooper and Green, 1995; Platzer, 1996; O’Hogain, 2003;

Obarska-Pempkowiak et al., 2004; Vymazal, 2005), or a

combination of the horizontal–vertical flows (with

recircu-lation) (Brix, 1998; Laber et al., 1999) Combination

wet-land systems of this nature are discussed in more detail in

STAGED TREATMENT

Another situation where a designer may elect to use multiple

wetland cells along the flow path is when each wetland cell

is intended to provide a different treatment function In many

situations, these wetland cells will be different types of

wet-lands, as discussed previously and in Chapter 24

However, in certain applications, the same type of

wetlands may be linked together solely based on

treat-ment function A good example of this approach is the

two-stage VF wetland systems in France (Molle et al.,

2005a) The first stage of the treatment process is often

three pulse-fed VF beds that are alternately loaded and

rested The primary function of these beds is BOD and

total suspended solids (TSS) removal The second stage

of the treatment process is typically two parallel pulse-fed

VF beds that are also alternately loaded and rested These

second-stage cells are intended primarily for BOD

polish-ing and nitrification

21.4 CLOGGING DYNAMICS

Both horizontal and vertical flow SSF wetlands rely on the ment of water through a porous media for proper functioning Both systems are susceptible to bed clogging, which restricts the flow of water through the bed media In some instances, this restriction in flow will create flooding and will be considered

move-to be a “failure” of the wetland treatment process However, the

definition of failure is qualitative and is often based on the

per-ception and expectations of local regulators, government cials, system operators, and nearby residents

offi-D EFINING F AILURE IN SSF W ETLANDS

Because failure is a subjective term in the treatment wetland

field, it is useful to define what the expectations of the tors, owners, and users of the system might be Several cases concerning hydraulic failure help to illustrate this point

regula-In Denmark, soil-based HSSF wetland technology was adopted early and spread rapidly throughout the country Although the design intent was sub-surface flow, these systems did not have sufficient hydraulic conductivity, and overland flow occurred instead (Brix, 1998) However, the treatment per-formance of these systems was satisfactory and the overland flow mode was tolerated

In the United States, regulations for single-home septic systems prohibit the surface exposure of

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TABLE 21.2

COD Reduction for Two Wetland Cells in Series; PTIS  3

Input Parameters Calculated Values

Flow rate, Q 137.1 m 3 /d Volume per tank 228 m 3

Precipitation, P 2.2 mm/d Area per tank 500 m 2

ET 0.13 mm/d Influent flow, Qi 137.1 m 3 /d

Infiltration 0.01 mm/d Effluent flow, Qo 140.2 m 3 /d

Area, A 1,500 m 2 Influent mass load 16,452 g/d

Porosity, E 0.38 Effluent mass load 1,563 g/d

Compliance C 20 mg/L HRT based on Qavg 4.93 d

Variability factor 1.78 HRT based on PTIS 4.88 d

Target C 11.2 mg/L Liner required 2,107 m 2

wastewater because of public health concerns (U.S

EPA, 2002c) HSSF wetlands that have clogged inlet

zones and overland flow (even for a small

percent-age of the wetland bed) are thus deemed “failed,”

and considerable effort and expense have gone into

maintaining these wetlands to eliminate overland

flow, even when treatment performance was

sat-isfactory A similar situation exists in the United

Kingdom (Cooper et al., 2006a).

VF wetland systems in France are highly loaded with

BOD and TSS (raw sewage) in the initial treatment

stage (Molle et al., 2004a) A large component of the

initial treatment is physical filtration and deposition

of organic matter on the surface of the beds In other

words, the beds are designed to clog However, these

high loads can only be tolerated for a short time; so,

a loading and resting regime (at least three parallel

beds in the first stage) is utilized so that there is

suf-ficient resting interval to recover from the clogged

condition via drying and cracking of the cake

(Figure 7.31) Therefore, in these systems, clogging

does not equate to failure because there is a routine

operational program to address the clogging issue

However, other VF wetlands have clogged and failed

This occurs when there are insufficient beds to allow

adequate rest intervals When these beds clog, water

ponds on the surface and air movement into the

bed ceases, resulting in poor treatment (Platzer and

Mauch, 1997; Cooper et al., 1997) In extreme cases,

ponded water may essentially fill the entire basin,

requiring the entire wetland to be bypassed

Thus, it can be seen that the definition of hydraulic failure

depends on the regulatory requirements, degree of treatment

flexibility, operational regime, and local expectations These

definitions of failure may or may not involve treatment

effi-ciency issues For design purposes in this book, the following

conditions are assumed to constitute a failure mode:

An inability to meet treatment standards on a

con-sistent basis based on the regulatory compliance

interval

For HSSF wetlands, the flow should be kept within

the wetland bed When clogging occurs in the inlet

region, any overland flow path should be kept to

a required minimum This may be zero exposed

water, or some minimal length that may be

consid-ered an inlet distribution zone This defines failure

based on length of overland flow, not the area of

the bed that is clogged

For VF wetlands, the inability of the system to

hydraulically pass the design flow This will

require sufficient beds to allow adequate resting

periods to resist clogging

For biosolids wetlands, not having sufficient beds

to allow adequate resting periods to stabilize the

CLOGGING IN HSSF WETLAND BEDS

The aspect ratio (L:W) of each wetland cell is an important design decision for HSSF wetlands Longer and narrower beds increase the organic loading applied to the cross-sectional area of the bed High cross-sectional loadings will result in

a greater length of the bed operating as an overland flow tem due to the clogging mechanisms described in Figure 7.25, Chapter 7 This is likely to be unacceptable from a regulatory standpoint in the United States, and if the overland flow chan-nelizes, treatment performance will be compromised

sys-At the present time, there is considerable interest in determining whether or not a sustainable balance between solids deposition and solids decomposition/resuspension can

be reached in the inlet zone of HSSF wetlands This issue

is of profound design importance If a steady-state criterion can be identified, then HSSF wetlands can be designed with

a sufficiently conservative inlet loading criteria so as to clude the need for bed-media maintenance or replacement

pre-If bed clogging is inevitable (and merely delayed by lower inlet loading rates), then the need for scheduled bed clean-ing/replacement in the inlet zone becomes an integral part of HSSF wetland technology operation and lifecycle costs

To achieve a steady state in the inlet region, the deposited inlet-suspended solids and the microbial biomat generated

by particulate/soluble organic matter must be in equilibrium with the combined rate of organic solids decomposition and the rate of particulate matter resuspension This implies that

the rate of accumulation, A, in the inlet zone is zero This

con-dition can be represented (using terms previously presented

in Equation 7.16, Chapter 7) as indicated in Equation 21.1:

wheregeneration rate, g/m ·dsettling ra

2

G S

Reduction in the hydraulic conductivity of the inlet zone can

be attributed to a variety of mechanisms, including

1 Accumulation of mineral (biologically inert) ments associated with influent TSS

sedi-2 Accumulation of particulate organic matter ject to biodegradation) within the inlet zone

(sub-3 Formation of chemical precipitates within the land bed

wet-4 Generation of microbial biofilms in response to the combined load of the particulate and soluble organic matter

These materials combine to form a mixture that is here termed

biosolids, and the degree of biosolid accumulation depends

on all four mechanisms Because a significant portion of the incoming organic load in domestic wastewaters is present as

decomposable particulate organic matter, depositions of this

particulate organic matter may result in organic loadings

ten-fold higher in the inlet region as opposed to the rest of the

wet-land bed (Puigagut et al., 2006) This has profound effects in

biosolid formation in the inlet zone Reductions in the

hydrau-lic conductivity from biosolid accumulations are a function of

the media size, as discussed in Figure 7.24, Chapter 7

Short-Term HSSF Inlet Zone Clogging

At the present time, there is insufficient knowledge to

quan-tify the generation G, decomposition D, and resuspension

R rates within HSSF wetland beds This is largely due to

the difficulties encountered when doing in situ sampling of

HSSF systems, as previously discussed in Chapter 7 Without

the ability to quantify the terms G, D, and R, the alternative is

to extract design information from the performance of

exist-ing HSSF wetlands, as shown in Figure 21.5

Data illustrated in Figure 21.5 suggests that HSSF

wet-land beds should be configured such that the cross-sectional

area (orthogonal to the flow) results in a BOD loading of

250 g/m2·d for bed medias with a d10  4 mm BOD has

been selected as the influent loading parameter in this case

because (1) the majority of degradable organic matter in

municipal effluents is present as particulates, (2) soluble

organic matter also contributes to inlet zone biosolids

for-mation, and (3) chemical precipitates (as represented by the

difference between COD and BOD) occur more uniformly in

the HSSF bed and are likely not restricted to the inlet zone

There are drawbacks to limiting performance

informa-tion to BOD, as illustrated in Figure 21.5 First of all, a HSSF

wetland with a high inlet flow of nonorganic (mineral) TSS

may plug at organic loads much lower than those indicated in

Figure 21.5 Secondly, the nature of the organic material must

be considered Some waste streams contain organic matter that is much more degradable than others HSSF wetlands treating soluble, rapidly degradable organic material can rea-sonably be expected to have a shorter clogged inlet zone

Long-Term HSSF Inlet Zone Clogging

Systems illustrated in Figure 21.5 generally have an tional performance history less than ten years Although reducing the cross-sectional organic loading may have short-term benefits in reducing bed clogging, the long-term viability of HSSF hydraulic conductivity remains uncertain Two factors argue against the ability to stabilize long-term hydraulic conductivity of HSSF beds These include

opera-Not all organic matter deposited in the inlet zone

is biodegradable The refractory (nondegradable) component represents a long-term accumulation of organic TSS

A certain component (variable between HSSF tems) of influent TSS consists of nondegradable (mineral) material This inert material is efficiently removed from the water column through the par-ticulate settling/filtration/interception mechanisms discussed in Chapter 7 Because these removal mechanisms are very efficient in HSSF beds, most

sys-of the TSS reduction occurs in the early stages sys-of the wetland bed (see, for example, Figure 7.26) Long-term loading of inert TSS results in an ongo-ing reduction of the hydraulic conductivity in the inlet zone

The current understanding of HSSF wetland design does not allow a quantitative determination of the region of inlet

•

•

0 100 200 300 400 500 600 700 800

FIGURE 21.5 Flooding status of HSSF wetlands as a function of cross-sectional organic loading and bed media size (Data from Steinmann

et al (2003) Water Research 37: 2035–2042; Wallace and Knight (2006) Small-scale constructed wetland treatment systems: Feasibility, design criteria, and O&M requirements Final Report, Project 01-CTS-5, Water Environment Research Foundation (WERF): Alexandria, Virginia; Puigagut et al (2006) Size distribution and biodegradability properties of the organic matter in horizontal subsurface flow con- structed wetlands Kröpfelová (Ed.), Paper presented at the 6th International Workshop on Nutrient Cycling and Retention in Natural and

Constructed Wetlands, 31 May–4 June 2006; Trebon, Czech Republic.)

clogging (biosolids penetration distance), although this is

believed to be related to the size of the bed media, as

indi-cated in Figure 21.5 The d10 of the media is often used to

approximate the pore volume, as only 10% of the bed media

is smaller than this size

Studies on sand clogging indicate that the clogging

dis-tance Dc, can be related to the d10 of the media: Dcy 150 d10

within the range of 5 r 10−3 mm  d10 3r10−2 mm

(Blaze-jewski et al., 1994) Because this d10 is much smaller than

the media sizes used in most HSSF wetlands, the usefulness

of this relationship is limited to soil-based HSSF systems

Blazejewski et al (1994) suggested a clogging thickness of

3 cm for fine-grained HSSF beds

Bavor et al (1989) noted that clogging within a series of

long, narrow HSSF trenches (L:W ratio of 25:1) was remedied

by excavating the first 5 m of the bed and replacing with coarse

rock Kadlec and Watson (1993) provided evidence from the

Benton, Kentucky, system that the farthest biomat-penetration

distance stretched 100 m into a 300-m bed But this type of

information does not help in design unless couched in terms

of organic loadings and decomposition rates

Based on the information currently available, it appears

that bed clogging (as evidenced by overland flow length) is

reduced, or at least delayed, by minimizing the cross-sectional

loading, and using coarser bed materials in the inlet zone

However, based on the current state of the art, there is

no available evidence that argues against the phenomenon of

long-term bed clogging (A 0), even if short-term clogging

pitfalls are avoided Long-term performance information

from the United Kingdom (Cooper et al., 2006b) indicates

that HSSF systems will require some degree of inlet zone

media replacement or cleaning during the course of their

operational lives Whereas the frequency of this maintenance

is dependent on the bed clogging factors described in this

book, it appears that even well-designed, well-operated HSSF

wetlands will require inlet zone maintenance approximately

every ten years to avoid the development of large regions of

overland flow At the present time, media replacement is the

most common (and most expensive) method of maintenance

(Wallace and Knight, 2006) although trials using hydrogen

peroxide as a cleaning agent have shown promising results

(Hanson, 2002b; Behrends et al., 2006).

C LOGGING IN VF W ETLAND B EDS

VF wetlands are also subject to solids accumulation, which

can fill the pore volume within the upper region of the

wet-land bed, potentially leading to ponding and hydraulic failure

Equations describing clogging of VF wetland beds have

pre-viously been presented in Equations 2.58–2.61 in Chapter 2

The organic loading applied to the VF wetland appears to

play a large role in the clogging process For a soil/sand VF

wetland at Merzdorf, Germany, Platzer and Mauch (1997)

observed a log-linear decrease of hydraulic conductivity as a

function of the cumulative applied organic load With resting

intervals, the hydraulic conductivity of the bed was restored

to the original values

At the present time, there are two operating regimes to address the clogging of VF wetland beds:

1 Systems that are continuously operated without resting These VF wetlands are loaded at relatively low organic loads to minimize biosolids accumu-lation and associated clogging of the bed

2 Systems that employ multiple beds in a rest regime These VF wetlands are designed to clog during the loading phase; hydraulic conduc-tivity is restored during the resting phase

load-and-Because clogging and organic loading appear to be mately related, it is not possible to specify an organic load-ing rate for a VF wetland system without also specifying the resting sequence associated with that loading, as previously discussed in Chapter 20

inti-Approximately 180 VF wetlands in North America have been designed as recirculating systems, which are function-ally very similar to recirculating gravel filters (Crites and Tchobanoglous, 1998) Recirculation ratios vary depending

on design goals, but ratios of 3 to 12 times the inlet flow rate are common

An array of distribution pipes results in a nonuniform application of influent TSS and organic matter This loading

is concentrated at the discharge orifices of the distribution pipes Biosolids then preferentially form at each discharge orifice in a zone about 30 cm in diameter In extreme condi-tions, these biosolids will plug the upper layer of the wetland bed because recirculating wetlands operate at hydraulic load-ings that are much higher than the inlet flow reductions in bed porosity and loss of hydraulic conductivity can rapidly lead to flooding of the wetland bed An example is shown in

Figure 21.6.Operational experience in the United States indicates that the organic loading (as represented by BOD) should be less than 15 g/d per orifice for buried pipes to avoid biosolids clogging with bed media greater than 4 mm The equivalent BOD loading in the biosolids clogging region at the orifice is approximately 200 g/m2·d

The uniformity of application is increased and the organic loading is reduced if there is a secondary redistri-bution of the influent by splashing or spraying Figure 21.7

shows a cold-climate method of achieving this Perforated pipes are placed in infiltration chambers that are then buried

in the upper portion of the wetland Water sprays out of the distribution orifices up into the infiltration chamber, where it splashes around and undergoes secondary distribution Ori-fice shields (resembling plastic pipe caps) are commercially available for this purpose

21.5 CELL CONFIGURATION

As discussed in the previous section, clogging dynamics will often determine the number of wetland treatment cells and, for HSSF wetlands, the aspect ratio (L:W) will be a function

of the cross-sectional loading if delaying or minimizing the

overland flow is a design goal (see Figure 21.5) Even if the

cell dimensions are governed by clogging considerations,

there are still a number of design decisions Several factors

of SSF wetland bed design warrant further consideration at

this stage; these include

The length-to-width (L:W) ratio of the wetland

cells

The depth of the wetland cells

The types and sizes of the bed media that will be

placed in the wetland cells

The overall size of the wetland cells

LENGTH-TO-WIDTH RATIO

For VF and biosolids wetlands, the aspect ratio (L:W) is

relatively unimportant, because these wetland beds are top

ing and flow velocity and, to some extent, the NTIS value.

When working with HSSF wetlands, the designer is faced with two mutually contradictory design goals:

The desire to spread the influent over a very wide planar area (This delays the time to clogging.)The desire to spread the influent as uniformly as pos-sible over the vertical cross section (This improves flow distribution.)

Spreading the influent over wide areas presents a number of hydraulic challenges, especially for small systems, where the flow rate is close to zero One approach, which has been implemented in Denmark, is to divide the inlet chamber into two or more segments that can alternately be loaded to gain

•

•

FIGURE 21.6 Example of biosolids clogging in a small recirculating VF wetland This system in central Minnesota experienced rapid

hydraulic failure in response to extremely excessive BOD loadings from illegal drug manufacturing.

FIGURE 21.7 Recirculating VF wetland under construction at The Ponds, Minnesota The black infiltration chambers are used as a means

of secondary flow distribution to reduce the organic loading at the distribution orifice (This photo was taken at an intermediate stage of construction; additional gravel was later placed to bury the influent distribution chambers.)

better control of the flow distribution (Brix, 1998) A

sche-matic of this is shown in Figure 21.8

For small-scale wetlands, flow-splitting approaches use

weirs, stoplogs, or rotating elbows if flow balancing is an

operations goal All such flow-splitting devices require

peri-odic maintenance to remove accumulated solids that may

interfere with the flow split, especially for small-scale

sys-tems with low flows

An alternate approach is to split the flow two ways using

a central effluent collection pipe with influent distribution

channels on either side of the wetland An example of this

design strategy is presented in Figure 15.5, Chapter 15

W ETLAND CELL DEPTH

HSSF wetlands were originally designed with a depth of 60

cm based on the belief that this was the maximum depth that

Phragmites root systems would penetrate (Kickuth, 1977)

However, analysis of many HSSF wetlands has indicated that

the root systems of emergent wetland plants grow

preferen-tially in the upper region of the bed; this concept is illustrated

typically being only about 30 cm Thus, deeper beds often

have a lower region without plant roots, and water flows

pref-erentially along the bottom of the wetland bed This lower

region is typically associated with more reducing conditions

and less efficient treatment

For HSSF wetlands, the designer is faced with choosing

the bed depth Some observations include the following:

1 Deeper beds promote vertical stratification of flow,

as discussed in Chapter 2 This allows bypassing

of the flow below the plant root zone (Fisher, 1990;

Pilgrim et al., 1992; Marsteiner et al., 1996; Garcia

et al., 2004b), with adverse impacts on treatment

performance

2 Shallower beds often result in better treatment (see Tables 8.14 and 9.33 for BOD and ammonia, respectively)

3 Deeper beds provide a greater cross-sectional area, which reduces cross-sectional loading (see Fig-ure 21.5) This theoretically reduces the flow velocity (in the absence of vertical stratification), resulting in less head loss through the wetland bed, which makes maintaining a uniform water surface profile easier

4 Deeper beds speculatively provide additional ume for storage of refractory and nondegradable (mineral) sediments This added storage volume would presumably decrease the time intervals between clogging and associated bed maintenance

vol-5 A deep bed speculatively allows for some ice formation at the top while still passing the water underneath the ice within the bed

From a treatment standpoint, the additional cost of the extra bed depth (e.g., 60 cm versus 30 cm) appears to add zero

or negative value to a fixed area of wetland, but there is no quantification across multiple systems Control of head loss

is better done through aspect ratio and media size If ment storage is a design consideration, more bed depth may

sedi-be warranted Ice formation can sedi-be adequately controlled via

a variety of other insulation techniques To date, virtually all HSSF wetlands have been designed with beds between 30 and 60 cm deep But, controlled field-scale research studies indicate that shallower depths provide better treatment

VF and biosolids wetlands are typically constructed to

a prespecified bed depth, 50–80 cm for VF wetlands and 50–60 cm for biosolids wetlands

Inspection port (typical)

Flow splitter to allow alternate loading and resting

Infiltration chamber

Perforated pipe

Louvered sidewall

FIGURE 21.8 Layout and design factors for a HSSF wetland utilizing an influent flow split to enhance flow distribution (From Wallace

and Knight (2006) Small-scale constructed wetland treatment systems: Feasibility, design criteria, and O&M requirements Final Report,

Project 01-CTS-5, Water Environment Research Foundation (WERF): Alexandria, Virginia Reprinted with permission.)

T YPE AND SIZE OF BED MEDIA

Whereas HSSF beds almost always use coarse stone in the

inlet and outlet regions, a variety of materials are used for

the main bed media

The original HSSF beds utilized sand in the Max Planck

Institute Process (MPIP) system (Seidel, 1966) and soil in the

root zone method beds (Kickuth, 1977) Many more of the

root zone method systems were constructed, despite

signifi-cant problems with the hydraulic conductivity and associated

problems of overland flow (Brix, 1998)

The response to the overland flow problem was to design

HSSF beds with coarser materials so that the water could flow

through the wetland bed Different countries took various routes

in the development of HSSF wetland technology, and there are

regional differences in the size and type of the bed media

Perusal of Table 10.14, Chapter 10, will provide an indication

of the wide variety of bed medias that have been evaluated In

certain cases, the use of media with specific chemical

proper-ties, such as phosphorus sorption or a high cation exchange

capacity (for ammonia retention), may be desirable Examples

of bed media and size gradations that have been used in HSSF

wetlands are summarized in Table 21.3

Finer materials have a greater surface area for

micro-bial biofilms and may result in better treatment, as indicated

more prone to clogging and associated hydraulic problems,

as illustrated in Figure 7.23, Chapter 7 The use of coarser

material in the first part of the HSSF wetland bed and finer

material closer to the outlet has been proposed as a

com-promise between these two competing design objectives

(Wallace and Knight, 2006) The type of wastewater may

also be a factor, because biosolids production and/or

distri-bution appears to be a function of the organic loading With

lower organic loadings, finer bed medias could conceivably

be used without clogging for the same time period as coarser materials with higher loadings, as proposed in the Austrian design guidelines (ÖNORM B 2505, 2005) and summarized

in Table 21.3

Economics, availability, and local design practices cally dictate what material will be preferred for the wetland bed To date, medias in the size range of 8 to 16 mm have been most commonly used in HSSF wetlands (IWA Specialist Group on Use of Macrophytes in Water Pollution Control, 2000)

typi-VF wetlands typically use a layered approach to the bed, with fine material (typically sand) at the top of the wetland bed and progressively coarser materials lower in the bed—down to the drainage layer It has long been recognized that the composition of the sand layer is critical; the material must be fine enough to trap TSS on the surface of the bed but coarse enough to allow water movement without clogging (Cooper, 2003) Depending on the level of pretreatment, finer bed media may be warranted; the French systems use coarser bed material in the first VF stage (which treats raw sewage) and finer bed material in the second VF stage, which pro-

vides COD polishing and nitrification (Molle et al., 2004a).

Biosolids dewatering wetlands typically have a fine layer (sandy loam or sand) with an underlying drainage layer (Nielsen, 2002) However, it has been noted that it is important

to maintain continuity of capillary suction to facilitate flow through the various drainage layers; otherwise, the result will

be “hanging water” as evidenced by a lack of sludge ing (Nielsen, 2004) So, large differences in the size range between adjacent drainage layers should be avoided

dewater-Different bed media and layering for VF wetlands are summarized in Table 21.4

EC/EWPCA (1990) 3–6; or 5–10 mm Gravel 86 m/d 50–200 mm stone

TVA (1993) 3–6 mm Gravel 2,600 m/d (secondary effluent)

260 m/d (primary effluent)

50–100 mm stone

ATV (1998) 0.2–1.0 mm Soil/sand

k(d10)2100

where k is in m/s and d10 is in mm U.S EPA (2000a) 20–30 mm Gravel 100,000 m/d 40–80 mm stone

5–20 mm top layer 10,000 m/d inlet region IWA (2000) 8–16 mm Gravel k D

P

 12 600 , 1 9

where k is in m/d and d10 is in cm use 10% of this value in the inlet region

75–100 mm stone

ÖNORM B2505 (2005) 0–4 mm (gray water)

1–4 mm (secondary influent) 4–8 mm (primary influent)

Sand Gravel Gravel

Use the saturated hydraulic conductivity;

head loss is sized on 1/10 of the daily flow being delivered in 1 hour

16–32 mm gravel or gradated bands of 8–16 and 4–8 mm gravel

Wallace and Knight (2006) 4 mm Gravel 260 m/d 25–50 mm gravel

SSF WETLAND CELL SIZE

Another factor enters into the design process when

determin-ing the footprint area of individual wetland cells, specifically

the construction equipment that will be used to place the

bed media Even large excavators have a maximum reach of

approximately 10 m (see Figure 21.1) Therefore, SSF

wet-land cells greater than about 20 m wide cannot be filled with

standard excavators, and the remaining construction options

consist of

Allowing equipment to operate inside the

wet-land basin (typically, skid-steer loaders for small

basins, or bulldozers for large basins)

Using a specialized piece of equipment, such as an

extendable conveyor, to place the bed media

The pros and cons of these different approaches are

dis-cussed under the SSF wetland construction section of this

chapter However, the designer should give thought to

con-struction methods and the type of concon-struction equipment

locally available when determining the dimensions of

indi-vidual wetland cells

The overall hydraulic profile for the treatment cess, including frictional losses within the wetland bed

pro-Distribution of the influent flowCollection of the effluent flowWater level control

Gravity flow through the treatment process is the preferred situation from an operations and reliability standpoint Pumping of the flow is less desirable because of the reliabil-ity, maintenance, and power consumption issues associated with wastewater pumps On very flat sites, grading the site

to achieve the “stair-step” configuration needed for gravity

Summary of Bed Materials Used in VF Wetlands

Source Wetland Type Layer Thickness Media Size Material

EC/EWPCA (1990 Pulse-fed Surface layer

Second layer Third layer Drainage layer

Sharp sand Pea gravel Round gravel Round gravel ATV (1998 Pulse-fed Main layer 80 cm 0.2–1.0 mm Soil/sand

Molle et al (2004a) Pulse-fed (First stage) Surface layer

Second layer Drainage layer

30 cm 10–20 cm 10–20 cm

2–8 mm 5–20 mm 20–40 mm

Fine gravel Gravel Coarse gravel

Molle et al (2004a) Pulse-fed (Second stage) Surface layer

Second layer Drainage layer

30 cm 10–20 cm 10–20 cm

0.25 d10 0.4 mm 3–10 mm 20–40 mm

Sand Gravel Coarse gravel ÖNORM B2505 (2005) Pulse-fed Surface layer

Second layer Third layer Fourth layer Drainage layer

10 cm

50 cm 5–10 cm Separation fabric

20 cm

8–16 mm 0–4 mm 4–8 mm

— 16–32 mm

Gravel Sand/gravel Gravel 2.5-mm mesh Gravel NAWE (2006) Recirculating Main layer

Drainage layer

75 cm

15 cm

12–15 mm 16–40 mm

Gravel Coarse gravel Nielsen (2002) Biosolids dewatering a Surface layer

Second layer Third layer Drainage layer

5–10 cm

15 cm Separation fabric 30–45 cm

— Sandy loam

Sand

— Pebble gravel

a Sludge dewatering effectiveness is dependent on maintaining continuity of the capillary suction under unsaturated flow Therefore, large transitions in the size range of adjacent drainage layers should be avoided, especially between the overlying biosolids material and the top layer of the filter (Nielsen, 2006).

flow must be weighed against the construction and

opera-tions costs of pumping For countries with a high level of

infrastructure support, such as the United States, pumping

is perceived as less expensive than extensive earthwork

dur-ing construction For developdur-ing countries, the inverse may

be true

The hydraulic profile for a wetland system results from

a number of different calculations Major components of the

hydraulic profile for SSF wetlands typically include:

The driving head needed to distribute the influent

flow

Frictional losses (head loss) in conveyance piping

Elevation differences between different stages of

the treatment process

Frictional losses in the wetland bed (HSSF

wetlands)

If the bed is intended to completely drain, the

depth of the bed

The elevation and hydraulic characteristics of

water level control devices such as weirs or

standpipes

Separate calculations are made for each unit process,

resulting in a stair-step diagram describing the relative

elevations of the water surface profile (and for pumped

systems, the hydraulic grade line) Typically, calculations

are made at different flow rates, such as the average daily

flow, the minimum flow rate, and the peak daily or hourly

flow rates The peak flows should include the capture of

water from rainfall events; often a 10-year or 25-year

storm event is used for this purpose, depending on the

regulatory jurisdiction

For very small wetland systems, the low flow at night

will be zero due to the diurnal fluctuation in water use

pat-terns This can create a number of design challenges in terms

of flow distribution and, in very cold climates, freezing of

pipes Specific design challenges for small-scale wetland

systems have been addressed in the literature (Wallace and

Knight, 2006)

Generally speaking, the larger the system, the more

important detailed hydraulic calculations become For SSF

wetlands, these calculations must also address anticipated

phenomena such as bed clogging (see Chapter 7) The

pre-ferred design practice for SSF wetlands is to make each bed

completely drainable and also to have a discrete elevation

drop between cells (often, a minimum drop of 0.3 m is

desir-able, unless the site cannot support this elevation change

or hydraulic calculations indicate that this differential is

insufficient)

Hydraulics of HSSF Wetlands

For HSSF wetlands, the water surface profile within the

wetland bed is assumed to be governed by frictional losses

within the wetland bed If flow is within the laminar range,

a simple relationship such as Darcy’s law can be used to

H k

 yydraulic conductivity, m/d

This is the one-dimensional version of Darcy’s law It is restricted to the laminar flow regime At higher flow veloci-ties, there will be a turbulent component to the flow, which must also be considered (see Equations 2.44, 2.46, 2.47, Chapter 2) The use of these friction-loss equations are only the start of the hydraulic design process, because what is of most use to the wetland designer is the longitudinal depth profile This is typically a nonuniform profile due to changes

in the flow rate (gains or losses from precipitation, tion, and ET) and hydraulic conductivity of the wetland bed Calculation procedures have been previously established in Equations 2.48–2.54, and the results of such calculations are illustrated in Figure 2.26

infiltra-These calculations are typically performed by dividing the wetland bed into subunits (each with its own hydraulic conductivity) and calculating the head loss across each sub-unit The standard procedure is to start at the effluent end of the bed (the tailwater condition) and work backwards to the inlet

Here, we explore the effects of design parameters on the water surface and its relation to the media surface The goals

of hydraulic design for HSSF wetlands are to convey the design flow within the wetland bed without surfacing while providing an acceptable hydrologic environment for the sur-vival of emergent wetland plants Two philosophies may be used to achieve these goals:

1 A management-intensive scheme, in which tors compensate for hydraulic upsets through oper-ational adjustments

opera-2 A design-intensive scheme, in which the design is made sufficiently conservative so as to eliminate the need for most operational adjustments

In either case, the overall wetland footprint area is presumed

to be set from the treatment requirements as previously cussed in Chapter 20

dis-Normally, the requirement for treatment performance will provide a specification of either the hydraulic load-ing rate or detention time In turn, these combine with the required volumetric flow rate to determine either the volume

of water in the bed or area of the bed However, it must be recognized that many wastewater sources create flows that vary considerably on both diurnal and seasonal time scales Neither the minimum nor the maximum anticipated flow should cause hydraulic failure

The requirements for stable water depth and controllable

water flow for creating proper vegetation conditions serve to

further restrict the geometry of the bed and the size of the

media

These requirements can be summarized as

1 Anticipated design flows must pass through the

bed without overland flow or flooding

2 Anticipated design flows must pass through the

bed without stranding the plants above water; i.e.,

there must not be protracted, excessive headspace

that results in desiccation and die-off of wetland

plants

3 Operation should remain acceptable in the

inevi-table event of changing hydraulic conductivity,

especially in the inlet zone As the bed clogs with

roots and microbial biofilms it should not flood

4 The bed should be drainable

5 Water levels within the system should be fully

controllable through the use of inlet and outlet

structures

6 The configuration must fit within the available site

area in terms of project boundaries and hydraulic

profiles

There are often serial and parallel arrangements of

individ-ual wetland cells within a system In the present discussion,

attention is restricted to individual cells and to simple

rectan-gular geometries The variables that may be chosen to satisfy

these considerations are: length (L), bottom slope (Sb), and

the media (D or k) Bed depth (D) is usually in the narrow

range of 30–60 cm and is therefore set by the desire for plant

root penetration within the HSSF bed rather than hydraulics

The width of the system is scaled by the need to carry the full

design flow and by clogging concerns.

There is no theoretical need for a slope to the top of the

bed If water level control is to include the ability to totally

inundate the bed for vegetation management, then a top slope

is detrimental

Because the media is likely to undergo significant changes

in hydraulic conductivity, its frictional resistance cannot be

relied upon for control of the water depth This implies that the conductivity of the media must be large enough, or the water travel distance short enough, to sustain a nearly level pool condition controlled by the elevation of the wetland out-let control structure

In certain cases, it may be desirable to be able to drain the HSSF wetland bed In this case, the bottom slope should

be set to provide for complete bed drainage Normally, a few centimeters of elevation differential allows for this require-ment The wetland bottom slope cannot be considered as the design driving force for water movement The reason is that designs based on bed slope are excessively sensitive to chang-ing conditions of flow and hydraulic conductivity Dry-out or flooding are virtually certain to occur with such designs

Headspace Considerations

Under conditions of very low flow, the water surface will be a horizontal level pool controlled by the elevation of the outlet structure It is necessary that this condition should not create

a dry zone of excessive depth (headspace, f ) (Figure 21.9),

which would occur due to a tilt of the upper bed surface If the top and bottom of the HSSF wetland are both sloped, it creates a condition very conducive to overland flow within the system, as illustrated in Figure 21.10

However, many old HSSF systems have been built to specifications that require uniform bed depth and a signifi-cant bed bottom slope Indeed, early design guidance docu-ments recommended high bottom slopes: 0.5–2.0% (TVA,

1993); 1–5% (Cooper et al., 1996) Another early guideline

was established for Danish systems, namely, restricting the bed surface slope to no more than a 30-cm drop, thereby ensuring that plant roots could reach water even under level pool conditions (Johansen, 1994)

The effect of bed slope is seen in Figure 2.26, Chapter 2, for the Benton, Kentucky, system There, a 0.1% bed slope over 334 m ($B  33 cm) results in a drained bed water ele-vation that is not compatible with the hydroperiod needs of emergent wetland plants at low flow conditions The result was unsuitable water conditions for the target wetland plant

species (Scirpus validus).

Low Flow or High Hydraulic Conductivity with Large Sb

Design intent S = SbActual water surface profile, S

Raising water level

to support plants at inlet causes outlet flooding

Dead plants at inlet due to drought stress

FIGURE 21.9 Water level problems occurring in sloped HSSF wetland beds under conditions of low flow (From Wallace and Knight

(2006) Small-scale constructed wetland treatment systems: Feasibility, design criteria, and O&M requirements Final Report, Project

01-CTS-5, Water Environment Research Foundation (WERF): Alexandria, Virginia Reprinted with permission.)