International Master of Science in Environmental...

Master’s dissertation submitted in partial fulfilment of the requirements for the joint degree of

International Master of Science in Environmental Technology and Engineering

an Erasmus Mundus Master Course

jointly organized by UGent (Belgium), ICTP (Prague) and UNESCO‐IHE (the Netherlands)

Academic year 2012 – 2014

Influence of recirculation in a pulse-fed Duplex Constructed Wetland used for domestic wastewater treatment

Host University:

UNESCO-IHE Institute for Water Education

Péter Pintér Promotor: Prof. P. Lens, PhD, MSc. (UNESCO-IHE) Co-promoter: JJA. van Bruggen, PhD, MSc. (UNESCO-IHE) This thesis was elaborated at UNESCO-IHE Institute for Water Education and defended at UNESCO-IHE

Institute for Water Education within the framework of the European Erasmus Mundus Programme “Erasmus Mundus International Master of Science in Environmental Technology and Engineering "

(Course N° 2011-0172)

© [2014] [Delft], [Péter Pintér], Ghent University, all rights reserved.

3

Influence of recirculation in a pulse-fed

Duplex Constructed Wetland used for domestic

wastewater treatment

Péter Pintér

MSc thesis ES.

August 2014

5

Influence of recirculation in a pulse-fed Duplex Constructed Wetland used for domestic wastewater treatment

Master of Science Thesis by

Péter Pintér

Supervisor Prof. P. Lens, PhD, MSc. (UNESCO-IHE)

Mentors JJA. van Bruggen, PhD, MSc. (UNESCO-IHE)

M. Zapater Pereyra MSc. (UNESCO-IHE)

Examination committee Prof. P. Lens, PhD, MSc. (UNESCO-IHE)

JJA. van Bruggen, PhD, MSc. (UNESCO-IHE) P. van der Steen, PhD, Msc. (UNESCO-IHE)

Ir. Drs. M. Bijlsma, MBA, LLB

This research is done for the partial fulfilment of requirements for the Master of Science degree at the UNESCO-IHE Institute for Water Education, Delft, the Netherlands

Delft

August 2014

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©2014 by Péter Pintér. All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without the prior permission of the author. Although the author and UNESCO-IHE Institute for Water Education have made every effort to ensure that the information in this thesis was correct at press time, the author and UNESCO-IHE do not assume and hereby disclaim any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from negligence, accident, or any other cause.

9

Abstract Several studies show that recirculation can improve the treatment performance and consequently

can reduce the horizontal extension of constructed wetlands (CWs). Besides, the intermittent

feeding regime has been described to improve passive aeration in the vertical flow CWs

(VFCWs). The combination of these two features was implemented in the Duplex-CW, which

consisted of a VFCW and a horizontal flow filter (HFF) in a stack arrangement.

The main objective was to enhance the treatment performance than what was achieved during the

application of batch feeding regime conducted by a previous MSc researcher at UNESCO-IHE

Institute for Water Education (Ilyas, 2013) and to assess the effect of recirculation at different

HLRs for the reduction of land requirements. Two Duplex-CWs were used. One served as the

control (C-Duplex) and in the other system recirculation was applied (R-Duplex). Both received

raw wastewater 12 times a day, each time in a pulse lasting for 15 min, then from the VFCW the

partially treated wastewater was drained to the HFF. In R-Duplex, the effluent of HFF was

recycled to the VFCW in 15 min pulses 12 times per day 45 min after the wastewater feeding

stopped.

In the first three experimental phases of this study, the hydraulic loading rates (HLRs) were 0.05,

0.08 and 0.16 m3 m-2 d-1. In the fourth experimental phase the HLR was kept 0.16 m3 m-2 d-1 but

the OLR was artificially increased to 88 g COD m-2 d-1. In the four experimental phases the

space requirements were calculated to be 9.0, 6.8, 2.1 and 1.2 m2 per PE. In the last two

experimental phases, besides the regular parameters the effluents were tested for pathogenic

indicators. The concentrations did not comply with the standards thresholds, therefore, the

effluent could not be considered for urban reuse.

In the first two experimental phases, the R-Duplex had slightly higher COD removal and from

the third experimental phase on the effect of recirculation became more pronounced. In the first

experimental phase, R-Duplex had higher total nitrogen (TN) removal due to the increased

simultaneous nitrification and denitrification in the VFCW. The TN removal trend in the VFCWs

was the same in the following periods. However, from the second experimental phase on, in C-

Duplex, due to the increased denitrification in the HFF the overall TN removal was higher.

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Taking into account the EU Council Directive 91/271/EEC concerning urban waste-water

treatment, in both systems area reduction could be achieved to 1.2 m2 per PE considering the

COD, BOD5 and TSS thresholds. Nevertheless, real area reduction could not be stated as even

with the lowest TN concentration, 21 mg L-1 the EU limit of 15 mg L-1 was not met. The HLR

and OLR of the first experimental phase was comparable to what Ilyas (2013) applied earlier in

the batch operated Duplex-CW. When the batch operated Duplex-CW was supplied with low

strength wastewater the TN concentration was below the EU threshold meaning that the overall

performance was better than during the intermittent feeding. However, the required area was still

too high, 8.6 m2 per PE. When higher strength wastewater was applied the TN concentration

reached the limit and the total phosphorus (TP) concentration was two times higher, than what is

required. In addition, during batch operation with the smaller load increase the performance was

deteriorating faster than in the intermittently fed Duplex-CWs.

Keywords: Constructed wetland, Recirculation, Intermittent feeding, Area reduction, Reuse,

Standards

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Acknowledgements I would like to thank the Erasmus Mundus program for providing me the scholarship because

without it I could not have started the IMETE Master programme.

I would like to thank my mentors Hans van Bruggen and Maribel Zapater Pereyra for their

supervision. However, special thanks to Maribel Zapater Pereyra for the fast corrections even in

the last minutes in the late hours. I am also grateful for the advices of Prof. Dr. Piet Lens during

the thesis work.

I acknowledge Eldon Raj and Prof. Wenxin Shi and all the colleagues in paper writing meeting

for correction and suggestions.

I would like to express my appreciation for the everyday help and assistance of the laboratory

staff at UNESCO-IHE: Fred Kruis, Peter Heerings, Berend Lolkema, Frank Wiegman, Lyzette

Robbemont and Ferdi Battes.

I would like to express my gratitude for the help and support of my IMETE groupmates and my

flatmates in Delft who became close to me but especially the advices of one person from the

IMETE programme.

Special thanks to Frank van Dien, who made it possible to see and work with real scale

constructed wetlands.

I also would like to thank my family for their support during the whole Master programme.

This way, I would like to say thanks to my friend, Rocio RegueiroFernandez, who financially

helped me out to start the IMETE Master programme.

Last but not least, I would like to express my thanks to the owners and staff of the LEF restaurant

for letting me work and making it possible to spend the night shifts in a friendly environment.

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Table of Contents

Abstract ......................................................................................................................................9

Acknowledgements ................................................................................................................... 13

List of figures ............................................................................................................................ 16

List of tables ............................................................................................................................. 18

List of Acronyms ...................................................................................................................... 19

1. Introduction ........................................................................................................................... 20

1.1 Background ..................................................................................................................... 20

1.2 Problem statement ........................................................................................................... 21

2. Literature review ................................................................................................................... 22

2.1 Constructed wetlands ....................................................................................................... 22

2.2 Types of constructed wetlands ......................................................................................... 22

2.3 Area requirements of constructed wetlands ...................................................................... 23

2.4 Feeding regime of constructed wetlands ........................................................................... 27

2.5 Role of recirculation ........................................................................................................ 28

2.6 Role of oxygen in constructed wetlands ........................................................................... 29

2.7 Removal of nitrogen ........................................................................................................ 30

2.8 Pathogenic and fecal indicator organisms and their removal ............................................. 31

3. Objectives ............................................................................................................................. 33

3.1 Overall objective.............................................................................................................. 33

3.2 Specific objectives ........................................................................................................... 33

4. Materials and Methods .......................................................................................................... 34

4.1 Experimental setup .......................................................................................................... 34

4.2 Experimental design ........................................................................................................ 36

4.3 Passive aeration capacity of the vertical flow constructed wetland ................................... 38

4.4 Analytical procedure ........................................................................................................ 39

4.5 Statistical analysis ............................................................................................................ 42

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5. Results .................................................................................................................................. 43

5.1 Passive aeration capacity of the Vertical Flow Constructed Wetland ................................ 43

5.2 First experimental period - HLR 0.05 m3 m-2 d-1 + OLR 13 g COD m-2 d-1 ....................... 44

5.3 Second experimental period - HLR 0.08 m3 m-2 d-1 + OLR 18 g COD m-2 d-1 ................... 48

5.4 Third experimental period - HLR 0.16 m3 m-2 d-1 + OLR 56 g COD m-2 d-1 ...................... 53

5.5 Fourth experimental period - HLR 0.16 m3 m-2 d-1 + OLR 88 g COD m-2 d-1 .................... 58

6. Discussion ............................................................................................................................. 64

6.1 Passive aeration capacity of the vertical flow constructed wetland ................................... 64

6.2 Effect of recirculation at different HLRs .......................................................................... 65

6.3 Possible reuse of the effluents considering various standards ........................................... 70

6.3.1 Land area requirements under different HLRs and OLRs ........................................... 73

7. Conclusions and recommendations ........................................................................................ 75

7.1 Conclusions ..................................................................................................................... 75

7.2 Recommendations............................................................................................................ 76

References ................................................................................................................................ 77

Annex 1. ................................................................................................................................... 84

Land area requirement calculation ......................................................................................... 84

Annex 2. ................................................................................................................................... 85

Passive aeration capacity of the vertical flow constructed wetland ......................................... 85

Annex 3. ................................................................................................................................... 86

Additional figures .................................................................................................................. 86

Annex 4. ................................................................................................................................... 88

Change in vegetation over time .............................................................................................. 88

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List of figures Figure 4.1: Duplex-CW setup ........................................................................................... 34

Figure 4.2: Setup of the passive aeration capacity test for the VFCWs .............................. 39

Figure 5.1: Change in the DO concentration (absolute values) of the dry and operational

VFCWs ................................................................................................................... 43

Figure 5.2: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of C-Duplex n=3) ....................................................................................... 45

Figure 5.3: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of C-Duplex n=3) ....................................................................................... 46

Figure 5.4 A: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of R-Duplex, n=3) .......................................................................... 47

Figure 5.4 B: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of R-Duplex, n=3 and C-Duplex n=2) ............................................. 48

Figure 5.5: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 2nd experimental phase (n=3, except EC n=2) ....................................................................................................................... 49

Figure 5.6: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 2nd experimental phase (n=3) ......... 51

Figure 5.7: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 2nd experimental phase (for COD n=3, for BOD5 n=2, except effluent R-HFF n=1) ............................................ 52

Figure 5.8: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=5, except effluent of C-VFF n=4) ............................................................................................ 54

Figure 5.9 A, B: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=5) ......... 55

17

Figure 5.9 C: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=5) ......... 56

Figure 5.10: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (for COD n=5, for BOD5 n=4 except effluent C-HFF n=4) ............................................. 57

Figure 5.11: Mean (±SEM) of the fecal indicator organisms of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=3) . 58

Figure 5.12: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (n=4) ......... 59

Figure 5.13: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (n=4) ......... 61

Figure 5.14: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (for COD n=4, for BOD5 influent n=3 effluent n=1) ....................................................... 62

Figure 5.15: Mean (±SEM) of the fecal indicator organisms of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (n=3) .....

....................................................................................................................... 63

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List of tables Table 2.1: Constructed wetlands with different organic loading rates and land requirements....

........................................................................................................................ 25

Table 2.2: Constructed wetlands with different hydraulic loading rates and land requirements . ........................................................................................................................... 26

Table 4.1: Characteristics of the settled domestic wastewater during this study.................... 35

Table 4.2: Operation of the Duplex-CWs ............................................................................ 35

Table 4.3: Operation modes and parameters of the Duplex-CWs ........................................ 36

Table 4.4: Analytical procedures ........................................................................................ 39

Table 6.1: Nitrogen balance of the Duplex-CWs in the four experimental phases ................ 66

Table 6.2: Effluent quality standards for different purposes in EU, United States and Japan ....

........................................................................................................................... 71

Table 6.3: Effluent quality of the batch operated Duplex-CW and C-Duplex concerning the EU

standards ............................................................................................................. 72

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List of Acronyms

APHA American Public Health Association

BOD Biochemical Oxygen Demand

COD Chemical oxygen demand

CW Constructed Wetland

C-Duplex Control Duplex-CW

C-VFCW Control vertical flow constructed wetland

C-HFF Control horizontal flow filter

DO Dissolved oxygen

EC Electrical conductivity

EPA Environmental Protection Agency

HF Horizontal flow

HFF Horizontal flow filter

HFCW Horizontal flow constructed wetland

HLR Hydraulic loading rate

HRT Hydraulic retention time

HSSF Horizontal subsurface flow

MDG Millennium Development Goal

OLR Organic loading rate

PE Population equivalent

R-Duplex Duplex-CW with recirculation

R-VFCW Vertical flow constructed wetland with recirculation

R-HFF Horizontal flow filter with recirculation

TN Total nitrogen

TP Total phosphorus

TSS Total suspended solids

VF Vertical flow

VFCW Vertical flow constructed wetland

VSSF Vertical subsurface flow

WWTP Wastewater treatment plant

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1. Introduction

1.1 Background Today the major challenge of poor sanitation is not only an issue in the developing world but

also in remote areas and in places where urbanization progresses with accelerated pace, and

resources and incentives for sanitation investments are scarce. In 2011, the world’s population

reached 7 billion and it is estimated to increase up to 9.6 billion by the year 2050. According to

the UN predictions, most of the growth will occur in developing regions, especially in Africa

(UN Press Release, 2013). In general, there is no adequate wastewater treatment in these regions

and the increasing amount of wastewater will further worsen the situation. However, the

conventional wastewater treatment technologies, usually associated with high costs and complex

operation, are not ideal solutions for these regions (Zhang et al. 2014). Solution must be found as

clean water and sanitation should be accessible to everyone as it is a human right. In order to

accelerate the realization of this human right, one of the Millennium Development Goals

(MDGs) aimed to halve by 2015 the proportion of people without access to safe drinking water

and basic sanitation (UN General Assembly, 2012). Alternative, usually decentralized treatment

technologies are implemented to achieve the sanitation related MDG. One option is the use of

constructed wetlands (CWs). Their principles come from natural wetlands but they are

intensified to improve the treatment capacity (Kadlec & Wallace, 2009).

In areas where land is relatively inexpensive CWs offer a competitive and appropriate solution

for the increasing amount of wastewater. Great advantages of the systems are the easy operation

and the low operational costs. However, in densely populated areas or in mountainous regions,

where the available space is limited the implementation of CWs can be hindered (Zhang et al.

2014). In general, subsurface flow CWs have their land requirements in the range of 2.2-10.0 m2

per population equivalent (PE) (Vymazal, 2002). These numbers show the obstacle to build CWs

for big settlements. Therefore, excessive amount of research has been conducted to optimize

CWs and reduce their horizontal extension. The main two intensification approaches are the use

of recirculation and artificial aeration (Foladori et al., 2013). With the use of intermittent aeration

and recirculation Foladori et al. (2013) managed to reduce the required area from 3.6 to 1.5 m2

per PE but Zhai et al. (2011) reported that their hybrid system required less than 1 m2 per PE

(Zhai et al., 2011, cited in Zhang et al., 2014).

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Besides, in the study of Prigent et al. (2013), a modification of the well-known French system

with two vertical flow CWs (VFCWs) was tested. This new design was a single stage compact

VFCW which could reduce the surface area to 1.2 m2 per PE (Prigent et al., 2013). Nevertheless,

the easy, stable and low cost operation and construction can be hindered in upgraded, more

compact systems where for example artificial aeration or recirculation is used.

At UNESCO-IHE, in order to fill missing gaps in the knowledge of CWs and to reduce the land

requirements the Duplex-CW was built in a stack design. One unit consisted of a VFCW and a

horizontal flow filter (HFF). This hybrid CW was designed to ensure oxic-anoxic conditions for

complete nitrogen removal and aerobic degradation of organic matter (Zapater-Pereyra, 2011).

1.2 Problem statement During the previous studies, the Duplex-CWs were operated in a batch mode with the same

hydraulic loading rate (HLR), at 0.05 m3 m-2 d-1 (Ilyas 2013; Lavrnić, 2013; Kyomukama, 2014;

Namakula, 2014). Ilyas (2013) was experimenting with artificially increased organic loading

rates (OLR), 40, 70 and 130 g COD m-2 d-1, in order to further decrease the space requirements.

However, at medium and high OLRs area reduction could not be stated as the total nitrogen

concentration in the effluent was higher than the EU limits. In this research, two Duplex-CWs,

R-Duplex with recirculation and C-Duplex, the control, were operated. Compared to the previous

operation, a more continuous, intermittent feeding was applied at increasing hydraulic loading

rates (HLRs), 0.05, 0.08 and 0.16 m3 m-2 d-1, and at artificially increased OLR at 88 g COD m-2

d-1, at the HLR 0.16 m3 m-2 d-1. The intermittent feeding was applied with the intention to

increase the aeration of the VFCWs and to provide more organic matter for denitrification in the

HFF due the shortened contact time in the VFCWs. Taking into account the abovementioned

HLRs and the average BOD5 concentration of the raw wastewater based on the results of Ilyas

(2013), the theoretical space requirements were calculated to be around 8, 5, 2.5 and 1.2 m2 per

PE. However, to state these values the effluent must comply with certain standards which are

detailed in a later chapter.

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2. Literature review

2.1 Constructed wetlands CWs are designed and constructed based on the properties of natural wetlands including soil

types, vegetation and microbial composition. However, these engineered systems are operated in

a more controlled way (EPA, 1999). CWs have many advantages including lower energy

requirements, long lifetime easy to operation and maintenance (García et al., 2010). Since the

1970s, significant amount of research has been carried out to test CWs for wastewater treatment

and to understand the mechanisms of pollutant removal (Kangas, 2005). Therefore, it can be

stated that CWs for wastewater treatment is a proven technology.

2.2 Types of constructed wetlands There are numerous CW designs, but there are two main types classified by literature and

practitioners (EPA, 1999; Vymazal, 2010). The first type is the Free Water Surface (FWS)

wetlands, also known as surface flow wetlands. They are comparable to natural wetlands because

in most cases the aquatic plants are rooted at the bottom of the CW and the wastewater flows

through the leaves and stems of the plants. The second main type is the Vegetated Submerged

Bed (VSB) systems which are also called subsurface flow (SSF) wetlands (EPA, 1999). They

have fewer similarities with natural wetlands. The bed material is sand, gravel or soil and aquatic

plants grow on it. However, the wastewater stays beneath the surface of the media only being

available for the roots and rhizomes (EPA, 1999; Zhang et al. 2014). The removal mechanisms in

these two types of CWs are similar; the particulates, including organic matter can go through

physical separation such as settling, filtration and in FWS CWs resuspension. The other main

process in the removal of organic matter is biological conversion which can be gasification or

mineralization (EPA, 1999). The main difference is the source of dissolved oxygen (DO) as in

FWS CWs it can also be due to the oxygen transfer through the open water surface while in SSF

CWs the DO concentration can increase mainly due to the operation. The principles of nitrogen

removal mechanisms are the same in both, however, the features of each design and operation

modifies the dominant process.

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SSF CWs are distinguished by the flow of the wastewater. In horizontal subsurface flow (HSSF)

CWs, the wastewater flows through the media horizontally from the inlet towards the outlet

(Vymazal, 2010). The advantage of the HSSF CWs compared to the FWS systems is that they

can operate at colder conditions due to the possibility to insulate the top of the bed (Kadlec &

Wallace, 2009). Vertical flow constructed wetlands (VFCWs) are fed at the top by a pump and

the w percolates through the system. They have the ability to provide higher level of oxygen

transfer compared to HSSF CWs (Kadlec & Wallace, 2009).

In order to use the advantages of the abovementioned CWs they can be built in series; these

systems are termed as hybrid CWs (Ávila et al., 2014, Vymazal, 2005, Zhao et al., 2011).

Besides the better effluent quality, the goal of these systems can also be the removal of heavy

metals, pharmaceuticals, personal care products, surfactants and herbicides (Ávila et al., 2014).

Hybrid CWs date back to the 1960s when they were introduced by Seidel in Germany (Seidel,

1965, cited in Vymazal, 2013). However, these configurations were not widely recognized at that

time (Vymazal, 2013). The first set-up of Seidel consisted of a VF bed intended for nitrification

and two or three HF beds for denitrification. The VF beds were mainly planted with Phragmites

australis and the HF beds were planted with other emergent macrophytes, such as Iris,

Schoenoplectus (Scirpus), Sparganium, Carex, Typha and Acorus (Vymazal, 2013). Nowadays,

several hybrid systems are composed of a VF and an HF CW, arranged in series (Vymazal,

2010). Apart from the VF-HF systems in the 1990s and early 2000s several FWS and SSF

wetland combinations were built in China and in some countries in Europe. They were used not

only for domestic sewage but for industrial wastewater and landfill leachate as well (Vymazal,

2013).

2.3 Area requirements of constructed wetlands Even if most CWs come with low costs, low maintenance requirements, offer good performance

and have ecological benefits, their space requirements are prohibitively large. Therefore, they are

more suitable for small communities, households where waste flows are not generated in large

quantities and where inexpensive land is available (EPA, 1999; García et al., 2010). Researchers

have been dealing with multi-stage hybrid CWs and altering operating conditions to increase the

efficiency of CWs and consequently reduce their horizontal dimension.

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Most authors give the required wetland area in m2 per PE. According to the EU Council

Directive 91/271/EEC concerning urban waste-water treatment, one PE is equal to the 5-day

BOD of 60 g of oxygen per day (EU Council Directive 91/271/EEC). However, in the UN-

HABITAT Constructed wetlands manual (2008) for developing countries, due to the

characteristics of the wastewater, 1 PE is considered 40 g of BOD5 and the wetland sizes are

given accordingly. In this document the EU definition of PE is used.

For secondary treatment, SSF CWs have their specific area in the range of 2.2-10.0 m2 per PE

(Vymazal, 2002). The “rule of thumb” is to use 5 m2 of land per PE for HSSF CWs (Vymazal,

2002); however, it was reported to be insufficient to remove nutrients (Babatunde et al., 2008).

According to the survey of 109 Danish CWs in 1990, between 15 and 30 m2 area per PE would

be necessary to obtain tn concentrations less than 8 mg L-1, and an area of 40-70 m2 PE-1 to

achieve less than 1.5 mg L-1 TP concentration (Babatunde et al., 2008). However, the sizing of

wetlands is greatly affected by the type of wastewater, the type of CW and the climatic

conditions, especially the temperature (EPA, 1999). Compared to HF CWs, VFCWs require less

area, usually 1-3 m2 per PE. VFCWs are generally built as one bed and are referred as “compact”

VFCWs (Vymazal, 2010). In order to reach these numbers, Foladori et al. (2013) used separate

and combined intermittent aeration and recirculation. Another way to achieve is to build different

stages in a stack design, i.e. to extend the system vertically.

2.3.1 Organic loading rate OLR is an important parameter to be considered during the design and operation of CWs and it is

expressed in g COD m-2 d-1 or g BOD m-2 d-1. For secondary treatment of domestic wastewater,

the recommended OLR is 8-10 g BOD5 m-2 d-1 (Chazarenc et al., 2007). However, the capacity to

treat higher organic load is preferable regarding the land requirements of CWs. Table 2.1 gives

the summary of lab-scale CWs, which were operated in different ambient conditions, and the

concentrations of two important parameters at relatively high OLRs.

Untreated domestic wastewater can be classified as low, medium or high strength wastewater

based on the concentrations of its constituents. For COD, the approximate values for low,

medium and high strength are 250, 430 and 800 mg L-1, respectively and for BOD5 are 110, 190

and 350 mg L-1, respectively (Tchobanoglous et al., 2003). Therefore, it is essential to identify

the type of wastewater to adjust the highest possible OLR.

25

Table 2.1 Constructed wetlands with different organic loading rates and land requirements

The drawbacks of highly loaded systems can be the lowered removal efficiency of organic matter

and nitrogenous compounds and faster clogging (Ghosh & Gopal, 2010). Therefore, it is

essential to ascertain the highest sustainable loading rate with the effluent quality which still

complies with the standards.

For the EU member countries, the effluent standards are set in the Council Directive 91/271/EEC

concerning urban waste-water treatment. However, the member countries have their own

national regulations based on the EU directive, which are generally stricter (Example: Hungary).

Besides, the standards are set according to the fate of the treated wastewater. An example is the

United States, as in each states, there are different requirements for the reclaimed water quality

depending on the end use, for example unrestricted urban reuse, agricultural reuse and so forth

(EPA, 2012).

2.3.2 Hydraulic retention time and hydraulic loading rate The hydraulics of CWs is greatly affected by two design and operation factors: the hydraulic

retention time (HRT) and the HLR. HRT is determined as the ratio of the available wetland

water volume to the flow rate. HLR is the volumetric flow rate divided by the surface area,

expressed in m3 m-2 d-1 (EPA, 1999).

CW type Area,

m2 PE-1

HLR,

m3 m-2 d-1

OLR,

g COD m-2 d-1

Effluent COD,

mg L-1

Effluent TN,

mg L-1 References,

VSSF 1.2 0.38 90 59-110 12-21 Prigent et al. (2013)

C1-VSSF 3.6 0.07 32 88 16 Foladori et al. (2013)

R2-VSSF 1.4 0.17 83 68 20 Foladori et al. (2013)

A3-VSSF 1.8 0.16 64 52 20 Foladori et al. (2013)

AR-VSSF 1.5 0.18 74 37 12 Foladori et al. (2013)

Duplex-CW 8 0.05 40 46 10 Ilyas (2013)

Duplex-CW 3.4 0.05 70 41 16 Ilyas (2013)

Duplex-CW 2.7 0.05 130 69 25 Ilyas (2013)

A-Duplex-CW 2.7 0.05 130 44 31 Ilyas (2013)

1 Control, 2 Recirculated, 3 Aerated

26

Table 2.2 Constructed wetlands with different hydraulic loading rates and land requirements

Typically, lower HLR or longer HRT results in higher removal efficiencies and offers some

buffer capacity against the fluctuation in wastewater characteristics. Besides, longer HRT can

also reduce the risk of rapid short-circuiting, especially in VFCWs, which could diminish the

biodegradation of organic matter (Foladori et al., 2013). The drawback is that it requires larger

wetland area for the treatment (Weerakoon et al., 2013). Applying the loading chart design

method, developed by Wallace & Knight (2006), the land requirements can be tremendous and

the HRT can be unnecessary long when the incoming wastewater is highly concentrated.

Therefore, the major goal of researcher and designers is to find the shortest HRT or the highest

HLR, reach the desired effluent quality and ensure the long operation without the breakdown of

the system.

From Table 2.2, it is quite apparent that by increasing the HLR, the OLR also becomes higher

and the retention time of the system is shortened. In addition, the BOD removal decreases

accordingly. From the listed systems, the HF Bio-rack performs the best at BOD removal as only

with 6 hours of HRT the removal is around 83%. However, in order to apply high hydraulic and

organic loads special designs, operation, regular monitoring and maintenance are needed.

CW type Area,

m2 PE-1

HLR,

m3 m-2 d-1

OLR,

g BOD m-2 d-1

HRT,

days

BOD

removal,

%

References,

HFCW 83.3 0.025 0.7 8 981 Weerakoon et al. (2013)

HFCW 59.5-16.7 0.035-0.125 1-3.6 5.7-1.6 901 Weerakoon et al. (2013)

HFCW 6.9 0.3 8.6 0.7 801 Weerakoon et al. (2013)

VFCW 1.7 0.06 5.1 4 942 Ghosh & Gopal (2010)

VFCW 11.4 0.08 5.2 3 822 Ghosh & Gopal (2010)

VFCW 7.6 0.12 7.9 2 702 Ghosh & Gopal (2010)

VFCW 2.9 0.24 20.5 1 302 Ghosh & Gopal (2010)

HF Bio-rack 4.0 0.07 15.1 0.75 891 Valipour et al. (2009)



1 BOD5 removal, 2 BOD3 removal

27

2.4 Feeding regime of constructed wetlands Wastewater treatment plants (WWTPs) and CWs can be operated in a continuous way and in

batches. In batch-operated CWs, typically in VFCWs, the wastewater is added and then drained

periodically resulting in a treatment cycle (Vymazal, 2010). The main advantage of batch

operation, compared to continuous flow systems, is the greater flexibility and control of

operating parameters. In batch systems, the volume of the influent and the outflow is controlled;

therefore, it is not flowrate dependent (Stricker & Béland, 2006). Besides, in CWs with

intermittent operation subsurface aeration accelerates the clog matter mineralization (Knowles et

al., 2011). On the other hand, in non-continuous systems an equalization or storage tank is

required and more frequent check up is needed.

2.4.1 VFCW hydraulics Due to the design and operation of VFCWs the oxygen transfer rate is high which results in

improved organic matter removal and nitrification capacity (Kadlec & Wallace, 2009; Vymazal,

2010). Regarding the hydraulics of the VFCWs, there are several variations but the main four

types are detailed below.

During intermittent downflow (pulse feeding) operation, the top of the bed is flooded for short

periods of time. In the draining period, air is drawn to the pores, resulting in aerobic conditions

in the biofilm. When no plants are used the systems are called intermittent sand filters (Kadlec &

Wallace, 2009). The variant termed unsaturated downflow is operated by distributing the

wastewater across the granular bed surface, and subsequently the wastewater trickles through the

bed in an unsaturated flow. These systems are commonly applied with recirculation (Kadlec &

Wallace, 2009). Saturated up- or downflow systems are continuously saturated in the plant root

zone. There are aerated and anaerobic types depending on the purpose of the VFCW (Kadlec &

Wallace, 2009). The anaerobic wetlands or also called alkalinity producing systems are used for

mine water treatment (Younger et al., 2002, cited in Kadlec & Wallace, 2009). Tidal flow (fill-

and-drain) systems operate in a filling and draining cycle. The wastewater is fed at the bottom of

the granular bed until the surface is flooded. Then, the wastewater is held in the system for the

time of the treatment. The next stage is drainage when air enters the pores. There are usually

parallel trains; while one train is filled the other one is draining.

28

The treatment performance of these VFCWs mainly depends on the loading rates, the frequency

of loading cycles, recirculation rates and length of resting periods between the loading cycles

(Kadlec & Wallace, 2009).

2.5 Role of recirculation In WWTPs, recirculation is applied to dilute the incoming wastewater in order to decrease the

load, and to sustain biological processes e.g. denitrification. In CWs, recirculation is considered

to improve the water-biomass contact time, provide buffering effect on the inlet load variations

and increase the dissolved oxygen concentration (Prost-Boucle & Molle, 2012). Nevertheless,

more specific effects of recirculation on the performance of CWs have also been reported in

literature (Ayaz et al., 2012; Foladori et al., 2013; Prost-Boucle & Molle, 2012; Tunçsiper, 2009;

Zhao et al., 2004). The emphasis of these studies was mainly on the removal of organics and

nitrogenous compounds.

In the United States, since 1889, vegetated reciculating gravel beds have been used as an

alternative of VFCWs. The hydraulic loading and the recirculation was applied with the intention

to increase the frequency of dosing, keeping the bed moist and to decrease the dose volume to

reach unsaturated flow and increase the oxygen diffusion (Kadlec & Wallace, 2009). Recycle

ratios of even 3 to 12 times the influent flow rate were common (Kadlec & Wallace, 2009).

However, in the case of large scale wetlands the pumping costs can be significant with high

recirculation ratios. On the other hand, Zhao et al. (2014) reported that the optimal recirculation

ratio for CWs was 1:1.

Ayaz et al. (2012) studied the effect of recirculation and temperature and assessed the

performance of a hybrid CW system. Operation with recirculation resulted in TKN removal as

high as 98%. The results showed that recirculation was necessary to have efficient removal of

nitrogen in the tested hybrid system. Tunçsiper (2009) also showed similar results with increased

recirculation rate (from 50% to 100%) and decreased HLR (from 0.1 m3 m-2 d-1 to 0.03 m3 m-2

d-1) the NH4+-N removal increased from 66% to 70% and the NO3

--N removal increased from

63% to 74%. The study of Sklarz et al. (2009) focused on a VFCW with recirculation treating

grey water from households. One of the most important advantages of this VFCW was the

increased passive aeration which resulted in higher organic matter degradation.

29

Other important effects were the constant moistening of the bed which was favorable for the

microbial community and the lowered fluctuations in the influent characteristics due to the

reintroduced diluted wastewater (Sklarz et al., 2009). Foladori et al. (2013) tested CWs

consisting of combined and separate aeration and recirculation. At high organic and nitrogen

loads, the intermittently aerated and recirculated VFCW was able to reduce the land

requirements up to 1.5 m2 per PE; however, optimization was still required to balance the energy

needs for aeration and recirculation.

2.6 Role of oxygen in constructed wetlands The presence and the amount of dissolved oxygen is an important factor for the biological and

biochemical processes occurring in WWTPs and in CWs. However, artificial aeration greatly

contributes to the operational costs of WWTPs. Due to this factor, treatment techniques requiring

artificial supply of high and constant concentration of dissolved oxygen are economically

limited.

The number of research articles dealing with aerated CWs has been increasing lately. In most

cases, the aim of these studies, just like in the case of recirculation, was to enhance organic

matter removal and nitrification in order to decrease the required area per PE. In one study

conducted by Tao et al. (2010), the effect of aeration in summer and winter conditions was

assessed. The results showed that aerobic degradation of organic matter and nitrification and

denitrification processes were enhanced. Nevertheless, the aeration could not fully compensate

the low temperature and plant dormancy under winter conditions (Tao et al., 2010).

However, the majority of reports dealing with artificial aeration or recirculation hardly

emphasize the expenses originating from these modifications. The aeration costs of small

systems can be negligible but it might be significant for large scale CWs. Thus, the lowered

investment costs can be counteracted by the increased operational costs, which is one of the

biggest advantages of these treatment systems. In addition, one alternative low cost option to

increase the oxygenation of the bed is frequent water level fluctuation or pulse feeding.

30

2.7 Removal of nitrogen Nitrogen has key importance in the life cycle of wetland plants; however, plant uptake of

nitrogen is not a significant removal mechanism. Nitrogen species such as ammonia, nitrite,

nitrate and organic nitrogen in water phase and nitrous oxides in the atmosphere have great

environmental and public health concerns (EPA, 1999; Mander et al., 2014). Therefore, it is

essential to know the transformation pathways and the mass balances of nitrogen species in order

to have an efficient treatment system and to inhibit the formation of some nitrogen species (EPA,

1999). During ammonification, the organic nitrogen forms are biologically, through

exoenzymatic activity, converted into ammonium nitrogen (García et al., 2010). In mixed liquor

around pH=7 ammonium is present in its ionic form (NH4+) and the reaction is given in equation

2.1 (van Haandel & van der Lubbe, 2012). The process is primarily temperature and pH

dependent and it occurs under aerobic and anaerobic conditions but slower in the latter condition.

RNH2 + H2O + H+ → ROH + NH4+ Eq. 2.1

Nitrification is responsible for the two-step transformation of ammonium nitrogen to nitrate as

end product. In the first step, ammonium is oxidized to nitrite by Nitrosomonas spp., and the

further oxidation of nitrite into nitrate is mediated by Nitrobacter spp. (Equation 2.2, 2.3). The

process is limited by dissolved oxygen, approximately 4.3 mg O2 is used to convert 1 mg

ammoniacal nitrogen to nitrate nitrogen (Vymazal, 2006). Therefore, it only takes place in well

aerated conditions. The minimum required temperature of nitrification is around 5oC and the

optimum pH varies from 6.6 to 8.0. These factors greatly determine the rate of transformation.

NH4+ + 3/2 O2 → NO2

- + H2O + 2 H+ Eq. 2.2

NO2- + 1/2 O2 → NO3

- Eq. 2.3

Dissimilatory nitrate reduction or denitrification happens in anaerobic (anoxic) conditions when

nitrate is present, serving as an electron acceptor and there is enough organic carbon functioning

as an electron donor (EPA, 1999). The products of denitrification are N2 and N2O gases which

are released to the atmosphere. The overall redox reaction is shown in equation 2.4 (van Haandel

& van der Lubbe, 2012).

CxHyOz + (4x+y-2z)/5 H+ + (4x+y-2z)/5 NO3-

→ x CO2 + (2x+3y-z)/5 H2O + (4x+y-2z)/10 N2 Eq. 2.4

31

In the study of Chiu et al. (2001), the optimal C/N ratios were determined for several initial

nitrate concentrations. The carbon source was sodium acetate and the optimal C/N ratios for

initial nitrate concentrations of 25, 50, 100, and 200 mg L-1 were around 5.5, 4.5, 4.0, and 2.6,

respectively. These ratios can be indicative for CWs but still different because of the carbon

source present in the system. Lu et al. (2009) artificially increased the available carbon source

for denitrification by providing glucose. The results showed that in the summer the nitrate

removal rates increased from 20% to 50% and from 10% to 30% in winter.

In the study of Foladori et al. (2013), the nitrified effluent was recirculated to the CW beds to

promote the use of residual biodegradable COD for denitrification. When intermittent aeration

and recirculation was applied simultaneous nitrification and denitrification occurred in the

VFCW (Foladori et al., 2013).

During nitrogen fixation nitrogen gas can be converted into organic nitrogen by organisms

containing nitrogenase enzyme. In natural wetlands it can be a significant source of nitrogen but

in constructed wetlands this factor is negligible.

2.8 Pathogenic and fecal indicator organisms and their removal Waterborne pathogens including helminths, protozoa, fungi, bacteria and viruses are great

concern to human health (EPA, 1999; Kadlec & Wallace, 2009). Nevertheless, in the developing

world it is a more severe issue due to the discharge of untreated wastewater to the environment

(García et al., 2013). In fact, as the measurement of pathogenic organisms is expensive and

technically challenging, indicator organisms are chosen to facilitate the easier monitoring. The

coliform bacteria group has been used among the first indicator organisms. Especially,

Escherichia coli is favored as it is easy to separate from other fecal groups and because several

strains can pose great risk to human health (Kadlec & Wallace, 2009).

Pathogens can be found in suspended solids and in suspensions of wastewater. Therefore, their

removal mechanism is greatly correlated with suspended particle removal which includes

sedimentation, interception and sorption (EPA, 1999). Besides predation, near the open water

surface of FWS CWs UV irradiation also contributes to the elimination of fecal indicators

(Kadlec & Wallace, 2009).

32

A survey conducted by Zhang & Farahbakhsh (2007), showed that conventional treatment

technologies consisting of primary, secondary treatment could achieve between 2 and 3 log units

removal for total and fecal coliform, somatic coliphage and F-specific coliphage. With the

addition of tertiary treatment (e.g. chlorination, sand filtration) it increased to 4 to 5 log units

removal and it was increased up to 6 to 7 log units for total and fecal coliform by applying

membrane bioreactor. However, this seemingly great solution is not applicable in developing

countries due to the high investments and operation costs.

CWs have also been studied to investigate pathogen removal mechanisms and assess their

removal rates. According to some authors SSF CWs have their pathogen removal rates in the

same range (García et al., 2013). However, in VFCWs and in HFFs the conditions such as

hydraulic retention time, the type of flow and resting periods are different and more varying in

VFCWs. Therefore, VFCWs are expected to have better performance in pathogen removal.

Besides, Morató et al. (2014) concluded that in HSSF CWs, the water depth, gravel size and

seasonal changes can greatly affect the microbial removal.

García et al. (2013), reached 3 to 4 log units removal for coliforms and E. coli by using a hybrid

CW system consisting of a VFCW and HSSF CW. Morató et al. (2014), testing HSSF CWs with

different water depth had 1.2-2.2 log units removal for total coliform and 1.4-2.3 log units

removal for E. coli.

33

3. Objectives

3.1 Overall objective The main objective of this study was to enhance the treatment performance compared to the

previous batch operation of Ilyas (2013) and to assess the effect of recirculation at different

HLRs considering the land requirements.

3.2 Specific objectives The specific objectives were:

1. To study the changes in the dissolved oxygen concentration in the VFCWs with pulse

feeding operation

2. To assess the performance of the Duplex-CWs with pulse feeding operation

3. To study the effect of intermittent pulse feeding of the primary settled wastewater and the

recirculated effluent on the performance of R-Duplex

4. To assess the effect of increasing HLR in the pulse-fed Duplex-CWs

5. To obtain higher denitrification rate in the HFF with pulse feeding operation

6. To reduce the land requirements of the Duplex-CW compared to the previous operation

7. To find a possible reuse for the effluent considering different standards

34

4. Materials and Methods

4.1 Experimental setup Two laboratory scale stacked type Duplex-CWs were used in this study. The setups consisted of

a VFCW (planted with Phragmites australis) and a HFF which was sealed to create

anoxic/anaerobic conditions, in time sequenced steps (Figure 3.1). The length, width and depth

of the VFCW were 0.6 m×0.4 m×0.8 m. The area of the HFF was identical to the VFCW’s and

the depth was 0.35 m. The media of the VFCW consisted of a 0.7 m fine sand section (1-2 mm)

and a 0.1 m thick gravel layer in the bottom for drainage (15-30 mm). These hybrid CW systems

were situated in the greenhouse of the UNESCO-IHE building (Delft, the Netherlands). The

ensured conditions in the greenhouse were: temperature at least 21oC and light intensity at least

85-100 µmol photons m–2 s–1 for 16 h d-1.

The first setup, abbreviated as R-Duplex, had the effluent of the HFF recirculated. The other,

referred as C-Duplex was the control without recirculation. Both systems were supplied with

domestic wastewater (settled primary effluent) from the Harnaschpolder WWTP in Delft. The

characteristics of this wastewater are given in Table 4.1. The wastewater was provided 5 days a

week and the weekends served as resting period. The wastewater was fed to the Duplex-CWs

from plastic tanks (volume = 50 L), where the bottom 5 L was kept for settling, and was not

used.

1. VFCW 2. HFF 3. Raw wastewater 4. Peristaltic pump -

influent 5. Peristaltic pump -

recirculation 6. Ball valve

Figure 4.1 Duplex-CW setup

35

Timer controlled Masterflex® peristaltic pumps (Cole-Parmer, United States) were used for

feeding the VFCWs and for recirculation. The influent was distributed on the surface of the

VFCW with the help of perforated pipes. Then, the partially treated wastewater from the VFCW

was drained to the HFF through a slightly open ball valve. The discharge time of the VFCWs

was around 30-40 min.

During the experimental periods, pulse feeding was applied to R-Duplex and C-Duplex,

respectively. The operation of the two set-ups was controlled with two timers (Egston Power

Supply, Austria) which were set to turn on the influent and the recirculation pumps for 15 min

every 2 h as shown in Table 4.2. In R-Duplex, the recirculation pump was always set to provide

a recirculation ratio of 0.8:1 in order to avoid significant water level fluctuations in the HFF.

Parameters Range

pH 7.0-7.6

EC (µS cm-1) 1381-1841

DO (mg L-1) 0.3-0.8

COD (mg L-1) 207-424

TSS (mg L-1) 56-189

NH4+‐N (mg L-1) 35-55

NO3‐‐N (mg L-1) 0.3-3

TN (mg L-1) 46-71

Hour 1 Hour 2 Hour 3 Hour 4 Hour 5 Hour 6 …

C-Duplex

Influ

ent f

eedi

ng fo

r

15 m

in

Influ

ent f

eedi

ng fo

r

15 m

in

Influ

ent f

eedi

ng fo

r

15 m

in

R-Duplex Recirculation

for 15 min

Recirculation

for 15 min

Recirculation

for 15 min

Table 4.2 Operation of the Duplex-CWs

Table 4.1 Characteristics of the settled domestic wastewater during this study

36

4.2 Experimental design The experimental phase was divided into four stages applying HLRs at 0.05, 0.08 and 0.16 m3

m-2 d-1. In the fourth experimental phase the HLR of 0.16 m3 m-2 d-1 was kept and the OLR was

increased to 88 g COD m-2 d-1 (Table 4.3). The second column of Table 4.3 shows the original,

two days per week feeding regime and its parameters in a previous study (Ilyas, 2013). The

OLRs were calculated based on the COD concentrations of each experimental phase, and the

land requirements were based on the BOD5 concentrations and the HLRs and were expressed in

m2 per PE. The calculations can be found in Annex 1.

Parameters Operation of

Ilyas, 2013

1st

Experimental

phase

2nd

Experimental

phase

3rd

Experimental

phase

4th

Experimental

phase

Feed volume (L d-1)

39 19 30 60 60

Feeding (d/week)

2 5 5 5 5

HLR (m3 m-2 d-1)

0.05 0.05 0.08 0.16 0.16

OLR (g COD m-2 d-1)

15 13 18 56 88*

Land requirement (m2 PE-1)

8.6 9.0 6.8 2.1 1.2

HRT HFF (d)

3-4 2 1.3 0.7 0.7

Duration of phase (week)

- 6 4 4 3

Table 4.3 Operation modes and parameters of the Duplex-CWs

*OLR was artificially increased with peptone

37

4.2.1 First experimental period - HLR 0.05 m3 m-2 d-1 + OLR 13 g COD m-2 d-1 In the first six weeks of operation, the peristaltic pumps were providing 19 L wastewater per day.

Taking into account the receiving surface of the VFCWs (0.24 m2), the HLR was 0.05 m3 m-2 d-1

which was the same as in the previous studies of the Duplex CWs (Ilyas 2013; Lavrnić, 2013;

Kyomukama, 2014; Namakula, 2014). Due to the natural variation of wastewater characteristics

the OLR was lower than the one Ilyas (2013) applied (Table 4.3). The required area was

calculated to be 9.0 m2 per PE.

4.2.2 Second experimental period - HLR 0.08 m3 m-2 d-1 + OLR 18 g COD m-2 d-1 In the second experimental phase, the pumps were set to provide 30 L wastewater per day. The

corresponding HLR was 0.08 m3 m-2 d-1, and the required are was 6.8 m2 per PE. The increase in

the feeding volume was chosen to be small as the Duplex-CWs had never been operated at

higher HLR.

4.2.3 Third experimental period - HLR 0.16 m3 m-2 d-1 + OLR 56 g COD m-2 d-1 In order to assess the performance of the Duplex-CWs in case of a shock load, the HLR was

chosen to be two times higher than in the second experimental phase. Therefore, each day 60 L

wastewater was fed to the systems, being equal to the HLR of 0.16 m3 m-2 d-1. This HLR is in the

medium category considering what other researchers (Ghosh & Gopal, 2010; Foladori et al.,

2013) were applying. Nevertheless, the OLR was more than two times higher compared to the

second experimental period, as the wastewater was more concentrated due to the variation in

weather. It caused significant area reduction to 2.1 m2 per PE.

4.2.4 Fourth experimental period - HLR 0.16 m3 m-2 d-1 + OLR 88 g COD m-2 d-1

In order to test the Duplex-CW with more concentrated wastewater, at the HLR of 0.16 m3 m-2

d-1 the composition of the wastewater was modified. Regarding the characteristics of the

Harnaschpolder wastewater (Table 4.1), it was classified as low strength domestic wastewater

(Tchobanoglous et al., 2003). Based on the preliminary experiments of Ilyas (2013), due to the

addition of peptone (0.3 g L-1 wastewater) the COD concentration in the fourth experimental

period was in the range of 443-633 mg L-1.

38

Implicating that in the fourth experimental period the wastewater was in the medium strength

domestic wastewater category (Tchobanoglous et al., 2003). As a result, the OLR was increased

from 56 g COD m-2 d-1 to 83 g COD m-2 d-1 and the corresponding required are was 1.2 m2 per

PE.

4.3 Passive aeration capacity of the vertical flow constructed wetland Anoxic water was prepared by adding CoCl2 (33 mg) as a catalyst and Na2SO3 (0.8 g) in 10 L

demineralized water and further by bubbling N2 gas. Annex 2. contains the details of the

calculations. The anoxic water was prepared in a container, which was connected to the

distribution pipes of the VFCWs through the original tubing (Figure 4.2). It was supplied by

mean of a peristaltic pump at flowrate of 315 mL min-1. Besides, care was taken to avoid

intrusion of oxygen into the system.

The experiment was performed on the dry R-VFCW after the two days resting period and on

both VFCWs during operation, replacing one feeding cycle. Samples were taken before the

distribution pipes to check if there was a change in the DO concentration due to the setup. In

order to assess the passive aeration capacity, 20 minutes after the pump was switched on samples

were taken in every 5 minutes. This experiment was conducted only once.

Figure 4.2 Setup of the passive aeration capacity test for the VFCWs

39

4.4 Analytical procedure During this study, samples were taken weekly from the inlet of the VFCWs, the outlet of the

VFCWs and the effluent of the HFFs. The sampling took place between 20 and 45 minutes after

the VFCWs were fed with wastewater. The samples were taken for analysis right after the

sampling or preserved at -4oC. Table 4.4 summarizes the methods used for the different analysis.

Ammonium nitrogen was measured according to the NEN Dutch standard. The rest of the

parameters were analyzed according to APHA (2012).

Parameter Method Equipment

Hydrogen potential Electrode method pH meter

Temperature (oC) DO meter

Electrical conductivity (µS cm-1) Electrode method EC meter

Total suspended solids (mg L-1) Gravimetric method Oven, filter papers

Dissolved oxygen (mg L-1) Electrode method DO meter

Biochemical oxygen demand (mg L-1) Electrode method

Total nitrogen (mg L-1) Persulfate method UV-VIS Perken Elmer spectrometer Lambda 20

Ammonium nitrogen (mg L-1) Colorimetric method

Nitrate nitrogen (mg L-1) 2,6-dimethylphenol method

4.4.1 Physical properties The physical parameters including dissolved oxygen (DO), electrical conductivity (EC),

temperature and hydrogen potential (pH) were measured right after sampling.

For the measurement of TSS, known volumes of samples were vacuum filtered and the filters

were dried at 105oC for 2 h. After cooling, the filters were weighed and the TSS concentration

was calculated according to equation 4.1.

TSS (mg L ) = Eq. 4.1 Where, A = weight of dry filter plus sample, g B = weight of dry filter, g V = sample volume, mL

Table 4.4 Analytical procedures

40

4.4.2 Nitrogen constituents TN was measured according to the Koroleff method. Known volumes of samples were placed in

100 mL Erlenmeyer flasks. 5 mL of digestion solution (5.0 g K2SO8 and 3.0 g of H3BO3

dissolved in 100 mL of 0.375 M NaOH solution) was added to each flask and were filled up to

30 mL with demineralized water. The Erlenmeyer flasks were covered with cotton plugs and

plastic lids and autoclaved for 30 min at 110oC. After cooling, the contents were transferred to 50

mL volumetric flasks and the absorbance of the samples was measured using the

spectrophotometer at 220 and 275 nm (Table 4.4). TN concentration was calculated according to

equation 3.2.

TN (mg L ) = ( ) × E Eq. 4.2

Where, A = absorbance at 220 nm B = absorbance at 275 nm C = calibration curve intercept D = slope of the calibration curve E = dilution factor

Ammonium nitrogen (NH4+-N) was measured according to the Dutch NEN standards. Up to 10.5

mL of the samples were placed in 20 mL plastic cups. Then, 1 mL of salicylate reagent (130 g

NaC7H5O3, 130 g Na3C6H5O7∙H2O and 0.970 g Na2Fe(CN)5NO∙2H2O dissolved in 1 L) and 1

mL of dichloroisocyanurate reagent (32.0 g NaOH and 2.00 g NaC3N3O3Cl2 dissolved in 1 L)

were added. The absorbance of samples was measured between 1 and 3 h with the

spectrophotometer at 655 nm (Table 4.4). The concentration of ammonium in the samples was

calculated as shown in equation 4.3.

NH − N (mg L ) = ( ) ×

Eq. 4.3

Where, A = absorbance at 655 nm C = calibration curve intercept D = slope of the calibration curve V = sample volume, mL

41

Nitrate nitrogen (NO3--N) was determined using the 2,6-dimethylphenol method. In 20 mL

plastic cups 7 mL of reagent 1 (0.040 g amidosulfonic acid in 10 mL H2O, 500 mL conc. H2SO4

and 500 mL conc. H3PO4) and up to 1 mL of the samples were placed. Then, it was shaken

manually and after 5-10 min 1 mL of reagent 2 (1.2 g 2,6-dimethylphenol in 1 L glacial acetic

acid) was added. The absorbance was measured between 10 and 60 minutes at 324 nm. The

calculation of nitrate concentration is given in equation 4.4.

NO − N (mg L ) = ( ) × E Eq. 4.4

Where, A = absorbance at 324 nm C = calibration curve intercept D = slope of the calibration curve E = dilution factor

4.4.3 Organic constituents BOD5 was determined with the oxygen electrode method and the concentration was calculated

according to equation 4.5.

BOD (mg L ) = ( )× Eq. 4.5

Where, A = initial DO concentration, mg L-1 B = final DO concentration, mg L-1 C = volume of sample bottle, mL D = sample volume, mL

COD was determined with the closed reflux method and the concentration was calculated

according to equation 4.6.

COD (mg L ) = ( )× × Eq. 4.6

Where, A = volume of FAS needed for titration of the blank, ml B = volume of FAS needed for titration of the sample, ml C = volume of the sample used, ml D = molarity of the FAS

42

4.4.4 Salmonella, E. coli and coliforms The pathogenic (Salmonella) and faecal indicator bacteria in water were enumerated by using

Chromo cult Coliform© (CCA) agar. The culture media was prepared according to the

manufacturer's instruction (Merck, Germany), where 26.5 g of CCA were dissolved in 1 L of

distilled water and was placed in a water bath at 99.9oC for 45 min. The liquid CCA was then

poured in 500 mL volumetric flasks and left to cool to about 50oC. After cooling it was poured

into 50 mm petri dishes and left to dry for one week before culturing the microbes.

Sample dilutions varied according to the sample type. Influent samples were diluted up to 200

times in most cases whereas the effluents were diluted up to 100 times. 0.1 ml of each diluted or

undiluted sample was added to the prepared petri dishes, spread aseptically and then incubated at

37oC. All plates were inoculated in triplicates.

After 24 h, the the different colonies of specific pathogenic and fecal indicator bacteria were

counted based on the characteristics (shape and color) of the colonies formed. The results were

expressed in colony forming units (CFUs)/100mL (Equation 4.7).

Bacteria Colonies (CFU/100mL) = ∗ E ∗ 100 Eq. 4.7

Where, A = colonies counted B = volume of sample plated, 0.1 ml E = dilution factor

4.5 Statistical analysis Mean and standard error of the mean (SEM) were calculated using Microsoft Office Excel 2007.

Normal distribution of the data was tested with RStudio (version 0.97.310) using the Bartlett test

of homogeneity of variances. One-way analysis of variance (ANOVA) was used to determine the

effect of recirculation in the Duplex-CWs. The test was done for the comparison of the two

setups using the mean effluent concentrations of the different parameters.

43

5. Results

5.1 Passive aeration capacity of the Vertical Flow Constructed Wetland The mean DO concentration of the anoxic water was 0.1 mg L-1 and the temperature was 23-

24oC. The DO concentration of the water before the distribution pipe was taken into account to

calculate the changes in the DO concentrations, these absolute values are presented for easier

interpretation (Figure 5.1). After the two days resting period, in the dry VFCW with recirculation

(R-VFCW) the DO concentration decreased sharply from 6.6 to 3.6 mg L-1 in the first 40

minutes. Then, there was a slow decrease and it leveled at 1.3 mg L-1 after 60 minutes. When, in

R-VFCW, the wastewater feeding cycle was replaced with the passive aeration capacity

experiment, the DO concentration increased to 2.6 mg L-1 in the first 45 minutes. After 60

minutes it stablized at around 1.6 mg L-1. In the control VFCW (C-VFCW), there was a slow

increase to 2.9 mg L-1 in the first 40 minutes. It was followed by a slow decrease and it leveled

after 60 minutes at 1.6 mg L-1.

01234567

0 20 25 30 35 40 45 50 55 60 65 70 75

DO (m

g L-1

)

Time (min)

Dry R-VFCW Operation R-VFCW Operation C-VFCW

Figure 5.1 Change in the DO concentration (absolute values) of the dry and operational VFCWs

44

5.2 First experimental period - HLR 0.05 m3 m-2 d-1 + OLR 13 g COD m-2 d-1

4.2.1 Physical properties – pH, EC, Temperature, DO and TSS The influent pH (Mean±SEM) of the two systems was 7.5±0.0. The effluent pH of the R-VFCW

and C-VFCW were slightly lower (6.7±0.0 and 6.6±0.1, respectively) than in the effluent. In

both systems the final effluents had a slight increase in the pH, 6.9±0.0 (Figure 5.2 A).

The EC of the influents was in the range of 1817-1841 µS cm-1. The effluents of the R-VFCW

and the C-VFCW were 1611±45.7 and 1632±57.4 µS cm-1, respectively. In the effluent of the

system with recirculation the EC increased, 1634±47.2 µS cm-1 and the control had a decrease to

1604±42.5 µS cm-1(Figure 5.2 B).

The temperature of the influent was in the range of 18-19oC, the effluents of the VFCWs were

20oC and the HFF effluents were around 20-21oC (Figure 5.2 C).

The DO concentration in the influent was 0.5-0.6 mg L-1 continuously. In the effluent of the

VFCWs, the sytem with recirculation had higher DO concentration 2.0±0.3 mg L-1, than the

control, 1.0±0.1 mg L-1. The R-HFF effluent’s DO concentration slightly decreased to 1.6±0.2

mg L-1 (Figure 5.2 D).

The influent TSS concentration was in the range of 75-98 mg L-1. The effluent concentrations of

the two VFCWs and the HFFs didn’t show significant difference. In the R-VFCW and in the C-

VFCW, the TSS concentrations were 12±2 mg L-1 and 13±2 mg L-1 and the corresponding

removals were 88 and 82%, respectively. Respectively, the TSS concentration in the final

effluent of R-Duplex and C-Duplex were 8±1 and 11±2 mg L-1. The recirculation increased the

TSS removal efficiency from 85% to 92% (Figure 5.2 E).

45

5.2.2 Nitrogen constituents The influent NH4

+-N concentration (Mean±SEM) was 48.7±1.0 mg L-1 for the two setups. The

removal rates were 88% and 83 % for R-VFCW and C-VFCW, respectively while the effluent

concentrations were 6.0±0.5 and 8.2±3.1 mg L-1, respectively.

0.01.02.03.04.05.06.07.08.0

R-Duplex C-Duplex

pH

A Influent VFCW effluent HFF effluent

0300600900

1200150018002100

R-Duplex C-Duplex

EC (µ

S cm

-1)

B

0.03.06.09.0

12.015.018.021.024.0

R-Duplex C-Duplex

Tem

pera

ture

(o C)

C

0.0

0.5

1.0

1.5

2.0

2.5

R-Duplex C-Duplex

DO (m

g L-1

)

D

0

20

40

60

80

100

0

20

40

60

80

100

120

R-Duplex C-Duplex

Rem

oval

(%)

TSS

(mg

L-1)

EVFCW removal Total removal HFF removal

Figure 5.2 Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of C-Duplex n=3)

46

0.0

20.0

40.0

60.0

80.0

100.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

R-Duplex C-Duplex

Rem

oval

(%)

NH

4+ -N

(mg

L-1)

A

Influent VFCW effluent HFF effluent

VFCW removal HFF removal Total removal

In overall the C-Duplex showed slightly better NH4+-N removal, 99% instead of 97% and the

final effluent had a concentration of 0.5±0.2 mg L-1 compared to the system with recirculation

which was 1.4±0.6 mg L-1 (Figure 5.3 A).

0.0

10.0

20.0

30.0

40.0

50.0

R-Duplex C-Duplex

NO

3- -N (

mg

L-1)

B

0

20

40

60

80

100

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

R-Duplex C-Duplex

Rem

oval

(%)

TN (m

g L-1

)

C

Figure 5.3 Mean (±SEM) of the nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of C-Duplex n=3)

47

0

20

40

60

80

100

04080

120160200240280

R-Duplex C-Duplex

Rem

oval

(%)

COD

(mg

L-1)

A



The wastewater entering the Duplex-CWs had a NO3--N concentration of 1.9-2.0 mg L-1. In both

systems, NO3--N increased with similar extent to 35.0±2.7 and 36.9±5.6 mg L-1 in the effluent of

R-VFCW and C-VFCW, respectively. Then, it further increased in the HFFs to 37.6±3.7 and

39.1±3.0 mg L-1, respectively (Figure 5.3 B).

The TN concentration (Mean±SEM) in the influent of R-Duplex and C-Duplex were 62.7±1.3

and 60.5±0.4 mg L-1, respectively. The VFCW with recirculation had slightly lower TN

concentration, 42.3±1.2 mg L-1 instead of 44.4±3.2 mg L-1, which corresponded with the removal

rate of 33% and 27%. The removal in the C-HFF was slightly better, 15% compared to the 12%

in R-HFF. For both systems the average concentration of the final effluent was 37.5±2.2 mg L-1

(Figure 5.3 C).

5.2.3 Organic constituents The COD concentration (Mean±SEM) of the influent was 247±13 and 258±13.7 mg L-1 for R-

Duplex and C-Duplex, respectively. Due to the removal in R-VFCW and C-VFCW, 81% and

78%, the effluent concentrations were 47±6 and 56±8 mg L-1, respectively. However, C-HFF

ensured higher removal than R-HFF, therefore, the average COD concentration in the final

effluents was 35 mg L-1 (Figure 5.4 A). In the two systems 86-87% of the incoming COD was

removed.

Figure 5.4 A Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of R-Duplex n=3)

48

The influents of R-Duplex and C-Duplex had a BOD5 concentration (Mean±SEM) of 152±18

and 116±24 mg L-1, respectively. In the effluents of the VFCWs, the BOD5 concentration was

20±5 and 24±2 mg L-1, respectively. The effluent of R-HFF and C-HFF had 6±1 and 4±0 mg L-1

BOD5, respectively. The total removal of BOD5 in both systems was 96-97% (Figure 5.4 B).

5.3 Second experimental period - HLR 0.08 m3 m-2 d-1 + OLR 18 g COD m-2 d-1

5.3.1 Physical properties – pH, EC, Temperature, DO and TSS The pH of both systems’ influent was stable during the second experimental phase, 7.3±0.0. The

effluent pH of the VFCWs dropped to 6.8±0.0. Then, in the HFFs the pH increased to 6.9±0.1

(Figure 5.5 A).

The EC of the influent for the system with recirculation and the control was 1405±3.0 and

1390±9.0 µS cm-1, respectively. In the effluents of the R-VFCW and the C-VFCW it decreased

to 1251±4.5 and 1287±18.0 µS cm-1, respectively. Similarly to the first phase, the EC increased

in the effluent of the system with recirculation, 1264±2.5 µS cm-1 and in the control it decreased

to 1253±10.0 µS cm-1 (Figure 5.5 B).

0

20

40

60

80

100

020406080

100120140160180

R-Duplex C-Duplex

Rem

oval

(%)

BOD 5

(mg

L-1)

B Influent VFCW effluent HFF effluent


Figure 5.4 B Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 1st experimental phase (n=4, except influent of R-Duplex n=3 and C-Duplex n=2)

49

0.01.02.03.04.05.06.07.08.0

R-Duplex C-Duplex

pH

AInfluent VFCW effluent HFF effluent

0200400600800

1000120014001600

R-Duplex C-DuplexEC

(µS

cm-1

)

B

The system with recirculation had slightly higher temperature in all stages compared to the

control. In R-Duplex, the influent was at 20.9±0.5oC, the effluent of the VFCW and the HFF

were 21.7±0.3 and 21.8±0.3oC, respectively. Respectively, the temperatures of C-Duplex were

20.2±0.3, 21.4±0.3 and 21.4±0.3oC (Figure 5.5 C).

0.03.06.09.0

12.015.018.021.024.0

R-Duplex C-Duplex

Tem

pera

ture

(o C)

C

0.0

1.0

2.0

3.0

4.0

5.0

R-Duplex C-Duplex

DO (m

g L-1

)

D

0

20

40

60

80

100

0102030405060708090

R-Duplex C-Duplex

Rem

oval

(%)

TSS

(mg

L-1)

EVFCW removal HFF removal Total removal

Figure 5.5 Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 2nd experimental phase (n=3, except EC n=2)

50

The DO concentration (Mean±SEM) in the influent of R-Duplex and C-Duplex was 0.6±0.1 and

0.7 ±0.1 mg L-1, respectively. The DO concentration in the VFCW of the recirculated system and

the control increased to 3.4±0.9 and 3.8±0.7 mg L-1. In R-HFF and C-HFF it dropped to 2.0±0.3

and 1.4±0.3 mg L-1, respectively (Figure 5.5 D).

The influent TSS concentration was 72±6 mg L-1. The R-VFCW was more effective at removing

TSS, 79% compared to the C-VFCW with 67% removal. However, there was no significant

difference in the effluent TSS of the VFCWs (ANOVA, F1,4=1.28, P=0.3). Respectively, the TSS

concentration in the effluent of R-VFCW and C-VFCW was 15±4 and 24±7 mg L-1. In the HFF

of R-Duplex and C-Duplex, there was further removal which resulted in 8±0 and 10±1 mg L-1

TSS. The overall TSS removal was slightly higher in R-Duplex than in C-Duplex with numbers

89% and 86%, respectively (Figure 5.5 E).


+-N was 36.8±0.6 mg L-1 which is slightly lower than in the first experimental

phase. In the VFCW, the recirculation increased the removal efficiency from 65% to 81%, the

corresponding effluent concentrations were 6.9±2.8 and 12.8±4.2 mg L-1. The total removal in R-

Duplex and C-Duplex was 98% and 93%. It resulted in 0.8±0.4 and 2.6±0.4 mg L-1 NH4+-N,

respectively (Figure 5.6 A).

The NO3--N concentration in the influent of R-Duplex and C-Duplex was 1.4±0.4 and 2.0±0.2

mg L-1. The increase was slightly higher in R-VFCW which resulted in 23.9±0.9 mg L-1

compared to the control with 22.2±3.1 mg L-1. In C-HFF, the NO3--N concentration decreased to

20.5±2.2 mg L-1, while in R-VFCW it increased to 32.2±2.6 mg L-1 (Figure 5.6 B).

In the influent of R-Duplex and C-Duplex the TN concentration was 48.6±0.9 and 47.6±0.7 mg

L-1, respectively. The VFCW with recirculation was more efficient in the removal than the

control 30% instead of the 19%. The corresponding concentrations were 34.0±0.1 and 38.8±1.7

mg L-1. However, in overall, the control performed better with 56% removal compared to 40% in

the recirculated system. The final TN concentrations in R-Duplex and C-Duplex were 29.1±1.8

and 20.8±1.7 mg L-1, respectively (Figure 5.6 C).

51

0

20

40

60

80

100

0.0

10.0

20.0

30.0

40.0

50.0

R-Duplex C-Duplex

Rem

oval

(%)

NH

4+ -N

(mg

L-1)

A



0.05.0

10.015.020.025.030.035.040.0

R-Duplex C-Duplex

NO

3- -N (m

g L-1

)

B

0

20

40

60

80

100

0.0

10.0

20.0

30.0

40.0

50.0

60.0

R-Duplex C-Duplex

Rem

oval

(%)

TN (m

g L-1

)

C

Figure 5.6 Mean (±SEM) of the nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 2nd experimental phase (n=3)

52

0

20

40

60

80

100

0

40

80

120

160

200

240

280

R-Duplex C-Duplex

Rem

oval

(%)

COD

(mg

L-1)



5.3.3 Organic constituents For R-Duplex and C-Duplex, the COD concentration in the influent was 224±15 and 219±7 mg

L-1, respectively. The removal rates in R-VFCW and C-VFCW were 70% and 63% and the

corresponding concentrations were 68±6 and 56±8 mg L-1. The final effluents of R-Duplex and

C-Duplex had 34±3 and 37±1 mg L-1 COD, respectively. The recirculation slightly improved the

removal from 83% to 85% (Figure 5.7 A).

The BOD5 concentration of the influent was slightly lower than in the first experimental phase;

for R-Duplex 121±6 and for C-Duplex 106±1 mg L-1. In R-VFCW and in C-VFCW the BOD5

concentration was reduced to 29±7 and 31±4 mg L-1, respectively. The overall removal was

increased to 98% from 94% in the system with recirculation. The BOD5 concentration in the final

effluent of R-HFF and C-HFF was 2±0 and 6 mg L-1, respectively (Figure 5.7 B).

0

20

40

60

80

100

0

20

40

60

80

100

120

140

R-Duplex C-Duplex

Rem

oval

%

BOD 5

(mg

L-1)

B

Figure 5.7 Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 2nd experimental phase (for COD n=3, for BOD5 n=2, except effluent of R-HFF n=1)

53

5.4 Third experimental period - HLR 0.16 m3 m-2 d-1 + OLR 56 g COD m-2 d-1

5.4.1 Physical properties – pH, EC, Temperature, DO and TSS The pH of both systems’ influent was 7.2±0.1. In the different compartments of R-Duplex the pH

was not varying, it was 6.6±0.1. However, in C-Duplex first it decreased to 6.7±0.1 and then, it

increased to 6.9±0.1 in the HFF (Figure 5.8 A).

The EC of the influent was 1816±6.5 µS cm-1. There was a gradual decrease in the EC in the two

stages in both systems. In the first stage of R-Duplex it decreased to 1657±24.8 and in the second

stage it went to 1624±17.2 µS cm-1. In C-Duplex, the EC concentrations were 1674±32.0 and

1629±20.2 µS cm-1, respectively (Figure 5.8 B).

For the two systems, the temperature of the influent was 20.2±0.5oC. The effluents of C-Duplex

had the same temperature 21.5±0.5oC. In R-Duplex, it increased from 21.3±0.7oC to 21.8±0.7oC

in the effluent of the VFCW to the effluent of the HFF (Figure 5.8 C).

In the influent of R-Duplex and C-Duplex the DO concentration was 0.4±0.0 and 0.3±0.0 mg L-1,

respectively. In R-VFCW and C-VFCW, it increased to 1.9±0.7 and 1.9±0.4 mg L-1,

respectively. In the HFFs, the decrease in DO was more pronounced in the C-Duplex, 1.0±0.0

mg L-1, while in the final effluent of R-Duplex it was 1.2±0.1 mg L-1 (Figure 5.8 D).

The TSS concentration in the influent of R-Duplex and C-Duplex was 140±18 and 127±21 mg L-

1, respectively. The TSS concentration in the effluent of C-VFCW was significantly higher than

in R-VFCW (ANOVA, F1,6=64.35, P=<0.001). The TSS concentrations were 34±2 mg L-1 and

16±2 mg L-1, respectively. In the final effluents the difference was insignificant, the TSS

concentration in C-HFF and R-HFF was 16±5 and 9±1 mg L-1, respectively. In overall the

system with recirculation had an increased TSS removal from 88% to 93% (Figure 5.8 E).

54

0.01.02.03.04.05.06.07.08.0

R-Duplex C-Duplex

pH


0200400600800

100012001400160018002000

R-Duplex C-Duplex

EC (µ

S cm

-1)

B

0.03.06.09.0

12.015.018.021.024.0

R-Duplex C-Duplex

Tem

pera

ture

(o C)

C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

R-Duplex C-Duplex

DO (m

g L-1

)

D

0

20

40

60

80

100

020406080

100120140160180

R-Duplex C-Duplex

Rem

oval

(%)

TSS

(mg

L-1)


Figure 5.8 Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=5, except effluent of C-VFCW n=3)

55

0

20

40

60

80

100

0.0

10.0

20.0

30.0

40.0

50.0

60.0

R-Duplex C-Duplex

Rem

oval

(%)

NH

4+ -N

(mg

L-1)

A



0.05.0

10.015.020.025.030.035.0

R-Duplex C-Duplex

NO

3- -N (m

g L-1

)

B


+-N concentration increased in the third period to 49.0±1.6 mg L-1. The

recirculation enhanced the removal of NH4+-N in the VFCW, it increased from 49% to 70% and

the corresponding effluent concentrations were 25.0±3.7 and 14.8±3.1 mg L-1. In C-HFF and R-

HFF, it further decreased to 19.2±2.6 and 7.7±1.8 mg L-1, respectively. The removal of NH4+-N

was significantly higher in R-Duplex (ANOVA, F1,8=13.45, P=<0.01)., the removal increased

from 61% to 84% (Figure 5.9 A).


mg L-1. The increase was more profound in R-VFCW as the NO3--N concentration in the effluent

was 27.6±1.9 mg L-1 while in the control it was 19.5±4.0 mg L-1. In the HFFs, the pattern was the

same as in the second experimental phase. In C-HFF, the NO3--N concentration decreased to

13.4±3.0 mg L-1, while in R-VFCW it increased to 30.4±1.5 mg L-1 (Figure 5.9 B).

Figure 5.9 A, B Mean (±SEM) of the nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=3)

56

The TN concentration in the influent of R-Duplex and C-Duplex was 65.4±2.2 and 64.5±1.7 mg

L-1, respectively. R-VFCW was slightly better at the removal of TN, resulting in 45.2±2.1 mg L-

1, while in the effluent of the control it was 49.5±2.1 mg L-1. Nevertheless, C-Duplex had

significantly higher removal (ANOVA, F1,8=15.05, P=<0.01), 51% compared to the 41% in the

system with recirculation. The final concentrations in R-Duplex and C-Duplex were 38.5±1.2

and 31.8±1.2 mg L-1, respectively (Figure 5.9 C).

5.4.3 Organic constituents For R-Duplex and C-Duplex, the COD concentration in the influent was 224±15 and 219±7 mg

L-1. The removal rates in R-VFCW and C-VFCW were 70% and 63% and the corresponding

concentrations were 68±6 and 56±8 mg L-1, respectively. The final effluents of R-Duplex and C-

Duplex had 34±3 and 37±1 mg L-1 COD, respectively. The recirculation slightly improved the

removal from 83% to 85% (Figure 5.10 A).

The BOD5 concentration of the influent was slightly lower than in the first experimental phase;

for R-Duplex 121±5.9 and for C-Duplex 106±0.9 mg L-1. In R-VFCW and in C-VFCW, the

BOD5 concentration was reduced to 29±7.3 and 31±4.2 mg L-1, respectively. The overall

removal was increased to 98% from 94% in the system with recirculation. The BOD5

concentration in the final effluent of R-HFF and C-HFF was 2±0.0 and 6 mg L-1, respectively

(Figure 5.10 B).

0

20

40

60

80

100

0.010.020.030.040.050.060.070.080.0

R-Duplex C-Duplex

Rem

oval

(%)

TN (m

g L-1

)

C

Figure 5.9 C Mean (±SEM) of the nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=3)

57

0

20

40

60

80

100

04080

120160200240280320360400

R-Duplex C-Duplex

Rem

oval

(%)

COD

(mg

L-1)

A



5.4.4 Fecal indicator organisms The concentration of E. coli (Mean±SEM) in the influent of both systems was 106CFU/100 ml.

The removal in the VFCWs was less than 1-log unit. In the HFFs of R-Duplex and C-Duplex, the

E. coli removal was 1-log unit. In overall, the recirculation improved the removal as it was

increased from 93% to 96% (Figure 5.11 A).

The fecal coliform concentration (Mean±SEM) was 106CFU/100 ml in the influent. Less than 1-

log unit was the removal in both VFCWs. R-VFCW performed better with 76% removal

compared to C-VFCW with 66%. In the HFFs, there was 1-log unit removal but the overall

performance of the two Duplex-CWs was very similar with 94-95% removal (Figure 5.11 B).

0

20

40

60

80

100

020406080

100120140160180200220

R-Duplex C-DuplexRe

mov

al (%

)

BOD 5

(mg

L-1)

B

Figure 5.10 Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (for COD n=5, for BOD5 n=4 except effluent of C-HFF n=3)

58

0

20

40

60

80

100

0

2

4

6

8

10

R-Duplex C-Duplex

Rem

oval

(%)

E. co

li(lo

g CF

U/1

00m

L)

A



Salmonella was detected in the influent and in the effluents of both systems but the concentration

was less than 101CFU/100mL.

5.5 Fourth experimental period - HLR 0.16 m3 m-2 d-1 + OLR 88 g COD m-2 d-1

5.4.1 Physical properties – pH, EC, Temperature, DO and TSS The pH in the influent of R-Duplex and C-Duplex was 7.1±0.1 and 6.9±0.3, respectively. In R-

Duplex, the pH decreased first to 6.5±0.2, then to 6.4±0.1. In C-VFCW it decreased to 6.4±0.2

and then, in C-HFF it increased to 6.7±0.1 (Figure 5.12 A).

0

20

40

60

80

100

0

2

4

6

8

10

R-Duplex C-DuplexRe

mov

al (%

)

Feca

l col

iform

(log

CFU

/100

mL)

B

Figure 5.11 Mean (±SEM) of the fecal indicator organisms of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 3rd experimental phase (n=3)

59

0.01.02.03.04.05.06.07.08.0

R-Duplex C-Duplex

pH


0300600900

1200150018002100

R-Duplex C-DuplexEC

(µS

cm-1

)

B

For R-Duplex and C-Duplex, the EC in the influent was 1847±101.2 and 1178±115.2 µS cm-1,

respectively. In R-Duplex, the decrease in EC was more visible, after the VFCW 1750±112.6 µS

cm-1 and after the HFF 1647±71.8 µS cm-1. In C-Duplex, it was more stable, in the two stages the

EC was 1754±121.9 and 1738±125.9 µS cm-1, respectively (Figure 5.12 B).

0.03.06.09.0

12.015.018.021.024.027.0

R-Duplex C-Duplex

Tem

pera

ture

(o C)

C

0.00.20.40.60.81.01.21.4

R-Duplex C-Duplex

DO (m

g L-1

)

D

0

20

40

60

80

100

020406080

100120140160180

R-Duplex C-Duplex

Rem

oval

(%)

TSS

(mg

L-1)


Figure 5.12 Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (n=4)

60

The influent temperature was stable, 22.7±0.3oC. In R-VFCW, it increased to 23.4±0.8, and in

the R-HFF it went to 24.1±0.6oC. In C-Duplex it was not varying in the different stages, it was

23.9±0.4oC (Figure 5.12 C).

In the influent of R-Duplex and C-Duplex, the DO concentration was 0.4±0.0 and 0.5±0.2 mg L-

1, respectively. The DO concentration in C-VFCW was slightly higher than in R-VFCW, it was

1.0±0.2 and 0.7±0.1 mg L-1, respectively. The pattern was the same in the HFFs, in C-Duplex,

1.1±0.1 mg L-1, while in R-Duplex it was 1.0±0.1 mg L-1 (Figure 5.12 D).

In the influent, the TSS concentration was 134±9 mg L-1. Recirculation improved TSS removal

in the VFCW from 70% to 82%, the corresponding concentraionts were 24± 4 and 39±9 mg L-1.

The overall removal was also higher in the R-Duplex, 92% instead of the 80% in the control.

However, the difference between the two effluents was not significant (ANOVA, F1,6=5.75,

P=0.05). The TSS concentration in the final effluents of R-Duplex and C-Duplex was 11±3 and

26±5 mg L-1, respectively (Figure 5.12 E).


+-N concentration for R-Duplex and C-Duplex was 63.1±4.8 and 64.1±4.8 mg

L-1, respectively. The control had better performance in the VFCW which resulted in 21.0±6.4

mg L-1 NH4+-N while in the recirculated system it was 23.3±6.6 mg L-1. However, in overall, R-

Duplex had an increased removal, 89% while in C-Duplex 67%. In the HFF of the control, there

was no NH4+-N removal. The final concentrations were 7.1±3.0 and 21.1±5.9 mg L-1,

respectively (Figure 5.13 A).


mg L-1, respectively. The conversion to NH4+-N was more remarkable in R-VFCW as the

concentration increased to 41.2±8.7 mg L-1 while in C-Duplex it was 29.8±7.2 mg L-1. In R-HFF

and C-HFF, it decreased to 36.6±4.9 and 16.3±6.4 mg L-1, respectively (Figure 5.13 B).

The TN concentration in the influent of R-Duplex and C-Duplex was 92.5±2.3 and 93.3±2.7 mg

L-1, respectively. In the VFCW, the removal was increased from 25% to 31% with recirculation,

resulting in 63.7±1.8 mg L-1 and 70.0±2.2 mg L-1 TN. In overall, C-Duplex had slightly higher

removal with 48% instead of the 47%. The final concentrations in R-Duplex and C-Duplex were

48.9±2.1 and 49.0±6.6 mg L-1, respectively (Figure 5.13 C).

61

0

20

40

60

80

100

0.010.020.030.040.050.060.070.080.0

R-Duplex C-Duplex

Rem

oval

(%)

NH

4+ -N

(mg

L-1)

A



0

20

40

60

80

100

0.0

20.0

40.0

60.0

80.0

100.0

120.0

R-Duplex C-Duplex

Rem

oval

(%)

TN (m

g L-1

)

C

0.0

10.0

20.0

30.0

40.0

50.0

60.0

R-Duplex C-Duplex

NO

3- -N (m

g L-1

)

B

Figure 5.13 Mean (±SEM) of the nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (n=4)

62

0

20

40

60

80

100

120

04080

120160200240280320360400

R-Duplex C-Duplex

Rem

oval

(%)

BOD 5

(mg

L-1)

B

5.4.3 Organic constituents The influent COD concentration was 546±24 mg L-1. In R-VFCW 83% while in C-VFCW 68%

of the COD was removed. The corresponding concentrations were 90±12 and 174±44 mg L-1.

The recirculation improved the overall removal from 81% to 91% resulting in 48±4 and 105±42

mg L-1 COD, respectively (Figure 5.14 A).

For R-Duplex, the BOD5 concentration of the influent was 309±30 and for C-Duplex it was

321±36 mg L-1. At the first sampling of the phase, the BOD5 concentration in R-VFCW and in

C-VFCW was reduced to 12 and 26 mg L-1, respectively. At that sampling, the concentration in

the final effluent of R-HFF and C-HFF was below 2 and 4 mg L-1, respectively (Figure 5.14 B).

0

20

40

60

80

100

070

140210280350420490560630

R-Duplex C-Duplex

Rem

oval

(%)

COD

(mg

L-1)

A



Figure 5.14 Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (for COD n=4, for BOD5 influent n=3, for effluent n=1)

63

0

20

40

60

80

100

120

0

2

4

6

8

10

R-Duplex C-Duplex

Rem

oval

(%)

E. co

li(lo

g CF

U/1

00m

L)



0

20

40

60

80

100

120

0

2

4

6

8

10

R-Duplex C-Duplex

Rem

oval

(%)

Feca

l col

iform

(log

CFU

/100

mL)

B

5.4.4 Fecal indicator organisms The concentration of E. coli in the influent of both systems was 106CFU/100 ml. In the first stage

of R-Duplex was less than 1-log unit but in the HFF there was 2-log unit removal. In the two

compartments of C-Duplex, the removal was less than 1-log unit. In overall, the recirculation

improved the removal as it was increased from 82% to 99% (Figure 5.15 A).

The fecal coliform concentration was 106/100 ml in the influent. Corresponding with the E. coli

removal in C-HFF, it was less than 1-log unit. In R-VFCW, the removal was also less than 1-log

unit and in R-HFF it was 1-log unit. In overall, R-Duplex had higher removal, 98% compared to

85% in C-Duplex (Figure 5.15 B).

Salmonella was detected in the influent, in both effluents of C-Duplex and in the effluent of R-

VFCW; the concentration was less than 101CFU/100mL. In the effluent of R-HFF the

Salmonella concentration was below the detection limit.

Figure 5.15 Mean (±SEM) of the fecal indicator organisms of influent, effluent of vertical flow constructed wetland and horizontal flow filter – 4th experimental phase (n=3)

64

6. Discussion

6.1 Passive aeration capacity of the vertical flow constructed wetland Anoxic water (DO ~0.1 mg L-1) was fed to the dry R-VFCW, after the two days resting period,

and to the operating R-VFCW and C-VFCW. The main purpose of the experiment was to assess

whether the system with recirculation, which had more frequent feeding, had greater oxygenation

capacity with higher diffused DO level. Besides, it was also conducted to see how the resting

period affected the DO level in the effluent of the VFCW and to have a comparison with the

results from the previous operation of Ilyas (2013). The VFCW compartment of the Duplex-CW

had been designed to be aerobic and the applied intermittent feeding ensured “resting” periods

when bed could get aerated due to atmospheric diffusion and due to the air which was drawn to

the pores as the wetland bed was drained (Kadlec & Wallace, 2009; Nivala et al., 2013). Besides,

Nivala et al. (2013) also pointed out the plant-mediated oxygen transfer which is the most

debated and in designs guidelines neglected source of oxygen in CWs.

The results show that the dry bed had higher oxygenation capacity, with the initial DO

concentration ~6 mg L-1, than when it was in operation, ~2 mg L-1 (Figure 4.1). The vegetation

of the two Duplex-CWs was comparable which could mean that the difference was due to

atmospheric diffusion. Rubol et al. (2013), expressed re-aeration in soil also as a function of soil

moisture, with increasing soil moisture the re-aeration was decreasing. Besides, the DO

concentrations in the effluent of the dry bed are comparable to the findings of Ilyas (2013),

which can be due to the similar length of resting period, 2-3 days in the VFCW. In the running

Duplex-CWs, the recirculation only slightly increased the DO concentration in the first 40

minutes which was the approximate discharge time of the VFCWs. Even if the DO concentration

was comparable in the two systems, the effluent quality was different which could have been due

to the different microbial composition thanks to the feeding regime. In addition, R-HFF received

two times more DO load due to the recirculation than C-HFF which can be the reason for the

different treatments (Table 4.2).

65

6.2 Effect of recirculation at different HLRs Intermittent feeding of the raw wastewater and recirculation of the effluent of the HFF was

applied to R-Duplex, while C-Duplex was only fed with raw wastewater. Recirculation was

applied as studies have shown enhanced organic matter and TN removal in CWs (Foladori et al.,

2013; Ayaz et al., 2012).

6.2.1 Nitrogen In the VFCWs, the removal of NH4

+-N was due to nitrification as the NO3--N concentration

increased and 1.4-2.7 mg L-1 DO was provided by passive aeration (Figure 4.1). Recirculation

resulted in elevated NO3--N concentration in the VFCW from the second experimental phase on

and as the loading was increased the positive effect of recirculation was more pronounced

(Annex 3.-Figure 1.). The increase in NO3--N concentration followed the increase in NH4

+-N

concentration in the influent to a certain extent. Due to the recirculation, the more frequently

wetted and aerated R-VFCW could have had more substantial nitrifier community. However, the

amount of NO3--N generated did not correspond with the NH4

+-N removals (Figure 5.3, 5.6, 5.9,

Annex 3.). During the whole study, in C-VFCW, the decrease in NH4+-N concentration

corresponded to the increment in NO3--N concentration to a certain extent. The nitrogen balance

gives the explanation by showing TN removal in both VFCWs (Table 6.1). It could have been

due to the simultaneous nitrification and denitrification which has been noted in several studies

including the ones of Foladori et al. (2013), Hu et al. (2014) and Mosquera-Corral et al. (2005).

Table 6.1 shows that R-VFCW had at least 20% higher TN removal per unit area than C-VFCW.

The recycled effluent with high NO3--N concentration and with the residual organic matter could

have been facilitating suitable conditions for the activity of denitrifiers as well. Similarly, in the

study of Ayaz et al. (2012) and Tunçsiper (2009), the recirculated NO3--N rich effluent improved

nitrogen removal in the compartment intended for denitrification.

During the whole study, the HFFs of both systems provided further NH4+-N removal indicating

the presence of nitrifiers. According to the design of the Duplex-CW, the HFF is the anoxic

compartment. However, due to the frequent incoming, partially treated wastewater with residual

DO the bed got partially aerated. This phenomen was also described by Kadlec & Wallace

(2009). The only exception was during the last experiment phase, when in the effluents of C-

Duplex the NH4+-N concentration showed higher variation.

66

It could have been due to the decreased flow and DO concentration from the VFCW originating

from the short partial clogging of the valve between the VFCW and the HFF. The increased

OLR, or the already 6 weeks of operation at the HLR of 0.16 m3 m-2 d-1 could have been the

reason for the partial clogging in the last experimental phase. Nevertheless, in the first

experimental phase, due to the low HLR and the excess DO from the VFCWs the NO3--N

concentration further increased in both HFFs.

In R-HFF, the trend of increasing NO3--N concentration was the same even at higher loads, while

in C-HFF the increase in load was followed by increased denitrification which generated a small

increase in the pH of the effluent (Figure 5.2 A, Figure 5.5 A, Figure 5.8 A, Figure 5.12A). The

increase in pH can be explained by equation 2.4 which shows that during denitrification the

reduction of NO3--N and consumption of H+ is in 1:1 ratio. It can also be expressed as production

of alkalinity (van Haandel & van der Lubbe, 2012). Firstly, the abovementioned trend in C-HFF

could have been due to the less frequent wastewater discharge into it which was providing DO

less frequently than to R-HFF. The second reason could be the higher COD/NO3--N ratio due to

the less efficient organic matter removal in C-VFCW.

Setup

Total Duplex-CW TN influent

(mg d-1)

VFCWs TN

effluent (mg d-1)

VFCWs TN

removal (mg d-1)/

(g m-2 d-1)

HFFs TN effluent (mg d-1)

HFFs TN removal (mg d-1)/

(g m-2 d-1)

Total Duplex-CW TN removal

(mg d-1)/ (g m-2 d-1)

First experimental - period HLR 0.05 m3m-2d-1 + OLR 13 g COD m-2 d-1 R-Duplex 771 520 251/(1.0) 456.3 64/(0.3) 315/(1.3) C-Duplex 744 546 198/(0.8) 464.9 81/(0.3) 279/(1.2)

Second experimental period - HLR 0.08 m3m-2d-1 + OLR 18 g COD m-2 d-1 R-Duplex 938 656 282/(1.2) 562 95/(0.4) 376/(1.6) C-Duplex 919 749 170/(0.7) 401 347/(1.4) 517/(2.2)

Third experimental period - HLR 0.16 m3m-2d-1 + OLR 56 g COD m-2 d-1 R-Duplex 2524 1745 780/(3.2) 1486 259/(1.1) 1038/(4.3) C-Duplex 2490 1911 579/(2.4) 1228 683/(2.8) 1262/(5.3)

Fourth experimental period - 0.16 m3m-2d-1 + OLR 88 g COD m-2 d-1 R-Duplex 3571 2459 1112/(4.6) 1887 571/(2.4) 1683/(7.0) C-Duplex 3601 2702 899/(3.7) 1891 811/(3.4) 1710/(7.1)

Table 6.1 Nitrogen balance of the Duplex-CWs in the four different experimental phases

67

As during denitrification, organic carbon serves as an electron donor the COD/NO3--N ratio is an

important parameter. This ratio in the wastewater discharged to C-HFF was 1.5, 3.7, 5.3 and 5.8

successively in the four experimental phases. In the last two phases, the ratio was optimal

considering the findings of Chiu et al. (2001), where for initial the 25 and 50 mg L-1 NO3--N the

optimal ratios were 5.5 and 4.5. However, it did not result in complete denitrification most

probably due to other limiting factors such as the oxygenation from the C-VFCW.

Regarding the overall TN removal per unit area, the control was more efficient than the R-

Duplex (Table 6.1). From the abovementioned results, it can be concluded that even R-VFCW

had higher TN removal than C-VFCW the overall denitrification was lower compared in R-

Duplex than in C-Duplex due to the abovementioned reasons. In addition, one-way ANOVA

showed that in the third experimental phase C-Duplex had significantly lower TN concentration

in the final effluent, meaning that in that phase the conditions were the most suitable for

denitrification. Namakula (2014) showed similar results during the batch operation of the

Duplex-CW, when aeration and recirculation was applied. In her research, from the three setups

the control performed the best with 2.0 g m-2 d-1 TN removal and the system with recirculation

only had 1.3 g m-2 d-1 TN removal. In this study, the removal rate in C-Duplex was more affected

by the change in HLR than in R-Duplex most probably due to the shorter contact time.

Indicatively in R-Duplex, the TN removal rates were consistent in the different experimental

phases: 41%, 40%, 41% and 47%.

6.2.2 Organic matter and suspended solids In all the experimental phases, R-VFCW was removing at least 70% of the incoming COD,

while in C-Duplex it ranged between 63% and 78%. The BOD5 removal was higher, especially

in R-VFCW with 76-93% removal while in C-VFCW 71-79%, showing that in the more

frequently fed bed the organic matter was degraded more efficiently. As mentioned in the

previous chapter, this tendency was observed in NH4+-N removal as well. Foladori et al. (2013)

and Sklarz et al. (2009) also reported that the constant wetting of the bed was beneficial for

organic matter removal. Besides, due to the recirculation air is drawn more frequently to the bed

facilitating the microbial communities requiring DO. Regarding COD removal, with increasing

load the performance of R-VFCW improved progressively compared to C-VFCW, meaning that

the effect of recirculation was more pronounced at increasing loads.

68

At the first two HLRs, the HFF worked as a buffer and the effluent COD concentration did not

change tremendously, in R-HFF 35 and 34 mg L-1 in C-HFF 35 and 37 mg L-1, respectively. This

buffer capacity kept the overall removal rate of both systems stable over the changes in the HLR,

in R-Duplex 86%, 85%, 89% and 91%, in C-Duplex 87%, 83%, 83% and 81% respectively. The

changes in NH4+-N and NO3

--N concentrations and the previously described DO load to the

HFFs indicates that in R-HFF the decrease was mainly due to aerobic degradation and only

partially due to the denitrification while in C-HFF it was on the contrary. Namakula (2014) had

comparable removal rate in the recirculated system but the control in the present operation

performed better than during, the previous batch operation which had 79% COD removal. The

difference can come from higher microbial activity due to the more frequent moistening in C-

Duplex. It received raw wastewater in every 2 hours while in the previous operation the control

received 2 days per week.

As the load was increased the system with recirculation had improved COD removal compared

to the control, which was mainly due to the performance of the VFCWs (Annex 3.). In addition,

the diluting effect of recirculation, which was also reported by Prost-Boucle & Molle (2013),

could have been the reason for the lower final effluent concentration of R-Duplex. However, the

statistical analysis showed that the difference in the two systems’ performance was not

significant. The biggest difference due to recirculation, 91% instead of 81% was in the last phase

with the HLR at 0.16 m3 m-2 d-1 and OLR at 88 g COD m-2 d-1. Besides, both systems were more

efficient at removing BOD5 than COD, meaning that the biodegradable organics were removed

more efficiently.

In CWs, besides sedimentation and sorption, the mineralization of organic solids also plays a role

in TSS removal. Mineralization can prevent clogging and can occur due to the intermittent

feeding which aerates the bed and creates aerobic conditions (Knowles et al., 2011). The overall

higher TSS removal in R-Duplex than in C-Duplex, removal rates 92%, 89%, 93%, 92% 85%,

86%, 88%, 80%, respectively could have been due to the abovementioned mineralization

process. As discussed earlier, the more frequent feeding could have drawn air more often to the

pores. Yet, these rates are higher than in a similarly, intermittently fed VSSF CW studied by

Foladori et al. (2013).

69

In their study the treatment cycles were 10.5 h including feeding, treatment and discharge and

the TSS removals in the control and in the recirculated systems were 73% and 76%, respectively.

It can indicate that the more frequent feeding and draining can enhance TSS removal.

Nevertheless, the last result of C-Duplex could indicate the clogging of C-HFF as the removal

decreased from 53% to 34% compared to the previous phase. Another explanation is, when the

valve was clogging, the DO concentration in the effluent of C-VFCW dropped and as the results

showed it could have affected the further aerobic degradation of TSS in C-HFF.

6.2.3 Fecal indicator organisms Fecal indicators had been monitored at the HLR 0.16 m3m-2d-1 and at the same HLR with

increased OLR to 88 g COD m-2d-1 to assess the scenario of high HLR and short HRT. The study

of García et al. (2003) showed that beside the bed material the HRT has a great impact on

microbial removal. HRTs ranging from 0.5 to 5 days were tested, and at the lowest HRT which

was 0.5 day the removal of fecal coliform and somatic coliphage was less than 1-log unit (García

et al., 2003). In the Duplex-CW, besides the fast discharge time of the VFCWs, 30-40 minutes, at

the highest HLR the theoretical HRT of the HFF was 0.7 day. It was calculated by dividing the

original pore volume of the HFF, 39 L by the daily wastewater input which was 60 L d-1.

Recirculation improved the E. coli removal in both phases and it increased with increasing load,

from 1.4-log unit to 2.2-log unit removal. The reason could be that the protozoa population

grazing on E. coli could have increased due to the recycled extra nutrients. The study of Puigagut

et al. (2007) also showed that for secondary treatment the OLR has an influence on the

abundance and diversity of ciliated protozoa species. In the Duplex-CW, Kyomukama (2014)

also observed significant increase in the protozoa population after the implementation of filtered

raw wastewater by-pass to provide carbon for denitrification. However, in her case it was not

certain whether it was due to the extra nutrients or it was added by the extra wastewater. In C-

Duplex, the additional nutrient in the last experimental phase did not enhance the removal

possible due to the partial clogging of the valve.

The difference of the setups in fecal coliform removal was comparable to the E. coli removal.

However, in R-Duplex the increased OLR enhanced the fecal coliform removal to smaller extent

(23% higher) than the E. coli removal (37% higher).

70

However, none of the coliform removal influencing parameter, such as HRT and porosity of the

bed was altered. It indicates that the smaller increment in the reduction of fecal coliform could

have been due to the additional nutrient which was serving as growth substrate for fecal

coliforms. The total removal of fecal coliform in R-Duplex was 1.3-log unit and 1.7-log unit

removal, while in C-Duplex it was 1.2-log unit and 0.8-log unit removal. During the previous

operation of the Duplex-CWs by Kyomukama (2014), the HLR was three times lower than in the

present study but the fecal coliform removal was 3-6 log units.

Regarding Salmonella, in the influents and in some cases in the effluents the concentration of it

was less than 101CFU/100 mL. However, it means that there was no total elimination of them

like during the batch operation with longer HRT (Kyomukama, 2014). Looking at the present

study, the previous batch operation of the Duplex-CW with 3-4 days HRT and the study of

García et al. (2003) the importance of HRT becomes clear.

6.3 Possible reuse of the effluents considering various standards The EU Council Directive 91/271/EEC concerning urban waste-water treatment gives the basic

discharge requirements of treated wastewater for the EU member countries (Table 6.2).

However, the authorities of each member countries can request more strict standards and

normally that is the case (Example: Hungarian thresholds concerning receiving water bodies,

Table 6.2). These EU standards have been used in the studies of Prigent et al. (2013) and Ilyas

(2013). On the other hand, in most member countries, there has been no explicit guidance for the

use of reclaimed water (Bixio et al., 2006) and the situation is still the same in some countries.

Nevertheless, there are some initiatives, one example is in Bussum, the Netherlands as the

wastewater treated by CW is used in on office building for flushing the toilets. In the United

States the difference, compared to the EU, is that there is a clear guideline for reclaimed water

reuse in the EPA Guidelines for Water Reuse (2012). Table 6.2 shows the standards for

unrestricted urban reuse, meaning that the treated wastewater is used for nonpotable applications

in municipal settings where public access is not restricted.

For this purpose, there are several treatment requirements, however when the effluent is intended

to be reused in the environment e.g. discharge to wetlands there are no limitation in California.

71

In Japan, even if the technological requirements are less stringent than in the US the guidelines

were established considering hygienic safety, appearance and acceptance, and risk of facility

malfunction.

EU

Hungary

PE/(Receiving

water*)

California

Unrestricted

reuse

Japan

Toilet flushing

Japan

Water for

landscape use

Parameters

Concentration

(Minimum %

of reduction)

Concentration

Concentration/

Required

treatment

Concentration/

Required

treatment

Concentration/

Required

treatment

BOD5 (mg L-1) 25 (70-90) 25/(15)

Oxi

datio

n, C

oagu

latio

n,

Filtr

atio

n, D

isin

fect

ion

Sand

filtr

atio

n or

equ

ival

ent

Sand

filtr

atio

n or

equ

ival

ent

COD (mg L-1) 125 (75) 125/(50)

TSS (mg L-1) 35 (90 35/(35)

TP (mg L-1) 2 (80) NS**/(0.7)

TN (mg L-1) 15 (70-80) NS**/(20)

NH4+-N

(mg L-1) NS NS/(2)

NO3--N + NO2

--N (mg L-1)

NS NS

pH NS NS/(6.5-8.5)

5.8-8.6 5.8-8.6

Total coliform (cfu/100 mL) NS NS 2.2 NS NS

Fecal coliform (cfu/100 mL)

NS NS NS N.D. 1000

Residual chlorine (mg L-1)

NS NS NS Free: 0.1

Combined: 0.4

Free: 0.1

Combined: 0.4

Table 6.2 Effluent quality standards for different purposes in the EU, United States and Japan

NS = not specified N.D. = not detectable

* Based on the 28/2004. (XII. 25.) KvVM directive about the discharge limits of water polluting substances and the rules of application - the receiving water body is the Balaton lake

** <10000 PE local authorities decided, 10001-100000 PE: TN 15-25 mg L-1, TP 2 mg L-1 (80% minimum reduction)

72

In most cases, the treatment requirement is sand filtration or any treatment technology equivalent

to that which could also mean CWs. The limits for this type of treatment, for example toilet

flushing water and water for landscape use are shown in Table 6.2.

Regarding the effluent quality of the Duplex-CWs, even if C-Duplex had better performance in

TN removal than R-Duplex, the lowest TN concentration was still higher than the EU limit 15

mg L-1. In the other experimental phases, it was two times higher than the limit (Table 6.3)

posing a burden for the application in the EU. During the previous operation of the Duplex-CW,

the TP concentration exceeded the 2 mg L-1 EU threshold (Table 6.3). Due to the more

continuous intermittent operation, TP removal was not expected to improve. Besides, no

additional treatment step was applied indicating that the effluent could not be discharged to

surface waters. This issue in CWs is well-known and hinders the spread of them.

Experimental phase TSS (mg L-1)

COD (mg L-1)

BOD5

(mg L-1) TN

(mg L-1) TP

(mg L-1) EU Council Directive 91/271/EEC concerning urban waste-water treatment

35 125 25 15 2

Batch operation of Duplex-CW- Ilyas, 2013

Low strength* 9 46 NA 10 2

Medium strength* 8 41 12 16 4

High strength* 16 69 44 25 5

Intermittent operation of C-Duplex - present study

1st experimental phase 11 35 4 38 ND

2nd experimental phase 10 37 6 21 ND

3rd experimental phase 16 57 11 32 ND

4th experimental phase 26 105 ND 49 ND

Table 6.3 Effluent quality of the batch operated Duplex-CW and C-Duplex concerning the EU standards

ND = no data

*The composition of wastewater was modified to increase the influent COD concentration from 319 to 597 and 797 mg L-1 at medium and high strength, respectively

73

Table 6.3 shows the effluent concentrations of the batch operated Duplex-CW and C-Duplex,

which, in operation was more comparable to Ilyas’ (2013) system as no recirculation was

applied. At the two lower HLRs, R-Duplex and C-Duplex could comply with the stricter

Hungarian standards regarding COD and BOD5 concentrations. However, as the load increased

the control setup could only keep the COD below the limit set by the EU, 125 mg L-1 (Table 6.3).

The suspended solid removal was around 90% and both systems and R-Duplex kept the TSS

concentration below 30 mg L-1 which is the threshold in the EU and in Hungary. The pH of the

effluent was always in the range stated in Table 6.2 and the fluctuation was minor. Some

legislation also have specific requirements about the temperature in order to reduce the

disturbance in the original temperature of the receiving water.

Table 6.2 also shows that for the use of reclaimed water the abovementioned parameters such as

COD and TN are not priority. Addition to the required treatment technologies the requested

parameters are indicators of pathogens and the residual chemical coming from disinfection. In

the last two experimental phases the effluent could not be used for the purposes shown in Table

6.2. However, it could be further discharged to a natural wetland in California where, according

to the EPA water reuse guideline (2012) the abovementioned parameters are not regulated by the

state. During batch operation of the Duplex-CWs, in the effluent of the system with recirculation

the fecal coliform concentration was around 103CFU/100 mL (Kyomukama, 2014). Therefore,

the effluent would be suitable for landscape use in Japan. At low HLRs the intermittent operation

could have resulted in the same concentration due to the longer HRT and lower load. However,

in order to use the effluent at high loads, even for the irrigation of the lawn in Japan, further

treatment e.g. chlorination or UV treatment would be necessary.

6.3.1 Land area requirements under different HLRs and OLRs One of the main issues of CWs is the amount of required space for the construction. According

to Vymazal (2002), for HSSF CWs the required land was 5 m2 per PE. Since then, more research

have been focusing on the topic and Foladori et al. (2013) stated that in their system 1.5 m2 land

per PE was sufficient. In the report it was not stated which discharge standard was taken into

account but as the tests were done in Italy the effluent should at least comply with the EU

standard.

74

After reviewing the effluent of the best performing system of Foladori et al (2013), where

intermittent recirculation and aeration was applied, the TP and TN concentrations in the effluent

were 4.8±2.1 mg L-1 and 18.2±6.2 mg L-1, respectively. It means that the effluent of the CW

could not be discharged in the EU.

Only considering the EU standards for organic matter and suspended solids (Table 6.2), both

Duplex-CWs could have required less area due to the increased load. At the HLRs of 0.05, 0.08,

0.16 and 0.16 m3 m-2 d-1 and at the corresponding OLRs at 13, 18, 56 and 88 g COD m-2 d-1 the

space requirements would be 9.0, 6.8, 2.1 and 1.2 m2 per PE. However, area reduction cannot be

stated as the requirements regarding TN and TP are not met by C-Duplex and R-Duplex. When

the Duplex-CW was operated in batch mode with low strength wastewater all the effluent

complied with all the EU limits listed in Table 6.2. However, when the strength of wastewater

was elevated to medium and high category based on the ranges from Tchobanoglous et al. (2003)

the TN concentration first increased to 16 mg L-1 then to 25 mg L-1.

Looking at these values the change in operation did not enhance the performance of the Duplex-

CW and no area reduction was achieved. However, if at the highest load C-Duplex was

optimized for nitrogen removal then the required area would be reduced to 1.2 m2 per PE and the

efficiency could be comparable to the Ecophyltre® system of Prigent et al. (2013), where the

TKN concentration in the final effluent was in the range of 12-21 mg L-1. However, operation at

high HLR and OLR can reduce the operation life of the CWs and malfunction can occur more

frequently.

75

7. Conclusions and recommendations

7.1 Conclusions 1. The intermittent operation with recirculation can offer a solution for the increasing OLR

as in the last experimental phase, with the HLR of 0.16 m3 m-2 d-1 and OLR at 88 g COD

m-2 d-1, R-Duplex provided 91% COD removal which was comparable to the removal in

batch operation at the OLR of 37 g COD m-2 d-1. In addition, in batch operation the COD

removal started to decline by increasing the OLR from 28 to 37 g COD m-2 d-1 (Ilyas,

2013).

2. Intermittent feeding can jeopardize denitrification, consequently TN removal due to the

too frequent DO supply to the HFFs. However, at the HLRs of 0.05, 0.08 m3 m-2 d-1, the

feeding regime in C-Duplex and R-Duplex increased NH4+-N removal, 93-99%

compared to the batch operation, where it was 82% (Ilyas, 2013).

3. At the highest load the removal of pathogenic indicators is not sufficient to reuse the final

effluent even for flushing toilets according to the standards from Japan and the United

States. Only taking into account the standards for reclaimed water use, where the

concentrations of TN or TP are not of concern, an additional disinfection step could be an

option for the Duplex-CW to deal with space limitations in a new way and also to close

the urban water cycle.

4. The results of the passive aeration capacity test showed that more frequent wetting of R-

VFCW compared to the control did not increase significantly the DO concentration. The

dry bed had higher DO concentration in the effluent meaning that longer resting periods

can be advantageous for the treatment.

5. Recirculation did not enhance significantly the overall performance of the Duplex-CW

and TN removal was even diminished as R-HFF was more aerated due to the more

frequent wastewater input.

6. Considering the standards from the EU, area reduction cannot be stated as the TN

concentration in all the experimental phases was above the 15 mg L-1 EU.

7. To reach the area of 1.2 m2 per PE with the highest HLR and OLR applied during the

intermittent feeding the system should be optimized for nitrogen removal and an

additional phosphorus removal compartment should be added.

76

7.2 Recommendations The following recommendations were made based on the experiences from the operation and

field works with CWs.

1. The distribution pipes providing wastewater on the top of the VFCWs should be

redesigned. Smaller diameter pipes should be chosen and placed more densely on the top

of the VFCWs. Besides the holes should have smaller diameter but still big enough to

avoid clogging, especially at flowrates less than 320 mL min-1 as with the present

distribution system the whole surface might not be used optimally.

2. To use recirculation, as it is described in this study is not advisable as it did not enhance

the performance significantly and it poses an additional operation cost originating from

the pumping.

3. If recirculation is decided to be applied it could be at the same time as the feeding of the

raw wastewater. Therefore, there would be less frequent DO loading to the HFF which

was impeding denitrification in the present study. Besides, due to the simultaneous

supply of NO3--N containing recycled effluent and the raw wastewater rich in organic

matter the already experienced simultaneous nitrification and denitrification could be

enhanced in the VFCW.

4. The results of this study and Ilyas (2013) could indicate that the optimum operation can

be with less than 12 feeding cycles per day but still more often than 2 times a week.

5. As the reuse of treated wastewater coming from CWs can pose health risks it can be

beneficial to conduct risk assessment studies concerning the operation of the Duplex-

CWs.

77

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84

Annex 1.

Land area requirement calculation

Land area requirement in the first experimental phase

Background information:

Number of feeding cycles in 5 days: 54

Volume of wastewater fed per cycle: 1.6 L

Area: 0.24 m2

BOD5 concentration: 0.13 g L-1

60 g BOD d-1 = 1 PE

Calculation:

Flowrate: 54 × 1.6 L / 7 d = 12.34 L d-1

BOD load = 0.13 g L-1 × 12.34 L d-1 = 1.6 g d-1

BOD load per area = 1.6 g BOD d-1 / 0.24 m2 = 6.7 g BOD m-2 d-1

Area per PE = 60 g BOD d-1 / 6.7 g BOD m-2 d-1 = 9.0 m2 per PE

The land area requirement in the first experimental period with HLR at 0.05 m3 m-2 d-1

was 9 m2 per PE.

In the other experimental phases the calculations remained the same just the actual BOD5

concentrations and flowrates were taken into account.

85

Annex 2.

Passive aeration capacity of the vertical flow constructed wetland Anoxic water was prepared by adding CoCl2 (33 mg) as a catalyst and Na2SO3 (0.71 g) in

demineralized water and by bubbling the solution with N2 gas.

Chemicals required: CoCl2, Na2SO3 and N2 gas

CoCl2 = 33 mg as a catalyst

Na2SO3 = 1.4 g

Chemical equation:

2Na2SO3 + O2 → 4Na+ + 2SO42-

To remove 10 mg O2 from 1 liter of demineralized water, 78.75 mg of Na2SO3 is required.

Calculations:

2Na2SO3 = O2

2(126) = 16 × 2

252 g = 32 g

Therefore,

252 mg = 32 mg

In this case:

32 mg O2 = 252 mg Na2SO3

1 mg O2 = 252/32 mg Na2SO3

10 mg O2 = 252×10/32 mg Na2SO3

10 mg O2 = 78.75 mg Na2SO3

There was 9 mg O2 in 1 liter of demineralized water, therefore, in the 10 liter tank of

demineralized water there was 90 mg O2.

To remove 90 mg O2, 0.71 g Na2SO3 was required, as indicated below:

10 mg O2 = 78.75 mg Na2SO3

90 mg O2 = 90 × 78.75 mg Na2SO3

90 mg O2 = 0.709 g Na2SO3

86

Annex 3.

Additional figures

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

R-Duplex C-Duplex R-Duplex C-Duplex R-Duplex C-Duplex R-Duplex C-Duplex

1st phase 2nd phase 3rd phase 4th phase

NO

3- -N (m

g L-1

)

Influent

VFCW effluent

HFF effluent

Figure 1. Variation in NO3--N concentration during the four experimental phases in R-Duplex and in C-Duplex

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0



NH

4+ -N

(mg

L-1)

Influent

VFCW effluent

HFF effluent

Figure 2. Variation in NH4+-N concentration during the four experimental phases in R-Duplex and in C-Duplex

87

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0



TN (m

g L-1

)

Influent

VFCW effluent

HFF effluent

Figure 3. Variation in TN concentration during the four experimental phases in R-Duplex and in C-Duplex

0

100

200

300

400

500

600



COD

(mg

L-1)

Influent

VFCW effluent

HFF effluent

Figure 4. Variation in COD concentration during the four experimental phases in R-Duplex and in C-Duplex

88

Annex 4.

Change in vegetation over time

25.03.2014 Before the intermittent feeding was

applied

20.05.2014 End of 1st experimental phase

05.06.2014 Close to the end of the 2nd

experimental phase

30.06.2014 Close to the end of the 3rd

experimental phase

28.07.2014 End of the 4th experimental phase

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