Study on potential filter materials for use as substrate in constructed wetlands to strengthen phosphorus treatment performance from swine wastewater001

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

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

First of all, I would like to express my heartfelt gratitude to my principalsupervisor, Dr Nguyen Thi An Hang for giving me a chance to explore an excitingresearch field – the constructed wetlands, for always inspiring me She has spentplenty of time for teaching, explaining hard questions as well as sharing her ownexperiences in approaching and solving research problems Thanks to that, I waswell equipped with essential knowledge and skills to fulfill my research I alsoexpress my deepest thanks to Assoc Prof Dr Sato Keisuke, who provided me agreat guidance during my internship Besides teaching, providing knowledge andenthusiastic support, he always treated me tenderly likes my father In addition, hehelped 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 JapanUniversity (VJU), Ritsumeikan University (RITs), Shimadzu Corporation andShigaraki Center for warm welcome and enthusiastic support during my internship

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

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

2018, Asean Research Center (ARC) research grant of Vietnam NationalUniversity, Hanoi (VNU), and Japan International Cooperation Agency (JICA).Last but not least, my profound gratitude goes to my family for their spiritualsupports during my thesis writing and my daily life as well This accomplishmentwould not have been possible without them

Hanoi, May 31th, 2019Nguyen Thi Thuong

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

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

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

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

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

LIST OF ABBREVIATIONS

EBPR Enhanced biological phosphorus removal

FTIR Fourier transform infrared spectroscopy

USEPA United States environmental protection agency

Swine breeding industry is an important part of agriculture sector in Vietnam Inrecent years, numerous large scales of pig farms have been developed to meet thepork demand in the market According to General Statistics Office of Vietnam(2018), the whole country has about 500,000 livestock households, over 29 millionpig heads, 3.8 million tons of meat Also, as the pig producer, Vietnam is the biggest

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

Despite the huge economic benefits, pig breeding industry makes manyenvironmental 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 isanaerobic digestion using biogas chamber However, according to several studies,the concentration of pollutants in the effluent after biogas treatment is still veryhigh, exceeding the permitted discharge standards (National Institute of AnimalHusbandry, 2015) Thus, further treatment is necessary to ensure the concentration

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

Constructed wetlands (CWs) have been applied as a green technology to treatvarious 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 isbecause CWs have many advantages, such as low cost, simple operation, high

removal efficiency, high biodiversity value, and great potential for water andnutrient reuse (Kadlec, 2009; Vymazal, 2007; Zhang, 2014)

In addition, Vietnam is a tropical country with the hot and humid climate,which is appropriate for the growth of plants and microorganisms in CWs Thus,

CW is a potential wastewater treatment method and can replace or support otherhigh-cost chemical and physiological technologies (Nguyen, 2015)

The main concern with the swine wastewater is high content of organic matter,nitrogen and phosphorus While CWs can remove organic matter, suspended solidsand nitrogen efficiently, its removal efficiency of phosphorus is still low, unlessspecial filter materials with high P sorption capacity are utilized (Almuktar et al.,2018) Therefore, in order to strengthen P removal from wastewater using CWs, it isnecessary to identify the potential filter materials with high P binding capacity(Vohla et al., 2011)

Previously, there have been many studies on P removal by CWs, whichutilized a wide range of filter materials, including natural materials (rock, gravel,mineral materials, apatite, etc.), industrial by-products (steel, ash, iron ore, etc.) andartificial products (Johansson, 2006) Research results showed that when absorptivematerials were used as substrates, the P removal efficiency of CWs wassignificantly improved in comparison with those using conventional filter materials,such as sand and gravel (Johansson Westholm, 2006) Therefore, the finding offilter 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 thepurification of several types of wastewater, such as sewage conveying river water(Nguyen, 2011), domestics wastewater (Ngo & Han, 2012; Nguyen, 2015); landfillleachate (Nguyen, 2012) However, to the best knowledge of the author, very fewstudies on swine wastewater treatment using CWs can be found (Le, 2012; Ngo,2013; Nguyen, 2016; Vu et al., 2014) Additionally, there is less of informationabout the role of filter materials in the CWs Meanwhile, Vietnam is known as acountry, which is rich in natural resource and, has large reserves of limestone andlaterite Also, with long coast (3260 km), it has a great potential for clam and coral

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

In brief, the use of special materials as filter media in CWs to intensify Premoval was reported somewhere However, there are no studies in Vietnam usinglocally available, adsorptive natural materials (laterite, limestone, coral), industrialby-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

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, adsorptionspeed, 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 adsorbentresults in double environmental benefits It not only enhances the ability of CWs inremoving P from wastewater but also reduces the WHC shell as a solid waste in acheap and environmentally friendly way Besides, it creates additional economicvalue for WHC

4

Thesis’ outline

The research’s outline is shown in the Figure 1 This thesis contains of 4chapters 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 fromswine wastewater

Chapter 2: Research materials and methodology describes the materials,

equipment, and methods used in this study Experiment setting-up is described indetail 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 andcolumn 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.

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 Experiment setting up Analytical and data statistical methods

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

Figure 1: Thesis‘s outline

CHAPTER 1 LITERATURE REVIEW

1.1 Phosphorus (P) pollution and its consequences

Phosphorus is a crucial nutrient that extremely needed for the growth of plantand 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 industrialprocesses, 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 (Ruzhitskayaand Gogina, 2017)

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

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 ischaracterized by excessive plant and algal growth The large consumption ofoxygen for the dead algae decomposition, resulted in the dissolved oxygen can bereduced dramatically in aquatic medium, and thus threatening the aquatic animalliving as discussed by Nguyen et al (2012) Consequently, the reducing waterquality, losing biodiversity, damning economic and recreational value and posingsignificant public health risks (Wilson et al 2006; Tillmanns et al 2008)

Therefore, P should be eliminated from wastewater before discharged into theenvironment

1.2 Regulations related to P removal

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

Table 1.1 Effluent discharge standards of different countries

Total phosphorus Country unless otherwise Year Source

indicated (mg/L)

and No85Colorado 1 (Existing plants)

8

Guidelines for Effluent

Treatment at FederalEstablishments

0.5-3

Emission Limit Guidelines forAustralia,

2001 Sewage Treatment Plants that

and Marine Waters June 2001

Northern

European 1 (> 100,000 PE)

1991 European Union Urban Waste

Standards

1 (Level B)

General Standards for

In the world, to prevent eutrophication of reservoirs, many countries haveregulated the level of phosphorus in the surface water is less of 0.05 mg/ L tocombat excessive algae growth (Nguyen et al., 2012) According to Ramasahayam,(2014), to prevent surface water pollution from eutrophication, the maximum

USEPA also recommended that the total level of phosphorus in the inflows to thelake and in the flow should be kept from 0.05 to 0.1 mg/ L, and EPA criterion for

9

the maximum concentration of P discharge into aqueous medium is 0.1 mg/ L(Ramasahayam, 2014).

In order to minimize the pollution burden on surface water as well as tocontrol phosphorus pollution at the source Some countries have developed andapplied national regulation of effluent discharge standards as shown in the Table1.1

It can be seen that the effluent discharge standards vary from one region toothers 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 phosphorusconcentrations in the water bodies in different regions and countries (Nguyen et al.,2013) To prevent P pollution from the consequences of rapid economicdevelopment, China also has developed strictly for phosphorus regulations with thelow-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 Industrialwastewater Discharge Standards (4-6 mg/ L), the effluent of aquatic ProductsProcessing industry (10-20 mg/ L), Health Care wastewater (6-10 mg/ L), domesticwastewater (6-10 mg/ L) Nevertheless, most of the effluent discharge standards arehigher 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 dischargedinto the water environment (Xu et al., 2011a)

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

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 ownmerits and demerits

Figure 1.2 P treatment technologies (Nguyen, 2015)

1.3.1 Physical methods

The physical technologies are membrane related processes They includemicrofiltration, reverse osmosis, and electrodialysis The mechanism ofmicrofiltration related to size exclusion, hence concentration and pressure are noteffect to its removal efficiency In contrast, initial concentration, water flux rate andpressure are affect to reverse osmosis due to its primary mechanism is diffusion Inthe electrodialysis method, ions are moved by an electric field on membrane, theytend to go to through the membrane and concentrate at one compartment, whiledecontaminated water remains in the other (Karachalios, 2012)

Magnetic separation

In 1970s, magnetic separation method was beginning investigated forphosphorus treatment This is considered as an attractive method because at thesame cost with other methods, the phosphorus level in effluent can be reached to0.1-0.5 mg/ L by magnetic separation

Magnetic separation may be applied as a reliable add-on technology forchemical removal Phosphates in solution are combined with reagent into insolublecompounds And after that, magnetic material is used to isolates phosphate-containing sediment The significant benefits of this process are simple process, andlow energy consumption However, it has low elimination efficiency (<10 %) incase of microfiltration and in term of high cost for RO and electrodialysis (Biswas,2008)

1.3.2 Chemical methods

Chemical method has been widely utilized for elimination of phosphorus, inresponse 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) andaluminium (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, andmixing conditions The treatment performance of chemical method is high.According to Nieminen, (2010), there are more 90 % of the total P might beeliminatedd by this method Nevertheless, this method still has several drawbacks,such as high chemical cost, potential sludge formation, insufficient efficiency forphosphorus 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, inresponse to more stringent regulations (Biswas, 2008) This method is based ongrowing phosphate crystals in wastewaters and later to be removed from the system

This process occurs on filters or in the suspended sludge, which include magnesiumammonium phosphate (MAP) and hydroxyapatite processes.

However, crystallization phosphorus removal method is not widely appliedbecause of very high cost (Biswas, 2008)

Adsorption

Among various phosphorus removal technologies, only adsorption methodholds the potential for phosphorus recovery (Nguyen et al., 2012; Zhang et al.,2014) P from wastewater is attracted by intermolecular forces onto surface of solidadsorbent and keeps in there This method is widely used for both high and lowlevel of phosphorus in various wastewater (Nguyen et al, 2012)

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

in particular in developing countries because the problems relate to high expenseand no renew ability (Karthikeyan et al., 2004) Therefore, using abundantavailability, low-cost materials (eg industrials by-products, agricultural by-products) with high efficiency, potential renewability and adaptation are trendingnew 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 ofphosphorus, 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 accumulationorganisms (PAO) Through the growth of PAO, a large amount of P has beeneliminated This method can be shown the P removal efficiency up to 85% fromwastewater (Bunce et al., 2018)

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

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

Constructed wetlands

Constructed wetlands (CWs) are engineered systems for decontaminatingwastewater 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 ofplants, it synthesized by uptake and assimilation of plants, and P is removed fromthe system when the plants are harvested (Karachalios, 2012) Microoganisms alsoconvert phosphorus from poorly soluble organic phosphorus to dissolved inorganicphosphorus which plants can easily to absorp In addition, P is also eliminated byadsorption 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 (Vymazal, 2007)

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

Table 1.2 Phosphorus removal efficiencies of different methods

P initial P removal Works efficiency (%) - Method concentration Types of wastewater Reference

application adsorption

(mg/L)

capacity (mg/g)

Secondary or

tertiary treatment

Tertiary treatment

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

1 91 % Synthetic wastewater Joko, 1985

Tertiary treatment

or recycle stream

5 91 % - (120 mg/ Synthetic wastewater Renman and Renman,

17

100 > 90 % - (4.75 Synthetic wastewater Nguyen et al., 2013

Adsorption

tertiary treatment 30 98.20 % Synthetic wastewater Vohla et al., 2010

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

or activatedsludge recycle

wetlands or activated

sludge recycle

18

1.4 Constructed wetlands (CWs) system for wastewater decontamination

1.4.1 Definition

Constructed wetlands (CWs) are engineered systems that have been designedand constructed to strengthen the natural processes for wastewater treatment(Vymazal, 2011) These systems comprised of major components such vegetation,substrates, soils, microorganisms and water The various contaminants fromwastewater are removed by natural microbial, biological, physical and chemicalprocess in CWs (Vymazal, 2011; Saeed and Sun, 2012)

1.4.2 Classification

There are various types of CWs that can be distinguished based on the

dominant vegetation type, hydrology and flow direction (Zheng et al., 2014)

Nevertheless, according to the majority of authors, most of CWs are typically

classified into the two types: free water surface flow and subsurface flow (Almuktar

et al, 2018)

Figure 1.4 The classification of CWs used in wastewater treatments (Wu et al., 2015)

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 naturalwetlands, they include an aquatic area with a variety of plants, a sealed basin filledwith 20-30 cm of substrates and about 40 cm for the depth of water (Stefanakis etal., 2014) These systems are also considered as expected habitats for many wildlifespecies

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

Thus, if phosphorus is the key contaminant of concern, the FWS systems are lesssuitable for treatment Additionally, the high possibility of human exposure topathogens 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 mediaand is generally invisible (Vymazal, 2007)

According to the flow direction, SSF might be classified into two types: verticalflow (VF) and horizontal flow (HF) (Almuktar et al., 2018) In HF constructedwetlands, the substrates are flooded by water, while VF constructed wetlands areapplied 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 ofconcern, subsurface flow constructed wetlands can be a greater treatment tool than

surface flow constructed wetlands Because they can be controlled by selectinghighly P adsorbable substrates as discussed by Pant, 2001 and White 2011.

In addition, the role of root-bed media for phosphorus sorption also is veryimportant 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 suchlimestone, oyster shells, crushed brick The viability of the substrate for P removaldepends on its maximum adsorption capacity The saturated substrates must beremoved and processed, then the new substrates need to be added periodically tomaintain P adsorption capacity This is a drawback of SSF constructed wetlands.Hence, the monitoring and evaluating the life of substrates are extremelyimportant and necessary to maintain the P-sorption capacity and to minimize the Pexport from the saturated (White et al., 2011)

Hybrid systems

Hybrid systems were developed to overcome the limitations of single stageCWs, 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 andanaerobic conditions simultaneously, so that its efficiency in nitrogen removal is not

high (Vymazal 2011) Thus, hybrid systems have the combination of HF and VFcan be obtained higher nitrogen removal efficiency (Zhang et al., 2014).

In recent years, the hybrid systems are used widely for many type ofwastewater According to Serrano et al., 2011 this combined system was usedeffectively for winery wastewater, and wastewater in oil field (Alley et al., 2013),industrial wastewater and grey water (Vymazal 2014; Comino et al., 2013), swinewastewater (Kato et al., 2013; Zhang et al., 2016), pharmaceuticals (Reyes et al.,2011)

In generally, constructed wetland is a potential technology, and the attractivealternatives to traditional wastewater treatment methods The improvements toenhance treatment efficiency for many wastewater types in CW are still beingconcerned 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 wasimplemented by Käthe Seidel at Germany in 1950s with the attempt aimed usingCWs to treat wastewater And lots of experiments were conducted and successfullyapplied in later At the initial stage, most the application of CWs were utilized totreat 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

effluents (Stefanakis, 2018) Additionally, they have been widely developed invarious climate conditions (e.g cold, tropical, warm etc) (Vymazal, 2015).

To improve the treatment performance of CWs, there are lots of studiesinvestigated the factors that effect to its treatment efficiencies Many studiesexamined the effect of hydraulic loading rate (HLR) and hydraulic retention time(HRT) Ramprasad et al (2017) found that elimination of phosphorus efficiencywas more at higher HRT In regard of biomass harvest and management, manyinternational studies showed the positive impacts of multiple biomass harvest on thephosphorus removal Specifically, Březinová and Vymazal (2015) observed that theamount of phosphorus removed through multiple biomass harvesting may increase

up to 43 % compared to a single harvest Concerning the seasonal variation,Ramprasad et al (2017) released that phosphorus removal rate in the summer washigher than that in other seasons Relating to evapotranspiration, Bakhshoodeh et al.(2017) reported that when evapotranspiration started to rise, phosphorus removalefficiency decreased Since the filter media is the main component of the CWs, itwas of great interests of many international researchers Barca et al (2014) foundthat, using steel slag as filter material can removed over 88 % total phosphorus ininfluent Andreo-Martínez (2017) obtained extremely high removal efficiency ofphosphorus (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 treatwastewater Specifically, in Europe possessed about 50,000 CWs and in NorthAmerica that of 10,000 CWs In addition, CWs is also a promising potentialtechnology that can be replaced to other technologies in developing countries (Zang,2014) Chen et al (2011) investigated that recently, the are many CWs have beenused successfully for wastewater treatment in China, India, and Southeast Asia

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, forinstance 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 swinewastewater 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 hydraulicretention time (HRT) (Ngo and Hans, 2012; Ngo, 2013; Vu et al., 2014), a fewstudies 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 Theinformation on role of microorganisms in the CWs is also lacking In contrast, theseare 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 inthe wetland The optimal HRT design plays a crucial role in the treatmentperformance of CWs According to Yan and Xu (2014), the contaminants can beremoved better at a longer HRT due to the great microbial community can beestablished In contrast, the ammonium and total nitrogen (TN) concentrationsdecreased significantly with increasing HRT for domestic wastewater treatment asdiscussed by Huang et al (2000) Lee et al (2009) also reported that a low HRT inCWs might be associated incomplete denitrification of wastewater Hence, toremove completely nitrogen, longer HRT is required

In addition, Avila et al (2014) investigated the possibility of removingemerging organic contaminants by using hybrid CW systems, this result pointedout 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 greaterHLR boosts quicker the movement of wastewater through the media, results in thecontact time of wastewater and media reduced An adequate microbial communityunable to establish with short contact time (Wu et al., 2015)

Cui et al (2010) reported that the ammonium and TN treatment efficiency wasreduced from 65 % to 60 % and 30 % to 20 % respectively in domestic wastewaterwith increasing HLR from 7 to 21 cm/ d However, Stefanakis and Tsihrintzis(2012) showed that the nitrogen and organics removal efficiencies in syntheticwastewater by VF CWs were increased as HLR increased

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

c Feeding mode of influent

The feeding mode is also a key design parameter (Vymazal, 2011) Thetreatment performance in CW also relies on the different feeding mode of influent.Many studies examined the treatment performance in CW using differentinfluent feeding modes Generally, the treatment performance can be better withbatch 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 fedsystems

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

Compare to FWS CWs, phosphorus can be treated greater in SSF CWsbecause 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 treatmentperformance 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 lowerthan compared to VSSF and HSSF CWs