(Winnie) Final Year Project Report

8/9/2019 (Winnie) Final Year Project Report

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QUOTES:

“Man has a fundamental right to liberty, equality and satisfactory living conditions in an

environment whose quality permits him to live in dignity and well-being. He has solemn

duty to protect and improve the environment for present and future generations............”

First principle of the United Nations Conference

On Human Environment, Stockholm, 1992.

(Royston, 1979)

“The earth does not belong to man: man belongs to the earth. This we know. All things

are connected like the blood which unites the family. All things are connected. Whatever

befalls the earth befalls the sons of the earth. Man did not weave the web of life; he is

merely a strand in it. Whatever he does to the web, he does it to himself.”

Chief Seattle, 1855.

(Royston, 1979)


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DECLARATION:

I, Mugera Winnie Nyaguthii, do declare that this report is my original work and to the

best of my knowledge, it has not been submitted for any degree award in any University

or Institution.

Signed ___________________________Date________________________

CERTIFICATION

I have read this report and approve it for examination.

Signed____________________________ Date________________________

Prof.Thumbi


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ACKNOWLEDGEMENT

I would like to thank God for giving me the opportunity to undertake this course and for

the grace that enabled this project to come to a completion. I would also like to thank my

family and friends for guiding me and being by my every step of the research.

I am greatly indebted to my mentor, J.N.Mburu for his guidance, commitment,

patience,believing in me and opening my eyes to the importance of constructed wetlands.

Special thanks to Prof.Thumbi, my Supervisor, for his guidance and positive criticism

through the research process.

To all I say thank you and God bless you.

NyaguthiiMugera

2010.


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ABSTRACT

The increasing application of Constructed Wetlands for Waste water treatment coupled

with increasingly strict water quality standards is an ever-growing incentive for the

development of better process design tool. The use of constructed wetlands in Kenya is

still limited despite there being a great need for inexpensive and reliable onsite/

decentralized wastewater treatment technology. Indeed, the major challenge facing the

country of Kenya is the need to ensure on-going pollution control of the environment and

water resources, in the face of increased wastewater volumes from domestic, agricultural

and industrial sources. Among the policy and decision makers, there is a strong desire for

the wastewater treatment constructed wetlands technology to be adapted locally with as

much confidence in their operability and pollutant removal levels as with comparable

conventional wastewater treatment technology.

This work evaluates the first order COD kinetics in a pilot scale horizontal subsurface

flow Constructed Wetlands (HSSF-CW) treating domestic wastewater.The HSSF-CW

was situated in JKUAT treatment plant. The first order kinetics evaluated in the study

were associated with area-based, volume-based 1st order plug flow models and tank-in-

series design models. The objective of the study was to calibrate these models and

thereafter use them to predict the effluent quality from the HSSF-CW. Further a

comparison of the rate constants calibrated from this wetland with those from literature

was undertaken. Samples were collected and dichromate laboratory tests for COD were

done. Influent COD was found to be between 326-52 mg/l. The effluent COD was found

to be between 30-140 mg/l. Hydraulic Loading Rate was found to between 8-

68m/yr.Average volume-based, area-based and Tank-in series rate constants were found

to be 0.229, 17.016 and 3.3045 respectively. The Ѳ values for the 3 models were found

to be 1.256,1.177 and 1.064 respectively. The k 20 values from the 3models were found to

be 0.0573, 5.789 and 2.683. These constants were lower compared with those from other

studies as reported in the literature were found. This was contrary to the hypothesis that

higher rate constants are expected in the tropics. This could have been caused by the fact

that this study was conducted on an under-developed system that had undergone

renovations four months before the study had commenced and had been compared with

values obtained from mature systems.


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TABLE OF CONTENTS:

QUOTES:......................................................................................................................... i

DECLARATION: ........................................................................................................... ii

CERTIFICATION .......................................................................................................... ii

ACKNOWLEDGEMENT ............................................................................................. iii

DEDICATION ............................................................................................................... iv

ABSTRACT.................................................................................................................... v

LIST OF TABLES .......................................................................................................... x

LIST OF GRAPHS ......................................................................................................... x

LIST OF PLATES ......................................................................................................... xi

LIST OF ABBREVIATIONS: ....................................................................................... xi

CHAPTER 1: ...................................................................................................................... 1

1.0.0 INTRODUCTION ................................................................................................. 1

1.1.0 PROBLEM STATEMENT .................................................................................... 2

1.2.0 PROBLEM JUSTIFICATION ............................................................................... 2

1.3.0 RESEARCH OBJECTIVES .................................................................................. 3

1.3.1 General objective ............................................................................................... 3

1.3.2 Specific objectives ............................................................................................. 3

1.4.0 RESEARCH HYPOTHESIS ................................................................................. 3

1.5.0 LIMITATIONS TO THE STUDY ........................................................................ 4

CHAPTER 2: ...................................................................................................................... 5


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2.0.0 LITERATURE REVIEW ...................................................................................... 5

2.1.0 CONSTRUCTED WETLANDS FOR DOMESTIC WASTEWATER

TREATMENT ............................................................................................................ 5

2.1.1 Discovery and evolving of constructed wetlands technology ........................ 5

2.1.2 Classification of constructed wetlands systems ............................................. 6

2.1.3 Design features of a constructed wetland ...................................................... 8

2.1.4 Maintenance of constructed wetlands .......................................................... 11

2.2.0 TREATMENT KINETICS .............................................................................. 11

2.2.1 Plug flow models: ........................................................................................ 13

a) Area- based plug- flow model ................................................................... 13

b) Volume –based plug flow model .............................................................. 13

c) Tank-in Series models: ............................................................................. 13

2.2.2 Drawbacks of plug flow models .................................................................. 14

2.3.0 Rate constants: Study and literature values ...................................................... 15

CHAPTER 3: .................................................................................................................... 16

3.0.0 RESEARCH METHODOLOGY ......................................................................... 16

3.1.0 The pilot scale subsurface horizontal flow constructed wetland ..................... 16

3.1.1 Site description: ........................................................................................... 16

3.1.2 Experiment’s set-up ..................................................................................... 19

3.2.0 SAMPLING ..................................................................................................... 20

3.3.0 WATER QUALITY ......................................................................................... 21


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3.3.1 TESTS .......................................................................................................... 21

CHAPTER 4: .................................................................................................................... 22

4.0.0 RESULTS AND DISCUSSION .......................................................................... 22

4.1.0 ENVIROMENTAL PARAMETERS RESULTS ............................................ 22

4.2.0 FLOW RATES................................................................................................. 23

4.2.1 Average flow rates, Hydraulic Retention Time and Hydraulic loading rate 23

4.3.0 COD RESULTS: .............................................................................................. 25

4.3.1 COD Removal:............................................................................................. 25

4.4.0 CALIBRATION OF VOLUME-BASED, AREA-BASED PLUG FLOW

MODELS ,TANK-IN-SERIES(TIS) MODEL RATE ............................................. 28

4.5.0 K20 AND Ѳ VALUES ...................................................................................... 30 4.6.0 COMPARISON OF THE KA/V/T K20 AND Ѳ VALUES FROM THISSTUDY AND THOSE OBTAINED FROM ............................................................ 31

4.7.0 INFLUENCE OF ENVIRONMENTAL PARAMETERS .............................. 33

4.7.1 AERATION PROCESS ............................................................................... 33

4.7.2 EFFECTS OF TEMPERATURE ON %COD REMOVAL......................... 34

4.7.3 EFFECTS HYDRAULIC RETENTION TIME(HRT) ON %COD

REMOVAL ........................................................................................................... 35

4.7.4 EFFECTS OF PH ON % COD REMOVAL ................................................ 36

CHAPTER 5 ..................................................................................................................... 37

5.0.0 PREDICTIONS OF EFFLUENT COD USING VOLUME- BASED, AREA-

BASED PLUG FLOW MODELS AND TANK-IN SERIES MODELS ..................... 37


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5.1.0 Volume-based plug flow model effluent COD predictions ............................ 37

5.2.0 Area-based plug flow model effluent COD predictions .................................. 39

5.3.0 Tank-in Series model effluent COD predictions .............................................. 40

CHAPTER 6 ..................................................................................................................... 43

6.0.0 CONCLUSIONS AND RECOMMENDATIONS .............................................. 43

6.1.0 Conclusions ...................................................................................................... 43

6.2.0 Recommendations ............................................................................................ 43

Bibliography ..................................................................................................................... 45

APPENDIX 1:............................................................................................................... 47

Laboratory tests procedures: ..................................................................................... 47

Open reflux method. ............................................................................................. 47

Dissolved Oxygen - titration method. ................................................................... 48

APPENDIX 2:............................................................................................................... 50

CALIBRATION VOLUME-BASED, AREA-BASED PLUG FLOW MODELS,

TANK-IN-SERIES(TIS) MODEL RATE CONSTANTS....................................... 50

EVALUATION OF K20 AND Ѳ VALUES .............................................................. 51 APPENDIX 3:............................................................................................................... 53

PLATES: ................................................................................................................... 53


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LIST OF TABLES

Table 2.1: Rate constants; values from literature .............................................................. 15

Table 4.1: Environmental parameters of the HSSF-CW ................................................... 22

Table 4.2: Inflow and Outflow rates ................................................................................. 23

Table 4.3:Average flow rates, Hydraulic Retention Time, Hydraulic Loading Rates ...... 23

Table 4.4: Measured influent and effluent COD............................................................... 23

Table 4.5: Percentage COD removal ................................................................................ 23

Table 4.6: Volume-based, Area-based ,Tank-in Series rate constants .............................. 23

Table 4.7: and Ѳ values ............................................................................................ 23 Table 4.8: Rate constant values from this study and other studies ................................... 31

The average of k v values was calculated from table 4.6. ................................................. 37

Table 5.1: Predicted effluent COD and Measured effluent COD .................................... 38

Table 5.2: Predicted effluent COD and Measured effluent COD ..................................... 39

Table 5.3: Predicted effluent COD and Measured effluent COD ..................................... 41

LIST OF GRAPHS

Graph 4.1: Influent COD trend ........................................................................................ .23

Graph 4.2:Effluent COD trend…………………………………………………………..23

Graph 4.3: Influent and Effluent COD comparison ........................................................ .23

Graph 4.4: Influent and Effluent DO comparison……………………………………….33

Graph 4.5: % COD removal vs Temperature……………………………………………34

Graph 4.6: % COD removal vs HRT…………………………………………………….35

Graph 4.7: % COD removal vs pH……………………………………………………...36

Graph 5.1: Measured effluent COD vs Predicted effluent COD………………………..38

Graph 5.2: Measured effluent COD vs Predicted effluent COD ...................................... 40

Graph 5.3:Measured effluent COD vs Predicted effluent COD ....................................... 41

Graph a: Ln kv vs Ln (T-20)……………………………………………………………..51

Graph b: Ln k A vs Ln (T-20) ............................................................................................. 52

Graph c: Ln k T vs Ln (T-20) ............................................................................................. 52


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LIST OF PLATES

Plate 3.1: Student collecting samples at the HSSF-CW .................................................... 20

Plate 3.2: Student carrying out tests in the Environmental laboratory .............................. 21

Plate a: Student collecting samples at the influent point of the HSSF-CW ...................... 53

Plate b: Student collecting samples at the effluent point of the HSSF-CW ...................... 54

Plate c: Student testing samples for COD in the Environmental laboratory ..................... 54

LIST OF ABBREVIATIONS:

HSSF-CW =Horizontal Subsurface Flow Constructed Wetland

COD = Chemical Oxygen Demand

DO= Dissolved Oxygen

HRT = Hydraulic Retention Time

CSTR = Continuously Stirred Tank Reactors

Mg/l = Milligrams per liter

TIS = Tank –in Series


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CHAPTER 1:

1.0.0 INTRODUCTION

Constructed wetlands for wastewater treatment can be defined as a designed and man-

made complex of saturated substrate, emergent and submergent vegetation, animal life,

and water that simulate natural wetlands for human use and benefits (Hammer & Bastian,

1989).

Among the treatment wetlands, horizontal subsurface flow (HSSF) constructed wetlands

are a widely applied concept. Pretreated wastewater infiltrates horizontally through the

artificial filter bed, usually consisting of a matrix of sand or gravel and rhizomes. This

matrix is colonized by a layer of attached microorganisms that forms a so-called biofilm.

Purification is achieved by a wide variety of physical, chemical and (micro) biological

processes, like sedimentation, filtration, precipitation, sorption, plant uptake, microbial

decomposition and nitrogen transformations.

Constructed wetlands have proven to be a very effective method for the treatment of

domestic wastewater. For a small community with limited funds for expanding or

updating wastewater treatment plants, constructed wetlands are an attractive option.

Constructed wetlands blend into a natural landscape setting. In addition, wetlands add

aesthetic value, and provide wildlife habitat and recreation opportunities, they are cheap

to construct and maintain no skilled manpower is required, odour- free, free from

mosquitoes and diseases.

A lot of research is therefore required in order to popularize these systems. The first order

kinetics evaluated in the study were associated with area-based, volume-based 1st order

plug flow models and tank-in-series design models

In first order kinetics, the rate of disappearance of reactant is proportional to the amount

reactant present.The reaction rate, k in waste water treatment is temperature dependent. Generally, as the

temperature increases, so does the rate at which the reaction occurs.

Plug flow models are used to describe reactions in a continuous flowing system. The

operating principle behind plug flow models is,’ what goes in first into the system is the


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first to go out’. Waste water flowing through the HSSF-CW is modeled as a series of

infinitely thin coherent ‘plugs’ each with a uniform composition travelling in an axial

direction of the system with each plug having a different composition from the ones

before and after it. As the waste water flows through the system, the residence time of the

plug is a function of its position in the system. Residence time is the average amount of

time a discrete amount of reactant spends inside the system.

1.1.0PROBLEM STATEMENT

Ironically, developing countries with a large burden of untreated wastewaters and

unsatisfactory sanitation conditions have continued to neglect inexpensive wastewater

management solution like the constructed wetlands. One of the contributing factors is the

lack of local research on constructed wetlands in these countries, as evidenced by the

scanty information and data in literature with regard to design parameters. To promote

this optimal alternative technology to conventional waste water treatment, it is important

to derive/evaluate design parameters in the tropical developing countries. This study

evaluates the first order kinetics for the subsurface flow constructed wetland under

tropical climatic conditions.

1.2.0 PROBLEM JUSTIFICATION

1. The existing wastewater treatment plants in the country are becoming

overwhelmed due to increase in population and waste water volumes. Attempts

to implement or upgrade the conventional systems of wastewater treatment are

met by unreasonably high initial costs as well as complex maintenance structures.

2.

Need for decentralized wastewater management systems due to mushrooming

townships, remotely located industries and agricultural enterprises. This calls for

an effective and relatively low maintenance method for providing onsite waste

water treatment.


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1.3.0 RESEARCH OBJECTIVES

1.3.1 General objective

• To evaluate the empirical design models (area-based and volume-based first

order, plug flow models) for COD using a pilot scale subsurface flow constructed

wetland performance in treating domestic waste.

1.3.2 Specific objectives

• To determine the organic matter content of both the influent and the effluent in

the HSSF-CW.

• To compare the rate constants calibrated from this HSSF-CW study with those

from the literature.

• To compare tank-in series (TIS) model for COD with plug- flow models in

describing the HSSF-CW performance.• To determine the influence of environmental conditions i.e., Temperature and

DO (dissolved oxygen) on the treatment performance of a horizontal subsurface

flow constructed wetland.

1.4.0 RESEARCH HYPOTHESIS

Larger constant values are expected in the tropics as compared to those in the temperateregions. It is therefore hypothesized that the Constructed Wetland systems would be more

efficient in the tropics.


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1.5.0 LIMITATIONS TO THE STUDY

• Limited financial resources and limited time factor will limit the extensiveness of

the study and therefore only COD will dealt with extensively in the study.

• Portable meters (thermometer and pH meter) for monitoring field water

temperature and pH cannot be carried to site as required. This will therefore be

determined in the lab, this way variations may be introduced.


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CHAPTER 2:

2.0.0LITERATURE REVIEW

2.1.0CONSTRUCTED WETLANDS FOR DOMESTIC WASTEWATER

TREATMENT

2.1.1Discovery and evolving of constructed wetlands technology

A lot of curiosity from ecologists over how much the wetlands were valuable arose in the

fifties and sixties. This initiated many studies on the topic. They seemingly stumbled over

the purification capabilities of these wetlands which set off the development of

constructed wetland technologies.

The technology of wastewater treatment by means of constructed wetlands with

horizontal sub-surface flow was pioneered in Germany based on research by the one Dr.

K. Seidel commencing in the 1960s and by Reinhold Kickuth in the 1970s. Her research

seemed heavily criticized since the investigations and calculations were mainly aimed at

nutrient removal through plant uptake which would require a regular harvesting and very

large surface areas.

The growing ‘green awareness’ in the seventies catalyzed the abandoning of dumping

wastewater in the natural wetlands in favor of constructed wetlands (CWs). Another

positive boost was possibly due to the first energy crisis in 1973. Energy-devouring

technologies suddenly lost their attractiveness to the advantage of low-energy ones.

Natural systems of wastewater treatment are characterized by the use of renewable,

naturally occurring energies such as solar and wind energy, as opposed to conventional

treatment technologies which are dependent on non-renewable fossil fuel energies. The

above mentioned stimuli soon outweighed the classic distrust against new technologies

and, from then on, constructed wetlands development took an exponential growth.

The use of wetlands for wastewater treatment was stimulated by a number of studies in

the early 1970s that demonstrated the ability of natural wetlands to remove suspended


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sediments and nutrients, particularly nitrogen and phosphorus, from domestic wastewater

(Mitsch & Gosselink, 1993).

The eighties were once again characterized by precaution and skepticism due to the

discovery of several drawbacks of the technology and failures of most prototypes. Further

research solved most of these problems and led to the maturity of the technology in the

nineties.

Constructed wetlands nowadays have many applications, ranging from the secondary

treatment of domestic, agricultural and industrial wastewaters to the tertiary treatment

and polishing wastewaters treated by means of activated sludge plants and even to the

treatment of storm waters. It has been adopted over the years in various parts of the world

for other various purposes. For example Constructed Wetlands at Milan Army

Ammunition Plant, Milan, Tennessee constructed in the World War II was used to treat

ground water contaminated with residue explosives. This was also used in Iowa Army

Ammunition Plant in Middletown, Iowa for the same purpose. In Fulton Regional Storm

Water Management facility in Edmonton, Alberta, for a storm water management system

(Jones, William.W., 1995).

In the developed countries, wetland treatment system has been identified as a treatment

system that features low operation cost, good reliability and effectiveness, in the face of

increased environmental awareness and enforcement of pollution prevention laws.

Besides, in many situations, a decentralized system of treating sewage wastes with

constructed wetlands, provide not only a more economical and energy efficient means of

achieving treatment objective but also a resource in form of reclaimed water available for

irrigation or creation of wildlife habitats (Campbell & M.H., 1999).

2.1.2 Classification of constructed wetlands systems

The following classification only considers the middle range ecosystems.i.e. the so-called

constructed wetlands, and is based on internationally accepted international water


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associations’ scientific and technical reports on constructed wetlands for pollution

control’(Kadlec et al.,2000b).

The various types are differentiated by water flow mode and plant species characteristics.

1) Above ground water: free-water-surface (FWS) constructed wetlands

With emergent macrophytes or helophytes, e.g. phragmitesaustralis (common reed),

typhaspp (cattails), scirpusspp, (bulrushes)

with floating-leaved, bottom rooted macrophytes, e.g. nymphaea spp.

(water lilies),nelumbospp,(lotus)

With free-floating macrophytes, e.g.eichhorniacrassipes (water hyacinth) , lemna spp.

(duckweed)

With submerged macrophytes, e.g.elodeaspp, (waterweed), myriophyllum spp. (water

milfoil)

With floating mats, e.g.phragimitesaustrils (common weed), typhaspp (cattails), glyceria

maxima (giant sweet grass)

2) Below-ground water: subsurface-flow (SSF) constructed wetlands.

Horizontal-flow systems (HSSF), planted with emergent macrophytes or helophytes, e.g.

phragmitesaustralis (common reed), typhaspp (cattails), scirpusspp, (bulrushes)

Vertical-flow systems(VSSF), planted with emergent macrophysics or helophytes,e.g.

phragmitesaustralis(common reed) , typhaspp(cattails),scirpusspp,(bulrushes)

3)Above ground water flow system consists of a relatively shallow basin (depth between

0.3 and 1.8 meters), isolated from the ground water by means of a plastic liner or by a

local clay layer. Length-width ratios >2 are to be preferred in order to obtain near plug-

flow conditions. The inlet distribution and effluent abstraction systems should run along

the entire width of the basin to avoid short-circuiting and the existence of dead volumes.

When using free-floating macrophytes, floating barriers are often used to avoid the piling

up of plants in one corner due to wind action.


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4)Below-ground water flow Horizontalsystem consists of the shallow(0.5-0.8m

deep)basin, isolated from the groundwater and usually filled with gravel although in

some cases local soils has been used. For the inlet and outlet zone, coarser gravel is

usually applied to allow a better spreading respectively collection of wastewater. In this

case the bed of impermeable material is sloped typically between 0 and 2 percent. It is

during the passage of wastewater through the rhizosphere that it gets cleaned through

microbiological degradation, and physical/ chemical process. The treated wastewater is

evacuated by means of a drainage tube at the bottom of the wetland. An appropriate

choice of filter material (c.q.hydraulic conductivity) and a correct length-width ratio are

indispensible to avoid above-ground water flow, which has detrimental effect on

treatment performance and can cause odour and insect nuisances.

5)Below- ground water Vertical flow systems consists of one or more filter layers of

coarse sand and/gravel with a total depth between 0.6 and 1.0 meter. Wastewater is

preferably spread equally over the top surface, then drains through the filter layers and is

collected at the bottom by means of drainage tubes. Loading often happens intermittently,

i.e. batch wise. Choosing the right filter material is a trade off between high respectively

low hydraulic conductivities.i.e. Less prone to clogging versus a longer hydraulic

retention time. (Risper, 2009)

2.1.3 Design features of a constructed wetland

There are a number of design features that can increase the efficiency ofconstructed

wetlands to trap and retain domestic wastewater pollutants.

Loading Rate: Proper sizing of a constructed wetland in relation to its watershed is

probably the most important factor affecting wetland performance. If the wetland is sized

too small, water flow through the system can be too rapid for effective treatment. Too

little water flowing through can result in stagnant water or temporary dry conditions.

Primary plant productivity and decomposition rates are both higher in flowing water but


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high velocities discourage plant growth. Permanently flooded wetlands perform better

than wetlands which dry out seasonally. Therefore, designing the hydraulic loading rate is

critical. Ideally, for optimal performance, the size of the constructed wetland should be

from 1% to 5% of the size of its drainage area. For example, a 25-acre watershed would

require a 1-acre wetland. Designing hydraulic

loading by analyzing existing channel discharge or watershed runoff coefficients is more

precise than the 5% rule above.

Hydraulic Retention Time: Hydraulic retention time refers to the length of time water

remains in the constructed wetland. It is closely related to hydraulic loading rate. The

treatment efficiency of a CW system is usually improved by decreasing the hydraulic

retention time; the longer the hydraulic retention time (HRT), the greater the nutrient

removal (Sakadevan and Bavor, 1999). The most effective HRT ranges from 4 to 15

d(Metcalf and Eddy Inc., 1991; Watson and Hobson, 1989). Gersberg et al. (1989) found

that even a short HRT of 3 to 6 d was effective in removing diseasecausing bacteria and

viruses. Operating with a shorter HRT means smaller land-area requirements.

The longer water remains in the wetland the greater chance ofsedimentation, adsorption,

biotic processing and retention of nutrients. Proper sizing of the wetland is important but

restricting the size of the wetland outlet is also effective. For wetlands with channel flow,

the outlet cross sectional area should be less than 1/3 that of the inlet.

Water Velocity: Peak water velocities through the wetland should not exceed1.5

feet/second. High velocities can wash out rooted vegetation and scour deposited

sediments. Keeps velocities low by regulating hydraulic loading, limiting the gradient

(slope) through the wetland, restricting the outlet size,

Creating sinuous edges and planting persistent emergent vegetation. Ideally, flow

velocities should be less than 0.6 feet/second.


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Water Depth: Water depths less than 40 inches result in greater resistance to flow and

shallow depths favor aquatic vegetation. The preferred depth range for emergent plants is

0–1.0 feet of water; for rooted surface plants, 1.0–2.0 feet of water; and for rooted

submersed plants, 1.5– 6.5 feet of water. Pools deeper

than 40 inches should also be included in the wetland design to maximize sediment

deposition and provide winter fish habitat.

Maximize Edge: Sinuous edges between the terrestrial and aquatic zones provide more

resistance to flow and more edge habitat for plants and animals. Round wetlands have the

least amount of edge per given surface area.

Minimize Edge Slope: The terrestrial-aquatic boundary should have a very gradual slope.

This allows for the establishment of a continuum of emergent species and reduces the

erosive effects of waves hitting a sharp shoreline boundary.

Persistent Emergent Vegetation: Persistent emergent vegetation has stems which persist

even after the growing season. This provides year-round resistance to water flow.

Persistent emergent plants include: cattail (Typha spp.),iris ( Iris pseudacorusor I.

versicolor ), rush ( Juncus spp.), cordgrass (Spartina

spp.), reedgrass (Calamagrostis spp.), sawgrass (Cladiumjamaicense), andswitch-grass

(Panicumvirgatum). Woody plants such as alder ( Alnus spp.),buttonbush

(Cephalanthusoccidentalis), black willow (Salix nigra), and othersare useful edge species

with persistent stems. Aquatic bed or submergent vegetation removes nutrients seasonally

but does not offer significant frictional resistance to suspended sediments.

Pre-sedimentation Basin: Many constructed wetland designs incorporate a

presedimentationbasin to trap sediments and large particulates before they enter the

wetland. This can extend the life of the constructed wetland and ultimately enhance

treatment efficiency.


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2.1.4 Maintenance of constructed wetlands

Constructed wetland planning should not overlook the need for long-term maintenance.

Additional vegetation planting may be required to speed plant coverage, replace damaged

plants or to try more suitable varieties. Perimeter fencing may be required if livestock

grazing is anticipated to be a problem.

Maintenance may be needed to control the spread of undesired plant species such as

purple loosestrife.

Inlets and outlets can become blocked with debris which will require periodic removal.

Inlet and outlet structures should be inspected weekly and especially following big storm

events. Most importantly, if the wetland functions well as a sediment and nutrient trap, it

may eventually require dredging to remove accumulated materials. Thus, vehicular

access to the site must be provided for maintenance vehicles and possibly dredging

equipment (Jones, William.W., 1995)

2.2.0TREATMENT KINETICS

The state-of-the-art in constructed treatment wetlands’ modeling consists of first-order

equations which in case of constant conditions (e.g. influent, flow and concentrations)

and an ideal plug-flow behavior predict an exponential profile between inlet and

outlet(D., Rousseau, 2005).

The wetland is designed as a channel with a limited width and can therefore be calculated

as a plug flow reactor where the wastewater moves as a front in one direction (along the

channel). Most systems in the US and Europe, whose design is based on kinetics, use a

first order plug flow model ((EPA), 1993).

The following first-order reaction equation (IWA, 2000)is used to describe pollutantremoval:

C e =Exp (-kv x HRT)……… (1)

C o


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Where: C e is the constituent concentration in the effluent (mg/l)

Co is the constituent concentration in the influent (mg/l)

k v is the volumetric decay rate (d-1)

The values of k v are obtained through regression of ln (Ce /Co) versus HRT.

Some reported kinetic data (IWA, 2000)(Kadlec, H, & Knight, 1996)are based on area:

K = kv x ε h……..( 2)

Where k is the area-based decay rate constant, ε is the porosity or space available for

water to flow through the wetland and h is the water depth(Reed et al., 1995);(IWA,

2000). Equation (1) can be modified by substituting kv from equation (2) giving; Where HLR is hydraulic loading rate.

The influent mass loading rates of constituentsis calculated as:

Where Q is the inflow rate and A is the surface area of the system

The removal efficiencies of (η%) of pollutants were calculated as:

% and the mass removal rate = IL x η


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n = number of tanks

t = HRT in the nth CSTR reactor

The tanks-in series (TIS) model is commonly used for modeling of pollutant removal in

ponds and wetlands (Kadlec, H, & Knight, 1996). The model represents a series of

continuously stirred tank reactors (CSTRs) where a substance is removed in each tank

according to first-order kinetics. The number

Of tanks, N represents the degree of mixing. High value of N means a small degree of

dispersion,

i.e., a plug flow reactor (PFR), while N = 1 defines completely mixed reactor.

2.2.2 Drawbacks of plug flow models

According to(D., Rousseau, 2005), after a research on Model based design for a pilot-

scale constructed reed bed belonging to Aqua fin NV and located in Aartselaar, Belgium;

• Calibration of the parameters k, C*, and is mostly done on the basis of inlet

and outlet concentrations, and not on the basis of transect data, although the latter

are to be preferred for calibration purposes. Because these parameters lump a

large number of other characteristics representing the complex web of

interactions in constructed treatment wetland as well as external influences like

weather conditions, a large variability can be observed in reported k, C*, and

values.

• The equations are based on the assumptions of plug flow and steady-state

conditions. However, small scale wastewater treatment plants under which most

treatment wetlands can be ranged are subject to large influent variations whereas

the larger ones are subject to hydrological influences, thus causing in both cases

non-steady state conditions.


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15

• Short-circuiting and dead zones are common phenomena in constructed treatment

wetlands causing non-ideal plug-flow conditions, thus jeopardizing the use of the

first-order model.

•

The so-called rate ‘constants’ do not seem to be constant at all but dependent on

the influent concentrations, the HLR and the water depth.

2.3.0 Rate constants: Study and literature values

Table 2.1: Rate constants; values from literature

STUDIES VOLUME-BASED 1st

ORDER MODEL

AREA-BASED 1st

ORDER MODEL

TIS MODEL

k v k 20(d-1) Ѳ k A(m/yr) k 20(d

-

1)

Ѳ k T k 20(d-1) Ѳ

Nebraska(2009) -- 0.256 1.011 24.6 -- 1.008 -- 0.353 1.015

Axler, et al(2000) -- -- -- 19.5 -- 1.071 -- -- --

IWA(2000) -- -- -- 21.9-113 -- -- -- -- --

Kadlec&

Knight(1996)

-- -- -- 180 -- -- -- -- --

Reed, et al(1995) 0.68 1.104 1.06 -- -- -- -- -- --

European

Cooper(1990)

-- 0.1 -- -- -- -- -- -- --

USEPA(1988) -- 0.86 1.06 -- -- -- -- -- --

Jing et al 0.40 -- -- -- 4.79 -- -- -- --


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16

CHAPTER 3:

3.0.0 RESEARCH METHODOLOGY

3.1.0 The pilot scale subsurface horizontal flow constructed wetland

3.1.1 Site description:

The pilot scale constructed wetland is sited within Jomo Kenyatta University of

Agriculture and Technology (JKUAT) sewage treatment works premises.(JKUAT is

located in Juja town, 40 Km North East of Nairobi City on Nairobi-Thika road at latitude

1o

10’S and longitude 37o

E, and at an altitude of 1463m above sea level).

JKUAT Sewage works treats domestic wastewater by use of a series of 5 wastewater

stabilization ponds: two primary facultative ponds, two secondary facultative ponds and

one maturation pond. The final effluent is discharged to a near-by river via a natural

water way.

A schematic diagram of the wetland set-up at the JKUAT sewage works is shown in

figure below.


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17

Figure 3.1: Schematic diagram of the pilot scale experiment set-up at the JKUAT

sewage treatment works

2

1

8

3

6

44

5

7

5

7

5

7

5

6

7

5

8

2

1

3

44

5

6

7

5

1. Raw sewa e

4. Primary facultative pond

3. Grit chamber2. Bar screens

7. Maturation pond

5. Secondary facultative pond

8. Treated effluent6. Pilot scale constructed wetland


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18

7.5m

A (vegetation and gra

B (gravel)

3m

End

Effluent

C (pond)

3m

3m

Figure 3.2: Plan layout of the pilot scale HSSF-CW.

Influent

Effluent

(Vegetation and gravel)

CELL A

(Gravel)

CELL B

(Pond)

CELL C

(Vegetation and gravel)

CELL D

3m

3m

3m

3m

Plastered reinforced

concrete base (Slope at

1%)

Gravel

materialWastewater

level

Outle

structu

Inlet structure

Vegetation


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19

3.1.2 Experiment’s set-up

Controlled quantities of wastewater and rain were the only inputs into the pilot HSSF-

CW system.

The wetland consists of four cells set in parallel, each 22.5m2

.

The wetland cells are 7.5m long and 3m wide. They have vertical sides and a bottom

horizontal slope of 1%. Cells A, B, and D were filled with gravel to a depth of 0.6m. The

gravel ranges in size from 9-37mm, with a porosity of 45 % and a hydraulic conductivity

Ks = 4050 m3

/m2

.d

The macrophyte Cyperus papyrus was introduced into cells A and D, using clumps at a

spacing of 0.75m by 0.75m. While cell B is unvegetated and cell C is a pond with no

gravel or vegetation.

The wastewater level in the wetland cells is maintained at an average depth of 0.55m

within the gravel and regulated by outlet pipes positioned at a height 0.54m from the

wetland floor.

Cells A was used in the measurements of the various parameters required for this

study.

The wetland receives a continuous feed of primary effluent from a primary facultative

pond at the JKUAT sewage works. This influent to the pilot wetland is tapped from the

primary facultative pond effluent stream at a manhole with the aid of a sluice valve. The

desired flow rate is maintained manually by regulating the sluice valve.


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20

3.2.0 SAMPLING

• Sample collection was carried out between 6:30am and 8:00am on a weekly basis

(on Wednesdays or Thursdays) during the study. Samples of the effluent and

influent flow were collected at their respective outflow and inflow points, intoclean sampling bottles.

• Care was taken not to collect deleterious materials {which would have otherwise

altered lab results} into the bottles.

• Safety gear such as an overall and safety gloves were worn to prevent contact

with the wastewater.

• The period between sampling and carrying out laboratory tests was maintained

below the allowable six hours; in so doing, preservation procedures were

unnecessary saving on both time and resources.

Plate 3.1: Student collecting samples at the HSSF-CW


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21

3.3.0 WATER QUALITY

3.3.1 TESTS

This included both on-site monitoring and laboratory tests

• Temperature: A Mercury thermometer was used on both the influent and

effluent samples collected.

• pH:A pH meter was used to monitor the pH of both the influent and effluent of

the samples collected.

• Flow rates: volumetric method was used to measure the flow rate of both

the influent and effluent.

• DO: Dissolved Oxygen titration method was carried out in the lab because a DO

meter was not available in the laboratory. See appendix 1 for procedure.

• Chemical Oxygen Demand (COD): Open reflux method was used to measure

the COD in both the influent and effluent. See appendix 1 for procedure.

Plate 3.2: Student carrying out tests in the Environmental laboratory


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22

CHAPTER 4:

4.0.0 RESULTS AND DISCUSSION

4.1.0 ENVIROMENTAL PARAMETERS RESULTS

Environmental parameters were monitored to determine their influence on COD removal.

The following environmental parameters were obtained:

Table 4.1: Environmental parameters of the HSSF-CW

Date Temp,T(0c) pH

DO

(Influent, mg/l)

DO

(Effluent, mg/l)

4/11/2009 23.5 7.3 0.38 0.42

5/11/2009 23 7.20.4 0.46

11/11/2009 21.5 7.00.48 0.68

12/11/2009 23 7.30.46 0.52

18/11/2009 23 7.20.38 0.4

19/11/2009 23 7.10.4 0.46

26/11/2009 21 6.90.44 0.56

27/01/2009 23 7.00.36 0.42

2/02/2010 22 7.20.4 0.44

3/02/2010 23 7.00.34 0.38

9/02/2010 23 7.30.42 0.48


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4.2.0 FLOW RATES

The following flow rates were obtained using the volumetric method from the

HSSF-CW:

Table 4.2: Inflow and Outflow rates

Date INFLOW (m3 /d) OTUFLOW (m

3 /d)

4/11/200913.13

7.39

5/11/2009 0.69 0.52

11/11/2009 1.04 2.25

12/11/2009 3.89 4.18

18/11/2009 0.95 1.67

19/11/2009 2.16 1.67

26/11/2009 1.91 1.86

27/01/2009 2.25 2.23

2/02/2010 1.77 1.71

3/02/2010 1.89 1.75

9/02/2010 3.55 2.59

4.2.1 Average flow rates, Hydraulic Retention Time and Hydraulic loading rate

Using table 4.2 the equations shown below were used to compute the Average

flow rates, Hydraulic Retention Time (HRT) and Hydraulic Loading Rate with the

help Ms Excel spreadsheet software:

, 2 where : inluent low rate and : efluent low rate , where :useable wetland water volume12.375m :porosity0.45 , where : wetland surface area 22.5


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24

The results obtained were as shown below:

Table 4.3:Average flow rates, Hydraulic Retention Time, Hydraulic Loading Rates

DateAverage flow rates

(Qav) (m3 /d)

Hydraulic

Retention Time

(HRT) d

Hydraulic Loading

Rate (m/yr)

4/11/2009 10.26 0.54 119.84

5/11/2009 0.60 9.21 8.41

11/11/2009 1.64 2.48 36.44

12/11/2009 4.03 1.33 67.74

18/11/2009 1.31 3.33 27.10

19/11/2009 1.91 2.91 27.10

26/11/2009 1.89 3.00 30.11

27/01/2009 2.24 2.50 36.20

2/02/2010 1.74 3.25 27.79

3/02/2010 1.82 3.18 28.37

9/02/2010 3.07 2.15 41.95


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4.3.0 COD RESULTS:

The following COD results were obtained using the standard laboratory test:

Table 4.4: Measured influent and effluent COD

Date COD in (mg/l) COD out (mg/l)

4/11/2009 326 140

5/11/2009 94 52

11/11/2009 156 114

12/11/2009 110 84

18/11/2009 84 72

19/11/2009 124 118

26/11/2009 116 94

27/01/2009 160 122

2/02/2010 52 30

3/02/2010 142 110

9/02/2010 104 88

4.3.1 COD Removal :

From Table 4.4 the following trends were be observed:

Graph 4.1: Influent COD trend

010020030000

/ 1 1 / 2 0 0

/ 1 1 / 2 0 0

1 1 / 1 1 / 2 0 0

1 2 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

2 / 0 1 / 2 0 1 0

2 / 0 1 / 2 0 1 0

2 / 2 / 2 0 1 0

3 / 2 / 2 0 1 0

/ 2 / 2 0 1 0

()

(/)


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26

The influent COD range of this system was found to be between 52 -326mg/l. The

highest COD value in the waste water could have been associated with technical

failure of the pond system (on this particular day) that pre-treats the waste water

before it gets to the constructed wetland. No particular trend was observed during

the sampling days.

Graph 4.2:Effluent COD trend

The effluent COD range was found to be between 140-50mg/l which means that

the waste water effluent had not reached the ministry of water requirements

(which is >50mg/l) and therefore required further treatment before being released

into any river. No particular trend was observed during the sampling days.

Comparing the influent COD and effluent COD the following was observed:

0

0

100

10

/ 1 1 / 2 0 0

/ 1 1 / 2 0 0

1 1 / 1 1 / 2 0 0

1 2 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

2 / 0 1 / 2 0 1 0

2 / 0 1 / 2 0 1 0

2 / 2 / 2 0 1 0

3 / 2 / 2 0 1 0

/ 2 / 2 0 1 0

()

(/)


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Graph 4.3: Influent

From the above co

each of the days s

COD removal.

The removal effici

Table 4.5: Percenta

Date CO

4/11/2009 326

5/11/2009 94

11/11/2009 156

12/11/2009 110

18/11/2009 84

19/11/2009 124

26/11/2009 116

27/01/2009 160

2/02/2010 52

3/02/2010 142

9/02/2010 104

0

0

100

10

200

20

300

30

/ 1 1 / 2 0 0

/ 1 1 / 2 0 0

and Effluent COD comparison

parisons, there was a significant amount of COD

mpling was done proving that the HSSF-CW was

ncies were as shown:

e COD removal

D in (mg/l) COD out (mg/l)

%

EFFICIENCY

140 57.06

52 44.68

114 26.92

84 23.64

72 14.29

118 4.84

94 18.97

122 23.75

30 42.31

110 22.54

88 15.38

1 2 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

2 / 0 1 / 2 0 1 0

2 / 0 1 / 2 0 1 0

2 / 2 / 2 0 1 0

3 / 2 / 2 0 1 0

/ 2 / 2 0 1 0

(

(

27

removal on

efficient in

/)

/)


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28

4.4.0 CALIBRATION OF VOLUME-BASED, AREA-BASED PLUG FLOW

MODELS ,TANK-IN-SERIES(TIS) MODEL RATE CONSTANTS

The volume- based area-based plug flow models, Tank-in-Series (TIS) model

were calibrated with the help of Ms Excel spreadsheet software. : ……………………(i) : …………………………….(ii): ………………………………(iii)

: ) Ѳ

⁄

3 ⁄ ⁄ (See appendix 2 for a step by step evaluation of the models.)

The rate constants obtained were as follows:


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29

Table 4.6: Volume-based, Area-based ,Tank-in Series rate constants

Date

Volume-based rate

constant (kV)

Area-based rate

constant (kA)

Tank-in series rate

constant (kit)

4/11/2009 1.5573 101.29265.9857

5/11/2009 0.0643 4.97894.4231

11/11/2009 0.1265 11.43023.1053

12/11/2009 0.2022 18.26812.9286

18/11/2009 0.0462 4.17712.5000

19/11/2009 0.0171 1.34402.1525

26/11/2009 0.0701 6.33132.7021

27/01/2009 0.1086 9.81482.9344

2/02/2010 0.1692 15.2877

4.2000

3/02/2010 0.0802 7.24452.8727

9/02/2010 0.0776 7.00722.5455


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30

4.5.0 K 20 AND ѲVALUESTo obtain k 20 and Ѳ values, the following equation was used and regression donewith the help of Ms Excel spreadsheet software.

// Ѳ………………………………………………………….(iv): Ѳ 20 (See appendix 2 for a step by step evaluation of the models)

The and Ѳ values obtained are as shown:Table 4.7: and Ѳ values

Constants Volume-basedrate constant(kava)

Area-based rateconstant(kA)

Tank-in-seriesrate constant(kit) 0.0573 5.789 2.683Ѳ 1.256 1.177 1.064

Significance of k 20 and ѳ values obtained:

From table 4.7 it was observed that the Ѳ values obtained were greater than 1. This

proved that the organic removal in the HSSF-CW is actually microbial. If the

value was= 1, the process usually is physical (Filtration, sedimentation).It was

further observed (using equation iv) that:

• On doubling/increasing the Ѳ value, a higher k V/A/T value was obtained.

Reducing the value significantly (0.12….values) a lower k V/A/T value was

obtained.

• On doubling/increasing the K 20 value, a higher k V/A/T value was obtained.

On halving the value, a lower k V/A/T value was obtained.

•

When the temperature value was increased, a higher k V/A/T value wasobtained and vies-versa.

This means that the temperature effects were very significant in the COD

removal process. (i.e. the higher the temperature the more the rate of

purification and vies-versa)


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4.6.0 COMPARISON OF THE K A/V/T K20 AND Ѳ ѲѲ Ѳ VALUES FROM THIS

STUDY AND THOSE OBTAINED FROM LITERUAURE

The comparison was as shown:

Table 1.8: Rate constant values from this study and other studies

STUDIES VOLUME-BASED

1st ORDER MODEL

AREA-BASED 1st

ORDER MODEL

TIS MODEL

k v k 20(d-

1)

Ѳ k A(m/y

r)

k 20(

d-1

)

Ѳ k T k 20(

d-1

)

Ѳ

This

study(2010)

0.22

9

0.057

3

1.25

6

17.016 5.78

9

1.17

7

3.304

5

2.68

3

1.06

4

Nebraska(200

9)

-- 0.256 1.01

1

24.6 -- 1.00

8

-- 0.35

3

1.01

5

Axler, et

al(2000)

-- -- -- 19.5 -- 1.07

1

-- -- --

IWA(2000) -- -- -- 21.9-113

-- -- -- -- --

Kadlec&

Knight(1996)

-- -- -- 180 -- -- -- -- --

Reed, et

al(1995)

0.68 1.104 1.06 -- -- -- -- -- --

European

Cooper(1990)

-- 0.1 -- -- -- -- -- -- --

USEPA(1988

)

-- 0.86 1.06 -- -- -- -- -- --

Jing et al 0.40 -- -- -- 4.79 -- -- -- --


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Lower kv, kA and their corresponding k20 values was obtained in this study as

compared to other studies. This was contrary to the hypothesis that a larger

kvalues would be expected in the tropics.

This could have been due to:

• The HSSF-CW being used for the study had undergone

renovations a few months before the study began and therefore

the system was not fully developed. Usually a minimum period of

SIX months of operation is recommended. During the course of

the research, the system was about FOUR months old.

The other studies on the other hand could have been carried out

on mature systems. Mature systems have a well developed root

system which provides sufficient room for biofill attachment.

Colonies are much larger and the biofilm is mature.

• The other studies could have also been carried out during the

summer period when microbial activities are much more

enhanced. (The temperatures for this study ranged between 20-

24oC. Summer temperatures on the other hand are much higher

than this.)

The rate constants obtained were however comparable with values from other

studies.

There was not much data available for comparison on studies done for Tank-in

Series models. The little data obtained however showed that the k value obtained

for TIS model in this study was higher. This is not very conclusive as more

studies are needed for concrete conclusions to be made.


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4.7.0 INFLUENC

4.7.1 AERATION

COD removal fro

higher the degrad

purification proces

A comparison of th

Graph 4.4: Influent

From the above co

on each of the d

effective.

0

0.1

0.2

0.3

0.

0.

0.

0.

( )

OF ENVIRONMENTAL PARAMETERS

ROCESS

waste water is aerobic. The higher the oxygen

ation rates. Plants add oxygen into the system

.

e influent DO and effluent DO was as shown:

and Effluent DO comparison

mparisons, there was a significant amount of oxyg

ys sampling was done proving that the system

/ 1 1 / 2 0 0

1 1 / 1 1 / 2 0 0

1 / 1 1 / 2 0 0

2 / 0 1 / 2 0 1 0

2 / 2 / 2 0 1 0

/ 2 / 2 0 1 0

(

(

33

levels, the

during the

en addition

was very

/)

/)


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34

4.7.2 EFFECTS OF TEMPERATURE ON %COD REMOVAL

A scatter plot of %COD removal against temperature was as shown:

Graph 4.5: % COD removal vs Temperature

The correlation between %COD removal and Temperature was found to be

10.6%. The temperature range was between 20-24oC. Temperature data was not

enough to reveal a trend.

0.10

0.00

10.00

20.00

30.00

0.00

0.00

0.00

0.00

0.00

20 21 22 23 2

%

%

1

(1)


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35

4.7.3 EFFECTS HYDRAULIC RETENTION TIME (HRT) ON %COD

REMOVAL

A scatter plot of %COD removal against HRT was as shown:

Graph 4.6: % COD removal vs HRT

The correlation between %COD removal and HRT was found to be 34.5%. This

was fairly correlated.

It was further observed, that an increase Hydraulic Retention Time (HRT) led to

an increase in %COD removal.

0.3

0.00

.0010.00

1.00

20.00

2.00

30.00

3.00

0.00

.00

0.00

0.0000 .0000 10.0000

%

, ()

%

1

(1)


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36

4.7.4 EFFECTS OF P H ON % COD REMOVAL

pH an important factor that affects the microbiological processes. The optimal

range fluctuates for the different processes but generally varies between 7.0 and

8.5. This treatment system buffers itself.

A scatter plot of %COD removal against pH was as shown:

Graph 4.7: % COD removal vs pH

The correlation between %COD removal and pH was found to be 15.7%. It was

further observed, that the pH levels were circum-neutral.

0.1

0.00

10.00

20.00

30.00

0.00

0.00

0.00

0.00

0.00

. . .0 .1 .2 .3 .

%

%

1

(1)


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37

CHAPTER 5

5.0.0 PREDICTIONS OF EFFLUENT COD USING VOLUME-

BASED, AREA-BASED PLUG FLOW MODELS AND TANK-INSERIES MODELS

The volume-based, Area-based plug flow models and Tank-in series models were

used to predict effluent COD using the obtained k v , k A and k T values.

5.1.0Volume-based plug flow model effluent COD predictions

The average of k v values was calculated from table 4.6.

Equation(i) was used to predict effluent COD (mg/l)

N/B: The average kv was kept constant, Cin (mg/l) and HRT (t) were varied

accordingly.

The predicted effluent COD (mg/l) values from the volume -based plug flow

model are as shown below and compared with the measured effluent COD (mg/l)

values:


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38

Table 5.1: Predicted effluent COD and Measured effluent COD

Date Predicted effluent

COD (mg/l)

Measured effluent

COD (mg/l)

4/11/2009 287.90 140

5/11/2009 11.41 52

11/11/2009 88.43 114

12/11/2009 81.05 84

18/11/2009 39.15 72

19/11/2009 63.72 118

26/11/2009 58.35 94

27/01/2009 90.35 122

2/02/2010 24.70 30

3/02/2010 68.49 110

9/02/2010 63.51 88

Graph 5.1: Measured effluent COD vs. Predicted effluent COD

Using Ms Excel software, the correlation between the predicted and measured

effluent COD values using the volume-based plug flow model was found to be

49.6%

0.0

0

100

10

200

0 0 100 10 200 20 300 30

( / )

(/)

()

()


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39

5.2.0Area-based plug flow model effluent COD predictions

The average of k A values was calculated from table 4.6.

Equation(ii) was used to predict Cout (mg/l)

N/B: The average k A was kept constant;Cin (mg/l) and Hydraulic Loading Time

(q) were varied accordingly. y=1

The predicted effluent COD (mg/l) values from the area -based plug flow model

are as shown below and compared with the measured effluent COD (mg/l) values:



COD (mg/l)

Measured effluent

COD (mg/l)

4/11/2009 282.85 140

5/11/2009 12.43 52

11/11/2009 97.80 114

12/11/2009 85.57 84

18/11/2009 44.83 72

19/11/2009 66.18 118

26/11/2009 65.92 94

27/01/2009 99.99 122

2/02/2010 28.19 30

3/02/2010 77.95 110

9/02/2010 69.32 88


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40

Graph5.2: Measured effluent COD vs. Predicted effluent COD


effluent COD values using the Area-based plug flow model was found to be

52.9%

5.3.0Tank-in Series model effluent COD predictions

The average of kT values was calculated from table 4.6 .

Equation(iii) was used to predict Cout (mg/l)

N/B: The average kT was kept constant,Cin (mg/l) and HRT (t) were varied

accordingly. n=3

The predicted effluent COD (mg/l) values from the Tank-in Series model are as

shown below and compared with the measured effluent COD (mg/l) values:

0.2

0 0 100 10 200 20 300

( / )

(/)

()

()


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41



COD (mg/l)

Measured effluent

COD (mg/l)

4/11/2009 189.00 140

5/11/2009 1.12 52

11/11/2009 22.38 114

12/11/2009 33.28 84

18/11/2009 7.64 72

19/11/2009 14.03 118

26/11/2009 12.50 94

27/01/2009 22.50 122

2/02/2010 4.93 30

3/02/2010 13.91 110

9/02/2010 18.11 88

Graph 5.3:Measured effluent COD vs Predicted effluent COD


effluent COD values using the Tank-in series model was found to be 31.1%

0.3110

0

100

10

200

0 0 100 10 200

( / )

()

() ()


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42

Significance of effluent COD predictions made from the 3 models

This showed that the Area-based 1st order plug flow model produced the best

predictions (the predicted effluent COD had a correlation of 52.9% with the

measured effluent COD). This could only mean one thing, that Hydraulic Loading

Rate (q) is a very important factor in the performance of the HSSF-CW. The

wetland surface area and inflow rates should be well taken care of during the

design and construction of the HSSF-CW.

It was further observed that the Measured effluent COD was generally higher than

the Predicted effluent COD. This meant that the models predicted cleaner effluent.

This could be disadvantageous during design of the systems using the models as

the probability of under designing would be high.


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CHAPTER 6

6.0.0 CONCLUSIONS AND RECOMMENDATIONS

6.1.0 Conclusions

From the results and subsequent discussion, the following has been concluded.

• The horizontal subsurface flow constructed wetlands have great

potential in secondary treatment of domestic wastewater because a

relatively significant removal of COD was observed during the study.

The k values obtained were also comparable to those found in

literature.

• All the three models produced good predictions.

The Area-based first order plug flow model however, produced the

best effluent COD predictions.

• The Area-based and Volume-based plug flow models were easy to

calibrate, unlike Tank-in series model where value of ‘n’ had to be

found first.

6.2.0 Recommendations

• Further research on the same is required in the tropics but this time on a

mature system. Annual evaluation of the tropics shows that they have a

higher energy turnover all season. The HSSF-CW should therefore be

very effective in the tropics as the purification process is temperature

dependent.

•

Further research should be done on the same but this time:

Using Higher Hydraulic Loading Rates than this study. (They

were between 120-8 m/yr for this study).

Using longer study duration.

Using different vegetation (Cyperus papyrus were used for this

study)


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Covering many seasons (dry- wet seasons) where a wide range

of temperature would be used.

• More studies should be done on the Tank-in series model. The 1st order

plug-flow models have a limitation as they ignore the mixing element in

the system assuming that what goes in first into the system is the first to

go out. Short-circuiting and dead zones are common phenomena in

constructed treatment wetlands causing non-ideal plug-flow conditions,

thus jeopardizing the use of the first-order model.


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Bibliography

1.

(EPA), U. S. (1993). Subsurface flow constructed wetland for wastewater

treatment:A technology assessment.

2.

Bruggen, J. (20008). Wetlands for Water Quality Module. Delft, Netherlands.

3.

Campbell, C., & M.H., O. (1999). Constructed wetlands in the sustainable

landscape. New York: J. Wiley.

4.

Connors, K. A. (1991). Chemical kinetics ,the study of reaction rates in solutions.

Michigan: VCH .

5. D, R., D, G., PA, V., & N, D. P. (2002). Short- term behaviour of constructed

reed-beds: pilot plant experiments under different temperature conditions. Eigth

International Conference on Wetland systems for water pollution, (pp. 5-19).

Arusha, Tanzania.

6.

D., Rousseau. (2005). Performance of constructed wetlands: Model based

evaluation and impact of operation and maintenance. Ghent, Belgium: Ghent

university.

7.

Hammer, & Donald. (1992). Creating Freshwater Wetlands. Michigan: Ann

Arbor.

8.

Hammer, D., & Bastian, R. (1989). Wetland Ecosystems: Natural water purifiers.

Michigan: Lewis publishers.

9.

IWA. (2000). Activated Sludge Models.

10.

Jones, William.W. (1995). Design features of constructed wetland for non-point

source treatment. Bloomington,Indiana: Indiana University.

11. Kadlec, H, R., & Knight, R. L. (1996). Treatment Wetlands. Michigan: Ann

Arbor.

12. M.F.Dahab, P., & Wenxin (Emy) Liu, P. (2009). Eavaluation o fFirst Order

Kinetics in Subsurface Flow Constructed Wetlands Treatment. Lincoln,NE:

University of Nebraska,Iowa Department of Natural Resourses.


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13. Marble, & D, A. (1992). A Guide to Wetland Functional Design. Michigan: Ann

Arbor.

14.

Mitsch, W., & Gosselink, J. (1993). Wetlands. Newyork.

15.

Persson, J. (2001). How hydraulic conditions enhance water quality in Wetlands.

16.

Risper, O. (2009). Evaluation of the performance of Horizontal Subsurface Flow

performing secondary treatment of domestic wastewater. Nairobi.

17.

Schmidt, & D., L. (1998). The Engineering of chemical reactions. New York:

Oxford University .

18.

Thunhorst, & A., G. (1993). Wetland Planting Guide for the Northeastern United

States-Plants for Wetland creation,Restoration and Enhancement. Maryland:

Environmental Concern Inc.


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APPENDIX 1:

Laboratory tests procedures:

Open reflux method.

Introduction: it is used as a measure of the oxygen equivalent of the organic matter

content of a s ample that is susceptible to oxidation by strong chemical oxidant. The test

is usually for monitoring and control of wastewater processes.

Procedure:

1. 50ml of the sample is placed in 500ml refluxing flask.

2. 1gm of mercuric sulphate, several glass beads are added. 5ml of

H2SO4 (concentrated sulphuric acid) was added slowly while mixing

to dissolve mercuric acid.

3.

The sample is mixed while cooling to avoid possible losses of

volatile materials.

4.

25ml of 0.0471M k 2cr2o7 solution is added and mixed thoroughly.

5. The samples are then transferred to the liebig condenser apparatus.

The remaining sulphuric acid reagent (70ml) is then added through

the open end of the condenser.

6.

The samples are continuously stirred while adding the acid reagent.

7. The reflux mixture is mixed thoroughly by applying heat to prevent

local heating of the flask bottom and a possible blow out of the flask

contents.

8.

The open end of the condenser is covered with a foil to prevent

foreign materials from entering the refluxing mixture.

9.

Refluxing is then carried out for 2hours.

10.

The condenser is thereafter cooled and washed down using distilled

water.

11.

The reflux is then disconnected and diluted to about twice its original

volume with distilled water.

12.

This mixture is then cooled to room temperature (due to the

exothermic nature of the reaction). The excess k 2cr2o7 is then titrated

using FAS using 0.1-0.15ml (2 to 3 drops) ferroin indicator.


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13. The same volume of ferroin indicator is used for all titrations.

14. The end point of the reaction is taken as the first sharp change in

colour, from blue-green to reddish brown.

15. Similarly a blank of equal volume to sample, distilled water is

titrated against FAS.

Calculations:

COD = ((A-B) x m x 8000)

ml of sample

Where: A = ml , FAS used for blank water

B = ml, FAS used for sample water

m = molarity of the FAS

Dissolved Oxygen-titration method.

Introduction: It is required for supporting fish and aquatic life in water. When it reaches

the level below 2mg/l some aquatic life die.

Reagents:

1.

Manganous sulphate solution. Mnso4.7h20.

2.

Alkaline azide iodide solution.

3.

Sulphuric acid solution.

4.

Starch indicator.

5. Standard sodium thiosulphate solution, 0.025N

Procedure

1.

200ml of the sample is collected in a beaker.

2.

1ml of manganous sulphate is added to the sample while stirring. A

pipette was used during the experiment.


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3. 1ml of alkaline azide iodide is then added to the sample whilst stirring.Some precipitates form.

4. 1ml of concentrated sulphuric acid is then added to the mixture. The

sulphuric acid dissolves the precipitates.

5.

5 drops of starch indicator are added to the mixture. At that point themixture turns blue-black colour.

6.

The mixture is then titrated against standard sodium thiosulphate

solution.

Calculations:

The 200ml of solution taken for titration corresponds to 200ml of the

original sample, because 1ml of 0.025N sodium thiosulphate solution titrantis equivalent to 0.2mg DO, each millilitre of sodium thiosulphate titrant is

equivalent to 1mg/ltr DO. When volume equal to 200ml of the original

sample is titrated therefore the DO concentration is ml of titrant used under

the above conditions.


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APPENDIX 2:

CALIBRATION VOLUME-BASED, AREA-BASED PLUG FLOW MODELS,TANK-

IN-SERIES(TIS) MODEL RATE CONSTANTS

The step by step evaluation was shown:

• : ……………………(i) : : 1

• : …………………………….(ii): : • : ………………………………(iii)

:

:


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EVALUATION OF K 20 AND Ѳ ѲѲ Ѳ VALUES

The step by step evaluation was shown:// Ѳ………………………………………………………….(iv)Introducing natural log ( Ln) on both sides of equation ( iv):

// Ѳ………………………………………….(v) Equation (v) was compared with where the Ln of k values from all themodels were plotted in the y-axis, T -20 valuesin the x-axis, Ln Ѳ as the slope of the

graph and as the y-intercept.The graphs are shown below:

Graph a: Ln kvvs Ln (T-20)

Using equation0.2282.860; . . Ѳ . .

0.22 2.0.0000

.0000

3.0000

2.0000

1.0000

0.0000

1.0000

0 1 2 3

()

()

1

(1)


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Graph b:Ln kA vs Ln (T-20)

Using equation0.1631.756; . . Ѳ . . Graph c:Ln kT vs Ln (T-20)

Using equation0.0620.987; . . Ѳ . .

0.13 + 1.0.0000

1.0000

2.0000

3.0000

.0000

.0000

0 1 2 3

(20)

()

1

(1)

0.02 + 0.0.0000

0.000

1.0000

1.000

2.0000

0 1 2 3

()

()

1

(1)


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APPENDIX 3:

PLATES:

Platea: Student collecting samples at the influent point of the HSSF-CW


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Plate b:Student collecting samples at the effluent point of the HSSF-CW

Plate c: Student testing samples for COD in the Environmental laboratory

(Winnie) Final Year Project Report

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