SCHOOL OF GRADUATES STUDIES

FACULTY OF SCIENCE

ENVIRONMENTAL SCIENCE PROGRAM

EVALUATION OF SELECTED PLANT SPECIES FOR THE

TREATMENT OF TANNERY EFFLUENT IN A CONSTRUCTED

WETLAND SYSTEM

By: Asaye Ketema

Advisor: Seyoum Leta (PhD.)

A thesis submitted to Graduate Studies of Addis Ababa University in Partial fulfillment

of the requirements for the Degree of Master of Science in Environmental Science

Addis Ababa, Ethiopia

August, 2009

ii

Acknowledgments

First and foremost, I would like to express my gratitude to Dr. Seyoum Leta, my research

advisor, for his continuous guidance, valuable suggestions and encouragement in making this

thesis possible.

I would also like to thank w/o Adey Feleke for her enormous support and help whenever I

needed her.

I would like also express my appreciation to BIO-EARN for providing me financial support

that enabled me to perform the hard tasks related with this research works in a sound way.

Sincere thanks to Modjo tannery industries for allowing me to construct the pilot plant and

take effluent sample throughout the study. I also extend my deep appreciation to staff

members of Applied Microbiology Laboratory of Biology Department, Addis Ababa

University, for their cooperation and permission to use their laboratory.

Last but not least, I would like to thanks my family and friends for their encouragement and

support from the beginning to end of my post graduate study.

iii

Table of Contents

Acknowledgments ...................................................................................................................... ii

Table of Contents ...................................................................................................................... iii

List of Figures ............................................................................................................................ v

List of Tables ............................................................................................................................. vi

List of Annex ............................................................................................................................ vii

Acronyms ................................................................................................................................ viii

Abstract ..................................................................................................................................... ix

1. Introduction ............................................................................................................................ 1

2. Literature Review ................................................................................................................... 5

2.1. Sources of wastewater in tanning industry ...................................................................... 5

2.2. Tannery wastewater characteristics ................................................................................. 8

2.3. Environmental impact ..................................................................................................... 9

2.4. Tannery wastewater treatment system .......................................................................... 11

2.4.1. Physico-chemical treatment system ....................................................................... 11

2.4.2. Biological treatment system ................................................................................... 12

2.4.2.1. Trickling filter ................................................................................................. 12

2.4.2.2. Activated sludge .............................................................................................. 12

2.4.2.3. Sequencing batch reactor (SBR) ..................................................................... 13

2.4.2.4. Wetland ........................................................................................................... 13

2.4.2.4.1. Constructed wetland technology .............................................................. 14

2.4.2.4.1.1. Free Water Surface (FWS) Wetland ................................................. 17

2.4.2.4.1.2. Subsurface Flow (SSF) wetland ........................................................ 18

2.4.2.4.2. Role of plants in constructed wetland ...................................................... 21

3. Materials and Methods ......................................................................................................... 23

3.1. Site Description ............................................................................................................. 23

3.2. Pilot constructed wetland establishment ....................................................................... 24

3.3. Plant material and experimental start-up ....................................................................... 24

3.4. Wastewater sampling and analysis ................................................................................ 26

iv

3.5. Statistical data analysis .................................................................................................. 28

4. Results and Discussion ......................................................................................................... 29

4.1. BOD5, COD and TSS removal ...................................................................................... 29

4.2. Nutrient Removal .......................................................................................................... 31

4.3. Sulfate and sulfide removal ........................................................................................... 34

4.4. Chromium removal ....................................................................................................... 36

4.5. TDS and electrical conductivity removal ...................................................................... 37

5. Conclusion and Recommendations ...................................................................................... 40

5.1. Conclusion ..................................................................................................................... 40

5.2. Recommendations ......................................................................................................... 41

6. Reference .............................................................................................................................. 42

Annex ....................................................................................................................................... 53

v

List of Figures

Figure 1.1. Point where Modjo tannery wastewater enters the Modjo River ............................. 2

Figure 2.1. General tanning process flow chart ......................................................................... 7

Figure 2.2. Free water surface wetland .................................................................................... 17

Figure 2.3. Subsurface flow wetland ........................................................................................ 18

Figure 3.1. Map of the study area. ............................................................................................ 23

Figure 3.2. Different stage of the subsurface wetland construction ......................................... 24

Figure 3.3. The different plants used for the constructed wetland after 3 months. .................. 25

Figure 3.4. Schematic diagram of the pilot scale subsurface flow constructed wetland .......... 27

vi

List of Tables

Table 2.1. Water consumption in individual tannery processing operations ............................ 8

Table 2.2. Summary of pollution load discharged in effluents from individual tannery

processing operations (Kg/tone of hide and skin) ...................................................................... 9

Table 3.1. Percentage ratio used to dilute the wastewater with tap water ................................ 25

Table 4.1. Average influent and effluent concentrations (Mean ± SE) of BOD5, COD and

TSS, and removal percentage of each cell ............................................................................... 29

Table 4.3. Average influent and effluent concentrations (Mean ± SE) of TN, NH4+ and NO3

-

and removal percentage of each cell ........................................................................................ 32

Table 4.5. Average influent and effluent concentration (Mean ± SE) of sulfate and sulfide, and

removal percentage of each cell ............................................................................................... 34

Table 4.7. Average influent and effluent concentration (Mean ± SE) of total chromium, and

removal percentage of each cell ............................................................................................... 36

Table 4.9. Average influent and effluent concentrations (Mean ± SE) of electrical

conductivity and TDS, and removal percentage of each cell ................................................... 38

vii

List of Annex

Annex 1. List of tannery industries around the Modjo River ................................................... 53

Annex 2. Modjo tannery influent characteristics during the study period ............................... 54

Annex 3. EEPA tannery wastewater emission standards to inland water. ............................... 55

viii

Acronyms

BOD5 Biological oxygen demand

COD Chemical oxygen demand

CW Constructed wetland

EEPA Ethiopia Environmental Protection Authority

FWS Free water surface

HLR Hydraulic loading rate

HRT Hydraulic retention time

SSF Subsurface flow

TDS Total dissolved solid

TN Total nitrogen

TSS Total suspended solid

USEPA United States Environmental Protection Authority

ix

Abstract

Constructed wetland is man made system aiming at simulating the treatment process in

natural wetland by cultivating emergent, submerged and floating plants on sand, gravel or soil

media for human use and benefits. In this study, the performance efficiency of emergent

plants in subsurface flow constructed wetland for treating tannery wastewater was evaluated

using six locally available selected plant species such as, Cyperus papyrus, Typha

domingensis, Cyperus alopcuroides, Schenoplectus corymbosus, Sesbania sesban,

Aeschynomene elaphroxylon. The seventh cell was unplanted and used as a control. The

treatment performances of each cell were assessed for selected parameters such as, BOD5,

COD, TSS, TN, NO3-, NH4

+, sulfide, sulfate, total chromium, TDS and electrical

conductivity. High removal efficiency for total Cr (98.4%), COD (68.7%) and sulfide (59.2%)

was observed in cell planted with Schenoplectus corymbosus. Whereas cell planted with

Sesbania sesban showed high removal efficiency for sulfate (96.3%), BOD5 (84.7%) and TN

(58.3%), while cell planted with Cyperus papyrus showed higher removal efficiency for NO3-

(73.2%) and NH4+ (26.2%). In addition, cell planted with Typha domingensis showed good

removal efficiency for BOD5 (84.2%), NO3- (69.1%), COD (57.9%), TN (53.2%), and NH4

+

(21.1%). Generally, with six months of operation, cell planted with Schenoplectus

corymbosus, Cyperus alopcuroides Typha domingensis and Sesbania sesban showed high

removal efficiency for the selected parameters indicating that these plants are a potential

candidate for large scale tannery wastewater treatment. However, evaluation of the system

over longer period is required before concluding whether these plants in subsurface

constructed wetland are efficient for primary treatment of tannery wastewater.

Keywords: Constructed wetland; wetland plants; wastewater treatment

1

1. Introduction

Leather tanneries, produces three different of wastes of which wastewater is the most

important challenge to the environment. The tanning process is almost wholly a wet process

that consumes high amount of water, that are estimated to be 34 – 56 m3 of water per tone of

hides or skin processed (Ludvick, 2000), where of the total water consumed, 85% is

discharged as a wastewater (Teodorescue and Gaidau, 2007). Apart from discharging high

BOD organic waste, the different chemical used in the tanning process characterized the

waste to be highly colored turbid and toxic.

Currently there are more than 20 tanning industries operating in Ethiopia and only 10% of the

existing tanning industries treat their wastewater to any degree, while the majority (90%)

discharges their wastewater into nearby bodies, streams and open land without any kind of

treatment ( EEPA, 2001; Seyoum Leta et al., 2003). Out of the tanneries existing in the

country, 14 are located along the Modjo river, where only the two, Colba and Ethiopia Share

company, treat their waste to certain level and the rest 12 discharge their wastewater into the

Modjo river.

The characteristics of the wastewater vary considerably from tannery to tannery depending

upon the size of the tannery, chemicals used for the specific process, amount of water used

and type of final product produced by a tannery. According to a study conducted by Seyoum

Leta et al., (2003), a composite tannery wastewater have BOD5 (1900 - 4800 mg/l), COD

(7900 - 15200 mg/l), sulfide (325 - 930 mg/l) and total chromium (12 - 64 mg/l). Another

study in Pakistan also indicated BOD5 (840 - 18620 mg/l), COD (1320 - 54000 mg/l), SS (220

- 1610 mg/l), TN (236 - 350 mg/l), sulfate (800 - 6480 mg/l), sulfide (800 - 6480 mg/l) and

chromium (41 - 133 mg/l) (Haydar et al., 2007). A study conducted by Aklilu Tilahun (2008)

at Modjo River indicated a BOD5 load of 204.52 ± 73.11 mg/l, COD of 261.25 ± 85.41 mg/l,

TSS of 295.25 ± 132.03 mg/l, electrical conductivity of 4543.5 ± 3719.07 µs/cm, which is

higher than the expected surface water quality standard, which is less than or equal to 5 mg/l,

50 mg/l and 1000 µs/cm for BOD5, TSS and electrical conductivity respectively (EEPA,

2003a).

2

Figure 1.1. Point where Modjo tannery wastewater enters the Modjo River

The discharges of untreated wastewater to the aquatic environment can result in the

accumulation of pollutants. The consequence of this accumulation could result in loss of

lively hood, loss of biodiversity and degradation of water quality, which in general affects the

ecosystem. In order to comply with environmental legislation and to improve the

competitiveness of the leather sector, the treatment of wastewater is not only desirable but

also necessary to correct wastewater characteristics in such a way that the use or final disposal

of the treated effluents can take place without causing an adverse impact on the receiving

water bodies.

Several methods have been used for the treatment of tannery wastewater. This includes

physico-chemical and biological treatment methods. The most commonly used physico-

chemical system includes screening, sedimentation, and the chemical system are chemical

precipitation, adsorption, disinfection dechlorination, whereas the biological includes pond

system trickling filter, activated sludge process and sequencing batch reactor. The chemical

system have inherent disadvantage of second pollution problems that will arise because of

chemical use. Generally, the above mentioned treatment system also require high initial or

operating cost, skilled man power and electricity.

3

Being low-cost and low technology system, eco-technological system, like constructed

wetland, are standing as the potential alternative on supplementary systems for the treatment

of municipal, industrial, agricultural wastewater (Kadlec and Knight, 1996). Constructed

wetland is based on natural processes involving complex and concerted interactions between

the plants, substrata and inherent microbial community to accomplish wastewater treatment in

a more controlled and predictable manner (Benham and Mote, 1999; Joseph, 2005).

Compared with conventional treatment system, constructed wetlands, which are of low cost,

easily operated and maintained, can be potentially applied in developing countries with

serious water pollution problems as an alternative treatment system. Study conducted in

Ireland by Reddy (2004) showed that the cost of a typical constructed wetland with a size of

4650 m2 is about $122000 which was cheaper by 30% than conventional treatment methods

of the same size considering the lifespan (which is 30 - 50 years) and replacement value of the

wetland. The above case studies also confirmed that maintenance cost for constructed wetland

was eight times lower than the conventional treatment system.

In constructed wetlands, vegetation plays a partial role during the treatment process, because

it helps in supplying oxygen to the microorganisms in the rhizosphere, reduce the amount of

nutrients in the system by uptake and perhaps provide more surface area in the rhizosphere for

the microorganisms (Brix, 1987). World wide, the most frequently used emergent wetland

plants are cattails (Typha latifolia), bulrush (Scripus lacustris) and reed (Phragmitus

australis) (Brix, 1993), but other species may also be more efficient under site specific

condition.

Generally, constructed wetlands are designed to maximize the physical, chemical and

biological abilities of natural wetland to reduce the biochemical oxygen demand (BOD), total

suspended solid (TSS), total nitrogen (TN), phosphorus and pathogen as wastewater flows

slowly through the vegetated surface (Reed et al., 1987; Reed, 1993). Different researchers

have investigated the wide use of constructed wetland for different type of wastewater,

including domestic (Kaseava, 2003; Joseph, 2005), industrial (Prabu and Udayasoorian, 2003;

Maine et al., 2006; Sohsalam et al., 2007; Cristina et al., 2007), agriculture run off (Forbes et

4

al., 2004), dairy (Pucci et al., 2000) and polluted river water (Jing et al., 2001; Li et al.,

2007), and have shown significant improvements in water quality in these systems.

While constructed wetlands have such a proven effectiveness for treatment of a variety of

wastewater (Hester and Harrison, 1995; Joseph, 2005; Muhammad et al., 2004), there is a

limited work in Ethiopia where the concept of constructed wetlands for wastewater treatment

is still a relatively new idea. The construction of constructed wetland requires a preliminary

efficiency study using small scale system that should be carefully designed and run to test the

capabilities in purifying tannery wastewater.

General objective

To evaluate the removal efficiency of six locally available selected plant species for the

treatment of tannery effluent using subsurface flow constructed wetland.

Specific objective

- Establishing a subsurface flow constructed wetland.

- Evaluating the treatment performance of the different plants based on selected

physico-chemical parameters such as, pH, temperature, BOD5, COD, TSS, TN,

NO3-, NH4

+, sulfide, sulfate, total chromium, TDS and electrical conductivity.

5

2. Literature Review

2.1. Sources of wastewater in tanning industry

The conversion of animal hide and skin into useful artifacts may be man’s oldest technology.

Untreated skins have limited value, because when wet they are susceptible to bacteria attack

and so they putrefy, but if they are dried they become inflexible and useless for purpose such

as clothing. Those effects are eliminated by tanning, a process by which putrefy able

biological material is converted into a stable material which is resistance to microbial attack

and has enhanced to wet and dry heat (Anthony, 1997).

Tanning consists of a series of successive operation converting raw hide and skin into leather.

The raw material in the production of leather is a byproduct of the meat industry. Tanners

recover the hide and skin from the slaughter houses and transform them into a stable material

that can be used in the manufacture of a wide range of products. Leather is an intermediate

industrial product with numerous applications in down stream sector. It can be cut and

assembled into shoes, clothing, leather good furniture and other items of daily use

(COTANCE, 2002). In addition, the produced products have distinct properties, which

include stability, appearance, water and abrasion resistance, temperature resistance and

elasticity, to help increase the shelf life.

The production process in a tannery can be split into four main categories, namely, beam

house, tanning (tan- yard), post-tanning and finishing operation. The beam house operation

consists of a sequence of process, soaking is the first stage where the hide and skin are soaked

in order to remove salt, restore the moisture content of the hide and skin and remove any

foreign material such as dirt and manure. Dehairing, liming and fleshing subsequently follows

the soaking step, which helps to remove the hair, open up the collagen structure by removing

interstitial material and remove access tissue from the interior of the hide and skin

respectively.

6

Tanning (Tan-Yard) operation is where the collagen fiber is stabilized by the tanning agents

so that the hide and skin would be no longer susceptible to putrefaction. In order to do so, the

hide and skin pass through different process in tanning operation. Those operations include

delimming, bating, pickling and tanning. Bating and pickling condition the skin and hide to

receive the tanning agent, where as the tanning process stabilize the material and impart basic

properties of the skin and hide.

Post tanning sometimes also called retanning operation involves, neutralization followed by

retanning, dyeing and fat liquoring. Neutralization and retanning are used to improve the feel

and handle of leather, where as dyeing and fat liquoring provide special property to the leather

(i.e. water resistance, abrasion resistance, flame retardancy and anti-electrostatic properties)

replenish oil to the hide and skin, and to give the leather different colors. Further more; the fat

liquoring help to lubricate the leather to achieve product specific characteristics and to re-

establish the fat content in the previous procedures.

After dyeing the leather from the post tanning operation, the finishing operation follows. The

overall objective of finishing operation is to attain final product specification by enhancing

the appearance of the end providing the performance characteristics with respect to color,

gloss, handle etc… The overall schematic diagram of the tanning process is presented in

Figure 2.1.

7

Figure 2.1. General tanning process flow chart (Gupta, 2003)

8

2.2. Tannery wastewater characteristics

The tanning process is almost wholly a wet process that consumes high amount of water,

estimated to be 34 – 56 m3 of water per tone of hide or skin processed (Table 2.1) (Ludvick,

2000), where out of the total water consumed, 85% is discharged as a wastewater

(Teodorescue and Gaidau, 2007).

Table 2.1. Water consumption in individual tannery processing operations

Operation Water consumption (m3/tone raw hide and skin)

Soaking 7 - 9

Liming 9 - 15

Deliming and Bating 7 - 11

Tanning 3 - 5

Post tanning 7 - 13

Finishing 1 - 3

Total 34 - 56

Source: Ludvick, 2000

The characteristics of the wastewater vary considerably from tannery to tannery depending

upon the size of the tannery, chemicals used for the specific process, amount of water used

and type of final product produced by a tannery. According to Seyoum Leta et al. (2003), a

composite tannery wastewater have BOD5 (1900 - 4800 mg/l), COD (7900 - 15200 mg/l),

sulfide (325 - 930 mg/l) and total chromium (12 - 64 mg/l). Another study in Pakistan also

indicated BOD5 (840 - 18620 mg/l), COD (1320 - 54000 mg/l), SS (220 - 1610 mg/l), TN

(236 - 350 m/l), sulfate (800 - 6480 mg/l), sulfide (800 - 6480 mg/l) and chromium (41 - 133

mg/l) (Haydar et al., 2007). The variations of effluent characteristics also occur through each

working day in a tannery. According to Cristina et al. (2007), average COD and pH analyzed

in one day were 2010 mg /l (± 516) and 6.98 (± 0.05), respectively, whereas 2068 mg/l (±

446) and 7.93 (± 0.08) respectively, in another day. Table 2.2 summarizes the pollution load

discharged from individual tannery processing operations.

9

Table 2.2. Summary of pollution load discharged in effluents from individual tannery

processing operations (Kg/tone of hide and skin)

Pollution Soaking Liming Deliming

and Bating

Tanning Post

tanning

Total

SS 11 - 17 53 - 97 8 – 12 5 - 10 6 - 11 83 - 149

COD 22 - 33 79 - 122 13 – 20 7 - 11 24 - 40 145 - 231

BOD5 7 - 11 28 - 45 5 – 9 2 - 4 8 - 15 50 - 86

Sulfate 1 - 2 1 - 2 10 – 26 30 - 55 10 - 25 52 - 110

Sulfide 3.9 - 8.7 0.1 - 0.3 4 - 9

TKN 1 - 2 6 - 8 3 – 5 0.6 - 0.9 1 - 2 12 - 18

NH3-N 0.6 - 0.9 0.4 - 0.5 2.6 - 3.9 0.6 - 0.9 0.3 - 0.5 4 - 6

Chromium 2 - 5 1 - 2 3 - 7

Source: Ludvick, 2000

2.3. Environmental impact

It could be agreed that the leather industry performs an environmentally important activity by

giving a new life to the left over of the meat industry. The transformation of this by-product

is, however, potentially pollution intensive and tanning is widely perceived as a consumer of

natural resources. Among various environmental pollutants of wastewater released from

different industries, tannery waste is the major challenging and devastating pollutant. Leather

industry is one of the most harmful to the environment for being responsible for extreme

pollution of water resources.

The quantities and qualities of emission and waste produced by tanneries depend on the type

of leather processed, the source of hides and skins, and the technique applied (COTANCE,

2002). This is indicated by the results from the analysis of tannery wastewater characterized

and presented in Table 2.2 above.

10

The addition of organic molecule to water typically increases BOD5 by increasing rates of

biological/chemical decomposition. Depletion of oxygen level caused by waste with high

BOD5 can produce both acute (killing) and chronic (e.g. reduced growth, fecundity and

disease resistance) impacts in aquatic biota (Allison, 1996).

Tannery wastewater is also characterized by being strongly alkaline with a high salt content,

one of which is chromium (Bajza and Vreck, 2001). Now a day’s chrome tanning is favored

by the majority of the leather industry because of the speed of processing, color of the leather

and greater stability of the resulting product. However; in the present chrome tanning practice

only 50 - 60 % of chromium applied is taken by the leather and the balance is discharged as

waste (Rajamanickam, 2000). Excess amounts of chromium uptake are very dangerous due to

its carcinogenic effect. Chromium in soils affects plant growth, it is non-essential for

microorganisms and other life forms and when in excess amounts it exerts toxic effect on

them after cellular uptake (Singanan, et al., 2007).

Further, the high amount of nitrogen in the effluent causes water bodies becoming enriched

with plant nutrient which result in a proliferation of water weeds and algae, which in turn,

leads to various water purification and health problem. In addition; nitrogen is a pollutant of

concern for a number of reasons. Nitrogen in the ammonia form is toxic to certain aquatic

organisms. In the environment, ammonia is oxidized rapidly to nitrate, creating an oxygen

demand and low dissolved oxygen in surface water. Organic and inorganic forms of nitrogen

may cause euthrophication (i.e. high productivity of algae) problem in nitrogen limited fresh

water lakes, in estuarine and costal waters. Finally, high concentration of nitrate can harm

young children when ingested (USEPA, 2008). These net effects reduce the aesthetic

appearance, recreational use and reuse of the water.

The sulfide content of tannery effluent results from the use of sodium sulfide, sodium

hydrosulfide and the breakdown of hair in the unhairing process. When the pH of the effluent

falls below 9.5 hydrogen sulfide is evolved from the effluent, the lower the pH the greater the

fate of evolution. This creates unpleasant smell (even in small quantity), smell of rotten egg,

and cause toxicity too for many forms of life (Buljan et al., 2000; Nazer et al., 2005).

11

Hydrogen sulfide gas is also fairly soluble, and when dissolved by condensation weak acids

can be formed, which can cause structural problem by corrosion. Sulfate is produced by the

use of sulfuric acid, or through using products with high sodium sulfate content. The sulfate is

broken down by anaerobic bacteria to produce sulfide and odor. Soluble sulfate also causes a

problem by increasing total salt concentration in the surface water and ground water (Bosnic

et al., 2003).

Furthermore; the untreated discharge of tannery effluent in addition to polluting the receiving

streams, if allowed to percolate into the ground for long periods affects the ground water table

of the surrounding locality to certain radius. Mondal et al. (2005) showed that a single tannery

can cause pollution of ground water about a radius of 7 and 8 Km. This will make the

surrounding water source unsuitable for drinking, irrigation and for general consumption.

2.4. Tannery wastewater treatment system

Today due to increased pollution as well as elevated public awareness and consequent

demand for protection of the world’s water resource, different types of treatment techniques

that remove organic matter and nutrients from waste water have been developed (Nicholas,

1996; UNEP, 1999; USEPA, 2004; Linda and Peter, 1999). Treatment of tannery effluent is a

challenge because it is a mixture of biogenic matter of hides, inorganic chemicals and a large

variety of organic pollutant with large molecular weights and complex structures (ESCAP,

1982; Elke, 1996; Thorsten, 1997).

22..44..11.. PPhhyyssiiccoo--cchheemmiiccaall ttrreeaattmmeenntt ssyysstteemm

Physico-chemical methods, such as adsorption, coagulation-flocculation and advanced

oxidation, are used for wastewater treatments. Oxidative degradation by chlorine and ozone

are the most common chemical processes for color removal, but chlorination has the

disadvantage of producing organochloride byproducts (Sarasa et al., 1998). Although

photocatalytic oxidation with H2O2 would be more attractive from an economic point of view,

photocatalytic processes are limited to post-treatment units because of the low penetration of

UV irradiation in highly colored wastewaters (Vandevivere et al., 1998; Yang et al., 1998).

12

All the processes which can be used for removal of both organic matter and nitrogen are

simple in principle; however, they are expensive (high operating and maintenance cost, and

consumption of chemicals) and also produced harmful products. Now a day, there is a

growing interest in the development of new technologies and procedures for the purification

of this waste. Among these procedures, biological methods have been recognized as a viable

possibility for the degradation of these wastewaters (Delpozo and Diez, 2003).

22..44..22.. BBiioollooggiiccaall ttrreeaattmmeenntt ssyysstteemm

In biological treatment, microorganisms convert the organic wastes into stabilized

compounds. Typical biological treatment processes make use of trickling filters, activated

sludge, Sequencing Batch Reactor (SBR) and wetland as polishing system.

2.4.2.1. Trickling filter

Trickling filter consist a bed of rocks over which the wastewater is gently sprayed by a

rotating arm. The microbial growth occurs on the subsurface of stone or plastic media and the

wastewater passes over the media along with air to provide oxygen (Benefield and Randall,

1985; Metcalf and Eddy, 2003). Water needs to be trickled several times over the rock before

it is sufficiently cleaned. The wastewater percolates over the biofilm growing on the carrier

material to achieve a very high biofilm specific area (Joseph, 2005). Using a trickling filter a

removal 85 - 90% for BOD and 60 - 70% for COD can be achieved (Kornaros and Lyberatos,

2006).

2.4.2.2. Activated sludge

Activated sludge is the most widely used biological treatment process because the

recirculation of the biomass allows microorganisms to adapt to changes in the wastewater

composition by a relatively short acclimation process (Doan and Lohi, 2001). Activated

sludge process is a continuous or semi continuous flow system containing a mass of activated

microorganisms that are capable of stabilizing organic matter. An active mass of

microorganisms mainly bacteria and protozoa aerobically degrade organic matter into carbon

dioxide, methane, water, new cells and other end products.

13

The unsettleable suspended solid and other constituents adsorbed on or entrapped by the

activated sludge floc (Cloete and Muyima, 1997). This process is based on the aeration of

wastewater with flocculation biological growth, followed by separation of treated water from

this growth. Part of this growth is then wasted and the remainder is returned to the system

(Dohse and Heywood, 1996). According to Haydar et al. (2007), a BOD and COD removal of

90% and 80% respectively can be achieve using activated sludge for tannery wastewater

treatment.

2.4.2.3. Sequencing batch reactor (SBR)

The sequencing batch reactor (SBR) is a fill and draw activated sludge system for wastewater

treatment. In this system wastewater is added to a single batch reactor, treated to remove

undesirable components and then discharged. Equalization, aeration and clarification can be

achieved using a single batch reactor (Mahvi, 2008). Sequencing batch reactors operate by a

cycle of periods consisting of fill, react, settle, decant, and idle. The duration, oxygen

concentration, and mixing in these periods could be altered according to the needs of the

particular treatment plant. The difference between SBR and activated sludge system is that the

SBR performs equalization, biological treatment and secondary clarification in a single tank

using a timed control sequence. In activated sludge system, these unit processes would be

accomplished by using separate tanks (USEPA, 1999). According to Dinesh et al. (2004) a

BOD and COD removal of 85 – 93% and 70 – 75% respectively can be achieved for tannery

wastewater using SBR. Andualem Mekonnen (2000) also reported a removal of 85%, 38%,

35% and 99.9% for COD, TN, NH4+ and sulfide respectively.

2.4.2.4. Wetland

Wetlands are commonly known as biological filters, providing protection for water resources

such as lakes, estuaries and ground water. According to Ramsar Convention (1997: 2)

wetlands are areas of marsh, fen, peatland or water, whether natural or artificial, permanent or

temporary, with water that is static or flowing, fresh, brackish or salt, including areas of

marine water the depth of which at low tide does not exceed six meters. They are also

important water bodies’ in-terms of ecological balance because they provide breeding areas

14

for different types of flora and fauna and act as active and natural treatment system, which in-

turn gives rise to improvement of water quality.

Natural wetlands have been used for wastewater treatment for many years. In many cases,

however, the reasoning behind this use was disposal, rather than treatment and the wetlands

simply served as convenient recipient that was closer than the nearest river or other

waterways (Vymazal, 1998). Uncontrolled discharge of wastewater led in many cases to an

irreversible degradation of many wetland areas. Wetlands have been considered for a long

time as "wastelands", were scientifically neglected and, therefore, the impact of wastewater

on different wetlands was not properly assessed.

However, there has been an explosive growth of knowledge about and radical change of

attitudes towards wetlands during the last few decades (Brodrick et al., 1988; Williams,

1990). Natural wetlands are characterized by extreme variability in functional components,

making it virtually impossible to predict responses to wastewater application and to translate

results from one geographical area to another. Although improvement in the quality of the

wastewater is generally observed as a result of flow through natural wetlands, the extent of

their treatment capability is largely unknown (Brix, 1993). While most natural wetland

systems were not designed for wastewater treatment, studies have led to both a greater

understanding of the potential of natural wetland ecosystem for pollutant assimilation and the

design of new natural water treatment systems (Pries, 1994).

2.4.2.4.1. Constructed wetland technology

Although natural wetlands have been used as wastewater discharge sites in some cases for

hundreds of years, recognition of the water quality treatment capacity of wetlands has

emerged only in the last 40 years as monitoring was initiated at existing discharge sites

(Kadlec and Knight, 1996). Studies of the feasibility of using wetland for wastewater

treatment were initiated during the early 1950’s in Germany, with the first operating

horizontal subsurface flow constructed wetland operating in 1974. In the United state,

wastewater- to- wetland research began in the late 1960’s and increased dramatically in scope

15

during the 1970’s. As a result, the use of wetlands for water and wastewater treatment has

gained considerable popularity world wide (Joseph, 2005).

Though, a better understanding of the benefit that wetlands provide has led to the use of

constructed wetlands to mimic the filtration process that takes place in the fragile ecosystem

of a natural wetland. Constructed wetland are planned systems designed and constructed to

employ wetland vegetation to assist in treating wastewater in a more controlled environment

than occurs in natural wetland (USEPA, 1993). Hammer (1990) defines constructed wetland

as a designed, man made complex of saturated substrate, emergent and sub merged

vegetation, animal life and water that simulate wetland for human uses and benefits.

Constructed wetlands can be built with a much greater degree of control, thus allowing the

establishment of experimental treatment facilities with a well-defined composition of

substrate, type of vegetation and flow pattern. In addition, constructed wetlands offer several

additional advantages compares to natural wetlands including site selection, flexibility in

sizing and most importantly, control over the hydraulic pathways and retention time.

In such systems, water goes through a series of purification process, which includes biological

degradation, filtration, sedimentation and adsorption resulting in the significant reduction of

organic compounds, suspended solids and also to some extent nitrogen compounds,

phosphorous and pathogens (Reed et al., 1987; Reed, 1993). Different researchers have

investigated the wide use of constructed wetland for different type of wastewater, including

domestic (Kaseava, 2003; Joseph, 2005, Berhanu Genet, 2007), industrial (Prabu and

Udayasoorian, 2003; Maine et al., 2006; Sohsalam et al., 2007; Cristina et al., 2007),

agriculture run off (Forbes et al., 2004) dairy (Pucci et al., 2000) and polluted river water

(Jing et al., 2000; Li et al., 2007), and have shown significant improvements in water quality

in these systems.

16

Compared with the conventional wastewater treatment system currently in use the constructed

wetland has its own advantage and limitations

a. Advantage of constructed wetland

Constructed wetlands are a cost effective and technically feasible approach for treating

wastewater. When compared to conventional treatment system, constructed wetlands are less

expensive to build as well as operational and maintenance costs are low. Study conducted in

Ireland by Reddy (2004) showed that the cost of a typical constructed wetland with a size of

4650 m2 is about $122000 which was cheaper by 30% than conventional treatment methods

of the same size considering the lifespan and replacement value of the wetland. The above

case studies also confirmed that maintenance cost for constructed wetland was eight times

lower than the conventional treatment system. Furthermore, operational and maintenance

require only periodic rather than continues, on-site labor

In addition, constructed wetland attracts wildlife such as bird, mammals, amphibians and

variety of dragon flies and other insects make the wetland home (Martha, 2003). For instance,

the recent USEPA (1999) publications indicated that more than 1,400 species of wildlife have

been identified from constructed and natural treatment wetlands, of these more than 800

species were reported in constructed wetland alone. Moreover, constructed wetland plants

provides a more aesthetically pleasing alternative than many other conventional wastewater

treatment systems (Richard, 1998)

b. Limitations of constructed wetland

- They generally require larger land areas than do conventional wastewater

treatment system. Wetland treatment may be economical relative to other options

only where land is available and affordable.

- Performance may be less consistent than in conventional treatment. Wetland

treatment efficiencies may vary seasonally in response to changing environmental

17

conditions including rainfall and drought. Wetland treatment cannot be relied upon

if effluent quality must meet stringent discharge standards at all times.

Generally; the vast majority of wetlands constructed for wastewater treatment are classified as

surface flow or free water surface (FWS) systems and subsurface flow (SSF) system.

2.4.2.4.1.1. Free Water Surface (FWS) Wetland

Free water surface (FWS) most resemble natural wetlands both in the way they look and the

way they provide treatment. Both designs can be used to treat wastewater from individual and

community sources, but FWS are usually more economical for treating large volumes of

wastewater (Sinclair, 2000).

Wetlands are areas on land where the ground maintains saturated conditions for much of the

year. FWS stay saturated enough to maintain a shallow level of water and wastewater (10 to

45 cm deep) above the soil (Figure 2.2). Wetland plants also are present in FWS, and natural

forces such as wind, sun, rain and temperature affect the plants, water and the treatment

processes in these systems (Pipeline, 1998).

Figure 2.2. Free water surface wetland (OWDP, 2001)

As soon as wastewater enters a FWS cell, natural processes immediately begin to break down

and remove the waste materials in the water (Renee, 2001; Kaseva, 2003). Before the

wastewater has moved very far in the wetland small suspended waste materials are physically

strained out by submerged plants, plant stems, and plant litter in the wetland (Hammer, 1992).

The roots, stems, leaves, and litter of wetland plants also provide a multitude of small surfaces

18

where wastes can become trapped and waste-consuming bacterial can attach themselves to the

plant (USEPA, 1993; Sinclair, 2000).

Bacteria provide the majority of wastewater treatment. Aerobic bacteria thrive in wetlands

wherever oxygen is present, especially near the surface. Wind, rain, wastewater and anything

else that agitates the water surface can add oxygen to the system. Anaerobic bacteria thrive

where there is little or no oxygen. In surface flow cells, oxygen is scarce in the lower substrate

and soil. When these bacteria consume waste particles in the water they convert them into

other substances, such as methane, carbon dioxide and new cellular material. Some of these

substances are used as food by plants and other bacteria (Christina, 2005).

2.4.2.4.1.2. Subsurface Flow (SSF) wetland

In SSF wetland, cell is filled with a treatment media, such as rock or gravel, which is placed

on top of the soil or lining on the cell bottom. The depth of the media layer is usually 30 to 60

cm. In properly functioning system, the wastewater flows just below the media surface and

remain unexposed to the atmosphere while it saturates the layer below (Figure 2.3). The

saturated media and soil, together with the wetland plants roots, create conditions below the

surface of the system that are conducive to treatment.

Figure 2.3. Subsurface flow wetland (OWDP, 2001)

Treatment in the SSF system is more efficient than in the FWS wetland because the media

provides a greater number of small surfaces, pores and crevices where treatment can occur.

Waste-consuming bacterial attach themselves to the various surfaces, and waste materials in

the water become trapped in the pores and crevices on the media and in the spaces between

media (USEPA, 1993). Vegetation in a wetland provides a substrate (roots, stems, and leaves)

19

upon which microorganisms can grow as they break down organic materials. This community

of microorganisms is known as the periphyton. The periphyton and natural chemical

processes are responsible for approximately 90% of pollutant removal and waste breakdown

(Wikipedia, 2007).

Commonly used plants are cattails, bulrushes and reeds. These plants are able to grow

extensive roots even in these anaerobic conditions. The area where the roots grow is called the

root zone and if cells are alternated or allowed to rest periodically, or if the water level is

regularly cycled, the roots can reach throughout the media layer (Pottir and Karathanosis,

2001). The vegetation on a SSF wetland bed is not a major factor in nutrient removal by the

system. According to Kvet et al. (1999) wetland plant can remove up to 20% of nutrients

found within treatment effluent depending on the type of vegetation. Maine et al. (2006) also

stated that, analysis of macrophyte biomass and tissue N concentration suggested that the

biomass N pool represented less than 10% of the N removed from the incoming wastewater.

The submerged plant roots do provide substrate for microbial processes and since most

emergent macrophytes can transmit oxygen from the leaves to their roots there are aerobic

micro-sites on the rhizome and root surfaces. Wetland macrophytes transport oxygen into the

root zone through lenticles, which are small openings on the above portions of these plants,

and aerenchymous tissue, which transport gases to and from the roots (Hammer, 1992; Brix,

1994; Kadlec and Knight, 1996; Newman et al., 2000). The remainder of the submerged

environment in the SSF wetland tends to be devoid of oxygen. This indicates biological

treatment in SSF wetlands is mostly anaerobic because the layers of media and soil remain

saturated and unexposed to the atmosphere (Pottir and Karathanosis, 2001). Furthermore, the

macrophytes contribute to wastewater treatment by providing additional surfaces where

bacterial can reside and where waste materials can become trapped (Faithfull, 1996; Joseph,

2005).

Generally, in subsurface constructed wetland, water goes through a series of purification

process, which includes biological degradation, filtration, sedimentation and adsorption

resulting in the significance reduction of organic compounds, suspended solids and also

20

nitrogen compounds, phosphorus and pathogen (Reed et al., 1987; Reed, 1993). Studies have

shown the use of SSF for different types of wastewater. According to Tchobanoglono (1997)

using subsurface constructed wetland a BOD5, TSS and TN concentration less than 25 mg/l,

15 mg/l and 12 mg/l can be achieved. Cristina et al. (2007) reported TSS removal of 92% and

57% at a HRT of 6.8 days and 3.4 days using Typha latifolia for tannery wastewater

treatment. Prabu and Udayasoorian (2003) showed removal of BOD5 (77%, 74%, 64%), COD

(62%, 55%, 44%) and TSS (77%, 67%, 72%) using Phragmitus australis, Typha latifolia and

Cyperus pangorei respectively for the treatment of pulp and paper industry.

However, to avoid clogging and to increase performance of subsurface constructed wetland,

for horizontal subsurface beds, some authors do not recommend the application of organic

loadings higher than 67.25 KgBOD5/ha/day (Metcalf and Eddy, 1991) or out of the range of

67 – 157 KgBOD5/ha/day, and for TSS 45 – 168 KgTSS/ha/day (USEPA, 2000b). Garcia et

al. (2004) have reported that in order to obtain a BOD5 removal of 90%, for urban

wastewater, an organic surface loading of 200 Kg/ha/day should not be exceeded (based on

the first year of operation system).

SSF constructed wetlands are recommended for areas where an exposed wastewater treatment

site may not be suitable due to potential health and safety concerns. Odors, mosquito and flies

are not a problem with proper system design, construction and maintenance. Due to the fact

that subsurface wetland has more surface area than SF wetland, it has higher reaction rates

and therefore can be smaller in area. All of the potential advantages of the SSF concept may

be offset by the relatively high cost to procure, deliver, and place the gravel media in the bed,

even though the total area required will be less than for a SF wetland (Reed et al., 1995).

A survey indicates that the capital costs for SSF wetland systems averaged around $200,000

per hectare ($87,000/ac) and the FWS were about $50,000 per hectare ($22,000/ac). The

major cost difference of the two systems is in the expense of procuring the rock or gravel

media, hauling it to the site, and placing it. Although the construction cost per hectare is

higher for SSF wetlands, the design flow rates at currently operating SSF systems are also

much higher than at the FWS type. As a result, for the systems included in the survey, the unit

21

cost is $163/m3 ($0.62/gal) of wastewater treated for the SSF type, and $206/m3 ($0.78/gal)

for the FWS type (USEPA, 1993).

2.4.2.4.2. Role of plants in constructed wetland

As reported by Greenway (2003), macrophytes are the dominant feature of both subsurface

and surface flow constructed wetlands. In SSF wetlands, emergent macrophytes grow in a

saturated substrate which may be intermittently flooded and drained. One of the most

important mechanisms for pollutant removal in wetlands is done by biological means

(Debusk, 1999a), in which plants play partial role. Plants can be involved, either directly or

indirectly, in the removal of pollutants present in wastewater. When plants directly uptake

contaminants into their root structures, this process is called phytodegradation, when plants

secret substances that adds to biological degradation, this process is called rhizodegradation.

The process from where contaminants entered the plant biomass and transpired through the

plant leaves is called phytovolatization (ITRC, 2003).

However, the choice of plants is an important issue in constructed wetland, as they must

survive the potential toxic effects of the wastewater and its variability. Wetland ecosystem

support plant communities dominated by the species that are able to tolerate either permanent

or periodic saturation. These hydrophytic species have adapted to environments that, for

atleast a portion of the growing season, are anaerobic. Additionally, plants in tidally

influenced wetlands have adapted to salinity levels that would be toxic to other species

(ITRC, 2003). Furthermore, plant species are assumed to be adequate as long as they have fast

growth rate, rapid establishment, large biomass with a well developed below ground (root

mat) system (Brisson and Chazarenc, 2009).

Aquatic plants have both structural and physiological adaptations to water logging, which

allows them to tolerate anoxia in saturated substrates (Mitsch and Gosselink, 2000). Emergent

macrophytes that have been used successfully in surface flow treatment wetlands and

subsurface flow treatment wetlands are adapted to cope with anoxia associated with

permanent water logging or saturated solids respectively.

22

Most importantly emergent plants stabilize the wetland bed surface, provide an attachment

surface for microbes insulate the bed and assist in decomposition of pollutants in the water by

providing oxygen to the microbes in the root zone and consuming nutrients to build additional

biomass. The diffused oxygen will be available on the surface of the smaller roots with in the

root zone in the bed. These aerobic microsites on the root hair provide potential contact

surfaces for the nitrification of ammonia (Ling, 2006).

However; plants capacity to supply oxygen to the root zone and nutrient uptake varies among

species due to the difference in vascular tissue, metabolism and root distribution (Gersberg et

al., 1986; Steinberg and Coonrod, 1994; Jackson and Armstrong, 1999). This also suggests

that, if the plant is expected to play a major role, the depth of the bed should not exceed the

potential root development for the plant species selected.

In addition, apart from providing attachment sites and diffusible oxygen to bacteria, root mats

increase wastewater residence time and retention of suspended organic particles, which upon

degradation avail nutrients to bacteria and plants (Joseph, 2005). Furthermore; during the

active growth period plants are able to significantly reduce pollutants than in the senescent

phase, even though it still contribute some (Myers et al., 2001).

According to Guntenspergen et al. (1989) 17 emergent species, 4 submergent species, and 11

floating species have been used in wetlands for treating municipal wastewater. Kadlec and

Knight (1996) also listed 37 families of vascular plants that have been used in water quality

treatment. These include cattails (Typha spp.), reeds (Phragmites communis), rushes (Juncus

spp.), bulrushes (Scirpus spp.) and sedges (Carex spp.) (Hammer, 1992; Knight et al., 2000;

Pontier et al., 2004). Constructed wetland usually requires time for plant development to be

fully operational. According to Kadlec et al. (2000), in general a complete root–rhizome

development for a newly constructed wetland may require 3–5 years. Wood (1990) also noted

that it takes six to twelve months for an adequate stand of vegetation to develop, though it

may be three to four years before the stand is fully developed with and active rhizosphere. For

instance, for Phragmites, three to four growing seasons are usually needed to reach maximum

standing crop but in some systems it may take even longer (Vymazal and Kr+opfelova, 2005).

23

3. Materials and Methods

3.1. Site Description

The study was conducted inside the Modjo tannery share company found in Modjo town, 75

Km South of Addis Ababa. Generally Modjo town is located 8-35° north and 39-10° east at

an attitude of 1825 m above mean sea level (EMA, 1988). The tannery produces various types

of leather from sheep and goat skin and cattle hide. The unit processed an average of 3399

goat skin, 2564 sheep skin and 255 cattle hide daily during the study period. On average, the

tannery processed 25 tones of skin to wet blue and crust leather and produced 250 m3 effluent

daily (Tadesse et al., 2003), in which the effluent is directly discharged into the adjacent

Modjo river that ends up in Koka reservoir, that is used for hydro-electric power generation

for some part of the country.

Figure 3.1. Map of the study area.

24

3.2. Pilot constructed wetland establishment

In order to establish the constructed wetland at Modjo tannery premises, seven parallel

subsurface flow wetlands were constructed with unit lengths of 4.25 m, width of 0.8 m and

height of 0.6 m at length to width ratio of 5:1. To avoid percolation (seepage) of wastewater

from the constructed wetland to the ground, the internal part of the wetland was lined with

polyethylene sheet. Then in order to support the root mat of the different plants used, gravel

of different size was used. Coarse size gravel was used near the inlet of the wetland, which

helps to avoid clogging and facilitate water distribution, and the rest was filled with medium

(03) and fine (00) size gravel (Figure 3.2). Each wetland accommodates a volume of 714

Liter, since the porosity of the gravel filled was 35% as estimated by the USEPA (1993).

Figure 3.2. Different stage of the subsurface wetland construction

3.3. Plant material and experimental start-up

To evaluate the treatment efficiency of constructed wetland, plants of different species were

selected based on literature and abundance around Modjo. Six plants were selected, namely,

Cyperus Papyrus L., Cyperus Alopecuroides Rottb., Typha domingensis Pers., Schenoplectus

corymbosus (Roem and Schult) Rayn., Sesbania sesban (L.) Merr and Aeschynomene

elaphroxylon (Guill. and Perr.) Jaub (Figure 3.3). Among the six species, Aechynomene spp.

was collected from Ziway and the rest five were collected from the swampy area of the

Awassa Lake. The plants specimen samples were taken to Addis Ababa University (AAU)

25

National Herbarium for identification. The remaining (one) cell was used as control (with no

plant) to compare the treatment efficiency of each plants.

a b c d e f

Figure 3.3. The different plants used for the constructed wetland after 3 months.

a. Cyperus papyrus c. Cyperus alopcuroides e. Sesbania sesban b. Typha domingensis d. Schenoplectus corymbosus f. Aeschenomene elaphroxylon

After transplantation, the plants in the constructed wetland cells were fully-grown with tap

water irrigation for nearly 3 months, for the plants stabilization. According to Davis (1995)

planting should be allowed to become well established before the wastewater is introduced

into the system since the plants need an opportunity to overcome the stress of planting before

other stresses are introduced. Then, the wastewater was diluted with tap water at different

percentage (Table 3.1) was introduced for the first one month, because gradual rather than

sudden increase in the concentration of the wastewater applied reduces shock to the

vegetation (Davis, 1995) and provide adaptation period for the plants..

Table 3.1. Percentage ratio used to dilute the wastewater with tap water

Days Tap water (%) Wastewater (%)

7 – 18 75 25

19 – 28 50 50

29 - 8 25 75

9 - onwards 0 100

26

The wastewater from the industry was diverted from the main wastewater stream using a

diverting channel. To reduce the substantial portion of the suspended solids, the wastewater

passes through two screens installed within the diverting channel. In order to collect and

distribute the wastewater to be treated for each cell, the water was pumped from the diverting

channel using a 5.5 hp pump into a tanker of 5000 L capacity collection tank which

subsequently flow to a 500 L equalization tank. The equalization tank helps for settlement of

suspended solids, and controls the flow rate to the constructed wetland. The influent flow rate

from the equalization tank distributed to each constructed wetland was 1225 l/day through a

polyvinylchloride (PVC) pipe of size 50 mm installed with flow control valves. The

perforated pipes of the same size were used for equal distribution of the wastewater into the

constructed wetland cells. The wastewater hydraulic retention time (HRT) was 5 days, which

was calculated based on Darcy’s law

T = Vp

Q

Where T is residence time (in days), V the volume of constructed wetland (in m3), p stands for

the porosity of the medium and Q is the flow rate through the constructed wetland (in m3/day,

which is calculated as (Qi+Qo)/2, where Qi is inflow and Qo is outflow. The inflow and

outflow rate was measured for five days using a stopwatch and measuring cylinder. The

average inflow and outflow rate 1225 l/day and 756 l/day respectively.

3.4. Wastewater sampling and analysis

The constructed wetland cells were irrigated with the wastewater starting from a mid August

up to February. Samples were taken for two consecutive months from February to April,

2009. The wastewater samples were collected from eight different places at an interval of five

days, a total of 80 samples, were taken during the study period.

As shown in Figure 3.4, the samples were taken from the equalization tank (as an influent,

SS0) and the point of the discharge from the constructed wetland (SS1, SS2, SS3, SS4, SS5,

SS6 and SS7). The samples were collected using sterile plastic sampling bottles and

transported to the Department of Biology, AAU, for analysis.

27

Figure 3.4. Schematic diagram of the pilot scale subsurface flow constructed wetland

Wastewater characterization was carried out for the following physico-chemical water quality

parameters; pH, T°, BOD5, COD, TN, ammonium, nitrate, TDS, electrical conductivity, TSS,

total Cr, sulfide, and sulfate. The parameters were measured using standard methods. COD,

TN, ammonium, nitrate, sulfide and sulfate were measured by spectrophotometer (DR/2010,

HACH, USA) according to HACH instructions. BOD5 was measured according to the

standards methods (APHA, 1998). Total Cr was also determined using Atomic Absorption

Spectrophotometer (Buck Scientific model 210 VGP, USA). TDS, T° and electrical

conductivity was measured using conductivity meter (ELEMETRON, CC401, Spain). pH was

measured using a pH meter. Finally, gravimetric method was used to analyze TSS by

evaporating the sample at 105°c and measuring the residue using balance (SCALTEC, SBA

32).

28

To calculate the percent removal of the different parameters the following formula was used

% removal ═ Co – Cf × 100

Co

Where Co is the initial concentration and Cf is the final concentration.

3.5. Statistical data analysis

Statistical analysis was performed using the SPSS program (SPSS Inc., Chicago, IL, USA;

version 14.0). One way ANOVA, using Tukey’s test, was used to compare the means between

influent and effluent, and treatment efficiency among each unit for the selected parameters.

29

4. Results and Discussion

In this study the removal efficiency of the different emergent plants in the different cells of

constructed wetland was recorded on the basis of selected phisico-chemical parameters. The

average temperature and pH of the whole system were 22.5 °c and 8.4, respectively.

4.1. BOD5, COD and TSS removal

In this study, the average BOD5, COD and TSS of the influent were 2505 ± 134.3 mg/l, 4647

± 168.7 mg/l and 94 ± 1.5 mg/l respectively. The average BOD5, value of the different cells at

the effluent were found to be between 383.5 ± 9.6 - 534.6 ± 33.6 mg/l, whereas the COD and

TSS values were between 1453.2 ± 111 – 2770.2 ± 113.5 mg/l and 54.1 ± 1.9 – 78.9 ± 2.6

mg/l respectively (Table 4.1).

Table 4.1. Average influent and effluent concentrations (Mean ± SE) of BOD5, COD and

TSS, and removal percentage of each cell

CW BOD5 COD TSS

Influent Effluent % Influent Effluent % Influent Effluent %

1 2505 ± 134.3

397.9 ± 8.7

84.1 4647 ± 168.7

2263.8 ± 87.6

51.3 94 ± 1.5

78.9 ± 2.6

16

2 2505 ± 134.3

395.1 ± 6.3

84.2 4647 ± 168.7

1954.2 ± 40.3

57.9 94 ± 1.5

72.7 ± 1.2

22.3

3 2505 ± 134.3

408.8 ± 9.0

83.7 4647 ± 168.7

2341 ± 114.5

49.6 94 ± 1.5

54.8 ± 1.4

41.7

4 2505 ± 134.3

403 ± 8.5

83.9 4647 ± 168.7

1453.2 ± 111

68.7 94 ± 1.5

60.3 ± 0.3

35.8

5 2505 ± 134.3

534.6 ± 33.6

78.7 4647 ± 168.7

2770.2 ± 113.5

40.4 94 ± 1.5

63 ± 0.9

32.9

6 2505 ± 134.3

383.5 ± 9.6

84.7 4647 ± 168.7

1884.2 ± 100.3

59.5 94 ± 1.5

63.5 ± 1.0

32.4

7 2505 ± 134.3

442 ± 11.4

82.1 4647 ± 168.7

2708.2 ± 84.3

41.7 94 ± 1.5

54.1 ± 1.9

42.4

Note; All units are in mg/l except removal efficiency (in %)

30

As shown in Table 4.1 the maximum BOD5 (84.7%) removal was observed in the cell planted

with Sesbania sesban followed by Typha domingensis (84.2%) and Cyperus papyrus (84.1%).

In case of BOD5 removal, no statistical difference among the units was observed however all

units were significantly higher (p < 0.05) from the control unit. COD the trend was as follow

Schenoplectus corymbsus (68.7%), Sesbania sesban (59.5%) and Typha domingensis (57.9%).

Statistical analysis showed that, the COD removal from the cell panted with Schenoplectus

corymbosus was significantly higher (p < 0.05) than the rest six plants. The cell planted with

Sesbania sesban was observed to be significantly higher (p < 0.05), than cells planted with

Cyperus alopcuroides, Aeschynomene elaphroxylon and control.

The maximum TSS removal was observed in cell planted with Aeschynomene elaphroxylon

(42.4%), Cyperus alopcuroides (41.7%) and Schenoplectus corymbosus (35.8%) in decreasing

order. The cells planted with Aeschynomene elaphroxylon and Cyperus alopcuroides were

statistically higher (p < 0.05) than cell planted with Cyperus papyrus, Typha domingensis,

Sesbania sesban and control, whereas cell planted with Shenoplectus corymbosus was

statistically higher (p < 0.05) than cell planted with Cyperus papyrus and Typha domingensis.

From the result (Table 4.1), it can be seen that all of the planted cells slightly improve organic

matter (BOD and COD) removal compared with the cell without plants. It is generally

accepted that planted in SSF wetlands improve organic matter removal due to combination of

mechanisms favored by the plants (Soto et al, 1999; Stottmeister et al, 2003), and this

improvement could be caused by factors such as the growth of biofilms on the root surface,

adsorption of certain organic pollutants or the aeration potential of the plants. According to

Brix (1994), plants contribute to the reduction of high level of organic matter due to the

oxygen transfer, by the aerenchymatic tissue, to their roots. In addition, organic matter can

also be degraded when taken up by plants (Renee, 2001).

The low TSS removal efficiency in this study could be due to the fact that the TSS

concentration in the influent was low, because most have settled in the collection and

equalization tank. Suspended solid removal is very effective in subsurface constructed

wetland (USEPA, 2000), but effective removal is observed when high concentration of

31

suspended solids is present in the influent. However; this is not recommended due to the high

likelihood of clogging (Kadlec, 2004). In addition, plants contribute little to suspended solids

removal in subsurface constructed wetland since the primary mechanisms involved are

filtration and sedimentation (IWA, 2000).

In this experiment, the COD, BOD5 and TSS the effluent from the constructed wetland do not

meet the discharge limit set by the EEPA for tannery industry. The provisional discharge

limit, set by the EEPA (2003b), to water bodies, is 500 mg/l, 200 mg/l or > 90% removal and

30 mg/l for COD, BOD5 and TSS respectively. This could be due to the fact that the organic

loading rate used in this study was 130 Kg BOD5/ha/day, which is higher than 67.25 Kg

BOD5/ha/day, the recommended organic loading rate recommended by Metcalf and Eddy

(1991),

4.2. Nutrient Removal

The average influent NH4+, NO3

- and TN concentration was 565 ± 15 mg/l, 5000 ± 112.2 mg/l

and 556 ± 10.9 mg/l, respectively. After 5 days of HRT, the constructed wetlands were able to

show some reduction of concentration in the effluent. Table 4.3 below summarizes the

average concentration of TN, NH4+ and NO3

- in the effluent and removal efficiency of the

system.

32

Table 4.3. Average influent and effluent concentrations (Mean ± SE) of TN, NH4+ and NO3

-

and removal percentage of each cell

TN NH4+ NO3

-

CW Influent Effluent % Influent Effluent % Influent Effluent %

1 556 ± 10.9

329 ± 11.69

40.8 565 ± 15

417 ± 12.2

26.8 5000 ± 112.2

1345 ± 81.78

73.1

2 556 ± 10.9

260 ± 14.45

53.2 565 ± 15

446 ± 9.8

21.2 5000 ± 112.2

1545 ± 112.46

69.1

3 556 ± 10.9

423 ± 18.89

24 565 ± 15

489 ± 15.45

14.2 5000 ± 112.2

1573 ± 70.18

68.5

4 556 ± 10.9

391 ± 26.36

29.7 565 ± 15

435 ± 11.95

22.7 5000 ± 112.2

1747 ± 143.5

65

5 556 ± 10.9

390 ± 20.76

29.9 565 ± 15

521 ± 12.33

9.7 5000 ± 112.2

1842 ± 121.39

63.2

6 556 ± 10.9

232 ± 9.64

58.3 565 ± 15

471 ± 8.75

18 5000 ± 112.2

1790 ± 126.1

64.2

7 556 ± 10.9

481 ± 18.1

13.5 565 ± 15

533 ± 9.67

6.4 5000 ± 112.2

1687 ± 138.5

66.3

Note; All units are in mg/l except removal efficiency (in %)

As shown in Table 4.3, the overall removal efficiency was in range of 13.5 - 58.3%, 63.2 -

73.1% and 5.7 - 26.2% for TN, NO3- and NH4

+ respectively. The maximum TN removal was

observed in cell planted with Sesbania sesban (58.3%) followed by Typha domingensis

(53.2%) and Cyperus papyrus (40.8%). Statistical analysis showed that, TN removal was

significantly higher (p < 0.05) between cell planted with Sesbania sesban and others except

for cell planted with Typha domingensis, which was significantly higher (p < 0.05) from cells

planted with Cyperus alopcuroides, Schenoplectus corymbosus, Aeschynomene elaphroxylon

and control. In addition, when comparing removal performance of cell planted with Cyperus

papyrus with the other cells, statistical difference (p < 0.05) was observed with cells planted

with Cyperus alopcuroides and Aeschynomene elaphroxylon.

In case of NH4+, Cyperus papyrus (26.8%), Schenoplectus corymbosus (22.7%) and Typha

domingensis (21.2%) showed the maximum removal in decreasing order. Statistical analysis

indicated that cell planted with Cyperus papyrus and Schenoplectus corymbosus were

statistically higher (p < 0.05) from cell planted with Aeschynomene elaphroxylon and the

control, when compared to the others. Whereas cell planted with Typha domingensis was only

statistically higher (p < 0.05) from the control. Regarding NO3-, cell planted with Cyperus

33

papyrus (73.1%) followed by Typha domingensis (69.1%) and Cyperus alopcuroides (68.5%)

showed maximum removal, however; no statistical difference (p < 0.05) was observed among

the different cells.

TN typically consists of varying proportion of particulate organic nitrogen, dissolved organic

nitrogen, ammonium nitrogen, nitrite nitrogen and nitrate nitrogen (Reddy and Patrick, 1984;

Kadlec and Knight, 1996). Organic matter mineralization represents an important source of

ammonium, which is not nitrified because low oxygen concentration limited nitrification. Due

to nitrate in the incoming water was much greater than ammonium, the overall N showed a

reduction of 13.5 – 58.3% of the incoming N.

The low NH4+ removal in the present study may be because the constructed wetland cells in

this study have only operated for short period of time, which is not enough for good root mat

development of the plants. This indirectly decreases the aerobic region in the constructed

wetland. According to Ling (2006), in SSF constructed wetlands oxygen is available only on

the surface of the smaller roots, with in the root zone in the bed, which provides potential

contact surface for the nitrification of ammonia. Vailant et al. (2004) reported that the

nitrification process is affected by the BOD level of the wastewater because of the

competition for available oxygen between the nitrifying bacteria and the microorganisms

removing biodegradable organic matter. In addition, the ionization of ammonia, since the pH

was lower than 8.5, may add NH4+ in the effluent which in contrary result in reduced removal

efficiency.

Nitrate removal was similar in all cells, because nitrates are almost totally removed by

denitrification. D’Angelo and Reddy (1993) indicated that most of the 15N-nitrate applied to

sediment water cores was lost by denitrification. Matheson et al. (2002) performed 15N

balances in wetland a microcosm estimating that denitrification accounted for 61% of the

nitrate load, 25% was retained in the soil while only 14% was assimilated by the vegetation.

In addition, since NO3- is the form of N taken up by plants, emergent plants use it during the

growing season (Renee, 2001), but this amount may be insignificant compared to the

34

wastewater inflow loading (Brix, 1994; 1997) which in this study was 5000 ± 112.2 mg/l. The

absorbed NO3- is then converted and stored in organic form of nitrogen in wetland plants.

However; this is a temporary removal, because a large portion of this nitrogen may later be

released and recycled, as plants die and decompose.

The result obtained from this study was higher when compared with that of the provisional

discharge limit set by EEPA (2003b) for tannery industry, which are 20 mg/l and 60 mg/l or >

80 % removal for NO3- and TN. The obtained result implies the need of further research after

the system reached maturity to conclude whether the system are efficient as primary treatment

system for tannery wastewater.

4.3. Sulfate and sulfide removal

Sulfur compound is mostly found in tannery wastewater in the form of sulfate and sulfide.

The influent sulfate and sulfide average concentration was 1355 ± 78.3 mg/l and 344.2 ± 18

mg/l respectively. Table 4.5 below summarizes the average concentration of sulfur and sulfide

in the effluent and corresponding removal efficiency from the constructed wetland cells.

Table 4.5. Average influent and effluent concentration (Mean ± SE) of sulfate and sulfide, and

removal percentage of each cell

CW

Sulfate Sulfide Influent Effluent % Influent Effluent %

1 1355 ± 78.3 75 ± 8.33 94.5 344.2 ± 18 245.5 ± 9.53 28.7

2 1355 ± 78.3 60 ± 16.33 95.6 344.2 ± 18 254.2 ± 10.37 26.1

3 1355 ± 78.3 75 ± 11.18 94.5 344.2 ± 18 192.6 ± 10.3 44

4 1355 ± 78.3 70 ± 11.06 94.8 344.2 ± 18 140.3 ± 10.42 59.2

5 1355 ± 78.3 80 ± 17 94.1 344.2 ± 18 211.1 ± 28.39 38.7

6 1355 ± 78.3 50 ± 12.9 96.3 344.2 ± 18 274.8 ± 12.6 20.2

7 1355 ± 78.3 60 ± 14.53 95.8 344.2 ± 18 170.9 ± 6.36 50.3

Note; All units are in mg/l except removal efficiency (in %)

As shown in Table 4.5, the maximum removal for sulfate was observed in cell planted with

Sesbania sesban (96.3%) followed by Aeschynomene elaphroxylon (95.8%) and Typha

domingensis (95.6%), but no statistical difference (p < 0.05) was observed between the seven

35

constructed wetland cells. The cells planted with Schenoplectus corymbosus (59.2%),

Aeschynomene elaphroxylon (50.3%) and Cyperus alopcuroides (44%) showed maximum

removal for sulfide in decreasing order. The cell planted with Schenoplectus corymbosus was

observed to be statistical higher (p < 0.05) than others except from cells planted with Cyperus

alopcuroides and Aeschynomene elaphroxylon. Cell planted with Aeschynomene

elaphroxylon, when compared to others, was significantly higher (p < 0.05) from cells planted

with Cyperus papyrus, Typha domingensis and Sesbania sesban, whereas cell planted with

Cyperus alopcuroides was significantly different (p < 0.05) from cells planted with Typha

domingensis and Sesbania sesban.

A study conducted by Berhanu Genet (2007) showed sulfide removal of 98.7% (Cyperus

papyrus), 98.7% (Cyperus alternifolus) and 99% (Phoenix canariensis) for domestic

wastewater using subsurface flow constructed wetland, which have operated for four years.

Likewise, the removal efficiency obtained for sulfate was 73.7% (Cyperus papyrus), 82.2%

(Cyperus alternifolus) and 77% (Phoenix canariensis). The possible explanation for the lower

removal efficiency of sulfide in this study may be due to the short operation time of the

constructed wetland used in this study, which provides short period for the accumulation of

metals essential for sulfide reduction. In contrary, low root mat development contribute in

increasing the anaerobic region within the subsurface constructed wetland, which is suitable

for sulfate reducing bacteria, resulting in increased sulfate reduction.

In wetland systems, sulfate reduction occurs due to the presence of sulfate reducing bacteria

in the substrate coupled with sufficient organic material to stimulate their activity (Martha,

2003). Sulfate reducing bacteria remove sulfate from the water column by metabolizing

sulfate into living tissue or by reducing sulfur to produce energy (Hsu, 1998; Simi and

Mitchell, 1999). These microorganisms are obligate anaerobes that utilize sulfate as terminal

electron acceptor in anaerobic respiration (Aisling and Marinus, 2006), eventually resulting in

sulfate reduction. The reduction of sulfate by the micro organisms conversely, results in the

production of sulfide through the transfer of electron produced by the simultaneous oxidation

of the organic compounds. In constructed wetland, sulfide, due to its unstable nature, readily

36

reacts with metal to form precipitate metal sulfide. Even though, there is some permanent loss

of sulfide in the form of hydrogen sulfide.

The average sulfate and sulfide concentration at this study was in range of 50 - 80 mg/l for

sulfate and 140.3 - 274.8 mg/l for sulfide, but the permissible discharge limit set by EEPA

(2003b) for tanneries, which is 1000 mg/l and 1 mg/l for sulfate and sulfide respectively. The

obtained effluent concentration was within the permissible discharge limit for sulfate,

whereas, the sulfide concentration was by far higher than that of the EEPA standard.

4.4. Chromium removal

Chromium average influent concentration was 48.78 ± 2.32 mg/l. Table 4.7 below

summarizes the average effluent concentration with their respective removal efficiency.

Table 4.7. Average influent and effluent concentration (Mean ± SE) of total chromium, and

removal percentage of each cell

CW Total chromium

Influent Effluent %

1 48.78 ± 2.32 2.43 ± 0.46 95

2 48.78 ± 2.32 1.45 ± 0.3 97

3 48.78 ± 2.32 1.1 ± 0.23 97.8

4 48.78 ± 2.32 0.79 ± 0.15 98.4

5 48.78 ± 2.32 0.95 ± 0.23 98

6 48.78 ± 2.32 1.6 ± 0.28 96.7

7 48.78 ± 2.32 2.38 ± 0.6 95.1

Note; All units are in mg/l except removal efficiency (in %)

Regarding the removal efficiency, the maximum removal was observed in cells planted with

Schenoplectus corymbosus (98.4%) followed by control (98%) and Cyperus alopcuroides

(97.8%), likewise, the minimum removal was for cell planted with Cyperus papyrus (95%).

Statistical analysis showed that, the Cr removal in cell planted with Schenoplectus

corymbosus was significantly higher (p < 0.05) from cells planted with Cyperus papyrus and

37

Aeschenomene elaphroxylon. Whereas no statistical difference (p < 0.05) was observed

between cells planted with Cyperus alopcuroides and control, with that of the other cells.

A study conducted by Maine et al. (2006) has shown a chromium removal efficiency of 86%

for metallurgic wastewater treatment, which support the result of this study. High removal of

chromium was obtained because in constructed wetland, heavy metals were removed using

different processes. In constructed wetland metals may tend to accumulate on the root

surfaces of plants, rather than being absorbed by the plant (Debusk, 1999b). Macrophytes

roots release oxygen to the rhizosphere (Reddy et al., 1989) and produce the precipitation of

iron to form the so called iron plaque. Metal binding affinity to iron oxyhydroxides cause

metal accumulation near the macrophytes roots (Otte et al., 1995), this also indicates that the

presence of well developed root mat is essential to have good chromium removal.

Metals can also be removed through a process called chemisorption. Chemisorption is a

process which enables metals, such as chromium, copper, lead and zinc to form strong

chemical complexes with the organic material that is present in the constructed wetland.

Furthermore, the metals, chromium and copper, can be chemically bound to clays and oxides

that finally can settle out (USEPA, 1999). This is the process through which most of the

chromium removal occurs in constructed wetland. This may explain the high removal

efficiency of chromium obtained in the control, which was 98.1%.

The effluent from the constructed wetlands in this study was compared with that of the EEPA

(2003b) standard for tanneries, which is 2 mg/l, to check if effluent from the pilot constructed

wetland cell meets the admissible limit. All cells used have resulted in the effluent chromium

concentration lower than 2 mg/l except for cell planted with Cyperus papyrus and

Aeschynomene elaphroxylon, which result in concentration of 2.43 ± 0.46 mg/l and 2.38 ± 0.6

mg/l respectively.

4.5. TDS and electrical conductivity removal

As summarized in Table 4.9, during the study period the average conductivity and TDS

concentration in the influent was 16700 ± 700 µs/cm and 10700 ± 200 mg/l respectively. The

38

maximum TDS and electrical conductivity removal was observed in cell planted with

Schenoplectus corymbosus followed by Cyperus alopcuroides and Cyperus papyrus whereas

the minimum observed in cell planted with Aeschynomene elaphroxylon.

Table 4.9. Average influent and effluent concentrations (Mean ± SE) of electrical

conductivity and TDS, and removal percentage of each cell

CW Electrical conductivity (EC) TDS Influent Effluent % Influent Effluent %

1 16700 ± 700 137800 ± 436 17.2 10700 ± 200 6.340 ± 210.7 40.6

2 16700 ± 700 14538 ± 371.8 12.7 10700 ± 200 6536 ± 166.8 38.7

3 16700 ± 700 13754 ± 499 17.4 10700 ± 200 6256 ± 228.9 41.4

4 16700 ± 700 13136 ± 593.8 21 10700 ± 200 5926 ± 268.7 44.4

5 16700 ± 700 14767 ± 417.3 11.3 10700 ± 200 6604 ± 171.5 38.2

6 16700 ± 700 14072.2 ± 683.9 15.5 10700 ± 200 6391 ± 284.1 40.2

7 16700 ± 700 14954 ± 461.3 10.2 10700 ± 200 6784 ± 211.1 36.5

Note; EC in µs/cm, TDS in mg/l and removal efficiency in %

However, statistical analysis showed that, TDS and conductivity removal among the different

constructed wetland cells were not significantly different (p < 0.05) (Table 4.10).

A wide variety of inorganic ions and organic substances contained in a molecular, ionized or

micro granular suspended form, many of which may not be considered contaminants,

contribute the sum total of dissolved solids. A number of these are biologically utilized or

chemically reactive in wetlands. However, TDS often includes relatively high concentration

of ‘conservative’ or relatively unreactive, dissolved compounds, which are not removed in

wetlands. For example, wetlands have very little effect on the concentration of sodium and

chloride ions, or more generally, on salinity levels (Debusk, 1999b).

In addition as plants decay, dissolved organic particles are released and contribute for TDS

concentration. Therefore reduction of TDS concentration in wetland is often insignificant

(low) despite high removal rates of target contaminants. Even though, there was decrease in

content of TDS as a result of different reactions taking place and the straining of the

39

particulates matter by the action of filtration and deposition of suspended solid as the

wastewater flowed through the subsurface flow constructed wetland.

On the other hand, during the study period, electrical conductivity range varied from 13163 –

14954 µs/cm, which was higher than the toxicity threshold level 404 µs/cm, for the growth of

aquatic plants (Sooknah and Wilkie, 2004). Conductivity measures the solution’s ability to

carry electrical current. Typically, this measures the amount of dissolved salts in a solution.

Most tanneries in Ethiopia use skin and hide preserved with salt (NaCl) as their raw material,

inevitably increasing the Na+ and Cl- ion concentration in the wastewater. This can explain the

reason why the conductivity was high during the study time.

The high concentration of electrical conductivity in the tannery wastewater may have negative

effect on the removal efficiency of the plants. Plants mostly have a toxicity range of

conductivity with in which they can tolerate and grow. In this study, cell planted with

Sesbania sesban and Aeschynomene elaphroxylon were highly stressed (almost dried) when

compared to the other plants used in the study. However; to be certain there is a need to

conduct further study on the tolerable electrical conductivity threshold range of the plants

used for the pilot constructed wetland.

When comparing the TDS discharge limit set by the EEPA for discharge to water, which is

3000 mg/l, with the result obtained from this study, which is in range of 5926 – 6784 mg/l,

was very high.

40

5. Conclusion and Recommendations

5.1. Conclusion

During the experimental period, the introduction of high strength tannery wastewater created

very poor growing conditions for the plants, with some plants even dying as a result. At the

first week of application of wastewater all units have shown sign of stress however Cyperus

spp., Typha domingensis and Schenoplectus spp. were able to withstand the wastewater from

the tannery. On the other hand Sesbania sesban and Aeschenomene elaphroxylon were highly

affected by the tannery wastewater, where the stress could be due to the presence of toxic

level of pollutant in the wastewater.

From the result, it is clear that constructed wetland can remove and retain nutrients and

pollutants from tannery wastewater. During the study period, high removal efficiency for total

Cr (98.4%), COD (68.7%) and sulfide (59.2%) was observed in cell planted with

Schenoplectus corymbosus. Whereas cell planted with Sesbania sesban showed high removal

efficiency for sulfate (96.3%), BOD5 (84.7%) and TN (58.3%), while cell planted with

Cyperus papyrus showed higher removal efficiency for NO3- (73.2%) and NH4

+ (26.2%). In

addition, cell planted with Typha domingensis showed good removal efficiency for BOD5

(84.2%), NO3- (69.1%), COD (57.9%), TN (53.2%), and NH4

+ (21.1%).

The difference in removal efficiency among the plants used was due to the difference in plant

structure, which includes above and belowground plant material. The removal efficiency

between the planted and the unplanted cell, during the study period, was not that much

different for some of the parameter tested. This performance of the planted and unplanted

beds obtained in the present study may be explained by the fact that the constructed wetland

may have not reached maturity. The other reason could be plants minimal uptake capacity for

some parameters.

Generally, with six months of operation, encouraging removal efficiencies for the studied

parameters were obtained. Cell planted with Schenoplectus corymbosus, Cyperus

41

alopcuroides, Typha domingensis and Sesbania sesban showed high removal efficiency for

selected parameters indicating that these plants have a potential to serve as a candidate for

large scale tannery wastewater treatment. However, evaluation of the system over longer

period is required before concluding whether these plants in subsurface constructed wetland

are efficient for primary treatment of tannery wastewater.

5.2. Recommendations

In order to introduce a subsurface constructed wetland system for the treatment of tannery

wastewater in developing countries like Ethiopia, further study should be done on

1. Determining electrical conductivity toxicity threshold for plants used in this study

and other potential locally available plants is essential, because plants develop

good root mat and take up nutrient when they grow in a given environment.

2. Subsurface flow constructed wetland using a combination of more than one type of

plant is essential, as a various types of plant tolerate wastewater differently and

their ability to remove nutrients from the effluent also differs.

3. Periodic harvesting of plants in constructed wetland is essential to avoid returning

of taken up pollutant to the system when decomposing. To ensure the safety of the

final use or disposal site of the harvested plant material, conducting study on the

chromium concentration at different parts of the plant is essential.

4. Study on root mat and above ground structure of the selected plant species.

5. Microbial dynamics inside the subsurface constructed wetland system.

6. Finally, the use of subsurface constructed wetland system as a secondary treatment

can also help in selecting the appropriate position of the system. (help in

comparing the efficiency, when used as primary or secondary treatment system)

42

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Annex

Annex 1. List of tannery industries around the Modjo River

No. Name of the Company

1 Modjo tannery Share Company

2 Shoa tannery

3 Colba tannery PLc.

4 Ethiopia tannery share Company

5 Hora tannery

6 Mohammed Abdulah tannery

7 Negash Mustefa tannery

8 Rashid Abdulah tannery

9 Nuru Hassen Leather processing

10 Gelan tannery

11 Lusi tannery

12 Geotraco tannery PLc.

13 Silash Haile Leather processing

14 Mesaco Global

Oromia Investment Commission (2005)

54

Annex 2. Modjo tannery influent characteristics during the study period

All are in mg/l except pH, temperature and electrical conductivity

Parameter Mean Standard error pH 8.4 0.14

Temperature 22.45 0.74

BOD5 2505.5 134.28

COD 4647 168.68

TSS 93.97 4.82

TN 556 10.87

NH4+ 565 15

NO3- 5000 112.25

Sulfide 344.2 17.98

Sulfate 1355 78.3

Total chromium 49.55 8.26

TDS 10,680 768.63

Electrical conductivity 16,652 698.9

55

Annex 3. EEPA tannery wastewater emission standards to inland water.

Parameters EEPA discharge standards

Tannery General

pH 40 °c

Temperature 6-9 pH unit

Sulfide 1

Sulfate - 1000

COD 500

BOD > 90% removal or 200

TN > 80% removal or 60

NO3- - 20

NH4+ -

Conductivity - -

TDS - 3000

TSS 50

Cr (as total Cr) 2