CONSTRUCTED WETLANDS: A LITERATURE...

Chapter 2

CONSTRUCTED WETLANDS: A LITERATURE REVIEW

Introduction

Wetlands are distinguished by wet soils, plants that are adapted to wet soils, and a

water table depth that maintains these characteristics. There is no single universally

acceptable and applicable definition. Majority of the wetlands are transitional areas as

they often occupy the interface between the land and deep open water. These interfaces

include the littoral zones (areas between the highest and lowest water level) of large

lakes and reservoirs, the riverine floodplains and the coastal areas that are regularly

flooded by tides. The soils that develop under the prolonged influence of waterlogging

or submergence are known as hydric soils, and the plants that are adapted to or require

such hydrological conditions are called 'hydrophytes'. On the basis of the dominant

plants, wetlands are classified into three groups: salt and freshwater swamps, marshes

and bogs. Swamps are areas dominated by flood-tolerant woody plants and trees,

marshes are dominated by herbaceous plants, and bogs are dominated by mosses and

acid-loving plants (Kadlec and Knight, 1996). Natural wetlands are estimated to cover

about 5.7 million km2, i.e. roughly 6% of the Earth's land surface, of which 30% are

bogs (Mitsch and Gosselink, 2000).

Among the many goods and services proYided by them, the wetlands are highly

Yalued for providing protection against pollution to water resources such as lakes,

estuaries and groundwater. Wetlands are commonly known as biological filters.

Although wetlands have always served this purpose. de,·elopment of wetland treatment

technology is a relatively recent phenomenon. Studies on the feasibility of using

wetlands for wastewater treatment were initiated during the early 1950's in Germany.

In the United States, wastewater to wetlands research began in the late 1960s and

increased dramatically in scope during the 1970s. By early 1990s, North America had

more than .200 natural and constmcted wetland treatment systems (Knight, 1993), and

Europe and Great Britain had more than 500 subsurface flow wetlands (Brix. 1993a).

Dozens of pollution control wetlands exist in other countries. Further, hundreds of

natural wetlands receive treated and untreated ,,·astewater.

Chapter 2. Literature Review

Here, I review the current knowledge on constructed wetland systems in terms

of their potential and efficiency for pollutant removal from various types of

wastewaters. First, some basic information on wetlands composition and function is

summarized, and is followed by the information on types and application of constructed

wetlands. Also several constraints of using constructed wetlands for wastewater

treatment and the problems likely to arise from such use are discussed briefly.

Wetland functions and values

Wetlands are characterized by high organic matter accumulation due to high rate of

primary productivity and a reduced rate of decomposition due to anaerobic conditions

(Hammer and Bastian, 1989). Inorganic nutrients support the growth of vegetation,

which converts inorganic chemicals into organic materials, the basis of the wetland

food chain. As a result of ample light, water and nutrient supply, the primary

productivity of wetland ecosystems is typically high (Brix, 1993a).

Natural wetlands are now recognized as ecosystems that perform many

ecological functions and provide a variety of valuable goods and services (Hammer and

Bastian, 1989, Mitsch and Gosselink 2000). Wetland functions are inherent processes

occurring in wetlands; while values are the attributes of wetlands that society perceives

as beneficial. Different wetlands help in groundwater recharge or flood abatement,

provide habitats to a large diversity of organisms representing all groups from

microorganisms to mammals, enhance the aesthetics of the landscape and perform

several important biogeochemical functions for which they are known as kidneys of the

earth and also as the agents of climate change (because they contribute significantly to

methane emission into the atmosphere).

Among various functions and ecosystem services. the wetlands are recognized

as 'natural purifiers of water' and most valued for their biogeochemical function that

results in the improvement of water quality. The functional role of natural wetlands in

water quality improvement has offered a compelling argument for wetland

conservation. By virtue of their locations, the wetlands receive nutrients and variety of

pollutants with the storm water, agricultural runoff and other overland and subsurface

tlows that invariably pass through them from upland areas before entering the open

waters. Wetlands reduce or remove these contaminants including organic matter,

inorganic matter, trace organics and pathogens from the intlowing water through a

7

Chapter? Literature Review

multitude of processes thereby improving the quality of water flowing out of the

wetland or infiltrating into the groundwater. Reduction or removal is accomplished by

diverse treatment mechanisms including sedimentation, filtration, chemical

precipitation adsorption, microbial interaction and uptake by vegetation (Watson et al,

1989).

Wetlands are capable of providing highly efficient physical removal of

contaminants associated with particulate matter in the water or waste stream. The major

physical process is the settling of suspended particulate matter. Surface water typically

moves very slowly through wetlands due to the characteristic broadsheet flow and the

resistance provided by rooted and floating plants. Sedimentation of suspended solids is

promoted by the low flow velocity and by the fact that the flow is often laminar (not

turbulent) in wetlands. Mats of floating plants in wetlands may serve, to a limited

extent, as sediment traps, but their primary role in suspended solids removal is to limit

resuspension of settled particulate matter. The reduction in the particulate organic

matter is the major cause of reduction in BOD of wastewaters.

Chemical processes such as adsorption, chelation and precipitation are

responsible for the major removal of phosphorus and heavy metals. Studies have shown

that the soil/litter compartment is the major long-term storage pool for phosphorus in

wetlands (Nichols, 1983; Verhoeven, 1986; Cooke, 1992; Hiley, 1995). Many

constituents of wastewater and runoff exist as cations, including ammonium (NH4 +) and

most traces metals, such as copper (Cu+2). The capacity of soils for retention of cations,

expressed as cation exchange capacity (CEC), generally increases with increasing clay

and organic matter content. A number of metals and organic matter can be immobilized

in the soil via chemisorption with clays, iron (Fe) and aluminium (AI) oxides, and

organic matter. Phosphates can bind with clays and Fe and AI oxides through

chemisorption, or precipitate with iron and aluminum oxides to form new mineral

compounds (Fe and AI- phosphates), which are potentially very stable in the soil,

affording long term storage of phosphorus. Wetlands that contain high concentrations

of calcium (Ca), phosphate can precipitate to form Ca-phosphate minerals, which are

also stable over a long period of time. Another important precipitation reaction in

wetland soils is the formation of metal sulfides. Such compounds are highly insoluble

and represent an effective means of immobilizing many toxic metals in wetlands.

0


Volatilization, which involves diffusion of a dissolved compound from the

water into the atmosphere, is another potential means of contaminant removal in

wetlands. Ammonia (NH3) volatilization can result in significant removal of nitrogen, if

the pH of the water is high (greater than about 8.5). However at a lower pH, ammonia

nitrogen exists almost exclusively in the ionized form (ammonium, NH4 +), which is not

volatile. Similarly many organic compounds are volatile and are readily lost from

wetlands and other surface waters to the atmosphere.

Among the biological processes, the most important are those mediated by the

microorganisms and include the oxidation or reduction of carbon, nitrogen and sulphur

depending upon the availability of oxygen. Nitrogen removal is due to reduction

reactions that dominates the systems due to high organic matter in the effluents and

most of the nitrogen is lost through denitrification (Howard- Williams,1985; Gale et al.

1993).

The hydrology of the place, vegetation and soil are reported to be the main factors

influencing water quality in wetlands. The hydrological cycle is the main factor, which

influence the type of vegetation, microbial activity and biogeochemical cycling of

nutrients in soil (Mitsch and Gosselink, 1993). The role played by wetland plants

(macrophytes) in influencing the treatment processes is well documented (Reddy and

DeBusk, 1985: DeBusk and Reddy, 1987; Brix, 1994. 1997; Greenway 1997; Koottatep

and Polprasert, 1997; Mars et al., 1999; Greenway and Wooley, 1999). Table I

summarizes the nutrient uptake capacities of commonly used macrophytes in wetlands.

Microorganisms play a central role in biogeochemical transformation of

nutrients (Hoppe et al, 1988; Madigan et al. I 997) and their capability in removing

toxic organic compounds added to wetlands has been reported (Pitter and Chudoba,

1990; Kadlec and Knight, 1996; Fliermans et al.. 1997; Orshanky and Narkis, 1997;

Reddy and D'Angelo, 1997; Suyama et al., 1998: Savin and Amador, 1998). Microbial

decomposers, primary soil bacteria, utilize the carbon in organic matter as a source of

energy, convertinF it to carbon dioxide (C02) or methane (CH4) gases. This provides an

important biological mechanism for removal of a wide variety of organic compounds,

including those found in municipal wastewater, food processing wastewater, pesticides

and petroleum products. The efficiency and rate of organic C degradation by

microorganisms is highly variable for different types of organic compounds.

9

Chapter l. Literature Review

Table I: Nutrient uptake potential (kg ha·' y( 1) of selected emergent, free-floating, and

submerged macrophytes (Brix, 1994)

Macrophyte Nutrient Uptake Potential (kg ha-1yr"1

)

Nitrogen Phosphorus Crperus paprrus 1100 50 Phragmites australis 2500 120 hpha latifo/ia 1000 180 Eichhornia crassipes 2400 350 Pisria stratiodes 900 40 Potamof!,eton pectinatus 500 40 Ceratophrlum demersum 100 10

Microbial metabolism also affords removal of inorganic nitrogen, i.e. nitrate

and ammonium, in wetlands. Specialized bacteria (Pseudomonas spp) metabolically

transform nitrate into nitrogen gas (N2), a process known as denitrification. The N2 is

subsequently lost to the atmosphere, thus denitrification represents a means for

permanent removal, rather than storage of nitrogen by the wetland. Removal of

ammonium in wetlands can occur as a result of the sequential processes of nitiification

and denitrification. Nitrification, the microbial (Nitrosomonas and Nitrobacter spp)

transformation of ammonium to nitrate, takes place in aerobic regions of the soil and

surface waters. The formed nitrate can then undergo denitrification on diffusing into the

deeper anaerobic zone. The key processes in the soil and water column as related to

nutrient retention and release by wetlands have been reviewed by Reddy and D'Angelo

(1994, 1997 ).

Natural Vs. Constructed Wetlands

Studies haw shown that natural wetlands are able to provide high levels of wastewater

treatment (Nichols, 1983; Knight et al. 1987: Kadlec and Knight, 1996; Mander and

Mauring, 1997). Long retention time and an extensive amount of sediment surface area

in contact with the flowing water provides for effective removal of particulate matter.

The sediment surfaces are !ilso where most of the microbial activity affecting water

quality occurs, including oxidation of organic matter and transformation of nutrients.

Natural wetlands are, however. characterized by extreme variability m

functional components, marking it virtually impossible to predict responses to

wastewater application and to translate results from one geographical area to another.

f{)


Although significant improvement in the quality of wastewater is generally observed as

a result of flow through natural wetlands, the extent of their treatment capability is

largely unknown. The performance may change over time as a consequence of changes

in species composition and accumulation of pollutants in the wetlands (Richardson and

Nichols, 1985). Therefore the treatment capacity of natural wetlands is unpredictable.

There are too few data from natural systems to allow confident predictions of the

treatment performance of the systems and the effects of wastewater discharge on

receiving ecosystems (Richardson and Davis, 1987). There has also been concern over

possible harmful effects of toxic materials and pathogens in wastewaters and long-term

degradation of wetlands due to additional nutrient and hydraulic loadings from

wastewater. Thus natural wetlands are not appropriate as large-scale wastewater

treatment systems, but should be preserved for environmental conservation.

Efforts have therefore been made towards using constructed wetlands (CWs) for

wastewater treatment (Hammer and Bastian, 1989). Constructed wetlands for

wastewater treatment involve the use of engineered systems that are designed and

constructed to utilize natural processes, by mimicking natural wetlands with much

degree of control, thus allowing the establishment of experimental treatment facilities

with a well defined composition of substrate, types of vegetation and flow pattern

(EPA, 1993). In addition, constructed wetland offer several additional advantages

compared to natural wetlands, including site selection, flexibility in sizing and most

importantly. control over the hydraulic pathways and retention time. Most constructed

wetlands emulate marshes because soft -stemmed plants in the marshes require the

shortest time compared to plants in bogs and swamps for full development and

operational performance (Hammer and Bastian, 1989). Constructed wetlands for

wastewater treatment in some locations have sewral advantages of low cost of

constmction and maintenance, low energy requirements. more flexible and less

susceptible to variations in loading rates compared to conventional secondary and

advanced wastewater treatment mechanisms.

Constructed wetlands are used to remove bacteria, enteric vimses, suspended

solids, biochemical oxygen demand. nitrogen (as ammonia and nitrate), phosphoms and

metals (Pinney et al., 2000). Two gc~.~ral forms of constructed wetlands are used in

practice: surface flow and subsurface flow. Surface tlow (SF) constmcted wetlands

most closely mimic natural environments and are usually more suitable for wetland

II

cnap1er L. Llferature Kevleu;

species because of permanent standing water. In sub-surface flow (SSF) wetlands,

water passes laterally through a porous medium (sand or gravel) with a limited number

of macrophyte species. These systems have often no standing water.

Constructed wetlands can be built above or below the existing land surface if an

external water source is supplied (wastewater). The grading of a particular wetland in

relation to the appropriate elevation is important for the optimal use of the wetland area

in terms of water distribution. Medium type and groundwater level must also be

considered if long-term water shortage is to be avoided. Liners can prevent excessive

desiccation, particularly where soils have a high permeability or where there is limited

or periodic flow.

Constructed wetlands are used for treating various types of wastewater e.g.

domestic wastewater (Cooper et al., 1997; Schreijer et al., 1997; Bays and Knight,

2000; Coleman et al., 2001), acid mine drainage (Kleinmann and Girts, 1987; Brodie et

al., 1989; Howard et al., 1989; Wenerick et al .. 1989; Woulds and Ngwenya, 2004;

Hallberg and Johnson, 2005), agricultural wastewater (Du Bowry and Reaves, 1994;

Rivera et al., 1997; Hunt et al., 2000; Smith et al .. 2006), landfillleachates (Dornbush,

1989; Trautmann et al., 1989; Staubitz et al., 1989), urban storm water (EPA, 1993) and

for polishing advanced treated wastewater effluents for return to freshwater resources

(Schwartz et al., 1994; Gschlobl et aL 1998).

Constructed wetlands are also used for treating eutrophic lake waters (D'Angelo

and Reddy, 1994b) and for the conservation of nature (WOITall et al., 1997).

Denitrification efficiency in wetlands treating high nitrate waters with low organic

carbon has been shown to depend on C: N ratios > 5: I (Baker, 1998). Constructed

wetland performance data vary with site, wastewater characteristics, wetland design,

application and water treatment goals. Therefore "a system approach" is often required

for successful management of wastewater.

Historical Background

The history of constructed wetlands can be traced back to 1950s when Kaithy Seidel

exan1ined in Northern Germany the possibility of wastewater treatment by wetland

plants, and Kickuth extended this work in Europe in the late 1970s (Table 2). Interest in

the subject started in the United States started in early 1980s with the studies of

Wolverton et al. (1983) and Gersberg et al. (1985).

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Table 2: Selected historical events of wetland treatment technology.

Date Location Description

1952-1970s Pion, Germany Removal of rhenols and dairy wastewater treatment with bulrush plants (Scidd and Kickuth).

1967-1972 Morehead city, NC Constructed estuarine ponds and natural salt marsh: for municipal effluent recvclin2 (H.T. Odum and associates).

1972-1977 Porter Ranch, MI Natural wetland treatment of municipal wastewater (Kadlec and associates).

1973-1976 Brookhaven, NY Meadow/ marsh/ pond svstems (Small and associates). 1973-1977 Gainesville, FL Studies on the use of cypress wetlands for municipal

wastewaters lOdum, Ewe! and associates). 1974-1975 Brillion, WI Constructed and natural marsh wetlands for phosphorus removal

(Span!!ler and associates). 1974--1988 NSTL Station, MS Gravel-based sub surface-flow wetland; for recycling municipal

waste water and priority pollutants (Wolverton and co-workers) 1975- 1977 Trenton, NJ Small enclosures in the Hamilton Marshes (Fresh Water Tidal )

were irri2ated with treated sewage (Whigham and Co- Workers) 1976-1979 Eagle Lake lA Natural marsh wetland: for assimilation of agricultural drainage

and municipal wastewater nutrients (Davis, van derValk and co-workers.

1976-1982 Southeast Florida Natural marsh wetlands receiving agricultural drainage waters; for nutrient removal (Davis and coworkers).

1981-1984 Santee, CA Subsurface flow wetlands. for municipal wastewaters (Gersberg and coworkers).

The concept developed by Seidel included a series of beds composed of sand or

gravel supporting emergent aquatic vegetation such as cattails (Typha), bulrush

(Scirpus) and reeds (Phragmites). In majority of her experiments on the use of wetland

plants for various types of wastewater (e.g. Seidel 1961, 1965, 1966, 1976), the flow

path was vertical through each cell to an under drain and then onto the next cell.

Excellent performance for removal of BOD5• TSS. nitrogen, phosphorus and more

complex organics was claimed. Although Seidel's experiments were heavily criticized

(Numann 1970), many researchers continued in her ideas (e.g. Tourbier & Pierson,

1976). The major reason for the criticism was the fact that investigations and

calculations were mostly aimed only at the use of plants for nutrient removal by plant

uptake. It took more than 20 years of research before the first operational full scale

constructed wetland for municipal sewage was built in Othfresen in Germany (Kickuth.

1977). Kickuth proposed the use of cohesive soils instead of sand or gravel; the

vegetation of preference was Phragmites and the design·flow path was horizontal

through the soil media. His theory suggested that the growth, development and death of

the plant roots and rhizomes would open up flow channels. to a depth of about 0.6m in

the cohesive soil, so that the hydraulic conductivity of a clay like soil would gradually

be converted to the equivalent of a sandy soil. This would permit flow through the

13

media at reasonable rates and would also take advantage of the adsorptive capacity of

the soil for phosphmus and other materials. Very effecti\·e removal of BOD5, TSS,

nitrogen, phosphorus and other complex organics \vas claimed. By 1990, about 500 of

these 'reed bed' or 'root zone' systems had been constructed in Germany, Denmark,

Austria and Switzerland. The type of systems in operation includes onsite single-family

units as well as larger systems treating municipal and industrial wastewaters.

Commencing in 1985, a number of reed bed systems were constructed in Great

Britain based on Kickuth's concept, but in many cases gravel was used as the bed

media rather than cohesive soil (Boon, 1985) due to concerns regarding soil hydraulic

conductivity. Many of these beds were built with a sloping bottom and a flat surface.

The purpose of the sloping bottom was to provide sufficient hydraulic gradient to

ensure subsurface flow in the bed. The flat upper surface would allow temporary

flooding as a weed control measure to kill undesirable plants. Some of these systems

also had an adjustable outlet, which permitted easy maintenance of the desired water

level in the bed (Cooper and Hobson, 1990).

Wolverton's work in Louisiana began with experimental bench-scale trays in a

greenhouse containing rock or gravel media and supporting a stand of emergent aquatic

vegetation (Wolverton et al., 1983). The trays were filled with wastewater, and then

drained after a certain number of hours (12 to 48 hours). In essence the procedure was a

fill and draw batch type process. Excellent performance was demonstrated for BOD5,

TSS and NH4 and moderate performance for phosphorus with a one day HRT. The

typical organic loading during these experiments (at I day HRT) was about 58kg ha. 1d. 1

and the hydraulic loading was about Scm d. 1• Design criteria based on this work

included one day HRT. about five acres of bed surface area per mgd and upto 15:1

aspect ratio (L: W). These criteria or variations have been widely applied and as of

1991, there were about 60 systems in operation or in various stages of design in the

south central U.S, based on these values. These systems range from onsite single

family units to large-scale municipal systems (Jones and Wolverton, 1990).

Gersberg et al. ( 1985) conducted large scale, continuous flow, field experiments

using 0. 76m deep gravel beds, in California. The removal of BOD, TSS and NH4 was

correlated with the depth of root penetration of the plants such as Typha, Scirpus.

Phragmites, with the best removals occurring with the deepest root penetration. The

14


organic loading was approximately 55kg ha-'d·' and the hydraulic loading about 5cm d. 1• The HRT in various systems varied between I and 6 days (Table 3.).

Table 3: Variation in design approaches in European and U.S. CW systems

Source Organic loading Hydraulic loading Area Hrt

(Kg ha"1d.1) (em d"1) (mz m-3 d.J) (day)

Europe

Boon (1985) 180 9 II 2.6·

Cooper (1990) 80 4 24 6

U.S.A.

Wolverton (1983) 58 8 13 I

Gersberg ( 1985) 55 5 20 6

Benton, KY ( 1989) 81 I4 7 2

Bear Cr, AL( 199 I) 4 3 33 3

Beginning in the mid 1980s, the Tennessee Valley Authority (TV A) began a program

of research and technical assistance on constructed wetlands for treatment of various

types of wastewater (municipal, acid mine drainage, agricultural wastes and runoff, etc)

(Hammer, 1989). More emphasis was given to subsurface flow constructed wetlands

for wastewater treatment, originally derived from the work of Kickuth (Steiner et al.,

1987), has been modified significantly in subsequent years. By 1990s there were some

80 subsurface flow systems, in operation in a number of states, based on criteria and

design provided from the previous research and studies. Table 2. presents some of the

events leading to the acceptance of the use of constructed wetlands for water quality

treatment and implementation of this technology has accelerated around the world since

about 1985.

This background perspective illustrates the worldwide spread of confidence in

this technology that originated from research conducted in the early 1950s and 1970s. It

is not surprising that the value of constructed wetlands for water quality improvement

was rediscovered during the last twenty years of heightened environmental awareness.

But it is important to remember that the knowledge gained over this relatively short

period is not adequate enough to use the conservative design for meeting stringent

water quality criteria. Many questions regarding the potential of CW technology in the

developing countries require further research and analysis. The following sections look

at the efforts made to explore this potential for various types of wastewater.

15

Chapter 2. Literature Reviell'

Constructed wetland in agricultural wastewater treatment.

Non-point and diffuse source pollution of waterways draining agricultural land have

become an increasing concern in many countries as farming practices intensify and

rural land uses diversify. Agricultural wastewaters characteristically contain high levels

of organic matter and nutrients. Typical composition of wastes from agricultural

sources is tabulated below (Knight et al., 2000).

fable 4: Typical waste characteristics of selected Agricultural Industries.

Parameter Units Dairy Poultry Cattle Swine

BOD5 mgL· 123-3400 473 300-12,000 104

COD mgL 1 722 2500-40,000

Total solids mgL- 1 1111 196 1000-13,400 128

NfL-N mgL 5.5 74 1-770 366

TKN mgL- 1 103 89 407

NOrN mgL- 1 0.1-1270

TP mgL· 16.2 20-480

TN mgL- 1 68

Agriculture wastewater has been widely managed using constructed wetland

;ystems in USA, Norway, Finland, Italy, New Zealand and UK (Maddox and Kingsley,

1989; Hammer et al., 1993; Du Bowy and Reeves, 1994; Cronk, 1996; Sun et al., 1998;

Kern and Idler, 1999; Knight et al., 2000; Newman et al., 2000; Nguyen, 2000;

~chaafsma et al., 2000; Koskiaho, 2003; Mantovi et al .. 2003; Poach et al., 2003).

CWs are often used as alternatives to, or components of, conventional nutrient

nanagement practices to reduce or eliminate contaminant and nutrient loads in

1gricultural wastewaters (Cronk, 1996; Peterson. 1998: Geary and Moore, 1999; Knight

~t al., 2000; Borin et aL 200 I; Hunt and Poach. 200 I; Szogi and Hunt, 2001;

Braskerud, 2002). Wetlands used to improve water quality within agriculture typically

ntercept and retain contaminants and nutrients from incoming waters through a series

)f vegetated ponds, before waterleaves or is reused in farm-scale operations (Knight et

11., 2000). Percent mass pollutant removal by SF-CW treatment of agricultural dirty

.vaters can varies between 48 and 95% of total suspended solids (Sievers, 1997;

\Jew man et al., 2000; Reddy et al., 2001 ); between 50 and 99% of nitrogen and

Jetween 30 and 94% of phosphorus inputs, depending on their forms (Cathcart et al.,

16


1994; Hunt et al., 1994; Humenik et al., 1997; Newman et al., 2000; Reddy et al.,

2001).

Several constructed wetlands have been tested for their ability to retain or

transform nutrient inputs specifically from dairy and swine wastewaters (Table 5 ).

Further, many other dairy wetlands have been constructed in several states of the USA

(Holmes et al., 1995, Chen et al., 1995, Kadlec and Knight, 1996).

Table 5: Average treatment wetland performance for removal of contaminants from

dairy wastewater. Percent reduction in concentration is given in parentheses.

Location Time BOD5 TSS TKN NH3 TP

Source Period mgL·1 mgL·1 mgL·1 mgL·1 mgL·1

Drointon, Nov-Apr 3.7 ( 17) 5.1 (57) Biddlestone et UK al., 1991. Sonoma Jut- Oct 7.4 (97) 95.4 (99) 4.5 (%) 1.4 (86) 2.1 (93) City of Santa Coi,CA Rosa, 1994 Fredrick Co, Aug-Apr 16.5 (70) 8.2 (90) 0.6 (78) 0.3 (78) 0.1 (54) Cronk et MD a1..1994 La Grange Jan-Dec 0.9 (78) 1.9 (89) 0.3 (66) 0.1 (66) 0.03 (64) Reaves et al., Co, IN 1994a Oregon Oct-Mar 8.7(61) 8.4 (73) 2.1 (57) 1.4 (54) 0.4 (66) Skarda et al., State Univ, 1994 OR Nova Apr-Jan 17.4 (99) 28.65 (94) 8.93 (98) 4.36 (89) Smith et al., Scotia, 2006 Canada Poggio I year 27.63 (75) 54.5 (30) 2.27 (65) Pucci et al., Antico. 1998. Florence Santa Lucia. 2 years 28 (94) 63 (91) 33.3 (49) 24.5 (-9) 5.0 (61) Mantovi and Italy Piccini Univ of 3 years 611(76) 130 (90) 73.48 (28) 130 (45) Newman ct al.. Connecticut. 2000. USA

In England. :.1 250m2 horizontal subsurface flow reed bed, which initially received only

dairy parlor washings, decreased BOD by 17% and total suspended solids (TSS) by

57% (Biddlestone et al., 1991 ). After 6 months, barnyard runoff was added to the waste

and the influent BOD and TSS concentrations more than doubled. The reed bed became

more effective and it reduced BOD by 49% and TSS by 70%. The enhanced retention

was probably because the reeds were better established or because of the difference in

the study period. The increased temperatures of the growing season may have enhanced

BOD removal. After 2 years of operation, the addition of two vertical flow reed beds

upstream of the original bed (Biddlestone et aL 1994) resulted in the reduction of BOD

concentration by 94% (from 1190 to 70 mg L- 1).

17

Chapter 2. Literature Revie1v

qther dairy wastewater treatment wetlands have also been reported to decrease

influent BOD, TSS, TKN, NH4, N03, TP and ortho-P by about 50% in the first year of

operation (Skarda et al., 1994, Reaves et al., 1994b ). Decrease of ammonia was

attributed to volatilization rather than nitrification and denitrification because oxygen

levels were too low for nitrification to occur.

In New Zealand, Tanner et al. (1995) compared the effect of influent loading

rates on removal of BOD, TSS, dissolved color and faecal bacteria from dairy farm

wastewaters, with planted (Sclzoenoplectus validus) and unplanted subsurface flow

gravel bed constructed wetland. Fortnightly observations over 20 months period on the

wetlands that were operated at 2 to 7 days retention times, showed that potential

nitrogenous oxygen demand (NBOD) was an important component of total BOD, and

that the mean mass removal of CBOD5 increased from 60-75% to 85-90%, total BOD

(CBOD5 + NBOD) from 50-80% and FC from 90-95% to >99% with increasing

retention time. Mean annual SS removal was 75%-85% irrespective of the loading

rates. High levels of dissolved humic color in the wastewaters were little affected by

passage through the wetland at short retention times, but were reduced up to 40% at

longer retentions. However mass removals of the above said parameters showed

monotonic relationships to mass loading rates with little difference between the

performance of a planted and unplanted wetlands, except for CBOD5 at high loadings

(>3m·2 d" 1). The planted wetlands showed significantly improved removal rates for

CBOD5 at higher loadings and 1.3 to 2.6 fold higher mass removals of total BOD.

In another paper, Tanner et al. ( 1995) emphasized the nutrient uptake by similar

wetland plants from dairy parlor wastewater. As retention time increased from 2 to 7

days, mean reduction of TN increased from 12 to 4l<k- and 48 to 75% in the unplanted

and planted wetlands respectively. TP removal increased from 12 to 36% and 37 to

74% respectively. The planted wetlands showed gradual increase in the annual removal

rates of TN (0.15-1.4 g m·2 d- 1) and TP (0.13-0.32 g m-2 d- 1

) with increased mass

loading rates whereas the unplanted wetlands showed a marked decline in the nutrient

removal at high loadings.

Two horizontal subsurface flow reed beds treating dairy parlor effluent (about

4.5m3) as well as domestic sewage (about 2m·\ were set up in I 999 to verify the

efficiency of this treatment system in reducing the polluting load (Mantovi et al.,2003).

High TSS (about 700 mg L- 1) and COD and BOD5 taverage 1200 mg 0 2 L- 1 and 450

18


mg 0 2 L" 1) characterized the influent waters. Whereas more than 90% SS and organic

load were removed, nitrogen and phosphorus removals were about 50% and 60%

respectively. The number of coliform bacteria, Escherichia coli and faecal streptococci

was reduced by 98-99%. Results demonstrated that reed beds could be appropriate

treatment for dairy parlor wastewater.

In Ireland, over a dozen dairy farms use integrated constructed wetlands to

manage farmyard dirty water (Dunne et al., 2005).

In the United States, the Ecological Treatment systems (ETS) have proven

effective for several other types of waste streams. Animal facility wastewater, for

example, has a high concentration of nutrients, solids and BOD (Midwest Plan Service,

1993; Geary and Moore, 1999; Knight et al., 2000). Lansing and Martin (2006)

explored the potential of an ETS in Columbus, OH, USA, to provide animal producers

with clean water that can be reused for dairy barn flushing, irrigation, or discharged

safely into surface waters. The system treated 1310 L d- 1 of diluted wastewater from a

dairy facility with over 99% removal of ammonium nitrogen (NR.-N) and

carbonaceous biochemical oxygen demand, and 79% removal of orthophosphate (P04-

P). Nitrate + nitrite (NOx - N) was produced and removed within the system and had

an average effluent concentration of 0.53 mg L- 1• The multiple anaerobic-aerobic

interfaces in the ETS design enhanced biological removal of nitrogen and phosphorus.

In Flanders (Belgium), Meers et al. (2005) evaluated the potential of

constructed wetlands planted with Phragmites australis to reduce nitrogen, phosphorus

and chemical oxygen demand in the liquid fraction of the pig manure processing plant

to Jeyels below discharge criteria. Removal efficiencies varied between 64-75% for

COD. 73-83lk for N and 71-92% for P. However. the effluent levels were still

significantly above the Flemish legal discharge criteria.

Hunt et al. (2000) assessed constructed wetland treatment of swine wastewaters

in both continual marsh and marsh-pond-marsh systems in North Carolina, USA. They

were used for treatment of swine manure effluent after it was treated in anaerobic

lagoons but prior to land application. The plant communities grown in the wetlands

were msh. bulrush, cattail, bur-reed, duckweed. and soybean, grown in saturated-soil

culture, and tlooded rice. Nitrogen loading rates ranged from 2 to 36 kg ha·' d- 1• TheN

removal rate for loadings of 2 to 27 kg N ha·' d- 1 were remarkably constant for all

19

uzaptl:r L. Literature Kev1ew

wetland types. However at a loading rate of 36 kg N ha- 1 d- 1, only 47% of the N was

removed indicating that the break point for loading was between 27 and 36 kg N ha- 1 d-

1. At the lower loading rates, plant and soil N accumulation constituted a significant

portion (- 40%) of the total amount applied, but at the higher loading rates, microbial

transformations were likely the more dominant treatment factors.

Table 6: Mass of contaminants retained per unit area in wetlands for swine wastewater

treatment (g m·2 d- 1). Percent reduction in mass between inflow and outflow is given in

parentheses.

Location Time

BOD5 TSS TKN NH3 TP Source Period

Auburn Univ .. AL Jul-Sept McCaskey et al., 1994

Low load BODs m-2d- 1

)

(0.23g - 0.1 (95) 0.3 (99) 0.2 (99) 0.2 (94) 0.2 (94)

Medium load (0.46g - 0.2 (90) 0.6 (96) 0.5 (97) 0.5 (98) 0.3 (84) BODs m-2d-1

)

High load (l.lg - 0.4 (72) 1.3 (94) 1.0 (82) 1.0 (89) 0.6 (79) BOD5 m-2d- 1)

MS State Univ. MS April, 0.3 (54) 0.8 (69) 1.2 (71) 0.2 (44) Cathcart et 1992 al., 1994

Duplin Co. NC Jun- Dec 0.3 (99) Hunt et al., 1994.

Some constructed wetlands have shown a high percent reduction in contaminant

levels (Table 6). However it is difficult to compare the study results since pretreatment,

substrate. vegetation and climatic conditions vary widely among the sites. Excellent

percent reductions have been noticed at constructed wetlands treating swine wastewater

in Alabama (McCaskey et a!.. 1994 ). North Carolina (Hunt, et al., 1994) and

Mississippi (Cathcart et al., 1994 ), perhaps because loading rates have tended to be

lower at swine operations than at dairy farms. Table 7 summarizes the wetland long

term and annual average operational performance data.

20


Table 7: Average treatment wetland performance for removal of 8005, TSS, NH4-N

and TN in the Livestock wastewater treatment wetland database

Average inflow A l'erage outflow Average Wastewater type Count (n) concentration concentration concentration

(mg L"1) (mg L"1

) reduction (%) BOD5

Cattle feeding 14 137 24 83 Dairy 374 442 141 68 Poultry 80 153 115 25 Swine 183 104 44 58

TSS Cattle feedin!! 12 291 55 81 Dairy 361 II II 592 47 Swine 180 128 62 52

NH4-N Cattle feedin!! 12 5.1 2.2 57 Dairy. 351 105 42 60 Poultry 80 74 59 20 Swine 183 366 221 40

TN Dairy 32 103 51 51 Poultry 80 89 70 22 Swine 164 407 248 39

Constructed wetlands for domestic wastewater treatment.

Many Appalachian households and rural communities lack centralized wastewater

collection and treatment facilities due to mountainous topography, low population

densities or a lack of financial resources. Typically onsite treatment consists of a septic

tank to settle solids, followed by a soil drain field. Recent surveys by the USEPA

indicate that the failing septic systems are the third most frequently cited source of

groundwater contamination in the United States (USEPA. 2000). Malfunctions may

occur when the systems are poorly constructed or not properly maintained, when the

soils are unsuitable, or from reuse. Releases to groundwater from household systems

can include bacteria, nitrates, viruses and phosphorus. Typical characteristics of

domestic wastewater are described in Table 8. Characteristic of this wastewater

considered includes septic tank effluent that are overloaded or poorly controlled

system. Such contaminants are of particular concern in rural areas, where domestic

septic systems are prevalent and 95% of the population receives their fresh water from ~ / ,,c .....

groundwater-recharged wells. ,~/~\ .~.!!...~,~ ):'\ (; ~"/ "v' ' It!/ ': -;:.] .t:.· / f ., .... ' ... _. (;,~- '-"vi;;J!Y/_~ ... ..... ,.... .. ~ ' ... } ' .. .., __ ___.,-"'.·\.~':>~

' . ... , +- ,• 21


Table 8: Characteristics of domestic wastewater (from EPA 2000)

Constituent (mg/1) Septic Tank effluent Primal)· effluent Pond effluent

BOD 129-147 40-200 11-35 COD 310-344 90-400 60-100 TSS 44-54 55-230 20-80 TN 41-49 20-85 8-22 NH4 28-34 15-tO 0.6-16 N03 0-0.9 0 0.1-0.8 TP 12-14 4-15 3-4 Ortho P 10-12 3-10 2-3 Fecal coli (logJIOOml) 5.4-6.0 5.0-7.0 0.8-5.6

In the 1980s, the United States Environmental Protection Agency (USEPA)

agreed that constructed wetlands were an appropriate management practice for

treatment of domestic wastewater (Bastian et al., 1987). Good aesthetic properties,

lower operational costs for homeowners and effective treatment capabilities make

surface or subsurface flow wetlands an appropriate choice for small scale, individual or

small group residential situations (Hiley, 1995; Knight, 1993; Knight et al., 1993;

Steiner and Combs, 1993; Mitsch and Gosselink, 2000).

Single-family constructed wetland systems in Ohio, USA were studied to

evaluate their effectiveness in improving water quality (Steer et al., 2002). Twenty-one

three-cell systems (septic tank with two wetlands) were found to meet USEPA effluent

load guidelines in 68% of the quarterly water quality samples collected from 1994 to

2001. These wetlands met the EPA standards for mitigation of BOD (89% below 30

mgL" 1), TSS (79% below 30 mgL" 1

) and fecal coliform (74% below 1000 counts/ 100

ml). Phosphorus and ammonia discharge met the guidelines less often (50% at I mg L·'

and 16% at 1.5 mg L" 1• respectively). These data also indicate that domestic treatment

wetlands can reduce output of fecal coliform 88 ± 27%, TSS 56 ± 53%, BOD 70 ±

48%. ammonia 56 ± 31% and phosphorus 80 ± 20lfe.

In West Virginia, a system was designed to simulate full-scale constructed

wetland for domestic wastewater treatment with intlow rates of 19 L d- 1 and frequency

of 3 times I day (Coleman et al., 200 I). The flow rates. frequencies and retention times

were selected to approximate those of household wastewater streams into full-scale

individual residential constructed wetland (Crites. 199-l). The average of all treatments

showed a 70% reduction in TSS and BOD. 50 to 600C reduction in nitrogen (TKN),

22


ammoma and phosphate, and a reduction of fecal coliforms by three orders of

magnitude.

Severe degradation of the water quality of the Texcoco river in central Mexico

as a result of discharges of raw sewage from communities into the watershed resulted in

construction of a pilot scale treatment wetland in the small community of Santa Maria

Nativitas in the Rio Texcoco watershed (Belmont et al., 2004). The pilot scale study

was directed at determining the effectiveness of a subsurface flow wetland for the

treatment of domestic sewage generated by small community. The system consisting of

sedimentation terraces, stabilization pond, subsurface flow wetland and vertical flow

wetland, removed > 80% of TSS, COD, and nitrate from dm:nestic sewage. Removal of

ammonium was less efficient at about 50%. The treated water was suitable for

irrigation, which could help to alleviate the scarcitY of water in the Rio Texcoco

watershed.

Official guidelines for the onsite treatment of domestic sewage have recently

been published by the Danish Ministry of Environment as a consequence of new

treatment requirements for single houses and dwellings in rural areas. Brix and Arias,

(2005) summarized the guidelines for vertical constructed wetland systems that will

fulfill demands of 95% removal of BOD and 90% nitrification.

In Czech Republic, there were 62 constructed wetlands treating domestic or

municipal sewage water by 1995 (Vyamazal, 1998). The data (Table 9) show that the

effluent concentrations of BODs were far lower than maximum concentrations set by

the European permissible limits (80 mg L" 1) and similar results were seen in case of

COD. The removal of suspended solids was also ,·ery high. The outtlow concentrations

rarely exceeded 2 mg L- 1 and were by far lower than the maximum allowable

concentrations set by 1992 law (Vyamazal et al, 1998). Total nitrogen and phosphorus

showed moderate removal with effluent concentrJtion usually <40mgL- 1• Other

domestic wastewater treatment wetlands exist in seYeral European countries (Austria,

Belgium. Denmark, France, Germany. Hungary. ;..;-orway, Poland, Portugal, and

Switzerland; Vyamazal et al., 1998). The treatment potential of these wetlands is

presented in Table 9.

23

Chapter 2. Literature Revieu:

Table 9: Average treatment wetland performance for removal of contaminants from

domestic wastewater. Percent reduction in concentration is given in parentheses.

Location BOD5 TSS TKN NH3 TP

Source (mgL"1) (mgL"1

) (mgL"1) (mgL"1

) (mgL"1)

Fort Deposit, Alabama 4.28 (90) 7.04 (93) 0.72(93) Bays and Knight, 2000

Queensland, Australia 20 (92) 9.0 (97) 13.5 (71) 13.0 (56) 11.0(13) Greenway and Bolton, 2000

Nova Scotia, Canada 21 (76) 17(40) 12.19(76) 1.88 (8 I) Hanson, 2000 Kolodege, Czech 9.5 (95) 10.8 (96) 29 (51.7) 22.4(36.5) 4.0 (55.6) Vyamazal, 2000 Republic Ondrejov, Czech 18.5 (91) 9.0 (96) 30.4(47.4) 23.6(20) 6.3 (14.9) Vyamazal, 2000 Republic Wuhan, China 3 (78.7) (80.2) 0.87(39.2) (16.5) 0.2 (25.8) Wu et al., 2000 Univ of lands and 19.41() 16.7 (22.4) Kaseva. 2004. agriculture, Tanzania Ohio, USA 16.27(82.2) 21.8(55.73) 16.09(58.65) 2.68(74.64) Steer et al.,

2002 West Virginia, USA Coleman ct al., Juncus sp 48.2 (65) 16.7(78) 7.7 (47.6) 6.1 (50) 2001. Scirpus sp 41.3 (69.8) 15.7 (78.2) 1l.G\26.4) 9.1(25.4) Typha sp. 33.0 (75.9) 18.3 (75.4) 5.6 (65) 4.7 (61.4) Santa Maria Nati\itas, 22.4 (94.4) 66.5(54.7) 42.4 (20.6) Belmont et al., Central Mexico 2004 Austria, Central Europe 4.0 (92.9) 10.0 (88.3) 44.6(51.0) 7.4 (90.5) 3.3 (70) Haberl et al.,

1998 Villers-la- Ville, 13(94) 7 (98) 16.7 (49) 3.5 (49) Cadelli et al., Belgium, Europe 1998. Prague central 21 (89.5) 5.7 (97.6) 26 (45.8) 3.3 (49.2) Vyamazal. I 998 wastewater treatment, Czech Republic. Europe Denmark. Europe <10 (80) 7.6 (92) 1.0 12 (52) 4.8 (35.13) Brix. 1998 Montromant plant. 16 (92.5) 12.0 (94.5) 10.2 (76) 7.42 (71.4) 5.6 (40) Lienard et al.. France. Europe 1998

Constructed wetlands for industrial wastewater treatment.

Although industrial wastewater quality varies among industries, it has a fairly

consistent intrasystem effluent quality. Table 10 summarizes the typical quality of raw

wastewater from a number of industries for which CW technology may be appropriate.

However raw industrial wastewater usually requires some level of pretreatment before

discharge to a wetland treatment system. If total concentrations of BOD, SS and NH4-N

in untreated industrial wastewater are in the I OOs or I OOOs of mg L- 1 concentration

range, it is generally not acceptable for wetland discharge without additional

pretreatment.

Chapter 2. Literature Revieu:

Table 10: Typical industrial concentrations in a variety of untreated industrial

wastewaters

Constituent Pulp and Landfill Petroleum Electroplating

Textile Breweries (mgL"1

) paper leachate refinery mills BOD5 100-500 42-10,900 10-800 75-6300 1500-3000 COD 600-1000 40-90,000 50-600 220-31,300 800-1400 TSS 500-1200 100-700 10-300 -l-600 25-24,500 100-500 N~-N 0.01-1000 0.05-300 TN 70-1900 10-120 10-30 25-45 TP <0.01-2.7 1-10 20-50 so4·-t s 10-260 ND-400 30-120

Although constructed wetland technology is well established, its application for

treating specific industrial effluents has not been well documented (Worall et al., 1997;

Kao and Wu, 2001; Garcia et al., 2004). Perhaps the most promising application of

constructed wetlands is for the control of many organic compounds in the landfill

leachates.

CWs have been used for advanced tertiary treatment of refinery wastewaters at

Amoco's Mandan, ND facility for over 20 years (Litchfield and Schatz, 1989;

Litchfield, 1990, 1993). Approximately 2500m3d- 1 of wastewaters passes through the

16.6 ha system, providing a hydraulic loading rate of I .5cmd· 1• Reasonable removals of

many refinery pollutants have been achieved by these wetlands (Table 1 I). High

reduction in hexavalent chromium, phenols, oil and grease, BOD and COD were

sustainably maintained. Similarly, Chevron utilized the surface flow wetlands to polish

the wastewater from their refinery in Richmond. CA. before being released into San

Francisco Bay (Duda, 1992). The 36 ha wetland treats 9500nY~ d- 1 of water emerging

from the oxidation ponds at the refinery with hydraulic loading of 2.6cmd-'.

Consequently, the removal of the ammonium (76lf). and nitrate (69%) over a 3-year

period were good. BOD dropped 51% and TSS -l-5C:C. but both these parameters were

fairly low in the influent (8 and 20 mg L" 1 respecli\·e]y).

Wetland wastewater treatment in the petroleum industry is not confined to U.S.

The Jinling Petrochemical Company, Nanjing in China. reported small reductions in

several parameters, including phenol and oil, in water hyacinth wetlands (Tang and Lu,

1993). Dong and Lin (l994a, b) report data and models for both a full-scale facility of

50 ha treating 100, 000 m~d- 1 and a research facility treating lesser amount of water.

25


Two bench scale wetlands were constructed in a greenhouse at the University of

Mississippi Field Station, Lafayette County, Mississippi in 1997 (Huddlestone et al.,

2000). The study evaluated the effectiveness of constructed wetlands for tertiary

treatment of a petroleum refinery effluent. Specific performance objectives were to

decrease 5 days BODs and ammonia by at least soq, and to reduce toxicity associated

with this effluent. From the values of the targeted parameters shown in table 11,

average BODs removal was 80%, while NH4-N decreased by an average of 95%.

Table 11: Average treatment wetland performance for removal of contaminants (mgL-1)

from refinery wastewater. Percent reduction in concentration is given in parentheses.

Oil TP Phenols Hex

Location BOD5 COD TSS NH4 TKN and pgL·I chrome Source

grease Amoco·s 3.0 37 5.0 130 5.0 3.6 Litschfield, Mandan. (88) (78.85) (85.71) (93.80) (93.75) (77.5) 1989. ND Yanshan. 15.3 47.5 41 3.5 5.8 0.43 290 10 Dong and Beijing, (59.73) (72.05) (77.34) (39.65) (41.41) (71.5) (65.47) (62.96) Lin, PRC 1994a,b Lafayette 2.0 0.25 Huddlestone County. (85.71) (94) et al., 2000. Mississippi

When many factories had to cut their production, and reduced wastewater flow

resulted in operational problems of wastewater treatment plants in Taiwan (Chen et al.,

2006), the Jess expensive wetland systems were proposed to replace the existing

wastewater treatment plants. Four parallel-scaled free water surface constructed

wetlands were installed in an industrial park receiving mixed wastewaters from a

variety of industries (electronic, steel, chemical. fcx"'d processing etc). Results showed

that the system with 5-days retention time and vesicles ceramic bioballs as the media

had the acceptable and optimal pollutant rem<..wal efficiency. If operated under

conditions of the above parameters, the pilot-plant wetland system can achieve removal

of61% COD, 89% BOD, 81% SS, 35% TP and 560C ~H3-N.

A pilot scale wetland was constructed to assess the feasibility of treating the

wastewater from a tool industry in Santo Tome. Santa fe. Argentina (Hadad et al.,

2006). The wastewater had high conductivity and pH. and contained Cr, Ni and Zn. The

system was mainly examined for nutrient and metal removal. Approximately 1000 L,r'

of wastewater passed through the system of 6x 3x OAm and residence time was 7 days.


The variables measured in the influent were significantly higher than those in the

effluent, except for HC03. and NH4 +.

Wetlands have been reported to effectively purify metal contaminated mme

drainage water, and to offer an economical, self-maintaining alternative to conventional

treatment of different types of wastewater (Erten et al., 1988; Gopal, 1999; Weis and

Weis, 2004). In P.R. China, the drainage from mining activities had caused serious

environmental problems, however little information is available concerning the use of

wetlands to treat metal contaminated mine wastewater. A wetland was constructed with

T)plza latifolia (cattail) in 1983, for the purpose of treating the metal contaminated

mine drainage discharged from metallifereous mine (mainly Pb/Zn) since 1985 at

Shaoguan, Guangdong Province, China. After 16 years of monitoring (from 1986 to

2000), it was observed that the water quality improved substantially after the

wastewater had passed through the wetland and a stabilization pond, with TSS reduced

by 99%, COD by 55%, Pb by 99%, Cd by 94% and Zn by 97% (Yang et al., 2006). It

was also found that there were no significantly annual or monthly variations in pH

values. The 16 year monitoring results showed a reciprocal relationship between

restorations of the wetland ecosystem, in other words the maturity of the wetland, and

the long-term efficiency and stability on purifying heavy metal contaminated

wastewater.

A comparison of metal accumulation in sediments and plants of constructed

wetlands for the treatment of acid mine drainage and natural wetlands (Mays and

Edwards, 200 1) showed that the load rates and removal efficiencies of most wetlands

were generally greater in the constructed wetlands. There were similar sediment and

plant metal concentration between one constructed wetland and the natural wetland and

greater metal concentrations in the sediments and plants in the other constructed

wetland compared to natural wetlands.

There is a considerable variation between metals and also between wetlands, in

degree to :vhich metal is removed by the wetland (NRA, 1992; Mays and Edwards,

200 I; Rousseau et al., 2004 ). Some studies indicated that constructed wetlands had a

finite ability with respect to metal retention and that they could eventually fail to

remove some elements (Noller et al., 1994; Scholes et al.. 1998). Furthermore factors

such as rain and temperature possibly affect long-term efficiency and stabilization for

metals removal (Sutu1a et al.. 2002). In addition to climatic and edaphic factors,

27


differences in the hydrological regime of vanous habitats create the diversity of

wetlands with different removal of metal contaminated \~.:astewater (Fink and Mitsch,

2004; Jing and Lin, 2004).

Other industrial wastewater treatment wetlands exist as well and these include

for various wastewater such as tannery wastewater (Calheiros et al., 2007), textile

waste (Mbuligwe, 2005), pulp and paper wastewater (Knight, 1993), acid mine

drainage (Kleinmann and Girts, 1987.) etc.

Wetland design

The increasing application of treatment wetlands coupled with increasingly strict water

quality standards has been an incentive for the development of better design tools. This

section reviews some simple as well as some more elaborate designs of constructed

wetlands and their effect on pollutant removal efficiency. The focus is on the standard

water quality variables such as COD, BOD, TSS, nitrogen and phosphorus.

Most existing designs for wastewater treatment wetlands are based on estimated or

measured BOD loadings. The equations used to calculate the desirable size and

hydraulic residence time take into consideration an average decay rate of BOD, average

temperature and the hydrology of the site. The assumptions within the equations are

derived from results from municipal wastewater treatment plants (Reed et al., 1988).

An alternative is to base the design on nitrogen loadings since many constructed

wetlands reduce BOD, TSS and bacteria concentrations. but nitrogen removal is

variable (Hammer and Knight, 1994). Designs. which feature a series of cells, each of

which enhances one process within the nitrogen cycle. may be desirable to maximize

nitrogen removal.

Before constmction of a wastewater treatment wetland. selection of a suitable site

includes consideration of the goals of the wetland. soil type, topography, climate, cost

and potential future changes in management. For example in case of animal

wastewater. if the herd size is going to increase. the wetlands need to be large enough

to handle the increased waste load ( McCaskey et al .. 1994 ).

28


Surface flow wetlands

Surface flow wetlands are the most common type used in the treatment of various types

of wastewater. A surface flow wetland (SF) typically consists of a basin or channels

with a barrier layer to prevent seepage, soil to support rooted vegetation, and relatively

shallow water flowing through the system. The water flows primarily horizontally and

above ground. The design may or may not include areas of open water in addition to

the vegetated areas. The near surface layer of water is aerobic while the deeper waters

and substrate are usually anaerobic. The vegetation is usually reeds, cattails, rushes and

sedges (Hurtado, 2004). The Savannah River site is a good example of SF constructed

wetland (Halverson, 2004). They are less expensiYe to construct than subsurface

wetlands because fewer construction steps and materials are involved. The substrate in

surface flow wetlands is usually compacted clay or hydric soils, if available. There are

more than twice as many as SF systems in the U.S. than SSF systems.

The design criteria and recommendations developed for SF wetlands can be

summarized as follows (Reed et al., 1988; Watson et al., 1989; Kadlec and Knight,

1996).

• Allowable BOD5 loading rate to a constructed wetland is 112 kg ha· 1 dai1•

• The residence time must be at least 12 days.

The residence time depends upon the average temper.nure and the length of time it

takes to degrade BOD.

• Hydraulic loading: Secondary= 1.2 to 4.7 em d·': Tertiary= 1.9 to 9.4 em d- 1•

• Aspect raio (L: W) > 10: I.

• Water depth: 0.2-0.4m

• Soils: 20- 30 em to support the growth of macrophytes.

The loading rates for the SF wetlands are modeled by first order kinetic equation (Reed

et al., 1988, Reed et al., 1995, Kadlec & Knight.I996). The general model developed

for the constructed wetland is given below.

Ce I Co= A exp [(-0.7K, (Av) 1.75 LWdn)]/Q eq (1).

where Ce is the effluent BOD concentration ~mg L" 1), Co IS influent BOD

concentration (mg C 1), A is the fraction of BOD which is not removed as settleable

29


solids near the headworks of the system (as decimal fraction). K1 is the reaction rate

constant (dai 1 ). A, is specific surface area for microbial activity (m2 m-3

), L is the

length of the system (m), W is the width of the system (m), dis the design water depth

in the system (m), n is the porosity of the system and Q is the average hydraulic loading

on the system ( m3 d- 1). A preliminary estimate of the land area required for an SF

wetland can be obtained from Table 12 of typical area loading rates presented below.

Table 12: Loading rates of different pollutants.

Constituent Typical Influent cone Target Effluent cone Mass loading rate

(mgL"1) (~L-1) (lb ac-1d-1)

Hydraulic 0.4-4_0 Load (ind- 1

)

BOD 5-100 5-30 9-89

TSS 5-100 5-30 9-100

NH.fNH4 2-20 1-10 1-4

N03 2-10 1-10 2-9

TN 2-20 1-10 2-9

TP 1-10 0.5-3.0 1-4

In case of animal wastewater, the Natural Resources Conservation Service

design guidelines (NRCS) provide two methods for determining the size of a

constructed wetland (Cronk, 1996). Both methods are based on BOD5 loadings as

described in the above model. The presumptive method. assumes that livestock produce

a certain amount of BOD:; per day. This method is useful if actual effluent cannot be

evaluated because the waste treatment svstem or one of the farm structures such as

dairy parlor has not been installed. The second i\RCS method, known as the field test

method, is based on measured BOD:; concentrations. The NRCS recommends that

several samples be collected during different seasons in order to obtain good estimate

of average BOD5.

Most of the animal wastewater treatment wetlands currently under study in the

U.S were designed according to guidelines of NRCS {1991 ). At three dairy wastewater

treatment wetlands built according to NRCS guidelines, BOD retention was from 61%

to 78% (Cronk et al., 1994; Reaves et al., 199-tb: Skarda et al., 1994).

Hammer in 1993 reported on marsh-pond marsh system constructed at the

Pontotoc Experiment Station in Mississippi to treat swine wastewater. Each of the two

cells had a 9 m long shallow emergent marsh at the intluent end. The cell then deepens

to 35 em for a 15 m long pond followed by a final 9 m long area 12 em deep. During

30

Chapter 2. Literature Revie~v

the entire monitoring period, ammonia removal was consistent and 71% of NH3 was

removed within the wetlands (Cathcart et al.. I 99-f). Ammonia reduction may have

been favored by the marsh-pond meadow design, which places an emphasis on nitrogen

removal. When the post wetland vegetated strips were included in the mass balance of

measured parameters, the entire system removed over 95% of the incoming mass of

BOD5, TSS, NH3-N and TP (Cathcart et al., 1994 ).

In another variation on this design type. constructed at Auburn University's

Agricultural Experiment Station in USA (Hammer, 1992), treatment of swine waste

first in a mixing pond, and then in five pairs of emergent marshes, showed high

removal ofTKN (82-99%) and NH3 (89-99lk) (McCaskey et al., 1994).

With domestic wastewater, Lim et al. (2001) assessed the potential of surface

flow wetland in removal of BOD and nitrogen under tropical conditions. A baffle was

installed in the middle of the unit to make a working dimension of 3.68 x 0.22 x 0.67

m, thus achieving an aspect ratio of 10: 1. The support media consisted of layers of sand

(< 4mm diameter), pebbles (4-25 mm diameter) and stones (> 25 mm diameter) at

depths of 0.2 m respectively. A water depth of 0.07 m was maintained above the media

in surface flow wetland. The performance of the SF wetland unit with cattail showed

85% reduction in BOD with mass loading of as high as 13.8 g m-2 d· 1• Based on

ultimate oxygen demand, the upper limit of BOD loading rate for SF should be limited

to 11.2 g m·2 d-1 (USEPA, 1988). Percent reduction of total nitrogen in SF wetland unit

was 62% with mass loading rate of 6.16 g m-2 d- 1•

SF wetlands installed at the sewage treatment plants (STPs) at South Lismore

and the neighboring town of Casino in the Richmond River Shire in the early 1 990s

experienced effluent quality compliance violations. and the respective councils

commissioned rehabilitation work. The Casino wetland of 3 ha received effluent

(about 2.5 MLd- 1) from a trickling filter plant with added aeration tanks followed by a

4 ha pond. The wetland was originally designed and built in about 1990 to operate at a

constant depth of 200 mm, regulated by weirs of gr~vel that, although designed to be

permeable, were causing effluent to back up. increasing the water depth to about 400-

500 mm and leading to the ultimate dieback of wetland vegetation. The hydraulic

residence time (HRT) of wastewater in the vegetated wetland cells was generally less

than 1 day. By early 2004, the Casino wetland had reached full vegetation cover with a

substantial litter layer. Monitoring of the system for 8 years showed better nitrification

31


in the treatment plant with subsequent denitrification in the shallow unvegetated ponds

from 1999 to 2000. The rehabilitating wetland began to increase denitrification in 2001,

with consequent declining TN concentrations as the wetland matured. Mean TSS

concentrations declined similarly over the 4-year period following the beginning of

rehabilitation (37 mgL· 1 in 2000 to 7.3 mgL" 1 in 2~), while BOD5 remained at about

5-6 mgL" 1 throughout. Table 13 summarizes actual perfonnance data for 27 SF systems

from a recent Technology Assessment (U.S. EPA 2000).

Table 13: Summary Performance for 27 SF Wetland Systems

Constituent Mean Influent (mgL.1l Mean Effiuent {mgL"1)

BOD 70 15

TSS 69 15

TKN 18 II

NHINH~ 9 7

N03 3 I

TN 12 4

TP 4 2

Dissolved P 3 2

Fecal coliforms (1100 ml) 73,000 1320

Sub- surface flow wetlands

Sub surface flow wetlands (SSF), used extensively to treat various types of wastewater,

are based on gravel or similar coarse substrate. SSF systems are susceptible to

clogging; therefore they are not recommended for waste with a high concentration of

total solids (Hammer, 1993 ). They may be best suited to treat septic tank effluent and

dilute chemical wastes (McCaskey et aL, 1994 ). The Tennessee Valley Authority had

conducted research on SSF wetlands. investigating mechanisms of nitrogen, carbon and

phosphorus uptake in an effort to provide design and operating guidelines (TV A, 1991 ).

From engineering point of view, SSF constructed wetlands follow some rules of

thumb design criteria (Table 14), based on observations from a wide range of systems,

climatic conditions and wastewater types.

Table 14: Rule of thumb design criteria for SSF constructed treatment wetlands (from

Wood, 1995 and Kadlec and Knight, 1996)

32


Criterion Value range

Wood, 1995 Kadlec and Knight, 1996

Hydraulic retention time (days) 2-7 2-4

Max BOD loading (Kg ha· d·') 75 n.g

Hydraulic loading rate (cmd- 1) 0.2-3.0 8-30

Area requirement (ha m·'d- 1) 0.001-0.007 n.g

n.g: not g1 ven

Considering the fact that the majority of the investigations on treatment

wetlands have mainly focused on input- output (110) data rather than on internal

processes data, regression equations seem to be a useful tool in interpreting and

applying these 110 data. However, important factors such as climate, bed material, bed

design (length, width, depth) etc. are neglected leading to a wide variety of regression

equations (Table 15) and thus a large uncertainty in the design. Most of these

regression equations rely on wastewater concentrations. Lc-oking for instance at the first

Table 15 entry of Brix, 1994, for a constant BOD intluent concentrations, the same

effluent concentration was predicted for a HLR of 0.8 as well as 22 em d·', which

suggests that the HLR is a non-limiting factor within certain boundaries. Only a limited

number of regression equations rely on both intluent concentration and HLR as inputs

to predict the effluent concentration.

The removal rates for BOD and nitrogen m SSF constructed wetlands are

estimated by the following general equation (2) (Reed et al, I 995), whereas for

coliforms and phosphorus by general equation (3) (Kadlec and Knight, 1996).

eq (2)

C.JC = e -K llhl eq (3)

Table 15: Regression equations for horizontal SSF constructed treatment wetlands according to different authors (q expressed as em d- 1

)

Reference System Equation Input range Output range q range

BOD

R2

Brix. 1994 Danish and Coul = (O.J J * C;.) + 1.87 I<C.n< .330 I<Cout< 50 0.8<q<22 0.74 UK soil-ba~d SSF

Knight et al. US gravel bed Couc = (0.33* C;.) + I A I<Cin< 57 I<Cout< 36 1.9<q<ll.4 0.48 1993 Griffin ct al.. US unplanted Coul = 502.20*exp (. IO<T< 30 0.69 1999. rock filter O.II*T) VymaLal SSF in Czech COlli = (0.099* C;.) + 3.24 5.::kC'"< .~28 1.3<C0111< 51 0.6<q<l4.2 0.33

33


1998 Republic Reed and 14 US SSF Lremo"'d =(o.653*Lin) -kl;0 < l.f5 4<Lr<nlO,<d<88 0.97 Brown 1993 +0.292 COD Vymazal, SSF in Czech Lout= (o.l7* L;0 ) + 5.78 15<L,.< 180 3<Lom<41 0.73 1998. Republic TSS Reed and 14 US SSF Cout =C;n 22<C1,< 118 :.kC0 m< 23 Brown 1993 (0.1058+0.00 I I *q) Knight et al, US gravel bed Cout = (0.09* C,.) + 4. 7 O<C,o< :.no O<Cout< 60 0.8<q<22 0.67 1993 Vymazal, SSF in Czech Cout = (0.021 * C;0 ) + 9.17 13<C;.< 179 1.7<C001< 30 0.6<q<l4.2 0.02 1998. Republic

TN Kadlec and NADB + Cout =2.6 + (0.46* C;0 ) + 5.1<C,.< 58.6 2.3<Com< 37.5 0.7<q<48.5 0.45 Knight 1996 others (0.124*q) Kadlec et al., Danish soil Cout = (0.52* C;0 ) + 3.1 ·kC~n< 142 5<Cout<69 0.8<q<22 0.63 2000 based SSF Vymazal, SSF in Czech Cout = (0.42* C;0 ) + 7.68 16.4<C;0 < 93 10.7<Cout< 49 L7<q<l4.2 0.72 1998. Republic TP Kadlec and US, Europe, Cout = 0.51 * C;n 0.5<C;.,< 20 0. I <Cou,< 15 0.64 Kni1rht, 1996 Australia SSF Kadlec and USSSF Cou, = 0.23*(q* C;.) 2.3<Cm< 7.3 O.I<C0111< 6 2.2<q<44 0.60 Knight19% Cm and COlli: mflent and effluent concentratiOns (mgL ): Lm and I....,..,: mfluent and effluent loads (kg ha 1d.1

); L..:move<~: Load removed (kg ha-•d- 1).

In these two equations: Ce (mgL' 1 of BOD, nitrogen, or phosphorus, or number of

feacal coliforms/100 ml) is the pollutant effluent concentration; Ci (mgL- 1 of BOD,

nitrogen, or phosphorus, or number of fecal colifonns/100 ml) is the pollutant influent

concentration: KT (d- 1) is a reaction rate parameter dependent on the water temperature

T (C), the pollutant of interest and the wetland type: Kl (m d' 1) is a reaction rate

constant dependent on the pollutant and wetland type: hi (m d' 1) is the hydraulic

loading rate: and t (d) is the hydraulic residence time in the system.

Typical values for the kinetic rate parJmeters in above equations are

summarized m Table 16 for the pollutants of interest in SSF constructed wetland

systems.

Table 16: Pollutant removal equations and rate constants for SSF constructed wetlands.

Pollutant Equation used Rate constant

BOD Eq 2 Kr= 1.104 (1.06) ·20

Fecal colifonns Eq 3 Kl = 0.30 md· Nitrogen Nitrification Eq 2 Kr = K,H (0.1779)T 0< T < I"C

Kr=K,H (0.6257)(1.15) -to I<T< IO''C Kr = K..,H ( 1.048)) · .. l T> I0°C

3-J


Denitrification Eq 2 KT = 0.023T 0< T < I°C ·KT=(I.I5) ·"U T> I°C

Phosphorus Eq 3 Kl = 0.0273 m d.

Csing the above equation (2), and appropriate reaction rate from table 16

followed by operational experience of several-constructed wetland systems in USA,

Reed et aL 1995 proposed that the organic loading in SSF constructed wetland systems

should not exceed 10 g m·2 d- 1 limit value.

SSF constructed wetlands are efficient m removmg bacteria and VIruses

(Khatiwada and Polprasert, 1999). Results from the work of Mandi et al., 1998, have

showed that constructed wetlands are also very efficient in removing helminth eggs (71

to 95%) in arid climates, even with high hydraulic application rates and the removal

efficiency of the parasitical load is favored during hot periods. The removal rate of

coliforms in SSF constructed wetlands is estimated from equation (3) using the

appropriate rate constant value listed in Table16 (Kadlec and Knight, 1996). There does

not appear to be any seasonal effect on removal perfonnance, since the removal is due

to physical separation of the particles and die-off (Kadlec and Knight, 1996; Reed et

al., 1995).

Greenway and Woolley (1999) reported that in constructed wetlands the predominant

forms of nitrogen are ammonia and nitrate; nitrification and denitrification are the

principal processes for nitrogen reduction together with some assimilation by biota. The

removal l)f nitrogen based on nitrification and denitrification is highly dependent on

water te:::perature (Reed et al., 1995; Kadlec and Knight. 1996; Huang et al., 2000).

Bl1th nitrification and denitrification processes are described by equation (2),

using the appropriate rate constant value from Table 16. A minimum design HRT of

about 6 to 8 days is advisable to guarantee adequate oxygen transfer for nitrification by

the plants to the roots, and maximize ammonia removal efficiency (Reed et al., 1995;

Reed and Brown, 1993) .

. -\ wetland system may show very effective phosphorus removal during the first

one or two years of operation. When a system reaches equilibrium, however the

phosphorus removal is likely to be reduced. Long-tem1 removal efficiencies are

belie,·ed to range between 30 and 50% (Reed et aL 1995). and can be estimated from

35

cnapler L. LtrPrawre Kel'lew

equation (3) using the appropriate rate constant listed in table 16. In SSF constructed

wetland, the area of the bed is calculated using the following equation:

eq (4)

where Ah is surface area of bed, K1 is a constant; Qd is the daily average wastewater

flow rate (m3 d- 1), Co is average BOD of inlet wastewater (mgL- 1

) and C1 is the required

average BOD of outlet water (mgL- 1). The constant K1 depends on the biodegradability

of the waste. For sewage, the value is 5.2; it should be determined experimentally for

other waste.

Table 17: Flow rates and concentration of inflow and outflow of selected SF wetlands.

System location Wastewater Q (m3d-t) Ci (mg/1) Co (mg/1)

Ah type (ha)

Denham Sprin!!s, LA, USA Municipal 2547.83 Ci. BOD= 25.5 Ct, BOD= 9.8 6.15 Utica North. MS, USA Municipal 140.36 Ci, BOD = 40.4 Ct, BOD = 13,5 0.61

Ci. n~ = 14.07 Ct, TN= 8.57 Utica South, MS, USA Municipal 228.68 Ci, BOD= 29.0 Ct, BOD= 9.9 0.81

Ci,TN = 13.0 Ct, TN= 6.2) Johnson City. TX. USA Municipal I 14.00 Ci. BOD = 20.0 Ct, BOD= 8.0 0.15

Ci, PHOS = 7.0 Ct, PHos= 5.0 Mandiville. LA. USA Municipal 523.94 Ci. BOD= 35.5 Ct, BOD= 12.5 1.85

Monterey, CA. USA Municipal 83.00 Ci, BOD = 38.0 Ct, BOD= 15.0 0.02

Ruakura Research Farm, Dairy 42.5 x w·'l Ci. BOD = 200.0 Ct, BOD = 22.0 0.0076 Hamilton, NZ Ci. TN= 38.2 Ct, TN= 18.3

Ci. TP= 11.2 Ci, TP= 6.8 Anne Valley. Waterford, Dairy 7.6 Ci.s00 = 1491.0 Ct, BOD = 18.2 0.48 Ireland. USA Ci. 'H~ + =147.0 Ct,NH4 + =8.1

Ci. TP = 44.4 Ci, TP= 4.0 Nova Scotia. Canada Dairy J X 10) 0.01 Wastewater treatment plant, Industrial 0.4 Ci. BOD= 80.0 Ct, BOD =24.0 4.0 Taiwan Ci. T-.: =35.0 Ct,TN=22.2

Ci.TP= 69.0 Ci, TP = 51.0

The majority of the wetlands engineered for livestock wastewater treatment are

small, with an average system size of 0.6 ha and a median size of 0.03 ha. The majority

of the swine. poultry and dairy treatment wetland systems are less than 0.1 ha. Many of

these systems listed in the table 17 were designed for research purpose and not as full

scale installations.

The influence of temperature on wetland unit is commonly determined via an

Arrhenius equation:

k e \T-20) d k k e<T-20) A.T = K A.2ll an V,T = v,:w eq (5)

36

Chapter 2. Literattire Revieu:

According to Kadlec and Knight ( 1996 ). removal of BOD, TSS and TP in

treatment wetlands is generally found to be independent of temperature (8 = 1.00)

whereas nitrogen removal is negatively influenced by lower temperature (8 = 1.05).

Vegetation

In constructed wetlands, plants provide a substrate and a carbon source for microbes.

Wetland plants oxygenate the substrate immediately adjacent to their roots and increase

the aerobic portion of an otherwise anaerobic zone (Brix, 1993). In addition, plants

remove nutrients from the incoming wastewater during the growing season. While plant

nutrient uptake is usually not the major pathway of nitrogen and phosphorus removal, it

has been credited with 16-75% of total N removal and 12-73% removal of total Pin

wastewater treatment wetlands (Reddy and DeBusk, 1987).

Common reed (Phragmites australis) is widely used in Europe, and NRCS

(1991) includes it as one of its recommended species. However, others in the US

discourage its use because it is an aggressive non-native species (Hammer, 1993). One

study of the oxygenation of the root zone showed the reed to be a poor oxygen

transporter and since root zone oxygenation is vital to oxidation processes, common

reed may not be as desirable as other macrophyte species. European studies contradict

these findings and have shown that the common reed can oxygenate the rhizosphere at

a rate of 15 g 0 2 m·2 d- 1 but an extensive root system takes 3 years to develop

(Biddlestone et al., 1991) so system effectiveness may be delayed. Plants may survive

best if they are planted before waste enters the wetlands and then allowed to establish

under wet conditions. Surrency recommends planting when the wetlands are dry and

adding 2.5 em of clean water per week until the desired depth is reached. At the end of

6 or 7 weeks, the wastewater can be added. If the plants are well established, they can

withstand high ammonia concentrations. In addition. the initial flush of wastewater is

diluted by the accumulated fresh water. McCaskey et al. ( 1994) suggest that plants

should be allowed to establish for two growing seasons before wastewater is applied;

however, this length of time before treatment may not be an option.

The potential of various wetland plant species (free floating or emergent) m

wastewater treatment in constructed \vetlands is discussed below.

37


Free floating macrophytes

These systems have been recommended for use in the tropics and subtropics (Vymazal,

1998). The most commonly used floating plant is \\later hyacinth. The capability of

water hyacinth to purify wastewater is well documented (Reddy and Sutton, 1984;

Reddy and DeBusk, 1985; DeBusk et al., 1989; Reddy and D'Angelo, 1990;

Vymazal, 1998). The extensive root system of the weed provides a large surface area for

attached microorganisms thus increasing the potential for decomposition of organic

matter. Plant uptake is the major process for nutrient removal from wastewater systems

containing water hyacinth plants. and it is related to nutrient loading to the system

(Reddy and Sutton, 1984). Nitrogen is removed through plant uptake (with harvesting),

ammonia is removed through volatilization and nitrification-denitrification, and

phosphorus is removed through plant uptake. Treatment systems with water hyacinth

are sufficiently developed to be successfully applied in the tropics and sub-tropics

(Vymazal, 1998) where climatic conditions favor luxuriant and continuous growth of

the macrophyte for the whole year. The recommendations for their applications are

shown below:

Table 18: Recommendations for application of water hyacinth systems (Vymazal,

1998).

Treatment Total detention time (da~·s) Depth (m) Hydraulic load (em d"1)

Secondary advanced >-lO <1.5 >2

Secondary 6 >0.9 8

Tcniary 6 <<0.9 8

Integration of water hyacinth-systems for wastewater treatment into methane:

carbon dioxide production projects as means of using excess water hyacinth biomass

has proved successful (Hayes et al.. 1987). At a sewage loading of 440 kg ha· 1 d- 1 and a

hydraulic retention time of 3 days. the water hyacinth system removed 81% of BOD5

and 80% of suspended solids. From a pond area of 0.75 ha. a biomass production of 68

Mg ha· 1 yr" 1 was achieved. A methane yield of 0.47 m3 kg- 1 VS added was obtained in

the anaerobic digester.

However. the concept of using water hyacinth for wastewater treatment in

developing countries is controversial. This is because the plant is exotic and has

invaded manv water bodies causmg a lot of ecological and economical problems

38


leading to negative image of the plant in areas like Lake Victoria region (Harley, 1990;

Cilliers, 1991; Mugasha, 1995; Greenway, 1997). Therefore, management efforts are

focused towards eliminating the weed rather than exploiting its potential use in

wastewater treatment (Cilliers, 1991 ). Despite this negative attitude towards the water

hyacinth, some research initiatives are being made to use the plant for wastewater

treatment. Under semi-arid conditions, E. crassipes, a water hyacinth species was tested

in CWs for purification of domestic wastewater for reuse in Morocco (Mandi, 1998). A

good reduction of organic load (COD, 78%; TSS. 90%) and a parasitic load

(helminthes eggs, 100%) under a retention time of 7 days was achieved. The effluent

satisfied WHO (1989) conditions for reuse to irrigate cereal crops, fodder crops, pasture

and trees. However, the system suffered evapo-transpiration loss of up to 60% during

summer as well as proliferation of mosquitoes.

Success of an integrated rural wastewater treatment with water hyacinth has

also been demonstrated in Brazil for a small community (Raquette et al., 1998). The

system is partly used to treat stream water for domestic use, and partly for treating

domestic wastewater, piggery, cattle-pen and poultry wastewater. Clean water is

returned into the stream. Excess plant biomass is used for biogas production (60%

methane), which is used for generating electricity. Excess biomass is also composted

for horticulture, and used as animal feed for the community.

Emergent macrophytes

These systems have been tested for treating vanous wastewaters under vanous

conditions in different countries. A study on the purification of domestic wastewater

under semi-arid conditions has been conducted (Mandi et al., 1998). At a hydraulic

application of 0.86-1.44 m3 d-1, reed beds with Phragmites australis, organic removal

of 48-62%. TSS of 58-67% and a parasitic removal of 71-95% were obtained in

Morocco. Further experiments to improve on the reed beds purifying efficiency were

carried out. In Egypt, Stotts et al. ( 1998) achieved a 100% removal of parasitic ova

from domestic wastewater intended for agriculture use. In Iran, a subsurface flow reed

bed (P. australis) of 150m2 was tested for treating municipal wastewater. At an organic

loading of 200 kg ha-td- 1 that is higher than previously recommended (<133 kg ha-td- 1)

(Metcalf and Eddy Inc, 1991 ), removal efficiencies of 86, 90, 89, 34, 56 and 99% for

COD, BOD. TSS, TN, TP. and fecal colifom1 bacteria. respectively, were obtained. No

39


clogging problems were experienced (Badkoubi et al .. 1998). In West Virginia, three

common Appalachian plant species, ]uncus effuses, Scirpus validus and Typha latifolia

were planted into small scale constructed wetland receiving domestic wastewater

(Coleman et al., 2001). Typha significantly outperfonned ]uncus and Scirpus both in

growth and effluent quality improvement. Removal efficiencies of 76, 75, 62, 61 and

81% for BOD,TSS, TN, NH3-N and fecal coliform bacteria respectively were obtained

for Typha than Scirpus (69, 25, 25 and 48% for BOD,TN, NH3-N and fecal coliforms

respectively) and ]uncus (64, 47, 50, 63% for BOD ,TN, NH3-N and fecal coliforms

respectively). In Turkey, domestic wastewater was treated with seven different wetland

plants namely Phragmites, Cyperus, Rush, Iris, Lolium. Canna and Paspalum (Ayaz

and Akca, 2001). COD and SS removal efficiencies were obtained as 90% and 95%

respectively for Cyperus with effluent COD concentrations at an average loading of

122g COD m-2d-1 was satisfactory for the Turkish Water Pollution Control Regulation.

Among the other species planted, the best performances were obtained by Iris for COD

(94% ), by Canna for ammonia nitrogen (98%) and by Iris for total nitrogen (90%) and

phosphorus (55%).

Okurut et al. ( 1998) demonstrated the viability of constructed wetlands with

indigenous C. papyrus and Phragmites mauritians in treating municipal wastewater, on

a pilot scale in Uganda. In the C. papyrus systems, average mass removal rates for

COD, TSS, NH4- N, TN and a-phosphorus were 15-.31. 6.62, 6.5, 1.06, 0.06 g m-2 d- 1,

respectively. In P. mauritianus systems, the rate for the same parameters was 2.25, 0_9,

0_66, 0.65, and 0_058 g m-2 d- 1, respectively. The level of BOD and TSS in the effluents

was below 20 and 25 mgL- 1_ A higher degree of fecal coliform removal was reported

for the papyrus systems.

A variety of plant species have been recommended for use in animal wastewater

treatment wetlands but not all of them may be appropriate for every wetland. Desirable

species are native, and therefore best suited to local conditions_ They should have high

productivity for rapid nutrient uptake, rhizome production, and colonization. They

should be perennial to avoid annual plantings (Hammer, 1993) and they should be

easily and inexpensively obtained_ Finally, in order to survive, they should be able to

tolerate high nutrient inputs. In particular, ammonia is often implicated in plant death in

animal wastewater treatment wetlands; therefore a tolerance of high ammonia

concentrations is necessary unless reliable pretreatment methods are used to control this

-tO


pollutant. In field trials, giant bulrush ( Scirpus califomicus) and softstem

bulrush(Scilpus validus) survived ammonia concentrations greater than 200 mg NH3-N

L·' while cattails were severely stressed by the same high ammonia concentration

(Surrency, 1993). Both giant and softstem bulrush rapidly produce rhizomes,and

continue to grow past the end of the growing season for other plants. Giant cutgrass

(Zizaniopsis milacea) has also been observed to surYive and grow rapidly in animal

wastewater treatment wetlands (Surrency, 1993).

In Thailand, wetland systems have been investigated for improving effluents of

lagoons treating industrial wastewater (Panswad and Chavalparit, 1997). The copper

rich (5.7mgL.1) effluent and difficult to degrade (COD:BOD = 17) was treated in a

Typha latifolia and Ipomes spp. SF beds at a retention time of 3 days and a hydraulic

loading of 30 cmd·1• BOD removal was very poor and sometimes increased to about

10% whereas TN removal was 29%, TP removal was 30% and copper removal was

61%. Similar to what was reported previously (Sinicrope et al., 1992), there was more

copper in the wetland sediments (7.6 g kg·' dry weight) than in the cattails and water

spinach (0.5 and 0.044 g ki 1 dry weight, respectively). In the same study a correlation

between filtered BOD and the presence of certain nematodes and protozoa was

established (Panswad and Chavalparit, 1997). These organisms have been suggested to

be bio-indicators of different water qualities in wetland systems. Experimentation on

the suitability of local and indigenous wetland emergent macrophytes for removal of

nutrients and heavy metals has been going on in several countries. Ojo and Mashauri

( 1996) demonstrated the capability of the wetland plants in Tanzania, Cyperus

exa/tatus, T. /atifolia and Phragmites australis. in uptake of heavy metals at laboratory

scale. In Thailand, Koottatep and Polprasert ( 1997) experimented with Typha

augustifolia on removal of nitrogen from prinury treated sewage in SF beds. With 8

weeks harvesting intervals at 5 days retention time. a maximum of 7.5 kg ha·' d- 1

nitrogen uptake by the plants was obtained. This was accompanied by a TN removal of

84-86%.

In Tanzania, cocoyam and cattails were assessed through comparing treatment

performance efficiencies of constructed wetlands receiving dye rich wastewater. The

system showed color concentration removal of 72.16Cf'c. COD removal of 68.24% and

SO/~ removal of 24.68% in cattail unit while cocoyam unit showed better color

removal of 77.13%, COD removal of 72A5lk and SO-t-~ removal of 58.53 %. These

-H


performance results were comparable to those reported by Nigam et al, 2000 who

observed color removals of 70-75% when they used com-cob shreds and wheat straw

as media.

Research on process optimization was done in Taiwan, Napier grass

(Pennisetum purpureum) which grows both in wetlands and dry land was tested on soil

and gravel based systems for sewage treatment (Yang and Chang, 1998). The gravel

based system performed best. Attempts were also being made to combine conventional

wastewater treatment systems with constructed wetlands to treat sewage. A septic tank ~

followed by a soil filter composed of juncus (Zizanopsis bonariensis bra) called a root

zone was tested in Brazil. The root zone system was demonstrated to increase the 32%

BOD, and 33% COD removal from the septic tank to 69% and 71%, respectively.

Further experimentation on the combination of a septic tank and constructed wetlands

with Typha and Eleocharis spp was recently proposed for treatment of domestic

wastewater and laboratory effluents. The system was to be monitored for BOD, COD,

TSS, pathogens, TKN, and TP (Valentim and Roston, 1998). Mexico has started

investigating the possibility of system combination. A two-stage system consisting of

an anaerobic digester followed by a gravel bed horizontal flow wetland system (600

m2) with a hydraulic retention time of 1.7 days, with P.australis and T. latifolia was

used for abattoir wastewater in Pachuca since 1994. The system has been reported to

remove 88.5% BOD. 87.4% COD, 89% TSS, and 99% faecal coliforms. The effluent is

suitable for irrigation of ornamental plants. However. the system showed a tendency of

phosphorus accumulation with time (Rivera et al.. 1997).

Table 19: Range~ of Phosphorus and Nitrogen content (mg g· 1) of some wetland plants

under high nutrient loads.

Plants P concentration N Concentration leaf root rhizome leaf root rhizome

Emergent species c_,perus im·olucrallls 2-5 1-7 2-7 15-43 11-45 5-21

Phrar:mites australis 2-4 1-3 1-3 10-40 15-:~ I 5-31

Typha spp 1-5 2-7 1-7 5-32 4-52 2-40

Scirpus tabemaemomani 2-4 2-8 'J.-7 6-25 4-21 9-18

Bolboschoenus spp 1-5 2-7 4-6 2-15 2-15 13-19

Baumea articulata 1-9 2-8 2-7 11-18 8-25 8-19

Floating species Eichhomia crassipes 1-12 10-40

Salvinia molesta 2-9 'J.0-48 Lemna spp 4-18 25-59 Pistia stratiores 2-12 12-40


Floating leaved species Altemanthera 2-9 I 15-35 philoxeroides Ludwigia peploides 4-6 25-45 Marselia mutica 5-7 23-36 Hvdrocleu mmphoides 5-10 14-50 Hvdrocot\·le umbellata 2-13 15-45 Nvmphoides indica 5-12 15-35

Submerged species Ceratophyl/wn 10-14 35-42 demursum Elodea canadenis 7-I I 40-4I Potamoxeton crispus 6-10 35-40 P.pectinatus 4-7 4-8 27-31 27-31

Trees Leaves Stem Leaves Stem wood wood

Acer rub rum 2-3 10-22 Magnolia vir~iniana 1-2 <I I9-25 4 Nnsa svlvatica I <I I9 I Taxodium disticlzum 1-3 <I <I <I T. ascendens I-2 I5-I8

Advantages of constructed wetlands

In developed countries and to some extent in developing countries, controlling water

pollution problems has almost exclusively focused on implementation of expensive

collection and treatment technologies. The conventional treatment methods used in

various countries include septic tanks, activated sludges, trickling filters, anaerobic

systems and reverse osmosis systems (Weber & Holtz. 1991; Metcalf and Eddy, 1991;

WEF, 1992: Williams, 1982; Von Sperling and l\1arcos, 1996). These conventional

treatment systems minimize the land area required for treatment per capita and shortens

the period that the wastewater effluent remains in retention ponds. In spite of the

development of various advanced technologies for wastewater treatment and

reclamation. economic, effective and rapid treatment at a large scale is still a major

problem in most of the developing nations. lntensiw use of chemicals, high capital and

maintenance cost for the infrastructure, no alternatives in the event of disaster and no

optimization for the recycling of nutrients and reuse of water, take these technologies

beyond the financial grasp of most developing countries. Further, these current waste

treatment procedures often do not provide effluents that meet stringent regulatory

requirements. Therefore, attempts for developing cost effective treatment approach

have revolved around using only the natural components devoid of any mechanical

requirements that use up energy. Most importantly. well-designed and well-operated

43


conventional systems can achieve almost total removal of helminthes (99.99%), enteric

bacteria and viruses (99% ), leaving an odor free eftluent, which is attractive for

agriculture (Shuval et al., 1986). However, residual N and P could become a problem

unless they can be directly utilized to support agriculture or fish farming. When the

effluents from the treatment plants are released without further treatment back into

environment, they can contaminate downstream ground and surface water making it

unsafe for drinking and other uses.

Thus, constructed wetland technology in combination with other treatment

systems, could be used to achieve a better removal of nutrients and pathogens from

wastewater prior to final release into the water supply. Watson et al. (1989) and Kadlec

and Knight ( 1996) have discussed the advantages of using wetland technology for

wastewater treatment. Compared to conventional treatment systems, wetland

technology is cheaper, more easily operated and more efficient to maintain. Minimal

fossil fuel is required and no chemicals are necessary. An additional benefit gained by

using wetlands for wastewater treatment is the multi-purpose sustainable utilization of

the facility for uses such as swamp fisheries, biomass production, seasonal agriculture,

water supply, public recreation, wild life conservation and scientific study (Santer,

1989; EPA, 1993; Knight, 1997). Being low-cost and low technology systems,

wetlands are potential alternative or supplementary systems for wastewater treatment in

developing countries.

Future Research Needs

Gopal ( 1999) has identified current limitations to widespread adoption of CW

technology for wastewater treatment in developing countries. These include large land

requirements, lack of knowledge of tropical v.-etland ecology and native wetland

species, prevalence of mixed domestic/industrial wastewaters, and limited knowledge

and experience with CW design and management.

Clearly, developing countries interested in implementing this technology must

identify specific research needs and develop appropriate strategies based on local

parameters. A clear understanding of the biological. hydraulic and chemical processes

involved are essential (Woods and Hensman, 1989: Denny, 1997). For instance,

information is limited concerning tropical plant species suitable for sustainable CW


development. Further investigations are needed to identify and characterize tropical

plant candidates in terms of their tolerance to high nutrient levels and suitability in

regional climatic conditions and wastewater types. Also, careful economic analysis is

required to determine whether CW treatment technology that is cost-effective,

environmentally sensitive, and technically reliable for a given project location, can be

developed (Kadlec and Knight, 1996; Batchelor and Loots, 1997; Gopal, 1999).

Cost

A number of questions regarding cost arise before a wetland is built. The amount of

funding available, and the limits and rules concerning its expenditure are important

considerations in the planning stages of a constructed wetland (Newling, 1982). An

estimate of a new wetland's cost should include (Tomljanovich and Perez, 1989):

I. engineering plan;

2. preconstruction site preparation;

3. construction (labor, equipment, matetials, supervision);

4. long-term management;

5. pest control;

6. contingency costs.

The cost of construction depends on location. type. treatment objectives, and the

maintenance required. Factors that can add to the cost are access to the site, substrate

characteristics, the addition of a cia~' or plastic liner. cost of protective structures, local

labor rates. and the availability of equipment (Newling. 1982). While constructed

\\"etlands may be an inexpensive alternative for waste management, the cost of

maintenance must be added to the construction cost. Maintenance includes active

management of the solid portion of the w·aste. The amount of solids entering the

wetland must be minimized in order for the wetland to effectively treat the waste.

Solids management is an ongoing problem for the farmer and this duty cannot be

ignored once wetlands have been installed (Healy and McLoud, 1994; Reaves et al.,

1994b ). In addition, water levels may need to be manipulated in order to maintain wet

conditions (Holmes et al., 1994) and the surrounding area may need to be cleared of

Yegetation periodically in order to allow access to the wetlands. Further, some wetlands

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fail to operate as originally designed and constructed. Workers must be willing to adopt

a flexible, iterative approach to develop an optimum design over a period of years.

Thus, all costs cannot be anticipated.

Need for Design Development and O&M Guidelines

Constructed wetlands have been implemented for wastewater treatment in many parts

of the world, but to date, these systems have not been developed successfully on a field

scale for regular treatment of wastewaters because there has been little effort at

understanding the design characteristics in relation to their treatment efficiencies.

Research is required for developing design criteria that are location-specific and related

to the wastewater characteristics. Equally important is the development of appropriate

O&M guidelines that take into consideration the hydrological variables and the plant

harvesting strategies in particular.

Type of wastewater and target water quality

In many developing countries municipal sewerage systems carry mixed wastewater rich

in inorganic and organic toxic pollutants, which may inhibit microbial processes, and

hence reduce the wetland treatment efficiency. Character of the wastewater to be

treated must be considered in the design.

The main wastewater treatment goal in developing countries is protection of

public health through control of pathogens. particularly if the water is to be reused.

Because Yiruses are hard to remove by facultatiYe ponds and the potential infective

dose is very low (< lO particles), they pose a bigger threat. On the other hand,

Yegetated wetlands have been shown to be effective in removal of viruses from

domestic sewage (Gersberg et al., 1989a). Since removal of viruses has been correlated

with removal of suspended solids from wastewater. design criteria that enhance

suspended solids removal are likely to enhance virus removal. Hence coupling of

\vetlands to treatment pond systems could achieve high virus removal efficiency.

Adequate water availability

Availability of water to maintain the required water regime is particularly important in

SF constructed wetlands in areas where evapotranspiration exceeds total water inflow

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during summer months. It is therefore crucial that appropriate design models to predict

wetland hydraulics be applied.

Selection and management of suitable macrophyte species

Appropriate choice of species adapted to tropical environments is of great significance.

In the tropics where growth rates are high, the frequency and hence the cost of

harvesting has to be considered. Use of very fast growing plants e.g. the water hyacinth

that requires frequent harvesting is not likely to be feasible. Economic utilization of

excess biomass and frequent harvesting costs should be well assessed before choosing

such a plant.

Control of noxious effects and disease vector

Bad odors are a possible problem associated with the constructed wetlands. All

wetlands, natural or artificial, have got their own characteristic odors. Depending on the

quality of the influent wastewater and dissolved oxygen, the odor levels vary. Under

anaerobic conditions, odorous compounds are produced and can be obnoxious.

Nuisance odors can be reduced by maintaining low BOD levels (Kadlec and Knight,

1996). Odor reduction strategies are required for constructed wetlands located near

habitation. Also wetlands being wet most of the time, are potential breeding habitats for

disease vectors such as mosquitoes and snails. In order to avoid public health risks,

vector control must be integrated in the design and oper,Hion procedures.

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CONSTRUCTED WETLANDS: A LITERATURE...

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