International Master of Science in Environmental Technology and Engineering

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1 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 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 ) [214] [Delft], [Péter Pintér], Ghent University, all rights reserved.

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3 Influence of recirculation in a pulse-fed Duplex Constructed Wetland used for domestic wastewater treatment Péter Pintér MSc thesis ES. August 214 3

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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 214 5

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7 214 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. 7

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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, 213) 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 () and in the other system recirculation was applied (). 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, 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.5,.8 and.16 m 3 m -2 d -1. In the fourth experimental phase the HLR was kept.16 m 3 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., 6.8, 2.1 and 1.2 m 2 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 had slightly higher COD removal and from the third experimental phase on the effect of recirculation became more pronounced. In the first experimental phase, 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. 9

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11 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 m 2 per PE considering the COD, BOD 5 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 (213) 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 m 2 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 11

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13 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. 13

14 Table of Contents Abstract...9 Acknowledgements List of figures List of tables List of Acronyms Introduction Background Problem statement Literature review Constructed wetlands Types of constructed wetlands Area requirements of constructed wetlands Feeding regime of constructed wetlands Role of recirculation Role of oxygen in constructed wetlands Removal of nitrogen Pathogenic and fecal indicator organisms and their removal Objectives Overall objective Specific objectives Materials and Methods Experimental setup Experimental design Passive aeration capacity of the vertical flow constructed wetland Analytical procedure Statistical analysis

15 5. Results Passive aeration capacity of the Vertical Flow Constructed Wetland First experimental period - HLR.5 m 3 m -2 d -1 + OLR 13 g COD m -2 d Second experimental period - HLR.8 m 3 m -2 d -1 + OLR 18 g COD m -2 d Third experimental period - HLR.16 m 3 m -2 d -1 + OLR 56 g COD m -2 d Fourth experimental period - HLR.16 m 3 m -2 d -1 + OLR 88 g COD m -2 d Discussion Passive aeration capacity of the vertical flow constructed wetland Effect of recirculation at different HLRs Possible reuse of the effluents considering various standards Land area requirements under different HLRs and OLRs Conclusions and recommendations Conclusions Recommendations References Annex Land area requirement calculation Annex Passive aeration capacity of the vertical flow constructed wetland Annex Additional figures Annex Change in vegetation over time

16 List of figures Figure 4.1: Duplex-CW setup Figure 4.2: Setup of the passive aeration capacity test for the VFCWs Figure 5.1: Change in the DO concentration (absolute values) of the dry and operational VFCWs Figure 5.2: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter 1 st experimental phase (n=4, except influent of n=3) Figure 5.3: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 1 st experimental phase (n=4, except influent of n=3) Figure 5.4 A: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 1 st experimental phase (n=4, except influent of, n=3) Figure 5.4 B: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 1 st experimental phase (n=4, except influent of, n=3 and n=2) Figure 5.5: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter 2 nd experimental phase (n=3, except EC n=2) Figure 5.6: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 2 nd experimental phase (n=3) Figure 5.7: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 2 nd experimental phase (for COD n=3, for BOD 5 n=2, except effluent R-HFF n=1) Figure 5.8: Mean (±SEM) of the physical properties of influent, effluent of vertical flow constructed wetland and horizontal flow filter 3 rd experimental phase (n=5, except effluent of C-VFF n=4) Figure 5.9 A, B: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 3 rd experimental phase (n=5)

17 Figure 5.9 C: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 3 rd experimental phase (n=5) Figure 5.1: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 3 rd experimental phase (for COD n=5, for BOD 5 n=4 except effluent C-HFF n=4) Figure 5.11: Mean (±SEM) of the fecal indicator organisms of influent, effluent of vertical flow constructed wetland and horizontal flow filter 3 rd 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 4 th experimental phase (n=4) Figure 5.13: Mean (±SEM) of nitrogen constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 4 th experimental phase (n=4) Figure 5.14: Mean (±SEM) of the organic constituents of influent, effluent of vertical flow constructed wetland and horizontal flow filter 4 th experimental phase (for COD n=4, for BOD 5 influent n=3 effluent n=1) Figure 5.15: Mean (±SEM) of the fecal indicator organisms of influent, effluent of vertical flow constructed wetland and horizontal flow filter 4 th experimental phase (n=3)

18 List of tables Table 2.1: Constructed wetlands with different organic loading rates and land requirements Table 2.2: Constructed wetlands with different hydraulic loading rates and land requirements Table 4.1: Characteristics of the settled domestic wastewater during this study Table 4.2: Operation of the Duplex-CWs Table 4.3: Operation modes and parameters of the Duplex-CWs Table 4.4: Analytical procedures Table 6.1: Nitrogen balance of the Duplex-CWs in the four experimental phases Table 6.2: Effluent quality standards for different purposes in EU, United States and Japan Table 6.3: Effluent quality of the batch operated Duplex-CW and concerning the EU standards

19 List of Acronyms APHA BOD COD CW C-VFCW C-HFF DO EC EPA HF HFF HFCW HLR HRT HSSF MDG OLR PE R-VFCW R-HFF TN TP TSS VF VFCW VSSF WWTP American Public Health Association Biochemical Oxygen Demand Chemical oxygen demand Constructed Wetland Control Duplex-CW Control vertical flow constructed wetland Control horizontal flow filter Dissolved oxygen Electrical conductivity Environmental Protection Agency Horizontal flow Horizontal flow filter Horizontal flow constructed wetland Hydraulic loading rate Hydraulic retention time Horizontal subsurface flow Millennium Development Goal Organic loading rate Population equivalent Duplex-CW with recirculation Vertical flow constructed wetland with recirculation Horizontal flow filter with recirculation Total nitrogen Total phosphorus Total suspended solids Vertical flow Vertical flow constructed wetland Vertical subsurface flow Wastewater treatment plant 19

20 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 211, the world s population reached 7 billion and it is estimated to increase up to 9.6 billion by the year 25. According to the UN predictions, most of the growth will occur in developing regions, especially in Africa (UN Press Release, 213). 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. 214). 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 215 the proportion of people without access to safe drinking water and basic sanitation (UN General Assembly, 212). 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, 29). 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. 214). In general, subsurface flow CWs have their land requirements in the range of m 2 per population equivalent (PE) (Vymazal, 22). 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., 213). With the use of intermittent aeration and recirculation Foladori et al. (213) managed to reduce the required area from 3.6 to 1.5 m 2 per PE but Zhai et al. (211) reported that their hybrid system required less than 1 m 2 per PE (Zhai et al., 211, cited in Zhang et al., 214). 2

21 Besides, in the study of Prigent et al. (213), 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 m 2 per PE (Prigent et al., 213). 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, 211). 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.5 m 3 m -2 d -1 (Ilyas 213; Lavrnić, 213; Kyomukama, 214; Namakula, 214). Ilyas (213) was experimenting with artificially increased organic loading rates (OLR), 4, 7 and 13 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, with recirculation and, the control, were operated. Compared to the previous operation, a more continuous, intermittent feeding was applied at increasing hydraulic loading rates (HLRs),.5,.8 and.16 m 3 m -2 d -1, and at artificially increased OLR at 88 g COD m -2 d -1, at the HLR.16 m 3 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 BOD 5 concentration of the raw wastewater based on the results of Ilyas (213), the theoretical space requirements were calculated to be around 8, 5, 2.5 and 1.2 m 2 per PE. However, to state these values the effluent must comply with certain standards which are detailed in a later chapter. 21

22 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., 21). Since the 197s, significant amount of research has been carried out to test CWs for wastewater treatment and to understand the mechanisms of pollutant removal (Kangas, 25). 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, 21). 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. 214). 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. 22

23 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, 21). 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, 29). 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, 29). 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., 214, Vymazal, 25, Zhao et al., 211). 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., 214). Hybrid CWs date back to the 196s when they were introduced by Seidel in Germany (Seidel, 1965, cited in Vymazal, 213). However, these configurations were not widely recognized at that time (Vymazal, 213). 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, 213). Nowadays, several hybrid systems are composed of a VF and an HF CW, arranged in series (Vymazal, 21). Apart from the VF-HF systems in the 199s and early 2s 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, 213). 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., 21). 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. 23

24 Most authors give the required wetland area in m 2 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 6 g of oxygen per day (EU Council Directive 91/271/EEC). However, in the UN- HABITAT Constructed wetlands manual (28) for developing countries, due to the characteristics of the wastewater, 1 PE is considered 4 g of BOD 5 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 m 2 per PE (Vymazal, 22). The rule of thumb is to use 5 m 2 of land per PE for HSSF CWs (Vymazal, 22); however, it was reported to be insufficient to remove nutrients (Babatunde et al., 28). According to the survey of 19 Danish CWs in 199, between 15 and 3 m 2 area per PE would be necessary to obtain tn concentrations less than 8 mg L -1, and an area of 4-7 m 2 PE -1 to achieve less than 1.5 mg L -1 TP concentration (Babatunde et al., 28). 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 m 2 per PE. VFCWs are generally built as one bed and are referred as compact VFCWs (Vymazal, 21). In order to reach these numbers, Foladori et al. (213) 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 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 8-1 g BOD 5 m -2 d -1 (Chazarenc et al., 27). 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 25, 43 and 8 mg L -1, respectively and for BOD 5 are 11, 19 and 35 mg L -1, respectively (Tchobanoglous et al., 23). Therefore, it is essential to identify the type of wastewater to adjust the highest possible OLR. 24

25 The drawbacks of highly loaded systems can be the lowered removal efficiency of organic matter and nitrogenous compounds and faster clogging (Ghosh & Gopal, 21). Therefore, it is essential to ascertain the highest sustainable loading rate with the effluent quality which still complies with the standards. Table 2.1 Constructed wetlands with different organic loading rates and land requirements CW type Area, HLR, OLR, Effluent COD, Effluent TN, m 2 PE -1 m 3 m -2 d -1 g COD m -2 d -1 mg L -1 mg L -1 References, VSSF Prigent et al. (213) C 1 -VSSF Foladori et al. (213) R 2 -VSSF Foladori et al. (213) A 3 -VSSF Foladori et al. (213) AR-VSSF Foladori et al. (213) Duplex-CW Ilyas (213) Duplex-CW Ilyas (213) Duplex-CW Ilyas (213) A-Duplex-CW Ilyas (213) 1 Control, 2 Recirculated, 3 Aerated 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, 212) 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 m 3 m -2 d -1 (EPA, 1999). 25

26 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., 213). The drawback is that it requires larger wetland area for the treatment (Weerakoon et al., 213). Applying the loading chart design method, developed by Wallace & Knight (26), 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. Table 2.2 Constructed wetlands with different hydraulic loading rates and land requirements CW type BOD Area, HLR, OLR, HRT, m 2 PE -1 m 3 m -2 d -1 g BOD m -2 d -1 removal, days % References, HFCW Weerakoon et al. (213) HFCW Weerakoon et al. (213) HFCW Weerakoon et al. (213) VFCW Ghosh & Gopal (21) VFCW Ghosh & Gopal (21) VFCW Ghosh & Gopal (21) VFCW Ghosh & Gopal (21) HF Bio-rack Valipour et al. (29) HF Bio-rack Valipour et al. (29) HF Bio-rack Valipour et al. (29) 1 BOD 5 removal, 2 BOD 3 removal 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. 26

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, 21). 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, 26). Besides, in CWs with intermittent operation subsurface aeration accelerates the clog matter mineralization (Knowles et al., 211). On the other hand, in non-continuous systems an equalization or storage tank is required and more frequent check up is needed 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, 29; Vymazal, 21). 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, 29). 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, 29). 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, 29). The anaerobic wetlands or also called alkalinity producing systems are used for mine water treatment (Younger et al., 22, cited in Kadlec & Wallace, 29). Tidal flow (filland-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. 27

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, 29). 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, 212). Nevertheless, more specific effects of recirculation on the performance of CWs have also been reported in literature (Ayaz et al., 212; Foladori et al., 213; Prost-Boucle & Molle, 212; Tunçsiper, 29; Zhao et al., 24). 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, 29). Recycle ratios of even 3 to 12 times the influent flow rate were common (Kadlec & Wallace, 29). 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. (214) reported that the optimal recirculation ratio for CWs was 1:1. Ayaz et al. (212) 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 (29) also showed similar results with increased recirculation rate (from 5% to 1%) and decreased HLR (from.1 m 3 m -2 d -1 to.3 m 3 m -2 d -1 ) the NH + 4 -N removal increased from 66% to 7% and the NO - 3 -N removal increased from 63% to 74%. The study of Sklarz et al. (29) 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. 28

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., 29). Foladori et al. (213) 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 m 2 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. (21), 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., 21). 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. 29

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., 214). 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., 21). In mixed liquor around ph=7 ammonium is present in its ionic form (NH + 4 ) and the reaction is given in equation 2.1 (van Haandel & van der Lubbe, 212). The process is primarily temperature and ph dependent and it occurs under aerobic and anaerobic conditions but slower in the latter condition. RNH 2 + H 2 O + H + + ROH + NH 4 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 O 2 is used to convert 1 mg ammoniacal nitrogen to nitrate nitrogen (Vymazal, 26). Therefore, it only takes place in well aerated conditions. The minimum required temperature of nitrification is around 5 o C and the optimum ph varies from 6.6 to 8.. These factors greatly determine the rate of transformation. NH / 2 O 2 NO H 2 O + 2 H + Eq. 2.2 NO / 2 O 2 NO 3 - 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 N 2 and N 2 O gases which are released to the atmosphere. The overall redox reaction is shown in equation 2.4 (van Haandel & van der Lubbe, 212). C x H y O z + (4x+y-2z)/5 H (4x+y-2z)/5 NO 3 x CO 2 + (2x+3y-z)/5 H 2 O + (4x+y-2z)/1 N 2 Eq

31 In the study of Chiu et al. (21), 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, 5, 1, and 2 mg L -1 were around 5.5, 4.5, 4., 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. (29) artificially increased the available carbon source for denitrification by providing glucose. The results showed that in the summer the nitrate removal rates increased from 2% to 5% and from 1% to 3% in winter. In the study of Foladori et al. (213), 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., 213). 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, 29). Nevertheless, in the developing world it is a more severe issue due to the discharge of untreated wastewater to the environment (García et al., 213). 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, 29). 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, 29). 31

32 A survey conducted by Zhang & Farahbakhsh (27), 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., 213). 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. (214) concluded that in HSSF CWs, the water depth, gravel size and seasonal changes can greatly affect the microbial removal. García et al. (213), 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. (214), testing HSSF CWs with different water depth had log units removal for total coliform and log units removal for E. coli. 32

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 (213) 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 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 33

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.6 m.4 m.8 m. The area of the HFF was identical to the VFCW s and the depth was.35 m. The media of the VFCW consisted of a.7 m fine sand section (1-2 mm) and a.1 m thick gravel layer in the bottom for drainage (15-3 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 85-1 µmol photons m 2 s 1 for 16 h d VFCW HFF Raw wastewater Peristaltic pump influent 5. Peristaltic pump recirculation 6. Ball valve Figure 4.1 Duplex-CW setup The first setup, abbreviated as, had the effluent of the HFF recirculated. The other, referred as 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 = 5 L), where the bottom 5 L was kept for settling, and was not used. 34

35 Table 4.1 Characteristics of the settled domestic wastewater during this study Parameters Range ph EC (µs cm -1 ) DO (mg L -1 ).3-.8 COD (mg L -1 ) TSS (mg L -1 ) NH + 4 N (mg L -1 ) NO 3 N (mg L -1 ).3-3 TN (mg L -1 ) 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 3-4 min. During the experimental periods, pulse feeding was applied to and, 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, the recirculation pump was always set to provide a recirculation ratio of.8:1 in order to avoid significant water level fluctuations in the HFF. Table 4.2 Operation of the Duplex-CWs Hour 1 Hour 2 Hour 3 Hour 4 Hour 5 Hour 6 Influent feeding for 15 min Recirculation for 15 min Influent feeding for 15 min Recirculation for 15 min Influent feeding for 15 min Recirculation for 15 min 35

36 4.2 Experimental design The experimental phase was divided into four stages applying HLRs at.5,.8 and.16 m 3 m -2 d -1. In the fourth experimental phase the HLR of.16 m 3 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, 213). The OLRs were calculated based on the COD concentrations of each experimental phase, and the land requirements were based on the BOD 5 concentrations and the HLRs and were expressed in m 2 per PE. The calculations can be found in Annex 1. Table 4.3 Operation modes and parameters of the Duplex-CWs Parameters Operation of Ilyas, st Experimental phase 2 nd Experimental phase 3 rd Experimental phase 4 th Experimental phase Feed volume (L d -1 ) Feeding (d/week) HLR (m 3 m -2 d -1 ) OLR (g COD m -2 d -1 ) Land requirement (m 2 PE -1 ) HRT HFF (d) Duration of phase (week) * *OLR was artificially increased with peptone 36

37 4.2.1 First experimental period - HLR.5 m 3 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 (.24 m 2 ), the HLR was.5 m 3 m -2 d -1 which was the same as in the previous studies of the Duplex CWs (Ilyas 213; Lavrnić, 213; Kyomukama, 214; Namakula, 214). Due to the natural variation of wastewater characteristics the OLR was lower than the one Ilyas (213) applied (Table 4.3). The required area was calculated to be 9. m 2 per PE Second experimental period - HLR.8 m 3 m -2 d -1 + OLR 18 g COD m -2 d -1 In the second experimental phase, the pumps were set to provide 3 L wastewater per day. The corresponding HLR was.8 m 3 m -2 d -1, and the required are was 6.8 m 2 per PE. The increase in the feeding volume was chosen to be small as the Duplex-CWs had never been operated at higher HLR Third experimental period - HLR.16 m 3 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 6 L wastewater was fed to the systems, being equal to the HLR of.16 m 3 m -2 d -1. This HLR is in the medium category considering what other researchers (Ghosh & Gopal, 21; Foladori et al., 213) 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 m 2 per PE Fourth experimental period - HLR.16 m 3 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.16 m 3 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., 23). Based on the preliminary experiments of Ilyas (213), due to the addition of peptone (.3 g L -1 wastewater) the COD concentration in the fourth experimental period was in the range of mg L

38 Implicating that in the fourth experimental period the wastewater was in the medium strength domestic wastewater category (Tchobanoglous et al., 23). 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 m 2 per PE. 4.3 Passive aeration capacity of the vertical flow constructed wetland Anoxic water was prepared by adding CoCl 2 (33 mg) as a catalyst and Na 2 SO 3 (.8 g) in 1 L demineralized water and further by bubbling N 2 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, 2 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 38