ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - NATURAL SYSTEMS FOR WASTEWATER TREATMENT pdf

TYPES OF NATURAL SYSTEMS Natural systems for effective wastewater treatment are divided into two major types: terrestrial, and aquatic systems.. 2,7 TABLE 1 Design features and expecte

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NATURAL SYSTEMS FOR WASTEWATER TREATMENT

INTRODUCTION

In the continual search for a simple, reliable, and

inexpen-sive wastewater treatment system, the natural systems are the

latest discovery They provide not only efficient methods of

wastewater treatment, but also provide some indirect

benefi-cial uses of the facility such as green space, wildlife habitat,

and recreational areas

In the natural environment, physical, chemical, and

bio-logical processes occur when water, soils, plants,

microor-ganisms, and atmosphere interact To utilize these processes,

natural systems are designed The processes involved in the

natural systems include: sedimentation, filtration, gas transfer,

adsorption, ion exchange, chemical precipitation, chemical

oxidation and reduction, and biological conversions plus other

unique processes such as photosynthesis, photooxidation,

and aquatic plant uptake. 1 In natural systems, the processes

occur at “natural” rates and tend to occur simultaneously in

a single “ecosystem reactor” as opposed to mechanical

sys-tems in which processes occur sequentially in separate

reac-tors or tanks in accelerated rates as a result of energy input

Generally, a natural system might typically include pumps

and piping for wastewater conveyance but would not depend

on external energy sources exclusively to maintain the major

treatment responses. 2

In this paper a general overview of natural systems for

wastewater treatment is presented The constructed

wet-lands are becoming a viable wastewater treatment

alterna-tive for small communities, individual homes, and rest areas

Therefore, in this paper, a great deal of information has been

presented on site selection, design of physical facilities,

per-formance expectations, hydraulic and organic loading rates,

and cost of the constructed wetlands

TYPES OF NATURAL SYSTEMS

Natural systems for effective wastewater treatment are

divided into two major types: terrestrial, and aquatic systems

Both systems depend on physical and chemical responses as

well as on the unique biological components. 2 Each of these systems are discussed below

Terrestrial Treatment Systems

Land application is the sole terrestrial treatment system used

to remove various constituents from the wastewater It uti-lizes natural physical, chemical, and biological processes within the soil-plant-water matrix The objectives of the land treatment system includes, irrigation, nutrient reuse, crop production, recharge of ground water and water reclamation for reuse There are three basic methods of land application:

(1) Slow-rate irrigation, (2) Rapid-infiltration–percolation, and (3) Overland flow Each method can produce renovated water of different quality, can be adapted to different site conditions, and can satisfy different overall objectives. 3,4 Typical design features and performance expectations for

Slow-rate irrigation Irrigation is the most widely used form

of land treatment systems It requires presence of vegetation

as a major treatment component The wastewater is applied either by sprinkling or by surface technologies In this pro-cess surface runoff is not allowed A large portion of water is lost by evapotranspiration whereas some water may reach the groundwater table Groundwater quality criteria may be a lim-iting factor for the selection of the system Some factors that are given consideration in design and selection of irrigation method are (1) availability of suitable site, (2) type of waste-water and pretreatment, (3) climatic conditions and storage needed, (4) soil type, and organic and hydraulic loading rates, (5) crop production, (6) distribution methods, (7) application cycle, and (8) ground and surface water pollution. 2,3,5

Rapid-infiltration – percolation In rapid-infiltration-percolation

the wastewater percolates through the soil and treated effluent reaches the groundwater or underdrain systems Plants are not used for evapotranspiration as in irrigation system The objectives of rapid infiltration—percolation are (1) ground-water recharge, (2) natural treatment followed by withdrawal the three basic terrestrial concepts are given in Table 1

by pumping or under-drain systems for recovery of treated

water, (3) natural treatment with groundwater moving

verti-cally and laterally in the soil, and (4) recharging a surface water

source. 5

Overland flow In the overland flow system, the wastewater

is applied over the upper reaches of the sloped terraces and

allowed to flow overland and is collected at the toe of the slopes

The collected effluent can be either reused or discharged into

a receiving water Biochemical oxidation, sedimentation,

fil-tration and chemical adsorption are the primary mechanisms

for removal of the contaminants Nitrogen removal is achieved

through denitrification Plant uptake of nitrogen and

phospho-rus are significant if crop harvesting is practiced. 3,5

Aquatic Treatment Systems

Aquatic system utilize lagoons and ponds, and wetlands The

lagoons and ponds depend on microbial life, and lower plants

and animals, while wetlands support the growth of rooted

plants Both pond systems and wetlands are discussed below

Pond Systems

Within the aquatic systems, pond systems are the most widely

accepted ones Basically pond systems can be of three types

based on oxygen requirement These are aerobic, anaerobic and facultative pond systems In all cases the major treatment responses are due to the biological components. 2,7 In most of the pond systems both performance and final water quality are dependent on the algae present in the system Algae are func-tionally beneficial, providing oxygen to support other biologi-cal responses, and the algae-carbonate reactions are the basis for the effective nitrogen removal in the ponds However, algae can be difficult to remove from the effluent As a result, the stringent limits for suspended solids in the effluent can not

be met The design features and performance expectations for

Aerobic ponds Aerobic ponds, also called high rate

aero-bic ponds, maintain dissolved oxygen (DO) throughout their entire depth They are usually 30 to 45 cm deep, allowing light to penetrate to the full depth. 3 Mixing is often pro-vided to expose all algae to sunlight and to avoid deposi-tion As a result, formation of anaerobic sludge layer can be reduced Oxygen is provided by photosynthesis and surface reaeration Because of the presence of sufficient dissolved oxygen, aerobic bacteria can stabilize the waste Detention time is short, usually 3 to 5 days Aerobic ponds are lim-ited to warm sunny climate and are used infrequently in the United States. 2,7

TABLE 1 Design features and expected performance for terrestrial treatment units

Typical criteria

Concepts Treatment goals Climate needs Vegetation Area, b ha

Organic loading,

kg BOD5/ha.d

Hydraulic loading, m/year

Effluent characteristics, mg/L Slow rate Secondary, or AWT a Warmer season Yes 23–280 — 0.5–6 BOD ⬍2

TSS ⬍1

TN ⬍3 c

TP ⬍0.1 c

FC 0 d

Rapid infiltration Secondary, or AWT,

or groundwater recharge

TP ⬍1 e

Overland flow Secondary, nitrogen

removal

TSS 10 f

a Advanced wastewater treatment.

b For design flow of 3800 m 3 /d.

c Total nitrogen removal depends on type of crop and management.

d FC ⫽ Fecal coliform, number per 100 ml.

e Measured in immediate vicinity of basin, increased removal with longer travel distance.

f Total suspended solids depends in part on type of wastewater applied.

Source: Adopted in part from Refs 2, 3, 5, and 6.

natural aquatic treatment units are summarized in Table 2

Anaerobic ponds Anaerobic ponds receive such a heavy

organic loading that there is no aerobic zone They are

usually 2.5 to 5 m in depth and have detention time of 20 to

50 days. 2,3 The principal biological reactions occurring are

acid formation and methane production Anaerobic ponds

are usually used for treatment of strong industrial and

agri-cultural wastes, or as a pretreatment step where an industry

is a significant contributor to a municipal system They do

not have wide application to the treatment of municipal

wastewater

Facultative ponds It is the most common type of pond

unit These ponds are usually 1.2 to 2.5 m (4 to 8 ft) in

depth with an aerobic layer overlaying an anaerobic layer,

which often contains sludge deposits The usual detention

time is 5 to 30 days. 2 Anaerobic fermentation occurs in the

lower layer, and aerobic stabilization occurs in the upper

layer The key to facultative operation is oxygen

produc-tion by photosynthetic algae and surface reaeraproduc-tion The

algae cells are necessary for oxygen production, but their

presence in the final effluent represents one of the most

serious performance problems associated with the facultative

ponds. 7

Wetland Treatment Systems

The wetlands are inundated land areas in which water table

is at or above the ground surface (usually 0.6 m or more)

This water table stands long enough time each year to

main-tain saturated soil conditions and also to support the growth

of related vegetation The vegetation provides surface for

attachment of bacteria films, and aids in the filtration and

adsorption of wastewater constituents Vegetation also

trans-locate oxygen from leaves to the root systems to support a

wide range of aerobic and facultative bacteria and controls

the growth of algae by restricting the penetration of sunlight. 9

The unique ability of wetland plants to translocate oxygen to

support a wide range of bacteria in the wetlands is shown

and constructed wetlands Both natural and constructed wet-lands have been used for wastewater treatment, although the use of natural wetlands is generally limited to the polishing

or further treatment of secondary or advanced wastewater treated effluent. 2

Natural wetlands From a regulatory standpoint, natural

wetlands are usually considered as part of the receiving waters Consequently discharges to natural wetlands, in most cases, must meet applicable regulatory requirements, which typically stipulate secondary or advanced wastewater treat-ment. 10 Furthermore, the principal objective when discharg-ing to natural wetlands should be enhancement of existdischarg-ing habitat Modification of existing wetlands to improve treat-ment capability is often very disruptive to the natural ecosys-tem and, in general, should not be atecosys-tempted. 10,11

Constructed wetlands Constructed wetlands offer all of

the treatment capabilities of the natural wetlands but with-out the constraints associated with discharging to a natural ecosystem Additionally, the constructed wetland treatment units are not restricted to the special requirements on influ-ent quality They can ensure much more reliable control over the hydraulic regime in the system and therefore per-form more reliably than the natural wetlands. 10 Two types

of constructed wetlands have been developed for wastewater treatment (1) free water surface (FWS) systems, and (2) sub-surface flow system (SF) The schematic flow diagrams of

eral description of both types of systems is provided below

The basic design considerations such as (a) site selection, (b) plants types, (c) physical facilities, (d) hydrologic fac-tors, (e) organic loading facfac-tors, and (f) performance expec-tations, that are presented under a separate section entitled

Basic Design Considerations of Constructed Wetlands

Free water surface (FWS) wetlands The free water surface

wetlands typically consist of a basin or channels with some type

of barrier to prevent seepage, soil to support the root systems

TABLE 2 Design features and expected performance for pond treatment units

Parameter

Aerobic (High Rate)

Aerobic–anaerobic (Facultative) Anaerobic Detention time, days 5–20 10–30 20–50

BOD5 loading, kg/ha.d 40–120 15–120 200–500 Soluble BOD5 removal, percent 90–97 85–95 80–95 Overall BOD5 removal, percent 40–80 70–90 60–90 Algae concentration, mg/L 100–200 20–80 0–5 Effluent TSS, mg/L 100–250 a 40–100 a 70–120 a

a TSS is high because of algae Effluent quality can be improved significantly if algae is removed.

Source: Adopted in part from Refs 3, 4 and 8.

both types of system have been shown in Figure 2 A

gen-in Figure 1 Wetlands can be of two types: natural wetlands

of the emergent vegetations, and water through the system at

a relatively shallow depth. 2,9 The water surface in FWS

wet-lands is exposed to the atmosphere, and the intended flow

path through the system is horizontal Pretreated wastewater is

applied continuously to such systems, and treatment occurs as

the water flows slowly through the stems and roots of emergent

vegetation Free water surface systems may also be designed

with the objective of creating new wildlife habitats or

enhanc-schematic flow diagram of free water surface wetlands

Subsurface flow (SF) wetlands The subsurface flow

wet-lands also consist of a basin or channel with a barrier to

pre-vent seepage The basin or channel is filled to a suitable depth

with a porous media Rock or gravel are the most commonly

used media types The media also supports the root systems of

the emergent vegetation The design of these systems assumes

that the water level in the bed will remain below the top of the

rock or gravel media The flow path through the operational

systems is usually horizontal. 12 The schematic flow diagram

of submerged flow wetlands is provided in Figure 2(b)

Comparing the two types of constructed wetland

sys-tems, the SF type of wetlands offer several advantages over

FWS type If the water surface is maintained below the

media surface there is little risk of odors, public exposure,

or insect vectors In addition, it is believed that the media

provides larger available surface area for attached growth

organisms As a result, the treatment response may be faster,

and smaller surface area may be needed for the same

waste-water conditions. 2,10 Furthermore, the subsurface position of

the water, and the accumulated plant debris on the surface of

the SF bed, offer great thermal protection in cold climates

as compared to the FWS type. 14 The reported disadvantage

of the SF type system however, is clogging of the media possible overflow

Basic Design Considerations of Constructed Wetlands

Constructed wetlands are relatively recent development, and are gaining popularity for treatment of wastewater from small communities, and residential and commercial areas In this section the basic design information and economics of constructed wetlands are compared

Site selection A constructed wetland can be constructed

almost anywhere In selecting a site for a free water surface wetland the underlying soil permeability must be consid-ered The most desirable soil permeability is 10E-6 to 10E-7 m/s. 10 Sandy clay and silty clay loams can be suitable when compacted Sandy soils are too permeable to support wetland vegetation unless there is an impermeable restricting layer

in the soil profile that result in a perched high ground water table Highly permeable soils can be used for wastewater flows by forming narrow trenches and lining the trench walls and bottoms with clay or an artificial liner In heavy clay soils, addition of peat moss or top soil will improve soil per-meability and accelerate initial plant growth

Plants Although natural wetlands typically contain a wide

diversity of plant life, there is no need to attempt to reproduce the natural diversity in a constructed wetland Such attempts

in the past have shown that eventually cattails alone or in combination with either reeds or bulrushes will dominate FIGURE 1 An enlarged root hair of wetland plants.

ing nearby existing natural wetlands Figure 2(a) provides a

in a wastewater system owing to the high nutrient levels. 2

The emergent plants most frequently found in the

waste-water wetlands include cattails, reeds, rushes, bulrushes and

sedges Some of the major environmental requirements of

Physical facilities The constructed wetlands behave

typ-ically like a plug flow reactor Constructed wetlands offer

significant potential for optimizing performance through

selection of proper system configuration, including aspect

ratios, compartmentalization, and location of alternate and

multiple discharge sites

The aspect ratio, defined as the ratio of length to width,

typically varies from 4:1 to 10:1 However, based on research

data developed on experimental constructed wetlands, the

aspect ratios approaching 1:1 may be acceptable. 13 Several

alternative flow diagrams and configurations of constructed

Hydrologic factors The performance of the constructed

wet-lands system is dependent on the system hydrology as well as many other factors such as precipitation, infiltration, evapotrans-piration, hydraulic loading rate, and water depth These factors affect the removal of organics, nutrients, and trace elements not only by altering the detention time but also by either con-centrating or diluting the wastewater. 9,10 For a constructed wetland, the water balance can be expressed by Eq (1) [d V /d t ] ⫽ Q i − Q e ⫹ P − ET (1) where,

Q i ⫽ influent wastewater flow, m 3 /d

Q e ⫽ effluent wastewater flow, m 3 /d

P ⫽ precipitation, m 3 /d

ET ⫽ evapotranspiration, m 3 /d [d V /d t ] ⫽ change in volume of water per unit time, m 3 /d

t ⫽ time, d

Influent

Effluent

Water surface Wetland Plants

Influent

Effluent

Water surface Wetland Plants

(a)

(b)

FIGURE 2 Schematic flow diagrams of constructed wetland systems, (a) Free water surface, (b) Subsurface flow 13

these plants are given in Table 3

wetlands are provided in Figure 3

Inf Effl.

Influent

Effluent

Recycle Recycle

Recycle

Inf.

Eff.

(d) (c)

FIGURE 3 Alternate flow diagram and configurations of constructed wetlands: (a) Plug flow with recycle; (b) Step feed with recycle;

(c) Step feed in wrap around pond; (d) Peripheral feed with center drawoff.

TABLE 3 Emergent aquatic plants for constructed wetlands

Common name

(Scientific name)

Temperature, °C

Maximum salinity tolerance, ppt b Optimum pH Survival Desirable a

Cattail

(Typha spp.)

Common reed

(Phragmites communis)

Rush

(Juncus spp.)

Bulrush

(Scirpus spp.)

Sedge

(Carex spp.)

a Temperature range for seed germination: roots and rhizomes can survive in frozen soils.

b ppt ⫽ parts per thousands.

Source: Adopted in part from Ref 15.

Hydraulic loading rate for FWS system is closely tied

to the hydrologic factors and conditions specific to the

site Typical hydraulic loading rate of 198 m 3 /d.ha (21,000

gpd/acre) is considered sufficient for optimum treatment

efficiency

BOD 5 loading rates There are two goals for organic load

control in the constructed wetlands The first is the provision

of carbon source for denitrifying bacteria The second goal

is to prevent overloading of the oxygen transfer ability of the

emergent plants High organic loading, if not properly

dis-tributed, will cause anaerobic conditions, and plants die off

The maximum organic loading rate for both type of systems

(FWS and SF) should not exceed 112 kg BOD 5 /ha.d. 2,9,10

Performance expectations Constructed wetland systems

can significantly remove the biochemical oxygen demand

(BOD), total suspended solids (TSS), nitrogen and

phos-phorus, as well as metals, trace organics and pathogens The

basic treatment mechanisms include sedimentation,

chemi-cal precipitation, adsorption, microbial decomposition,

as well as uptake by vegetation Removal of BOD 5 , TSS,

nitrogen, phosphorus, heavy metals and toxic organics have

been reported. 2,17 The performance of many well known

con-structed wetlands in terms of BOD 5 , TSS, ammonia nitrogen

Mosquito control and plant harvesting are the two

opera-tional considerations associated with constructed wetlands for

wastewater treatment Mosquito problems may occur when

wetland treatment systems are overloaded organically and

anaerobic conditions develop Biological control agents such

as mosquitofish ( Gambusia affins ) die either from oxygen

starvation or hydrogen sulfide toxicity, allowing mosquito

larvae to mature into adults Strategies used to control

mos-quito populations include effective pretreatment to reduce

total organic loading; stepfeeding of the influent wastewater

stream with effective influent distribution and effluent

recy-cle; vegetation management; natural controls, principally

mosquitofish, in conjunction with the above techniques; and

application of approved and environmentally safe chemical

control agents

The usefulness of plant harvesting in wetland treatment

systems depends on several factors, including climate, plant

species, and the specific wastewater objectives Plant

harvest-ing can affect treatment performance of wetlands by alterharvest-ing

the effect that plants have on the aquatic environment

Further, because harvesting reduces congestion at the water

surface, control of mosquito larvae using fish is enhanced It

has been reported in the literature that a total of 29,000 kg/ha

dry weight of harvestable biomass of Phragmites shoots can

be harvested for single harvesting in a year Higher yield is

achievable with multiple harvesting

The BOD 5 , TSS, nitrogen and phosphorus removal

effi-ciencies of constructed wetlands are discussed below

BOD 5 Removal in FWS wetlands In the free water surface

constructed wetlands the soluble BOD 5 removal is due to

microbial growth attached to plant root, stems, and leaf litter

that has fallen into the water BOD 5 removal is generally expressed by a first order reaction kinetic (Eq 2). 2,3,10 [ C e / C o ] ⫽ exp(− K T t ) (2) where,

C e ⫽ effluent BOD 5 , mg/L

C o ⫽ influent BOD 5 , mg/L

K T ⫽ reaction rate constant, d −1

t ⫽ hydraulic retention time, d

BOD 5 Removal in SF wetlands The major oxygen source

for the subsurface components (soil, gravel, rock, and other media, in trenches or beds) is the oxygen transmitted by the vegetation to the root zone In most cases, there is very little direct atmospheric reaeration as water surface remains below the surface of the media. 17 Removal of BOD 5 is expressed by Equation (3) This is also a first order equation and can be rearranged to calculate the area required for the subsurface flow system

log( C e / C o ) ⫽ −[ A s K t d e]/ Q (3)

where,

C e ⫽ effluent BOD 5 , mg/L

C o ⫽ influent BOD 5 , mg/L

K t ⫽ reaction rate constant, d −1

Q ⫽ flow rate through the system, m 3 /d

d ⫽ depth of submergence, m

e ⫽ porosity of the bed,

A s ⫽ surface area of the system, m 2

Suspended solids removal Suspended solids removal is

very effective in both types of constructed wetlands Most

of the removal occurs within few meters beyond the inlet

Control dispersion of the inlet flow will enhance removal near the inlet zone Proper dispersion of solids can be achieved by low inlet velocities, even cross sectional load-ings, and uniform flow without stagnation. 10

Nitrogen removal Nitrogen removal is very effective in

both the free water surface and submerged flow constructed wetlands, and the nitrification/denitrification is the major path of nitrogen removal Total nitrogen removals of up to

79 percent are reported at nitrogen loading rates of (based

on elemental N) up to 44 kg/(ha.day) [39 lb/(acre.day)], in a variety of constructed wetlands If plant harvesting is prac-ticed, a higher rate of nitrogen removal can be expected. 9,20

Phosphorus removal Phosphorus removal in many

wet-lands is not very effective because of the limited contact opportunities between the wastewater and the soil The exceptions are in the submerged bed design when proper soils are selected as the medium for the system. 21 A signifi-cant clay content and the presence of iron, and aluminum will enhance the potential for phosphorus removal. 9,20,21 and total phosphorus is summarized in Table 4

TABLE 4 Performances of some well known constructed wetlands

Record

Reedy Creek FL FWS 11.2 35.24 12058 5.3 1.9 8.9 2.4 2.98 0.72 1.4 1.78 Reedy Creek FL FWS 11.2 5.87 2423 5.8 1.6 10.9 2.4 3.29 0.12 1.8 0.79 Ironbridge FL FWS 1 486 34254 4.8 2.1 10.5 65.9 3.99 0.94 0.53 0.11 Apalachicola FL FWS 3 63.7 3936 15.2 1.1 107 8 3.62 0.09 3 0.21

West Jackson MS FWS 0.5 8.91 1953 21.6 10.5 65.9 24.5 2.69 0.19 5.13 3.56 Leaf River 1 MS FWS 1.2 0.13 225 15.8 14 54.8 30.1 9.91 7.21 8.91 8.16 Leaf River 2 MS FWS 1.2 0.13 254 15.8 15.7 54.8 34.9 9.91 6.33 8.91 8.19 Leaf River 3 MS FWS 1.2 0.13 220 15.8 13.9 54.8 25.5 9.91 6.79 8.91 5.9 Cobalt Canada FWS 1 0.10 49 20.7 4.6 36.2 28 2.95 1.04 1.68 0.77 Pembroke KY FWS 0.75 1.48 188 67.4 9.4 91.9 8.2 13.8 3.35 6.03 3.16 Central Slough SC FWS 4 32 5372 16.3 6.5 27.7 14.8 7.49 1.38 4.09 1.46

Source: Adopted in part from Ref 13.

© 2006 by Taylor & Francis Group, LLC

Cost Cost is often a significant factor in selecting the type of

treatment system for a particular application Unfortunately,

the availability of reliable cost data for wetland treatment

systems is limited The cost of wetland treatment systems

varies depending on wastewater characteristics, the type of

wetland system, and the type of bottom preparation required

Subsurface flow systems are generally more expensive

than free water surface systems It has been reported that

the Tennesee Valley Authority (TVA) wetland

construc-tion costs ranged from $3.58/m 2 to $32.03/m 2 . 22 Estimates

are that the construction, operation, and maintenance costs

of constructed wetland systems are quite competitive with

other wastewater treatment options

REFERENCES

1 Mitsch, W.J and J.G Gosselink, Wetlands, Van Nostrand and Reinhold

Company, New York, 1993

2 Reed, S.C., E.J Middlebrooks, and R.W Crites, Natural Systems for

Waste Management and Treatment, McGraw-Hill, Inc., New York, NY,

1988

3 Qasim, S.R., Wastewater Treatment Plants: Planning, Design, and

Operation, Technomic Publishing Company, Inc PA, 1994

4 Middlibrooks, E.J., C.H Middlebrooks, J.H Reynolds, G.Z Watters,

S.C Reed, and D.B George, Wastewater Stabilization Lagoon Design,

Performance and Upgrading, Macmillan Book Co., New York, 1982

5 US Environmental Protection Agency, Process Design Manual for

Land Treatment of Municipal Wastewater, Office of Water Program

Operation, EPA/COE/USDA, EPA 625/1-77-008, October 1977

6 Sanks, R.L and T Asano (Eds.), Land Treatment and Disposal of

Municipal and Industrial Wastewater, Ann Arbor Science Publisher,

Ann Arbor, Mich 1976

7 Metcalf and Eddy, Inc., Wastewater Engineering: Treatment, Disposal

and Reuse, McGraw-Hill, Inc., New York, NY, 1991

8 Bastian, R.K and S.C Reed (Eds.), Aquaculture Systems for

Waste-water Treatment: Seminar Proceedings and Engineering Assessment,

US Environmental Protection Agency, Office of Water Program

Opera-tions, Municipal Construction Division, EPA 430/9-80-006,

Washing-ton, D.C 20460, September 1979

9 Hammer, D.A., Constructed Wetlands for Wastewater Treatment, Lewis

Publishers, Ann Arbor, Michigan, 1989

10 Water Pollution Control Federation, Natural Systems for Wastewater

Treatment, Manual of Practice No FD-16, Water Pollution Control

Federation, 1990

11 Dinges, R., Natural Systems for Water Pollution Control, Van Nostrand

and Reinhold Company, New York, 1982

12 Reed, S.C., Constructed Wetlands for Industrial Wastewaters, Presented

at Purdue Industrial Waste Conference, May 10, 1993, Purdue University, West Lafayette, IN

13 Moshiri, G.A., Constructed Wetlands for Water Quality Improvement,

Lewis Publishers, Florida, 1993

14 US Environmental Protection Agency, Subsurface Flow Constructed

Texas at El Paso, August 16 and 17, El Paso Texas, 1993

15 Stephenson, M., G Turner, P Pope, J Colt, A Knight, and G

Tcho-banoglous, The Use and Potential of Aquatic Species for Wastewater

Treatment, Appendix A, The Environmental Requirements of Aquatic

Plants, Publication No 65, California State Water Resources Control Board, Sacramento, California, 1980

16 Tchobanoglous, G., Aquatic Plant Systems for Wastewater Treatment:

Engineering Considerations, in Aquatic Plants for Water Treatment and

Resource Recovery K.R Reedy and W.H Smiths, Eds Magnolia

Pub-lishing, Orlando, FL, 1987, pp 27–48

17 US Environmental Protection Agency, Subsurface Flow Constructed

Wetlands for Wastewater Treatment — A Technology Assessment, Office

of Water, EPA 832R-93-001, July 1993

18 Cueto, A.J., Development of Criteria for the Design and Construction of

Engineered Aquatic Treatment Units in Texas, Chapter 9, Constructed

Wetlands for Water Quality Improvements, G.A Moshiri, Lewis

Pub-lishers, FL 1993, pp 99–105

19 Hammer, D.A., B.P Pullin, and J.T Watson, Constructed Wetlands for Livestock Waste Treatment, National Nonpoint Conference, St Louis,

MO, April 1989

20 US Environmental Protection Agency, Design Manual on Constructed

Wetlands and Aquatic Plant Systems for Municipal Wastewater Treat-ment, Office of Research and DevelopTreat-ment, Center for Environmental

Research Information, Cincinnati, OH, September 1988

21 Brix, H., Treatment of Wastewater in the Rhizosphere of Wetland

Plants—The Root-Zone Method, Water Science Technology, Vol 19,

1987, pp 107–118

22 Brodie, G.A., D.A Hammer, and D.A Tomljanovich, Constructed

Wetlands for Acid Drainage Control in the Tennessee Valley, in Mine

Drainage and Surface Mine Reclamation, Bureau of Mines Information

Circular 9183, 1988, pp 325–331

MOHAMMED S KAMAL SYED R QASIM

The University of Texas at Arlington