Study on potential filter materials for use as substrate in constructed wetlands to strengthen phosphorus treatment performance from swine wastewater

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN THI THUONG STUDY ON POTENTIAL FILTER MATERIALS FOR USE AS SUBSTRATE IN CONSTRUCTED WETLAND TO STRENGTHEN PHOSPHORU

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN THI THUONG

STUDY ON POTENTIAL FILTER

MATERIALS FOR USE AS SUBSTRATE

IN CONSTRUCTED WETLAND

TO STRENGTHEN PHOSPHORUS TREATMENT PERFORMANCE

FROM SWINE WASTEWATER

MASTER'S THESIS

Hanoi, 2019

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN THI THUONG

STUDY ON POTENTIAL FILTER

MATERIALS FOR USE AS SUBSTRATE

IN CONSTRUCTED WETLAND

TO STRENGTHEN PHOSPHORUS TREATMENT PERFORMANCE

FROM SWINE WASTEWATER

MAJOR: ENVIRONMENTAL ENGINEERING

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ACKNOWLEDGMENTS

First of all, I would like to express my heartfelt gratitude to my principal supervisor, Dr Nguyen Thi An Hang for giving me a chance to explore an exciting research field – the constructed wetlands, for always inspiring me She has spent plenty of time for teaching, explaining hard questions as well as sharing her own experiences in approaching and solving research problems Thanks to that, I was well equipped with essential knowledge and skills to fulfill my research I also express my deepest thanks to Assoc Prof Dr Sato Keisuke, who provided me a great guidance during my internship Besides teaching, providing knowledge and enthusiastic support, he always treated me tenderly likes my father In addition, he helped me not to be confused when I first arrived in Japan My special thanks go to

Dr Vu Ngoc Duy, who gave me valuable supports in developing research methods, implementing experiments, and deepening my research

The second, I want to send my sincere thanks to VNU Vietnam Japan University (VJU), Ritsumeikan University (RITs), Shimadzu Corporation and Shigaraki Center for warm welcome and enthusiastic support during my internship

in Japan Without their precious supports, I would not be able to complete this research Especially, I would like to convey my devoted appreciation to Prof Dr Jun Nakajima, Assoc Prof Dr Hiroyuki Katayama, for teaching and supporting me during my study at VJU

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 105.99-2018.13,

2018, Asean Research Center (ARC) research grant of Vietnam National

University, Hanoi (VNU), and Japan International Cooperation Agency (JICA)

Last but not least, my profound gratitude goes to my family for their spiritual supports during my thesis writing and my daily life as well This accomplishment would not have been possible without them

Hanoi, May 31th, 2019 Nguyen Thi Thuong

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TABLE OF CONTENTS

ACKNOWLEDGMENTS i

TABLE OF CONTENTS ii

LIST OF TABLES iv

LIST OF FIGURES iv

LIST OF ABBREVIATIONS v

INTRODUCTION 1

CHAPTER 1 LITERATURE REVIEW 7

1.1 Phosphorus (P) pollution and its consequences 7

1.2 Regulations related to P removal 8

1.3 Phosphorus treatment technologies 11

1.4 Constructed wetlands (CWs) system for wastewater decontamination 19

1.4.1 Definition 19

1.4.2 Classification 19

1.4.3 Application of CWs in wastewater treatment 23

1.4.4 Factors influencing the CWs treatment performance 25

1.4.5 Mechanisms of P removal in CWs 29

1.5 Removing P by substrates in CWs 31

1.6 Overview of research objects 33

1.6.1 Swine waste water 33

1.6.2 Ca-rich bivalve shell as the substrate in CWs 35

CHAPTER 2 MATERIALS AND RESEARCH METHODOLOGY 41

2.1 Materials and equipment 41

2.2 Experiment setting up 45

2.2.1 Modification of materials 45

2.2.2 Characterization of the developed material 46

2.2.3 Adsorption experiments 49

2.2.4 Removal of P from synthetic wastewater using the integrated CWs- adsorption system 51

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2.3 Analytical methods 53

2.3.1 Phosphorus analysis 53

2.3.2 Other parameters analysis 53

2.4 Data statistical analysis 53

CHAPTER 3 RESULTS AND DISCUSSION 55

3.1 Screening of filter materials for use as substrate in CWs 55

3.1.1 Comparing potential materials based on P adsorption capacities 55

3.1.2 Comparing filter materials based on their permeability 57

3.1.3 Comparing filter materials based on their side effects 58

3.1.4 Selection of potential filter materials 62

3.2 Intensive investigation of the selected filter materials –white hard clam (WHC) 64

3.2.1 Identification of the optimal modification conditions of WHC 64

3.2.2 Physicochemical properties 66

3.2.3 Batch experiment 70

3.2.4 Column experiment 80

3.2.5 Comparing the P removal efficiency of modified white hard clam (WHC-M800) in the synthetic and real swine wastewater 82

3.3 The P treatment performance in the integrated CWs – adsorption system 83

CHAPER 4 CONCLUSION AND RECOMMENDATION 88

4.1 CONCLUSION 88

4.2 RECOMMENDATION 89

REFERENCES 90

APPENDICES 108

Appendix 1: Visiting some CW systems during internship in Japan 108

Appendix 2: Preparing WHC as the substrate in CWs 109

Appendix 3: Designing and operating the integrated CW-adsorption system 110

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LIST OF TABLES

Table 1.1 Effluent discharge standards of different countries 8

Table 1.2 Phosphorus removal efficiencies of different methods 17

Table 1.3 Mechanism of phosphorus removal in constructed wetland system 30

Table 1.4 Some filter media used for P removal 32

Table 1.5 The main composition of swine wastewater after anaerobic digestion by biogas chamber 34

Table 1.6 The main chemical compositions of bivalve shells and limestone 37

Table 1.7 Some studies used bivalve shell for P removal 39

Table 3.1 Phosphorus adsorption capacity of different materials 57

Table 3.2 Permeability constant (K) of investigated materials 58

Table 3.3 The concentration of heavy metals released from materials 61

Table 3.4 Summary of the obtained scores for investigated materials Error! Bookmark not defined Table 3.5 Effect of calcination temperature 65

Table 3.6 Effect of the calcination time 66

Table 3.7 Brunauer Emmett Teller (BET) analysis 67

Table 3.8 Elemental content of WHC 68

Table 3.9 Elemental content of WHC-M800 68

Table 3.10 Langmuir and Freundlich adsorption isotherm constants 78

Table 3.10 P adsorption capacity at different conditions 81

Table 3.11 Parameters of real post-biogas swine wastewater in Chuong My, Hanoi 83

Table 3.12 The phosphorus concentrations before and after treatment with horizontal flow lab-scale constructed wetlands 85

Table 3.13 The phosphorus removal efficiency and pH after treatment with horizontal flow lab-scale constructed wetlands 86

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LIST OF FIGURES

Figure 1: Thesis‘s outline 6

Figure 1.1 Eutrophication from phosphorus contamination 7

Figure 1.2 The treatment technologies for phosphorus removal 11

Figure 1.3 Metabolic pathways of PAO under aerobic and anaerobic conditions 15

Figure 1.4 The classification of CWs used in wastewater treatments 19

Figure 1.5 The schematic surface flow constructed wetland 20

Figure 1.6 The schematic vertical flow constructed wetland 21

Figure 1.7 The schematic horizontal flow constructed wetland 21

Figure 1.8 The schematic hybrid constructed wetland 22

Figure 1.9 Phosphorus cycle in constructed wetland 29

Figure 1.10 The main clam species in Vietnam 37

Figure 2.1 Images of investigated filter materials 41

Figure 2.2 The routine to Thai Binh shellfish Co., Ltd, Tien Hai Thai Binh 42

Figure 2.3 Procedure to prepare WHC as phosphorous adsorbent 43

Figure 2.4 The pig farm in Chuong My, Hanoi 44

Figure 2.5 Equipments used in this study 45

Figure 2.7 The experiment setting according to Darcy law 47

Figure 2.8 Procedure for determine of porosity 47

Figure 2.9 Small column adsorption test 51

Figure 2.10 Integrated CWs-adsorption systems 52

Figure 2.11 Calibration curve for phosphorus analysis 53

Figure 3.1 Comparison of P adsorption capacity of investigated filter materials 56

Figure 3.2 pH of post-adsorption solutions 59

Figure 3.3 Images of raw WHC and WHC modified at different temperatures 65

Figure 3.4 SEM observation for WHC 67

Figure 3.5 SEM observation for 67

Figure 3.6 EDX spectrum of WHC 68

Figure 3.7 EDX spectrum of WHC-M800 68

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Figure 3.8 FTIR analysis for WHC 69

Figure 3.9 FTIR analysis for WHC WHC-M800 69

Figure 3.10 Effect of pH of WHC on phosphorus removal 71

Figure 3.11 Effect of pH of WHC-M800 on phosphorus removal 71

Figure 3.12 Effect of dosage of WHC on phosphorus removal 73

Figure 3.13 Effect of dosage of WHC-M800 on phosphorus removal 73

Figure 3.14 Effect of temperature of WHC on phosphorus removal 74

Figure 3.15 Effect of temperature WHC-M800 on phosphorus removal 74

Figure 3.16 The fitting of isotherm models to P adsorption onto WHC 77

Figure 3.17 The fitting of isotherm models to P adsorption onto WHC-M800 77

Figure 3.18 Linear form of adsorption isotherm following Langmuir of WHC 77

Figure 3.19 Linear form of adsorption isotherm following Freundlich of WHC 77

Figure 3.20 Linear form of adsorption isotherm following Langmuir of WHC-M800 78 Figure 3.21 Linear form of adsorption isotherm following Freundlich of WHC-M800 78

Figure 3.22 Kinetic test of WHC 79

Figure 3.23 Kinetic test of WHC-M800 79

Figure 3.24 Breakthrough curve of WHC-M800 for P removal under the different flowrate 81

Figure 3.25 Breakthrough curve of WHC-M800 for P removal under the different initial concentration 81

Figure 3.26 Breakthrough curve of WHC-M800 for P removal under the different weight of material 81

Figure 3.28 P adsorption capacity of WHC-M by real wastewater and synthetic wastewater 83

Figure 3.29 The change of phosphorus in the effluent over the time 85

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LIST OF ABBREVIATIONS

EBPR Enhanced biological phosphorus removal

FTIR Fourier transform infrared spectroscopy

MAP Magnesium ammonium phosphate hexahydrate

PAOs Polyphosphate accumulating organisms

USEPA United States environmental protection agency

in ASEAN and the seventh biggest in the world The swine breeding industry has promoted the economic development as well as the GDP of the country

Despite the huge economic benefits, pig breeding industry makes many environmental problems, which negatively affect to human health and ecosystems That is because swine wastewater normally contains high concentration of nutrients, such as phosphorus (P) and nitrogen (N) that are main reasons for eutrophication (Wang et al., 2013)

Currently, the most common method for swine wastewater treatment is anaerobic digestion using biogas chamber However, according to several studies, the concentration of pollutants in the effluent after biogas treatment is still very high, exceeding the permitted discharge standards (National Institute of Animal Husbandry, 2015) Thus, further treatment is necessary to ensure the concentration

of P in the effluent meets requirements (Ngo, 2013; Nguyen, 2016) Among several technologies utilized for swine wastewater treatment, constructed wetland has shown a promising technology

Constructed wetlands (CWs) have been applied as a green technology to treat various kinds of wastewater This technology is gaining much attention of scientists

in all over the world, especially in developing countries (Wu et al., 2015) That is because CWs have many advantages, such as low cost, simple operation, high removal efficiency, high biodiversity value, and great potential for water and nutrient reuse (Kadlec, 2009; Vymazal, 2007; Zhang, 2014)

Previously, there have been many studies on P removal by CWs, which utilized a wide range of filter materials, including natural materials (rock, gravel, mineral materials, apatite, etc.), industrial by-products (steel, ash, iron ore, etc.) and artificial products (Johansson, 2006) Research results showed that when absorptive materials were used as substrates, the P removal efficiency of CWs was significantly improved in comparison with those using conventional filter materials, such as sand and gravel (Johansson Westholm, 2006) Therefore, the finding of filter materials, which are capable in P decontamination as CWs substrates, continues to be of the great concern (Vohla et al., 2011) In Vietnam, the CW model

is still quite novel and not widely applied So far, CWs have been applied for the purification of several types of wastewater, such as sewage conveying river water (Nguyen, 2011), domestics wastewater (Ngo & Han, 2012; Nguyen, 2015); landfill leachate (Nguyen, 2012) However, to the best knowledge of the author, very few studies on swine wastewater treatment using CWs can be found (Le, 2012; Ngo, 2013; Nguyen, 2016; Vu et al., 2014) Additionally, there is less of information about the role of filter materials in the CWs Meanwhile, Vietnam is known as a country, which is rich in natural resource and, has large reserves of limestone and laterite Also, with long coast (3260 km), it has a great potential for clam and coral

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production As a result, a huge amount of solid waste can be generated, creating the environmental burden Besides, the large amount of agricultural and industrial by-products (okara, coal slag, steel slag) discharged from the food processing and fuel manufacturing… This is a great potential for the development of the constructed wetlands based on the indigenous materials

In brief, the use of special materials as filter media in CWs to intensify P removal was reported somewhere However, there are no studies in Vietnam using locally available, adsorptive natural materials (laterite, limestone, coral), industrial by-products (steel slag, coal slag, white hard clam) and agricultural by products (okara) for enhancing phosphorus removal efficacy from swine wastewater

In that context, this research “Study on potential filter materials for use as

substrate in constructed wetland to strengthen phosphorus removal from swine wastewater” is necessary to strengthen P removal by CWs from swine wastewater

as well as to reduce solid waste

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Research objectives and scope

This study has three main objectives as follows:

(1) To determine potential filter materials for use as substrate in CWs for P removal

• To compare filter materials (based on adsorption capacity and other selection criteria)

(2) To understand the physio-chemical and adsorptive characteristics of the selected material

• To understand physicochemial properties of selected material;

• To clarify adsorption behaviors (adsorption capacity, adsorption speed, adsorption efficiency in synthetic and real wastewater) of the selected material;

• To evaluate the suitability of the selected material for use in CWs (3) To evaluate the applicability of the selected material as the substrate in CWs for

P elimination

• To evaluate the P treatment performance in CWs

• To evaluate the contribution of different components (substrates, plant) in CWs

• To evaluate the lifespan of the CWs

Research significance

The recycling of white hard clam shell (WHC) as a phosphorus adsorbent results in double environmental benefits It not only enhances the ability of CWs in removing P from wastewater but also reduces the WHC shell as a solid waste in a cheap and environmentally friendly way Besides, it creates additional economic value for WHC

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Thesis’ outline

The research’s outline is shown in the Figure 1 This thesis contains of 4 chapters The main content of each chapter is presented as follows:

Introduction provides the research background, identifies research

objectives, research scope, main tasks, and research significance

Chapter 1: Literature review provides information about phosphorus

pollution and consequences, the relevant regulations and treatment technologies The focus is placed on the role of substrate in CW for removing phosphorus from swine wastewater

Chapter 2: Research materials and methodology describes the materials,

equipment, and methods used in this study Experiment setting-up is described in detail The analytical methods as well as instruments are also introduced

Chapter 3: Result and discussion provides results on phosphorus

adsorption capacity, physicochemical properties of materials, isotherm, kinetics and column studies, and P treatment performance in the CW-adsorption system

Chapter 4: Conclusion and recommendation summaries the main

findings, limitations of this research and further research directions

Appendices: includes some pictures of research activities of this study

6 Figure 1: Thesis‘s outline

Introduction

Chapter 1: Literature Review

P pollution and its consequences

P removal technologies

P removal by substrate in CWs

Chapter 2: Materials and Methods

Materials and equipment

Analytical and data statistical methods

Experiment setting up

Chapter 3: Results and Discussion

Screening potential filter material

Intensive investigation of selected material

P treatment performance in CW system

Chapter 4: Conclusions and Recommendations

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LITERATURE REVIEW

1.1 Phosphorus (P) pollution and its consequences

Phosphorus is a crucial nutrient that extremely needed for the growth of plant and animals (Han et al, 2015) It is an abundant element in the earth’s crust and also a vital component of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine triphosphate (ATP), phospholipids, teeth and bones in animal bodies (Nguyen et al., 2012) In addition, phosphorus also plays a key role in industrial processes, it is a major material for several principal industries (e.g fertilizers, metallurgical industry, detergents, paints, and pharmaceuticals) (Nguyen et al., 2012)

Nevertheless, the excessive loading of P in water bodies is a major cause lead

to eutrophication, this process is a serious threat to water resources (Ruzhitskaya and Gogina, 2017)

Figure 1.1 Eutrophication caused by P contamination (Chislock et al., 2013)

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The concentration of P in the aqueous medium reaches 0.02 mg/ L can cause

to eutrophication (Ismail, 2012; Nguyen et al., 2012) This phenomenon is characterized by excessive plant and algal growth The large consumption of oxygen for the dead algae decomposition, resulted in the dissolved oxygen can be reduced dramatically in aquatic medium, and thus threatening the aquatic animal living as discussed by Nguyen et al (2012) Consequently, the reducing water quality, losing biodiversity, damning economic and recreational value and posing significant public health risks (Wilson et al 2006; Tillmanns et al 2008)

Therefore, P should be eliminated from wastewater before discharged into the environment

1.2 Regulations related to P removal

The excessive amount of phosphorus in aqueous medium due to both of natural sources and human activities can cause in negative impacts on ecosystems Therefore, several guidelines and standards that have been published for protecting and controlling phosphorus pollution in natural water bodies and wastewater effluents (Gibbons, 2009)

Table 1.1 Effluent discharge standards of different countries

Country

Total phosphorus unless otherwise indicated (mg/L)

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Guidelines for Effluent Quality and Wastewater Treatment at Federal Establishments

1972 Environment and Heritage

Service (EHS) European

In the world, to prevent eutrophication of reservoirs, many countries have regulated the level of phosphorus in the surface water is less of 0.05 mg/ L to combat excessive algae growth (Nguyen et al., 2012) According to Ramasahayam, (2014), to prevent surface water pollution from eutrophication, the maximum allowable concentration of P should be lower than 0.01mg/L For the same purpose, USEPA also recommended that the total level of phosphorus in the inflows to the lake and in the flow should be kept from 0.05 to 0.1 mg/ L, and EPA criterion for

It can be seen that the effluent discharge standards vary from one region to others in a country (USA) as well as one country to another This can be explained

by the variation in the level of treatment technologies and background phosphorus concentrations in the water bodies in different regions and countries (Nguyen et al., 2013) To prevent P pollution from the consequences of rapid economic development, China also has developed strictly for phosphorus regulations with the low-acceptable P concentrations (0.5-1 mg/ L) (Wang et al., 2013)

In Vietnam, P regulation is applied in some types of water such as Industrial wastewater Discharge Standards (4-6 mg/ L), the effluent of aquatic Products Processing industry (10-20 mg/ L), Health Care wastewater (6-10 mg/ L), domestic wastewater (6-10 mg/ L) Nevertheless, most of the effluent discharge standards are higher than developed countries and much higher than EPA criterion

Although, each country has the effluent discharge standards is different, however, the most stringent regulations have suggested that the total concentration

of P in the effluent only should be kept from 0.5 to 1 mg/ L before being discharged into the water environment (Xu et al., 2011a)

Therefore, in order to meet these stringent limits, the search for technologies treatment of phosphorus is required to protect water bodies from eutrophication (Nguyen et al., 2013)

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1.3 Phosphorus treatment technologies

There are many technologies for phosphorus removal as shown in Figure 1.2

(Nguyen, 2015) Each method has distinctive characteristics and presents its own

merits and demerits

Figure 1.2 P treatment technologies (Nguyen, 2015)

1.3.1 Physical methods

The physical technologies are membrane related processes They include

microfiltration, reverse osmosis, and electrodialysis The mechanism of

microfiltration related to size exclusion, hence concentration and pressure are not

effect to its removal efficiency In contrast, initial concentration, water flux rate and

pressure are affect to reverse osmosis due to its primary mechanism is diffusion In

the electrodialysis method, ions are moved by an electric field on membrane, they

tend to go to through the membrane and concentrate at one compartment, while

decontaminated water remains in the other (Karachalios, 2012)

Magnetic separation

In 1970s, magnetic separation method was beginning investigated for phosphorus treatment This is considered as an attractive method because at the same cost with other methods, the phosphorus level in effluent can be reached to 0.1-0.5 mg/ L by magnetic separation

Magnetic separation may be applied as a reliable add-on technology for chemical removal Phosphates in solution are combined with reagent into insoluble compounds And after that, magnetic material is used to isolates phosphate-containing sediment The significant benefits of this process are simple process, and low energy consumption However, it has low elimination efficiency (<10 %) in case of microfiltration and in term of high cost for RO and electrodialysis (Biswas,

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1.3.2 Chemical methods

Chemical method has been widely utilized for elimination of phosphorus, in response to increasing concern over eutrophication (Ruzhitskaya and Gogina, 2017)

5Ca2+ + 3 PO43- + OH- → Ca5(PO4)3(OH)↓

The most common chemicals employed for this method are iron (II, III) and aluminium (Thistleton et al., 2001)

This treatment performance is influenced by some parameters such as pH, TSS, dissolved organics, type of the precipitant, location of dose application, and mixing conditions The treatment performance of chemical method is high According to Nieminen, (2010), there are more 90 % of the total P might be eliminatedd by this method Nevertheless, this method still has several drawbacks, such as high chemical cost, potential sludge formation, insufficient efficiency for phosphorus with low concentration (Biswas, 2008; Mallampati and Valiyaveetttil, 2013) The sludge handling will increase the treatment cost and require much space (Lanning, 2008) In addition, the end-products of chemical methods are non-reusable, due to high impurities and low bioavailability (Nieminen, 2010) Besides,

it is hard to identify the optimal dosing conditions (Biswas, 2008)

Crystallization

The crystallization technology has been developed since the 1970s, in response to more stringent regulations (Biswas, 2008) This method is based on growing phosphate crystals in wastewaters and later to be removed from the system

According to Biswas (2008), the feasibility of the phosphorus adsorption process mostly relied on the preparation of adsorbents Formerly, activated carbon was commonly applied to remove phosphorus However, its application is not wide,

in particular in developing countries because the problems relate to high expense and no renew ability (Karthikeyan et al., 2004) Therefore, using abundant availability, low-cost materials (eg industrials by-products, agricultural by-products) with high efficiency, potential renewability and adaptation are trending new approach (Biswas, 2008, Ning et al., 2008)

1.3.3 Biological methods

Biological method for phosphorus removal was developed in the late 1950s, and it has shown to be a firm technology This method ensures the best removal of phosphorus, as they help to maximize the biological potential of activated sludge (Ruzhitskay and Gogina, 2017)

Enhanced biological phosphorus removal (EBPR)

Phosphorus removal was removed by using polyphosphate accumulation organisms (PAO) Through the growth of PAO, a large amount of P has been eliminated This method can be shown the P removal efficiency up to 85% from wastewater (Bunce et al., 2018)

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Figure 1.3 Metabolic pathways of PAO (Bunce et al., 2018)

EBPR is also a green approach to the elimination of phosphorus However, the phosphorus treatment performance is limited (≤30 %) Additionally, microorganisms are less adapted with the variation of environment Moreover, this method could not treat effectively with trace levels of phosphorus (Bunce et al., 2018)

Constructed wetlands

Constructed wetlands (CWs) are engineered systems for decontaminating wastewater based on natural functions of filter media, plant and organisms (Vymazal, 2007)

In CWs, phosphorus is removed by microbial decomposition, plants up take, sedimentation, adsorption and precipitation of substrates P is a vital element of plants, it synthesized by uptake and assimilation of plants, and P is removed from the system when the plants are harvested (Karachalios, 2012) Microoganisms also convert phosphorus from poorly soluble organic phosphorus to dissolved inorganic phosphorus which plants can easily to absorp In addition, P is also eliminated by adsorption and precipitation of substrates The combination of phosphorus with

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Ca2+, Al3+, Mg2+, Fe3+, and Mn2+ ion in substrates is an important way to remove P

in constructed wetland system (Vymazal, 2007)

This method has many advantages such as low cost, simple operation and maintaince, high biodiversity value (Babatunde et al., 2010) However, removal phosphorus will be limited when the substrates has low adsorption or reach to saturation On the other hand, it can create a large amount of sediment (Vymazal, 2007)

P removal efficiency (%) - adsorption capacity (mg/g)

Membrane Secondary or

tertiary treatment

6-13 > 90 % Real wastewater Gnirss et al., 2003

5 94.1 % Domestic wastewater Smith et al., 2014 Magnetic

500 50 mg/ g Synthetic wastewater Martin et al., 2009

50 51.52 mg/ g Synthetic wastewater Ren et al., 2014

90 % Synthetic wastewater Seo et al., 2013

Precipitation

Primary, secondary or tertiary treatment

or activated sludge recycle

2014

Crystallization Tertiary treatment

or recycle stream

25 91.30 % Synthetic wastewater Xuechu et al., 2009 30-120 96.10 % Synthetic wastewater Menglin et al., 2016

5 91 % - (120 mg/

g) Synthetic wastewater

Renman and Renman,

2010

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Adsorption Secondary or

tertiary treatment

100 > 90 % - (4.75

mg/ g) Synthetic wastewater Nguyen et al., 2013

30 98.20 % Synthetic wastewater Vohla et al., 2010

5 - 25 95% (3.11 mg/ g) Synthetic wastewater Vohla et al., 2010

EBPR

Secondary or tertiary treatment

or activated sludge recycle

16.67 99 % Synthetic wastewater Ong et al., 2016

Constructed

wetlands

Secondary or tertiary treatment

or activated sludge recycle

0.5 - 2 40-75 % landfill leachate Koiv et al., 2009a

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Free surface water flow:

Figure 1.5 The schematic surface flow constructed wetland (Almuktar et al., 2018) Free water surface flow (FWS) systems are designed similar to natural wetlands, they include an aquatic area with a variety of plants, a sealed basin filled with 20-30 cm of substrates and about 40 cm for the depth of water (Stefanakis et al., 2014) These systems are also considered as expected habitats for many wildlife species

In free surface flow systems, organic compounds in wastewater are effectively removed through primarily the process of sedimentation, filtration and decomposition of microorganisms Nitrogen is effectively treated by denitrification and ammonia volatilization However, phosphorus is unable to effectively removed because the water does not tend to come in contact with soils particles (which adsorb and precipitate with P) as discussed by Taylor et al., 2006

Thus, if phosphorus is the key contaminant of concern, the FWS systems are less suitable for treatment Additionally, the high possibility of human exposure to pathogens and the large area requirement are also disadvantages of these systems

Subsurface water flow (SSF)

In subsurface flow (SSF) constructed wetlands, water come directly to media and is generally invisible (Vymazal, 2007)

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According to the flow direction, SSF might be classified into two types: vertical flow (VF) and horizontal flow (HF) (Almuktar et al., 2018) In HF constructed wetlands, the substrates are flooded by water, while VF constructed wetlands are applied intermittently to gain the high rate of oxygen transfer (Stefanakis et al., 2014)

Figure 1.6 The schematic vertical flow constructed wetland (Almuktar et al., 2018)

Figure 1.7 The schematic horizontal flow constructed wetland (Almuktar et al.,

2018)

For wastewater treatment, if phosphorus is the primary contaminant of concern, subsurface flow constructed wetlands can be a greater treatment tool than

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surface flow constructed wetlands Because they can be controlled by selecting

highly P adsorbable substrates as discussed by Pant, 2001 and White 2011

In addition, the role of root-bed media for phosphorus sorption also is very important They facilitate better to remove P from aqueous media for longer time (Pant et al., 2001; Seo et al., 2005) White et al (2011) reported that the P removal

of root-bed substrates could be reached around 74 % by using substrates such limestone, oyster shells, crushed brick The viability of the substrate for P removal depends on its maximum adsorption capacity The saturated substrates must be removed and processed, then the new substrates need to be added periodically to maintain P adsorption capacity This is a drawback of SSF constructed wetlands Hence, the monitoring and evaluating the life of substrates are extremely important and necessary to maintain the P-sorption capacity and to minimize the P export from the saturated (White et al., 2011)

Hybrid systems

Hybrid systems were developed to overcome the limitations of single stage CWs, because many wastewaters could be difficult to treat in individual systems (Vymazal 2005, 2007)

Figure 1.8 The schematic hybrid constructed wetland (Almuktar et al., 2018) Due to the ability of individual systems could not to provide both aerobic and anaerobic conditions simultaneously, so that its efficiency in nitrogen removal is not

In generally, constructed wetland is a potential technology, and the attractive alternatives to traditional wastewater treatment methods The improvements to enhance treatment efficiency for many wastewater types in CW are still being concerned and developed (Zhang et al., 2014)

1.4.3 Application of CWs in wastewater treatment

In the world

CWs have been used for quite a long time The first experiment was implemented by Käthe Seidel at Germany in 1950s with the attempt aimed using CWs to treat wastewater And lots of experiments were conducted and successfully applied in later At the initial stage, most the application of CWs were utilized to treat conventional municipal and domestic wastewater (Wu et al., 2015)

To date , they have been used to various wastewater such as mine drainage (Smith, 1997), landfill leachates (Bulc et al., 1997), seafood processing wastewater (Xeybouangeun, 2011), lake waters (Cui et al., 2011), domestics wastewater (Andreo-Martínez, 2017), compost leachate (Bakhshoodeh et al., 2017), grey water (Ramprasad et al., 2017), dairy wastewater (Adhikari et al., 2015), eutrophic water (Hernández-Crespo et al., 2016), rainfall runoff from a motorway (Shutes at al., 1999) and agricultural runoff (Wang et al., 2018), and industrial and sewage

up to 43 % compared to a single harvest Concerning the seasonal variation, Ramprasad et al (2017) released that phosphorus removal rate in the summer was higher than that in other seasons Relating to evapotranspiration, Bakhshoodeh et al (2017) reported that when evapotranspiration started to rise, phosphorus removal efficiency decreased Since the filter media is the main component of the CWs, it was of great interests of many international researchers Barca et al (2014) found that, using steel slag as filter material can removed over 88 % total phosphorus in influent Andreo-Martínez (2017) obtained extremely high removal efficiency of phosphorus (96.9±1.7 %) by applying blast furnace slags as filter media in the HF-CWs for reclaiming domestic wastewater

From the 1960s to the present, there are many CWs have been built to treat wastewater Specifically, in Europe possessed about 50,000 CWs and in North America that of 10,000 CWs In addition, CWs is also a promising potential technology that can be replaced to other technologies in developing countries (Zang, 2014) Chen et al (2011) investigated that recently, the are many CWs have been used successfully for wastewater treatment in China, India, and Southeast Asia

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In Vietnam

In Vietnam, the CW model is still quite novel and not widely applied So far, CWs have been applied for the purification of several kinds of wastewater, for instance sewage conveying river water (Nguyen et al., 2011), domestics wastewater (Ngo & Han, 2012; Nguyen, 2015; Nguyen, 2016); landfill leachate (Nguyen, 2012), However, to the knowledge of the author, very few studies on swine wastewater treatment using CWs can be found (Le, 2012; Ngo, 2013; Nguyen, 2016; Vu et al., 2014)

It is recognized that the performance of CWs is affected by many factors Several studies examined the effect of hydraulic loading rate (HLR) and hydraulic retention time (HRT) (Ngo and Hans, 2012; Ngo, 2013; Vu et al., 2014), a few studies implemented to show the role of filter materials in the CWs (Ngo and Hans, 2012; Nguyen, 2008; Nguyen, 2015) So far, there has been no studies in Vietnam

on mass balance study to interpret phosphorus removal mechanisms The information on role of microorganisms in the CWs is also lacking In contrast, these are the focus of international studies

1.4.4 Factors influencing the CWs treatment performance

a HRT (Hydraulic retention time)

The hydraulic residence time (HRT) is the average time that water remains in the wetland The optimal HRT design plays a crucial role in the treatment performance of CWs According to Yan and Xu (2014), the contaminants can be removed better at a longer HRT due to the great microbial community can be established In contrast, the ammonium and total nitrogen (TN) concentrations decreased significantly with increasing HRT for domestic wastewater treatment as discussed by Huang et al (2000) Lee et al (2009) also reported that a low HRT in CWs might be associated incomplete denitrification of wastewater Hence, to remove completely nitrogen, longer HRT is required

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In addition, Avila et al (2014) investigated the possibility of removing emerging organic contaminants by using hybrid CW systems, this result pointed out that the elimination of phosphorus reduced as the HRT decreased

b HLR (Hydraulic loading rate)

Hydraulic loading rate (HLR) is the amount of loading water per unit area It

is an important factor in monitoring and operating CWs (Wu et al., 2015) A greater HLR boosts quicker the movement of wastewater through the media, results in the contact time of wastewater and media reduced An adequate microbial community unable to establish with short contact time (Wu et al., 2015) Cui et al (2010) reported that the ammonium and TN treatment efficiency was reduced from 65 % to 60 % and 30 % to 20 % respectively in domestic wastewater with increasing HLR from 7 to 21 cm/ d However, Stefanakis and Tsihrintzis (2012) showed that the nitrogen and organics removal efficiencies in synthetic wastewater by VF CWs were increased as HLR increased

Thus, depend on the types of CW and wastewater that need to have appropriate HLR Therefore, to ensure the treatment performance in CWs, the optimal design of HLR is extremely necessary

c Feeding mode of influent

The feeding mode is also a key design parameter (Vymazal, 2011) The treatment performance in CW also relies on the different feeding mode of influent Many studies examined the treatment performance in CW using different influent feeding modes Generally, the treatment performance can be better with batch feeding mode, due to having more oxidized conditions (Vymazal, 2011) Zhang et al (2012) reported that the ammonium treatment efficiency is 95.2 % with batch flow mode, meanwhile it only reached 80 % with the continuously fed systems

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However, Saeed and Sun, (2012) indicated that the nitrogen and organic compounds removal efficiencies could be enhanced with intermittent feeding mode Additionally, Caselles-Osorio and García (2007) investigated the contaminants treatment efficiency in SSF CWs by the continuous and intermittent feeding modes, and noted that continuous s feeding modes is lower than to intermittent feeding mode for ammonium removal efficiencies Jia et al (2010) also reported that the COD and TP removal efficiencies in VF CWs by intermittent feeding modes are slower than continuous feeding modes

Generally, the feeding modes influence to the treatment efficiency of CW Thus, depending on the primary contaminant of concern, has suitable selection for feeding mode

Compare to FWS CWs, phosphorus can be treated greater in SSF CWs because the wastewater tend to comes into contact with filter media (Zhang et al., 2014) VSSF CWs can remove nitrogen more effective than HSSF CWsVymazal (2007)

Li et al., (2008) carried out a comparative study to evaluate the treatment performance of FWS and SSF system for purifying eutrophic water of Taihu Lake The results obtained shown that the treatment efficacy in the FWS CWs was lower than compared to VSSF and HSSF CWs

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Generally, beside other factors, the type of CW also influences to the treatment performance of CW Thus, to select an appropriate type of CW, it is necessary to determine the primary of the concern

e Substrates (medium)

The substrate is a key component of the CWs, it is very important in SSF CW, because it can provide an appropriate growing media for plants as well as keep various pollutants (Nguyen, 2015) Furthermore, filter materials adsorption plays extremely an important role in the removal of numerous contaminants such as phosphorus (Ju et al., 2014) Therefore, selection of appropriate substrate using in

f Plant

Plant is another important component and could play a strategic role in CWs Many international studies focused on plant selection by evaluating and comparing their treatment efficiencies (Zhang et al., 2014)

Ong et al (2010) revealed that the total phosphorus elimination efficacy in CWs planted with Manchurian wild rice (52 %) was considerably higher than that

in CWs planted with Phragmites australis (34 %) Adhikari et al (2015) reported

the P removal rate of duckweed in the surface flow CWs was 7.4 g N/ m2/ yr

In addition, Calheiros et al., 2012 indicated that Arundo donax and

Sarcocornia can be treated at high concentration of contaminant and can remove

effectively nitrogen as well as phosphorus P.australis could remove antibiotics

from wastewater as discussed by Liu et al (2013)

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Moreover, the biomass of plant is important for removing of pollutants Specifically, Březinová and Vymazal (2015) observed that phosphorus removal efficiency via multiple biomass harvesting might be increased up to 43 % compared to a single harvest This is the same with the finding of Hernández-

Crespo et al (2016) that a multiple harvest of Iris pseudacorus increased the

nutrients removal efficiency up to 50% for N and 100 % for P compared with a single harvest Similarly, Luo et al (2017) discovered that multiple harvest of

Myriophyllum aquaticum from SFCWs contributed up to 22.5-59.6 % of total

removed phosphorus mass

1.4.5 Mechanisms of P removal in CWs

Figure 1.9 Phosphorus cycle in constructed wetland (Reddy et al., 1999)

Mechanism of phosphorus removal in CWs includes plant uptake, accumulation of microorganisms, absorption and precipitation of substrates… (Vymazal, 2007)

P is an important element of essential component in plants such as DNA, RNA, ATP, phospholipids They are synthesized by uptake and assimilation of plants, and phosphorus is eliminated from the system as the plants are harvested Additionally, thanks to microorganisms, phosphorus is converted from poorly

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soluble organic phosphorus to dissolved inorganic phosphorus which plants can easily to absorp Phosphorus is also removed by adsorption and precipitation of substrates The combination of phosphorus with Ca2+, Al3+, Mg2+, Fe3+, and Mn2+ion in substrates is an important way to remove P in constructed wetland system However, it will be limited when the substrate has low adsorption or reach to saturation

In generally, long-term removal of phosphorus from a given ecosystem can only occur when vegetation is harvested or when sediment bound phosphorus is removed from the system (Vymazal, 2007)

Table 1.3 Mechanism of phosphorus removal in constructed wetland system

Sedimentation Particulate phosphorus settles out of the water

column

Adsorption Chemical bonding of phosphorus to iron, aluminum,

and calcium on soil particle exchange sites

Precipitation Phosphorus binds to dissolved iron, aluminum, and

calcium to form a solid or semi-solid

Uptake by Vegetation Orthophosphates and some organic phosphorus

taken up by plants and algae

Immobilization

Plant available and some organic forms of phosphorus are consumed by microbial communities and stored in their tissues

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1.5 Removing P by substrates in CWs

1.5.1 The role and mechanism of P removal by substrates

Substrate plays an important role for phosphorus removal The contaminants can be removed from wastewater through exchange, adsorption, precipitation and

complexation process (Wu et al., 2015)

Depend on the physicochemical properties of filter materials that the mechanism will be dominant The phosphorus removal capacities are depended on the contents of calcium, iron, aluminum and their oxides and hydroxides (Yang et al., 2018) such as phosphorus was removed effectively by the use of steel slag (44.3 % of CaO , 16.9 %

of Al2O3 and 24.2 % of Fe2O3) (Park et al., 2016), limestone and scallop shells (mainly CaCO3) (Nguyen et al., 2014), BOF-slag (47.08 of CaO, 36.12 % of Fe2O3 8.04 % of SiO2 , 4.94 % of MgO) (Han et al., 2015)

1.5.2 Selection criteria

The selection of substrate with high efficiency, abundant availability, durability, appropriate cost, less side effects has been a crucial issue in constructed wetlands

The P adsorption capacity of filter materials depends on its physicochemical properties The chemical composition of materials plays a crucial role for P removal because the phosphorus is mainly removed via adsorption and precipitation process, it means that the Ca, Al, Fe contents is important for the effective P removal However, the chemical composition may also be to side effects when they can affect to the electrical conductivity, pH, and heavy metal release Hence, the filter media should be selected very careful (Vohla et al., 2011)

Besides, the hydraulic conductivity is also considered an important issue for selection of substrate To avoid clogging, the material must have durability, porosity and reasonable size (Vohla et al., 2011)

In addition, CWs are generally characterized as cost-effective technology due

to the minimum cost of construction, operation and maintenance To minimize the