**3. Constructed wetlands for water pollution management**

These man-made wetlands are used to treat aquaculture and municipal water, to regulate the water quality of shrimp ponds and manage pollution from pond effluents. The wetland treated effluents satisfy standards for aquaculture farms. Since the technology to use the constructed wetlands to treat waste water of high BOD5 is limited, these are generally used to polish secondary effluents. Other applications of constructed wetlands are (a) to treat acid mine drainage, (b) to treat storm water, and (c) the enhancement of existing wetlands.

The suggestion to use wetland technology for waste water treatment is attractive for both ecological and economic reasons. Constructed wetlands are efficient in removing pathogens [2]. It performs better than conventional waste water treatment methods although the lack of knowledge of principles of pathogen removal in plants hampers optimum performance. Interactions between soil matrix, micro-organisms and plants and higher retention time of the waste water in these biologically complex systems make phyto-remediation more effective than conventional systems. Phyto-remediation involves complex interactions between plant roots and micro-organisms in the rhizo-sphere. The efficient functioning of wetland systems is hampered due to following factors:


Wetland systems efficiently treat water polluted by heavy metals, chromium and magnesium.

The metal removal in these systems involves following mechanisms:

• Filtration and sedimentation of suspended particles,

Wetlands for Water Quality Management – The Science and Technology 171

Fig. 6. A cross – sectional view of a Free Water Surface Wetland system (reprinted from the

The hydrologic budget is an important part of design of constructed wetlands. The

, *i o dV Q Q P ET dt*

Ground water inflow and infiltration are excluded from the above equation as impermeable barriers are used. Historical climatic records can be used for estimating the precipitation and evapo-transpiration. Infiltration losses can be estimated by conducting infiltration tests [3].

Divide the width into individual cells for control of hydraulic loading rate. Vegetation used in United States is Cattails, reeds, rushes, bulrushes, and sedges. Physical presence of this vegetation transports oxygen deeper than it would reach through diffusion. Submerged portions serve as home for microbial activity. The attached biota is responsible for treatment

Horizontal Subsurface Flow systems (submerged horizontal flow) consist in basins containing inert material with selected granulometry with the aim to assure an adequate hydraulic conductivity (filling media mostly used are sand and gravel). These inert

**3.2 Constructed wetlands with horizontal subsurface flow (HF)** 

SOIL OR GRAVEL WATERTIGHT MEMBRANE

− +− = (5)

EFFLUENT OUTLET HEIGHT VARIABLE

CATTAILS

SLOTTED PIPE FOR WASTEWATER DISTRIBUTION

SLOPE 1%

RHIZOME NETWORK

US Environmental Protection Agency design manual)

following water balance equation is generally used

*Qi* influent waste water flow, volume/time *Qo* effluent waste water flow, volume/time

*ET* evapo-transpiration, volume per unit time

*P* precipitation, volume/time

Typical dimensions of a FWS are:

*V* volume of water, and

• Length ≈ 64 meters • Bed width = 660 meters. • Bed depth = 0.3 meters, • Retention time is 5.2 days.

INLET STONE DISTRIBUTOR

Where:

*t* time.

that occurs.


Constructed wetlands are either free water surface systems with shallow water depth or subsurface flow systems with water flowing laterally through the land and gravel. These wetlands have been used for wastewater treatment for nearly 40 years and have become a widely accepted technology available to deal with both point and non-point sources of water pollution. They offer a land-intensive, low-energy, and low-operational-requirements alternative to conventional treatment systems, especially for small communities and remote locations. Constructed wetlands also prove to be affordable tools for wastewater reclamation, especially in arid and semi-arid areas. Although the emission of N2O and CH4 from constructed wetlands is found to be relatively high, their global influence is not significant towards their contribution to global warming.

Three main components of an artificial wetland are as follows:

### **(a) Construction practices**

While design should be kept as simple as possible to facilitate ease of construction and operation, the use of irregular depths and shapes can be beneficial to enhance the wildlife habitat. The site for construction should be properly chosen so as to limit damage to local landscape by minimizing excavation and surface runoff during construction and, at the same time, maximize flexibility of the system to adapt extreme conditions.

### **(b) Soil**

The chosen soil must not contain a seed bank of unwanted species. The permeability of the soil should be carefully controlled as highly permeable soils may allow infiltration and possible contamination of ground water. High permeability is not conducive for development of suitable hydrological conditions for wetland vegetation. Use of impermeable barriers may be suggested in certain instances.

### **(c) Selection of vegetation**

Plant species among native and locally available species should be chosen keeping in mind water quality and habitat functions. The use of weedy, invasive and non-native species should be avoided. Plants ability to adapt to various water depths, soil and light conditions should also be taken into consideration.

In the following, design and construction of two kinds of artificial wetlands which are used for water purification will be described.

## **3.1 Free water surface (FWS) wetland systems**

These systems consist of basins or channels with subsurface barrier to prevent seepage, soil or another medium to support the emergent vegetation and water at a shallow depth flowing through the unit. The shallow water depth, low flow velocity and presence of plant stalks and litter regulate the water flow [3]. The soil permeability is an important parameter. The most desirable soil permeability is 10-6 to 10-7 meter per second. The uses of highly permeable soils are recommended for small waste water flows by forming narrow trenches and lining the trench walls and bottom with clay or an artificial liner.

Fig. 6. A cross – sectional view of a Free Water Surface Wetland system (reprinted from the US Environmental Protection Agency design manual)

The hydrologic budget is an important part of design of constructed wetlands. The following water balance equation is generally used

$$Q\_i - Q\_o + P - ET = \frac{dV}{dt} \,\prime \tag{5}$$

Where:

170 Current Issues of Water Management

Constructed wetlands are either free water surface systems with shallow water depth or subsurface flow systems with water flowing laterally through the land and gravel. These wetlands have been used for wastewater treatment for nearly 40 years and have become a widely accepted technology available to deal with both point and non-point sources of water pollution. They offer a land-intensive, low-energy, and low-operational-requirements alternative to conventional treatment systems, especially for small communities and remote locations. Constructed wetlands also prove to be affordable tools for wastewater reclamation, especially in arid and semi-arid areas. Although the emission of N2O and CH4 from constructed wetlands is found to be relatively high, their global influence is not

While design should be kept as simple as possible to facilitate ease of construction and operation, the use of irregular depths and shapes can be beneficial to enhance the wildlife habitat. The site for construction should be properly chosen so as to limit damage to local landscape by minimizing excavation and surface runoff during construction and, at the

The chosen soil must not contain a seed bank of unwanted species. The permeability of the soil should be carefully controlled as highly permeable soils may allow infiltration and possible contamination of ground water. High permeability is not conducive for development of suitable hydrological conditions for wetland vegetation. Use of

Plant species among native and locally available species should be chosen keeping in mind water quality and habitat functions. The use of weedy, invasive and non-native species should be avoided. Plants ability to adapt to various water depths, soil and light conditions

In the following, design and construction of two kinds of artificial wetlands which are used

These systems consist of basins or channels with subsurface barrier to prevent seepage, soil or another medium to support the emergent vegetation and water at a shallow depth flowing through the unit. The shallow water depth, low flow velocity and presence of plant stalks and litter regulate the water flow [3]. The soil permeability is an important parameter. The most desirable soil permeability is 10-6 to 10-7 meter per second. The uses of highly permeable soils are recommended for small waste water flows by forming narrow trenches

• Precipitation by microbial mediated biogeochemical processes, and

significant towards their contribution to global warming.

Three main components of an artificial wetland are as follows:

impermeable barriers may be suggested in certain instances.

same time, maximize flexibility of the system to adapt extreme conditions.

• Incorporation into plant material,

• Adsorption on the precipitates.

**(a) Construction practices** 

**(c) Selection of vegetation** 

should also be taken into consideration.

for water purification will be described.

**3.1 Free water surface (FWS) wetland systems** 

and lining the trench walls and bottom with clay or an artificial liner.

**(b) Soil** 


Ground water inflow and infiltration are excluded from the above equation as impermeable barriers are used. Historical climatic records can be used for estimating the precipitation and evapo-transpiration. Infiltration losses can be estimated by conducting infiltration tests [3].

Typical dimensions of a FWS are:


Divide the width into individual cells for control of hydraulic loading rate. Vegetation used in United States is Cattails, reeds, rushes, bulrushes, and sedges. Physical presence of this vegetation transports oxygen deeper than it would reach through diffusion. Submerged portions serve as home for microbial activity. The attached biota is responsible for treatment that occurs.

### **3.2 Constructed wetlands with horizontal subsurface flow (HF)**

Horizontal Subsurface Flow systems (submerged horizontal flow) consist in basins containing inert material with selected granulometry with the aim to assure an adequate hydraulic conductivity (filling media mostly used are sand and gravel). These inert

Wetlands for Water Quality Management – The Science and Technology 173

velocity. A greater surface allows capture of particles with smaller settling velocities.

The aspect ratio defines the length to width ratio. This is considered to be of critical importance for the adequate flow through the wetland. Constructed wetlands are designed with an aspect ratio of less than 2 to optimize the flow and minimize the clogging of the inlet.

Wetland systems significantly reduce biological oxygen demand (BOD5), suspended solids (SS), and nitrogen, as well as metals, trace element, and pathogens. The basic treatment mechanisms include sedimentation, chemical precipitation, adsorption, and microbial degradation of organic matter, Suspended solids and nitrogen, as well as some uptake by

Microbial degradation (also expressed as biological oxygen demand BOD5) in a wetland can

exp( ) *<sup>e</sup> <sup>T</sup>*

*<sup>C</sup>* = − (6)

*<sup>Q</sup>* <sup>=</sup> (7)

*<sup>V</sup>* <sup>=</sup> (8)

*<sup>C</sup> K t*

. *LWd <sup>t</sup>*

In a FWS wetland, a portion of the available volume will be occupied by the vegetation; therefore, the actual detention time is a function of the porosity (n). The porosity is defined

*Vv <sup>n</sup>*

The ratio of residence time from dye studies to theoretical residence time calculated from

*o*

Equation (7) represents hydraulic residence time for an unrestricted flow system.

KT temperature-dependent first-order reaction rate constant, d-1

Typical hydraulic rates in subsurface flow wetlands vary from 2 to 20 cm per day.

**3.3 Performance evaluation** 

*Co* influent BOD5, mg/L *Ce* effluent BOD5, mg/L

*t* hydraulic residence time, d

be described by a first-order degradation model

Hydraulic residence time can be represented as

*Q* average flow rate = (flow in + flow out) ÷ 2

as the remaining cross-sectional area available for flow.

the physical dimensions of the system should be equal to the ratio.

the vegetation.

Where:

Where: *L* length *W* width *d* depth

With:

Vv volume of voids, V total volume.

materials represent the support for the growth of the roots of emerging plants (cf. Fig. 7). The bottom of the basins has to be correctly waterproofed using a layer of clay, often available on site and under adequate hydro-geological conditions or using synthetic membranes (HDPE or LDPE 2 mm thick). The water flow remains always under the surface of the absorbing basin and it flows horizontally [11]. A low bottom slope (about 1%) obtained with a sand layer under the waterproof layer guarantees this.

During the passage of wastewater through the rhizo-sphere of the macro-phytes, organic matter is decomposed by microbial activity, nitrogen is denitrified. In the presence of sufficient organic content, phosphorus and heavy metals are fixed by adsorption on the filling medium. Vegetation's contribution to the depurative process is represented both by the development of an efficient microbial aerobic population in the rhizo-sphere and by the action of pumping atmospheric oxygen from the emerged part to the roots and so to the underlying soil portion, with a consequent better oxidation of the wastewater and creation of an alternation of aerobic, anoxic and anaerobic zones. This leads to the development of different specialized families of micro-organisms. It also leads to nearly complete disappearance of pathogens, which are highly sensitive to rapid changes in **dissolved oxygen content.** Submerged flow systems assure a good thermal protection of the wastewater during winter, especially when frequent periods of snow are prevented.

Fig. 7. Sketch of a subsurface flow wetland showing the working principles (reprinted from reference [10])

Key design parameters of horizontal subsurface flow constructed wetlands


Hydraulic linear loading rate is the volume of waste water that the soil surrounding a waste water infiltration system can transmit far enough away from the infiltration surface such that it no longer influences the infiltration of additional waste water. It depends on the soil characteristics. In principle, the hydraulic loading rate is equal to the particles settling velocity. A greater surface allows capture of particles with smaller settling velocities. Typical hydraulic rates in subsurface flow wetlands vary from 2 to 20 cm per day.

The aspect ratio defines the length to width ratio. This is considered to be of critical importance for the adequate flow through the wetland. Constructed wetlands are designed with an aspect ratio of less than 2 to optimize the flow and minimize the clogging of the inlet.

### **3.3 Performance evaluation**

Wetland systems significantly reduce biological oxygen demand (BOD5), suspended solids (SS), and nitrogen, as well as metals, trace element, and pathogens. The basic treatment mechanisms include sedimentation, chemical precipitation, adsorption, and microbial degradation of organic matter, Suspended solids and nitrogen, as well as some uptake by the vegetation.

Microbial degradation (also expressed as biological oxygen demand BOD5) in a wetland can be described by a first-order degradation model

$$\frac{C\_e}{C\_o} = \exp\left(-K\_T t\right) \tag{6}$$

Where:

172 Current Issues of Water Management

materials represent the support for the growth of the roots of emerging plants (cf. Fig. 7). The bottom of the basins has to be correctly waterproofed using a layer of clay, often available on site and under adequate hydro-geological conditions or using synthetic membranes (HDPE or LDPE 2 mm thick). The water flow remains always under the surface of the absorbing basin and it flows horizontally [11]. A low bottom slope (about 1%)

During the passage of wastewater through the rhizo-sphere of the macro-phytes, organic matter is decomposed by microbial activity, nitrogen is denitrified. In the presence of sufficient organic content, phosphorus and heavy metals are fixed by adsorption on the filling medium. Vegetation's contribution to the depurative process is represented both by the development of an efficient microbial aerobic population in the rhizo-sphere and by the action of pumping atmospheric oxygen from the emerged part to the roots and so to the underlying soil portion, with a consequent better oxidation of the wastewater and creation of an alternation of aerobic, anoxic and anaerobic zones. This leads to the development of different specialized families of micro-organisms. It also leads to nearly complete disappearance of pathogens, which are highly sensitive to rapid changes in **dissolved oxygen content.** Submerged flow systems assure a good thermal protection of the

wastewater during winter, especially when frequent periods of snow are prevented.

Outlet box Inlet structure

Fig. 7. Sketch of a subsurface flow wetland showing the working principles (reprinted from

Hydraulic linear loading rate is the volume of waste water that the soil surrounding a waste water infiltration system can transmit far enough away from the infiltration surface such that it no longer influences the infiltration of additional waste water. It depends on the soil characteristics. In principle, the hydraulic loading rate is equal to the particles settling

Capped towers @ 1m crs. Across width of bed. Drill 12 m holes below water line

level control

Large stones Outlet structure

macrophytes

substrate

Key design parameters of horizontal subsurface flow constructed wetlands

Overflow outlet in case of blockage in spreader pipe holes

• hydraulic loading rate (HLR),

• size of the granular medium, and

 Normal entry pathway via holes in bottom of spreader pipe

reference [10])

• aspect ratio,

• water depth.

obtained with a sand layer under the waterproof layer guarantees this.

$$
\mathbb{C}\_{\diamond} \quad \text{influence BOD}\_{5} \text{ mg/L}
$$

*Ce* effluent BOD5, mg/L

KT temperature-dependent first-order reaction rate constant, d-1

*t* hydraulic residence time, d

Hydraulic residence time can be represented as

$$t = \frac{LW d}{Q}.\tag{7}$$

Where:

*L* length

*W* width

*d* depth

*Q* average flow rate = (flow in + flow out) ÷ 2

Equation (7) represents hydraulic residence time for an unrestricted flow system.

In a FWS wetland, a portion of the available volume will be occupied by the vegetation; therefore, the actual detention time is a function of the porosity (n). The porosity is defined as the remaining cross-sectional area available for flow.

$$m = \frac{V\_v}{V} \tag{8}$$

With:

Vv volume of voids,

V total volume.

The ratio of residence time from dye studies to theoretical residence time calculated from the physical dimensions of the system should be equal to the ratio.

Wetlands for Water Quality Management – The Science and Technology 175

*<sup>W</sup> Ac d* =

( )( ) <sup>20</sup>

<sup>20</sup> 1.1 *<sup>T</sup> K K <sup>T</sup>*

Constructed wetlands are a cost-effective technology for the treatment of waste water and runoff. Operation and maintenance expenditure are low. These systems can tolerate high fluctuation in flow; with wastewaters with different constituents and concentration. Free water systems (FWS) are designed to simulate natural wetlands with water flow over the soil surface at shallow depth. FWS are better suited for large community systems in mild climates. The treatment in subsurface flow (SF) wetlands is anaerobic because the layers of media and soil remain saturated and unexposed to the atmosphere. Use of medium – sized gravel is advised as clogging by accumulation of solids is a remote possibility. Additionally, medium – sized gravel offers more number of surfaces where biological treatment can take place. Thus SF types of wetlands perform better than FWS. A properly operating constructed wetland system should produce an effluent with less than 30 mg/L BOD, less than 25 mg/L of total suspended solids and less than 10,000 cfu per 100 mL, fecal coliform

In sum, we note that artificial wetlands are known to perform better as far as removal of nitrogen is concerned. The removal of phosphorous and metals depend critically on contact opportunities between the waste water and the soil. Performance of both kinds of constructed wetlands is poor as contact opportunities are limited in both of them. The submerged bed designs with proper soil selection are preferred when phosphorous removal is the main objective. In contrast to this, removal of suspended solids is excellent in both types of artificial wetlands. Constructed (artificial) wetlands assume special significance as natural wetlands are degrading at a rate faster than the other ecosystems. Two primary

The water in the wetland must be shielded from sunlight in order to control algae growth problems. Algae is known to contribute to suspended solids and cause large diurnal swings

ecological agents which cause degradation of natural wetlands are

*Q kAS* = *S S* (13)

<sup>−</sup> = (14)

The bed width is calculated by the following equation

The value of *KT* is calculated using

<sup>20</sup> *K* = 1.28 d-1 for typical media types.

**4. Conclusion** 

bacteria.

1. eutrophication and

in oxygen levels in the water.

2. introduction of invasive alien species.

Cross – sectional area and bed width are established by Darcy's law

Combining the relationships in Equations (7) and (8) with the general model (Equation 6) yields

$$\frac{C\_e}{C\_o} = A \exp\left[-0.7 K\_T \left(A\_v\right)^{1.7} \frac{L\mathcal{W}d\eta}{Q}\right] \tag{9}$$

Where:


$$K\_T = K\_{20} \left( 1.1 \right)^{\left( \left[ \text{\$T \rightarrow 20} \right] \right)},\tag{10}$$

where *K*20 is the rate constant at 20°C.

Other coefficients in equation (5)

*A*= 0.52 K20 = 0.0057 d-1 *Av*= 15.7 m2/m3 *n*= 0.75

In most of the SFS wetlands, the system is designed to maintain the flow below the surface of the bed where direct atmospheric aeration is very low. The oxygen transmitted by the vegetation to the root zone is the major oxygen source. Therefore, the selection of plant species is an important factor. The required surface area for a subsurface flow system is given by

$$A\_S = \frac{Q \left(\ln C\_o - \ln C\_e\right)}{K\_T dm} \tag{11}$$

The cross – sectional area for the flow for a subsurface flow is calculated according to

$$A\_c = \frac{Q}{k\_S S} \text{ \AA} \tag{12}$$

Where *Ac* = *d W*× , cross – sectional area for wetland bed, perpendicular to the direction of the flow, m2,

*d* bed depth, m

*W* bed width, m

*ks* hydraulic conductivity of the medium, 2 3 *m m* d

*S* slope of the bed, or hydraulic gradient.

The bed width is calculated by the following equation

$$\mathcal{W} = \frac{A\_c}{d}$$

Cross – sectional area and bed width are established by Darcy's law

$$Q = k\_S A\_S S \tag{13}$$

The value of *KT* is calculated using

$$K\_T = K\_{20} \left( 1.1 \right)^{(T-20)} \tag{14}$$

<sup>20</sup> *K* = 1.28 d-1 for typical media types.

### **4. Conclusion**

174 Current Issues of Water Management

Combining the relationships in Equations (7) and (8) with the general model (Equation 6)

( )1.7 exp 0.7 *<sup>e</sup> T v*

*<sup>C</sup> LWdn A KA C Q* <sup>⎡</sup> <sup>⎤</sup> = −⎢ <sup>⎥</sup> <sup>⎣</sup> <sup>⎦</sup>

*A* fraction of BOD5 not removable as settling of solids near head works of the system (as

( )( ) <sup>20</sup>

<sup>−</sup> = , (10)

<sup>−</sup> <sup>=</sup> (11)

*k S* <sup>=</sup> , (12)

<sup>20</sup> 1.1 *<sup>T</sup> K K <sup>T</sup>*

In most of the SFS wetlands, the system is designed to maintain the flow below the surface of the bed where direct atmospheric aeration is very low. The oxygen transmitted by the vegetation to the root zone is the major oxygen source. Therefore, the selection of plant species is an important factor. The required surface area for a subsurface flow system is

(ln ln *o e* )

*T QC C <sup>A</sup> K dn*

> *S <sup>Q</sup> <sup>A</sup>*

Where *Ac* = *d W*× , cross – sectional area for wetland bed, perpendicular to the direction of

2 3 *m m* d

*S*

The cross – sectional area for the flow for a subsurface flow is calculated according to

*c*

(9)

*o*

*Av* specific surface area for microbial activity, m2/m3 *L* length of system (parallel to flow path), m

*Q* average hydraulic loading of the system, m/d *n* porosity of system (as a decimal fraction).

where *K*20 is the rate constant at 20°C.

Other coefficients in equation (5)

yields

Where:

*A*= 0.52 K20 = 0.0057 d-1 *Av*= 15.7 m2/m3

*n*= 0.75

given by

the flow, m2,

*d* bed depth, m *W* bed width, m

*ks* hydraulic conductivity of the medium,

*S* slope of the bed, or hydraulic gradient.

decimal fraction),

*W* width of system, m *d* design depth of system, m

> Constructed wetlands are a cost-effective technology for the treatment of waste water and runoff. Operation and maintenance expenditure are low. These systems can tolerate high fluctuation in flow; with wastewaters with different constituents and concentration. Free water systems (FWS) are designed to simulate natural wetlands with water flow over the soil surface at shallow depth. FWS are better suited for large community systems in mild climates. The treatment in subsurface flow (SF) wetlands is anaerobic because the layers of media and soil remain saturated and unexposed to the atmosphere. Use of medium – sized gravel is advised as clogging by accumulation of solids is a remote possibility. Additionally, medium – sized gravel offers more number of surfaces where biological treatment can take place. Thus SF types of wetlands perform better than FWS. A properly operating constructed wetland system should produce an effluent with less than 30 mg/L BOD, less than 25 mg/L of total suspended solids and less than 10,000 cfu per 100 mL, fecal coliform bacteria.

> In sum, we note that artificial wetlands are known to perform better as far as removal of nitrogen is concerned. The removal of phosphorous and metals depend critically on contact opportunities between the waste water and the soil. Performance of both kinds of constructed wetlands is poor as contact opportunities are limited in both of them. The submerged bed designs with proper soil selection are preferred when phosphorous removal is the main objective. In contrast to this, removal of suspended solids is excellent in both types of artificial wetlands. Constructed (artificial) wetlands assume special significance as natural wetlands are degrading at a rate faster than the other ecosystems. Two primary ecological agents which cause degradation of natural wetlands are


The water in the wetland must be shielded from sunlight in order to control algae growth problems. Algae is known to contribute to suspended solids and cause large diurnal swings in oxygen levels in the water.

**Part 4** 

**Politics, Regulation and Guidelines** 

### **5. References**

