The behavior of humic Substance in iron electrolysis Process and its influence on Phosphorus removal

1) Iron coagulation proceeds not only under aerobic condition by forming ferric floc but also under anaerobic conditions by forming ferrous floc, showing a new p[r]

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY

HA THI DIEP ANH

THE BEHAVIOR OF HUMIC

SUBSTANCE IN IRON ELECTROLYSIS PROCESS AND ITS INFLUENCE ON

PHOSPHORUS REMOVAL

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY

HA THI DIEP ANH

THE BEHAVIOR OF HUMIC

SUBSTANCE IN IRON ELECTROLYSIS PROCESS AND ITS INFLUENCE ON

PHOSPHORUS REMOVAL

MAJOR: ENVIRONMENTAL ENGINEERING CODE: 8520320.01

RESEARCH SUPERVISOR: Prof Dr JUN NAKAJIMA Associate Prof Dr LE VAN CHIEU

ACKNOWLEDGMENT

First and foremost, I would like to sincerely thank my instructor, Prof Jun Nakajima for helping and always encouraging me, because of his patience, motivation, and immense knowledge His generosity and devoted guidance contributed greatly to my dissertation completion and developed myself There is no unmatched honor to work with him

Second, I would like to thank my co-supervisor, Associate Prof Dr Le Van Chieu a lot because of his thoughtfulness and kindness He is always enthusiastic about reading and revising my research carefully

Third, I would like to express my sincere thanks to all MEE Department for your valuable support in the process of implementing the thesis as well as my stay at VJU And I would also like to thank JICA for its support Thanks for all that we have been through together

I would like to express my appreciation to all Ritsumeikan University professors, staff, and doctors, for their warm and enthusiastic welcome during my internship They gave me access to labs and research facilities Without their valuable support, it would not be possible to this research

Finally, I would like to thank my family and friends who have supported me spiritually throughout the process of writing this thesis in particular, and my life in general

Hanoi, August 7th, 2020

TABLE OF CONTENT

ACKNOWLEDGMENT i

INTRODUCTION

1 Background

2 Objectives

3 Structure of thesis

CHAPTER LITERATURE REVIEW

1.1 Phosphorus removal technologies

1.1.1 Phosphorus (P) pollution

1.1.2 Phosphorus removal technologies

1.2 Electrocoagulation/Iron electrolysis 15

1.2.1 Definition 16

1.2.2 Advantages and drawbacks of EC 17

1.2.3 The principle of electrocoagulation 18

1.3.4 Application of EC 19

1.3 Iron electrolysis application for phosphorus removal in Johkasou systems 19

1.3.1 Johkasou systems for decentralized domestic wastewater treatment 19

1.3.2 Phosphorus removal in Johkasou and application of iron electrolysis 20

1.3.3 Interference of phosphorus removal using iron electrolysis 23

1.4 Humic substance 24

1.4.1 General description 24

1.4.2 Chemical characteristic 26

CHAPTER MATERIALS AND METHODOLOGY 28

2.1 Materials 28

2.1.1 Synthetic test liquor (phosphate solution) 28

2.1.2 Humic substance sample liquor 28

2.1.3 Humic acid sample liquor 29

2.2 Iron electrolysis experiment set-up 30

2.3 Operational condition of experiment 31

2.3.1 Iron electrolysis with or without oxygen supply 31

2.3.2 Iron electrolysis with HS addition 32

2.3.3 Iron electrolysis with humic acid addition 33

2.4 Chemical analysis 34

2.4.1 Suspended solid (SS) 34

2.4.2 Iron analysis 35

2.4.3 Phosphorus analysis (PO4-P) 36

2.5 Fluorescence spectroscopy analyses by three-dimensional excitation-emission matrix 36

3.1.1 Iron electrolysis with aeration 38

3.1.2 Iron electrolysis without aeration 39

3.1.3 Discussion 41

3.2 The effect of humic substance on iron electrolysis 43

3.2.1 Iron coagulation decrease by humic substance addition 43

3.2.2 Decrease of phosphorus insolubilization by iron coagulation decrease 44

3.2.3 Discussion 45

3.3 The effect of fulvic acid to iron electrolysis 47

3.3.1 Iron electrolysis with humic acid addition 47

3.3.3 Discussion 50

CONCLUSION 52

LIST OF TABLES

Table 1.1 Vietnam national technical regulations on effluent discharge

Table 2.1 Preparation of synthetic test liquor 28

Table 2.2 Operational experiment condition 32

Table 2.3 Preparation chemicals to iron analysis 35

Table 2.4 Preparation chemicals to phosphorus analysis 36

Table 3.1 Effluent parameters after electrolysis performed in aeration condition 38

Table 3.2 Effluent parameters after electrolysis performed in humic substance addition experiment 43

LIST OF FIGUREURES

Figure Iron electrolysis reactor (Fayad, N (n.d.)., 2017)

Figure Structure of thesis

Figure 1.1 Changes in structure of phosphorus compounds in municipal wastewater between year 1971 and 1991 (Rybicki, n.d.)

Figure 1.2 Phosphorus removal technologies

Figure 1.3 One – point chemical addition 10

Figure 1.4 Two – point chemical addition 10

Figure 1.5 Metabolic pathway of PAO under aerobic and anaerobic conditions (Bunce et al., 2018) 14

Figure 1.6 Iron electrolysis principle 18

Figure 1.7 Combination process of BOD and nitrogen removal type Johkasou and phosphorus adsorption column (Ebie et al., 2008) 22

Figure 1.8 Johkasou for phosphorus – BOD – Nitrogen removal (Kumokawa, n.d.) 23

Figure 1.9 Hypothetical humic acid structure according to Stevenson (1982) 26

Figure 1.10 The hypothetical model structure of fulvic acid (Buffle's model) 26

Figure 1.11 Chelation of Cu and Zn in top examples with simple complexation of Zn by an amino acid (Hd, n.d.) 27

Figure 2.1 The map of Hanoi and Nam Son landfill 29

Figure 2.2 Humic acid, Nacalai Tesque, Japan 29

Figure 2.3 Schematic diagram of the laboratory-scale experiment 30

Figure 2.4 The types of equipment used to set-up experiments 30

Figure 2.5 Synthetic test wastewater preparation 31

Figure 2.6 Set – up experiments 32

Figure 2.7 Humic substance experiment set-up 33

Figure 2.8 Humic acids addition experiment set-up 34

Figure 2.9 Procedure iron calculate 34

Figure 10 Fluorescence Spectrophotometer F-7000 (Hitachi, Tokyo, Japan) 37

Figure 3.1 Phosphorus insolubilization 39

Figure 3.2 Iron coagulation ……… 39

Figure 3.3 Iron coagulation (without aeration) 39

Figure 3.3 Iron coagulation (without aeration) 39

Figure 3.4 Iron coagulation (N2 gas bubbling) 39

Figure 3.5 Iron coagulation under aerobic condition (a) and anaerobic condition (b)……… 37

Figure 3.6 Phosphorus insolubilization (without aeration)……… 41

Figure 3.7 Phosphorus insolubilization (N2 gas bubbling) 41

Figure 3.8 The existing pathway models 41

Figure 3.10 Iron coagulation (Humic substance addition) 44

Figure 3.11 Phosphorus insolubilization (HS addition) 45

Figure 3.12 Molar ratio of ΔFe / ΔP 46

Figure 3.13 Soluble complex formation of ferrous ion and HS 47

Figure 3.14 Iron coagulation (Humic acid addition) 48

Figure 3.15 Phosphorus insolubilization (HA addition) 49

Figure 3.16 EEMs Fluorescence spectra of humic substance sample (leachate sample) 49

Figure 3.17 EEMs Fluorescence spectra of humic acid sample 50

LIST OF ABBREVIATIONS

BOD DC DOC EBPR EC EEM FDOM HA HS MBR PAO SBR SWTPs SS TDS WWTP

Biochemical oxygen demand Direct current

Dissolved organic carbon

Enhanced biological phosphorus removal Electrocoagulation

Excitation emission matrix

Fluorescent dissolved organic matter Humic acid

Humic substance Membrane bioreactor

Phosphorus accumulation organisms Small-scale wastewater treatment plants Sequencing batch reactor

Suspended solids Total dissolved solid

INTRODUCTION

1 Background

Some serious environmental problems such as eutrophication are due to the direct discharge of phosphorus into the water source The abundance of these nutrients will spur the development of algae, mosses, and mollusks in the water and will ultimately affect the biological balance of water In addition, phosphorus is also a limited resource, so we need to remove and recover P effectively from wastewater before discharging it into the water source

In order to remove phosphorus from wastewater sources, there are several methods being applied, including adsorption, chemical precipitation (using metal salts), biological processes, and ion-exchange methods ion (Omwene et al., 2018) Among the methods in the two most used methods are chemical precipitation and biological processes Chemical precipitation and adsorption are currently the best methods for efficiency By adding metal salts (aluminum salts or iron salts) most of the phosphorus is removed Biological methods can also eliminate up to 90% of total phosphorus but this method is only suitable for wastewater with low phosphorus concentrations And when there is a change in the chemical composition, high phosphorus concentration, and changes in the temperature of the wastewater, the treatment efficiency is not high Moreover, many of the above methods have long operating times, eliminating ineffective and costly (Wysocka and Sokolowska, 2016) Therefore, electrocoagulation (EC) to remove phosphorus has been used as an alternative process (especially chemical precipitation) Electrochemical (electrolysis + coagulation) combining coagulation, flotation, and electrolysis is a process of destabilizing suspended pollutants or dissolving in water environments using electric current (Fayad, N (n.d.)., 2017)

their removal is mainly accomplished by destabilization and adsorption” Coagulation is a traditional physicochemical treatment via phase separation for the decontamination of wastewaters before discharge to the environment EC is causally related to the conventional coagulation process, which has been used as a method for water clarification and stabilization, and nowadays, it is still extensively used (Garcia-Segura et al., 2017)

Moreover, this technique has the advantages to be able to overcome the drawbacks of the above methods such as simple equipment, easy operation, and only use electric current so there is no need to add chemicals and reduce time retention time, settling speed is also faster and creates less sludge (Moussa et al., 2017)

In addition, the EC does not use chemicals, so it does not raise water or aquatic organisms The EC only uses electricity for operation without adding any chemical, so it is suitable for domestic scale facilities EC applied in small-scale wastewater treatment Johkasou (domestic, small-scale, on-site, decentralized) (Fayad, N (n.d.)., 2017)

EC can be applied to treat wastewater containing heavy metals, organic substances, and other ions such as PO43- and AsO2-,

EC reactor is composed of an electrolytic cell and connected externally to a direct current power supply

Figure Iron electrolysis reactor (Fayad, N (n.d.)., 2017)

This Fe2+ ion will be oxidized with dissolved oxygen in the water to trivalent iron ion (Fe3+) Fe3+ will combine with PO

43- in water to form a precipitate and settle to the

bottom of the device (Morrizumi et al., 1999) This precipitate can be removed by pumping out of the system or by using the flotation method to remove the sludge

EC has been applied to industrial wastewater treatment plants or small wastewater treatment models The small-scale wastewater treatment plants (SWTPs) are called Johkasou and this model treats domestic wastewater on-site for about 10 households, so it is widely applied in Japan But it is difficult to remove phosphorus by the activated sludge method because it is dependent on the input parameters Therefore iron-electrolysis was developed and used in this model to remove phosphorus more effectively According to previous studies, it has achieved good performance although some examples showed a slightly lower phosphorus removal (Mishima et al., 2017)

A study on the effects of calcium in increasing phosphorus removal efficiency has been conducted and results of countermeasures have been reported

In addition, testing of such cases shows that the DOC (humic substances are imported from sewage or produced in Johkasou tanks), causing low performance Regarding the effect of humic substances, a hypothesis has been obtained that it forms a chelate compound with iron ions provided by iron electrolysis (Mishima et al., 2018)

Testing to verify this idea has been started but has not ended Previous research using EDTA, a typical chelate-making material, shows the potential for interfering with phosphorus removal by forming a chelate with supplied iron The mechanism of the effects of humic compounds on phosphorus removal is still unclear, especially the sequencing batch reactor activated sludge processes, which are still poorly understood Next, a test using humic substances is needed to clarify this mechanism of intervention

2 Objectives

treatment Johkasou However, there are still problems remain that affecting the removal of phosphorus There have been many previous studies on factors affecting the phosphorus removal process, such as the influence of electric current, the effect of initial pH, the effect of initial phosphorus concentration No research has been done to study influence the DOC co-substance or co-ions present in wastewater on phosphorus removal It is very necessary to improve this method to clear the interference problem Because in real sewage not only phosphorus but also many other compounds such as DOC coexist under some condition It may increase or decrease processing efficiency

Therefore, the action of phosphorus, iron, and organic substances coexisting in wastewater must be thoroughly investigated to clarify the factors that influence the phosphorus removal process Moreover, it is also necessary to determine the optimal and stable reaction conditions in the actual model

Based on this study focused on investigating the impact of a high molecular organic compound capable of complexing with Fe, particularly humic substance (HS) However, humic substances including humic acid (HA), fulvic acid, and humin, can also affect the removal of phosphorus by electrolysis of iron Therefore, in this study, the effect of HA is the main object of study, by adding HA to the electrolysis process and conducting related analyzes to evaluate the effect This study focuses on clarifying the mechanism of the phenomenon occurring during electrolysis under the presence of HS and developing a model describing this process

To achieve the above objective, I operated a laboratory scale batch experiments with simulated wastewater and prepared HA (commercial humic acid or humic acid from humic substance sample) was operated

Summary of research object and scope: Research question:

(2) What is main DOC factor that interferes phosphorus removal in iron electrolysis process?

Research objective:

(1) To clear the mechanism of phenomena occurring in iron electrolysis process under the condition of abundant of DOC

(2) To clarify specific factor in DOC that interferes phosphorus removal in iron electrolysis process used in Johkasou

3 Structure of thesis

The structure of this thesis is shown in the Figure This thesis contains of chapters The main content of each chapter is presented as follows:

Introduction: Introduction Briefly summarize the foundational knowledge causally related to the research and identify the main research subjects and tasks

Chapter 1: Literature review provide background knowledge of phosphorus pollution and its consequences, history of phosphorus removal technologies Focus on EC's role in phosphorus removal

Chapter 2: Material and Methodology Describe materials, equipment, and methods used in the study Detailed description set-up experiments Analytical methods as well as equipment were also introduced

Chapter 3: Results and discussion

3.1 Iron electrolysis without oxygen supply

3.2 The effect of Humic substance to iron electrolysis 3.3 The effect of Fulvic acid to ion electrolysis

Figure Structure of thesis Introduction

Briefly summarize the foundational knowledge causally related to the research and identify the main research subjects and tasks

Chapter 1: Literature review

Provide background knowledge of phosphorus pollution and its consequences, history of phosphorus removal technologies

Chapter 2: Material and methodology

Describe materials, equipment, and methods used in the study Detailed description Set-up experiments Analytical methods as well as equipment were also introduced

Chapter 3: Results and discussion

3.1 Iron electrolysis without oxygen supply

3.2 The effect of Humic substance to iron electrolysis

3.3 The effect of Fulvic

CHAPTER 1: LITERATURE REVIEW

1.1 Phosphorus removal technologies

1.1.1 Phosphorus (P) pollution

Phosphorus and nitrogen are crucial nutrient that extremely needed for growth of plant and animals (Yan et al., 2015) In addition, phosphorus plays an important role in several industries (e.g fertilizers, detergents, paint .) Increasing input of nitrogen and phosphorus compounds to receiving surface waters, especially to lakes and artificial reservoirs lead to increase of primary production of water born organisms and finally its consequence is lack of oxygen in waters The removal of phosphorus from domestic wastewater is primarily to reduce the potential for eutrophication (Dunne et al., 2015)

The excessive amounts of phosphorus in the aquatic environment due to human activity can negatively affect aquatic ecosystems Therefore, several technical standards for the quality of wastewater effluent have been made public to control phosphorus pollution

To minimize surface water pollution and to control pollution sources, each country has issued its own standards on effluent standards The following are some of Vietnam's effluent discharge standards that specify a limit for phosphorus effluence

Table 1.1 Vietnam national technical regulations on effluent discharge for Phosphorus

No Regulations Unit Maximum value allowed

1

QCVN 40:2011/BTNMT

National technical regulation on Industrial wastewater

mg/L -

National technical regulation on the effluent of aquatic Products Processing industry

3

QCVN 14-MT:2015/BTNMT

National technical regulation on domestic wastewater

mg/L - 15

4

QCVN 08-MT:2015/BTNMT

National technical regulation on surface water quality

mg/L – 0.5

1.1.2 Phosphorus removal technologies

Phosphorus enters water derived from urban sewage, chemical fertilizers, washed away from the soil, rainwater, or phosphorus sediments dissolved again Phosphorus in water usually exists in the form of orthophosphate (PO43-, HPO42-,

H2PO4-, H3PO4) or polyphosphates [Na3(PO3)6] and organic phosphates

Phosphorus exists in wastewater soluble form That is why most of the applied methods based on a general principle of converting phosphorus compounds from soluble to insoluble The basic principle for removing phosphorus in water is to convert phosphorus from soluble form to insoluble form by precipitating with ions of aluminum, iron, calcium, or forming biomass by chemical methods There are many methods of handling phosphorus but can be classified into two main groups: physical-chemical method and biological method

Figure 1.1 Changes in structure of phosphorus compounds in municipal wastewater between year 1971 and 1991 (Rybicki, S M (n.d.)., 2004)

Figure 1.2 Phosphorus removal technologies

Physical-chemical technologies

Physical and chemical processes have been applied to remove and control phosphorus for many years This method clearly shows the processing efficiency, but they still have some limitations Physical-chemical treatment of phosphorus removal involves the addition of trivalent metal salts to react with dissolved phosphates and remove by sedimentation or filtration Metal salts are commonly used in the form of

Phosphorus removaltechnologies

Physical – chemical Biological

Electrolytical method

Magnetic separation

Crystallization Adsorption Enhanced biological P removal (EBPR)

alum and the most common is salt of iron or aluminum Depending on the dosage point, this method can be used in various technology schemes (Graziani et al., 2006): • primary precipitation in mechanical wastewater treatment plants (older constructions)

• primary precipitation before further biological treatment

• simultaneous precipitation (adding chemicals to final zones of activated sludge reactor)

• final precipitation

Because the amount of precipitate produced is causally related to the amount of phosphorus removed, hence study to find the quantitative optimization point is extremely important in chemical treatment Contact filtration is also a widely integrated method with physical-chemical methods to ensure a stable phosphorus output Investigations on other physical-chemical methods containing many processes most will be described in the following processes:

• Electrolytical method • Precipitation

• Crystallization • Magnetic separation • Adsorption

Electrolytical method

Electricity has been used for water treatment for a long time, around the 1860s electricity was used to treat sewage in England Development of the direct use of electricity for treatment is carried out in subsequent years The basic principle of the process is that the chemical precipitation of iron compounds is formed on an electrode Operation of the plant showed positive results In the following years, the technology continued to be researched and developed, and until 1950 the first experiments on electrolyte treatment were directed at removing nutrients During this process, the phosphorus content was reduced to 1.0 mgP/L

• Aluminum electrode for phosphorus removal • Carbon electrodes for electrochemical

For decades, this technology has been increasingly used to treat industrial wastewater containing metals It is also used to treat pulp and paper industry wastewater, metal processing, and mining EC is also applied to treat many types of wastewater containing food waste, dyes, organic matter from leachate Studies are often carried out on the EC to optimize key operational parameters such as amperage, effluent flux (Fayad, N (n.d.)., 2017)

Precipitation

Chemical methods have been widely used in phosphorus removal This method removes phosphorus by adding metal salts to the wastewater so that it reacts with the phosphorus in a soluble form The produced precipitate will then be removed by sedimentation or filtration The most used metal salts are trivalent metal salts (iron, aluminum): aluminum sulfate, ferric chloride, ferric sulfate, ferrous sulfate, and ferrous chloride These chemicals combine with phosphorus as shown by the following reactions (Graziani et al., 2006)

Al3+ + PO

43- → AlPO4↓

Fe3+ + PO

43-→ FePO4↓

Depending on the design of each specific treatment plant, the chemical addition point is designed differently But there are two main scenarios for chemical additions:

Effluent polishing in the secondary process Chemicals added right before the

secondary settling tanks

Two – point chemical addition Chemicals are added in both primary and

Figure 1.3 One – point chemical addition Figure 1.4 Two – point chemical addition

Crystallization

This method has been developed and applied for phosphorus removal since the 1980s This method was specifically presented by Joko, who showed the long-term operation of the installation to remove phosphorus The phosphorus from wastewater is biologically treated by crystallizing hydroxyapathyte Ca5(OH)(PO4)3

This method also shows relatively good handling efficiency Joko completed tests on Yamato (Japan) WWTP, which confirmed the decrease of P level from 1-4 mgP/L in biologically treated wastewater down to 0.3 - 1.0 mgP/L after crystallization

This method has the advantage that the product after crystallization can be used for fertilizer production, but this method is not widely applied because it is quite complex and high processing cost (Rybicki, S M (n.d.)., 2004)

Magnetic separation

In the 1970s, magnetic separation technology was investigated by De Latour and reported that it was an effective method if applied after adding iron or aluminum salts This method can remove most of the phosphorus in the water, the amount of

phosphorus in the output can reach 0.1 - 0.5mgP/L compared to other methods with equivalent costs (Velsen et al.1991)

The principle of this method is to separate particles that are removed by a magnetic field Therefore, it can remove all impurities

Adsorption

Around the 1970s there were trials of phosphorus adsorption using fly ash The phosphorus in the wastewater will be attracted to the molecular binding force and trapped on the adsorbent surface This method is widely used for both high and low concentrations of phosphorus (Rybicki, S M (n.d.)., 2004)

Adsorbents are the most important factor affecting phosphorus removal efficiency In the past, activated carbon was the most widely used adsorbent, but it also revealed some disadvantages such as regeneration and high cost Therefore, a lot of research has been done to reduce the production costs of these adsorbents, and there are several solutions that are proposed to use by-products in agriculture and industry

Biological technologies

Biological methods for handling phosphorus have been studied and applied for a long time This method is associated with the use of activated sludge to remove pollutants in the water environment, which proved to be quite effective with organic pollutants Current biological methods are developing in two directions:

• Optimizing wastewater treatment plants using activated sludge technology • Dealing with pollutants by constructed wetlands

Enhanced biological phosphorus removal (EBPR)

skills, making it difficult to control the process (Seviour et al., 2003) This is a technological barrier when applied to decentralized treatment facilities (Brown and Shilton, 2014) However, the understanding of biochemical mechanisms involved in P uptake is increasing The phosphorus uptake process is dependent on phosphorus accumulation organisms (PAO) for EBPR The application of this method is subject to strict operating conditions for carbon source, glycogen, and electron acceptor When good operating conditions can be assured, 80% of the phosphorus can be removed from the wastewater by this method (Bunce et al., 2018)

Figure 1.5 Metabolic pathway of PAO under aerobic and anaerobic conditions (Bunce et al., 2018)

This method can be used in different designs for each type of wastewater plant Recent EBPR applications include a combination of a membrane bioreactor (MBR), a sequencing batch reactor (SBR), and an activated sludge reactor This combination has been shown to be effective in removing phosphorus from municipal sewage, particularly the MBR proving highly effective in capturing suspended solids in the tank

Constructed wetland

Using natural cycles to remove phosphorus is particularly suitable for small communities and local systems because it is easy to operate and the cost is quite cheap:

• Using artificial and natural wetlands can apply treatment without the use of chemicals

An artificial wetland is an engineering system comprising of filter materials, plants, and microorganisms Phosphorus will be removed by decomposing organisms, plants that absorb, settle, or adsorb on filter materials Microorganisms in the system also have the role of metabolizing phosphorus from the form of poorly soluble organic to dissolved inorganic phosphorus which plants can easily to absorb (Vymazal., 2007)

1.2 Electrocoagulation/Iron electrolysis

Electrocoagulation (EC) is a technique that has been used and successfully for treating various types of wastewater The technology uses direct current between a pair of metal electrodes submerged in water Metal ions at the right pH will produce precipitates and metal hydroxides The resulting precipitate will destabilize and synthesize particles or adsorb dissolved pollutants This method was also started to apply in the late 19th century:

1880: in US – first document on the use of EC for the treatment of effluents 1880: in UK, WWTP apply this patent to treat sewage

1930: due to high operating costs and replace by chemical coagulant

1947: small size installations, EC is more competitive than conventional process

1.2.1 Definition

Electrolysis process in which current is passed between electrodes through an ionized solution (electrolyte) to deposit positive ions on the negative electrode (cathode) and negative ions on the positive electrode (anode) (Yousuf et al., 2001)

Electrolyte:

• positive ions → move to cathode (occurring oxidation process) • negative ions → move to anode (occurring reduction process)

EC is a process of destabilizing suspended emulsified or dissolved contaminants in an aqueous medium

Connected externally to a direct current power supply (DC) Electrochemical dissolution of the sacrificial anode (+)

The dissociation of the ions from the anode follows Faraday ‘s law 𝑚 = ì ì

ì (g) Where:

ã I: current (A)

• t: time of operation (s) Coagulating ions

Coagulant

Metallic hydroxide

• M: molecular weight of the anode material (g/mol) • F: Faraday’s constant (96,500 C/mol)

• Z: number of electrons involved in the reaction • m: mass of anode dissolved (g)

1.2.2 Advantages and drawbacks of EC

Advantages of EC

• Requires simple equipment and is easy to operate

• EC cell has no moving parts and requires little maintenance as the electrolytic processes are controlled electrically

• The treated solution gives palatable, pleasant, clear, colorless, and odorless water

• The formed sludge is mainly composed of metallic oxides/hydroxides, so it is readily settable and easy to de-water

• The formed flocs are much larger than those produced by chemical coagulation, contain less bound water and are acid-resistant and more stable • Does not require the use of chemicals, so there will be less risk of secondary pollution, contrary to chemical coagulation Where chemical substances are added at high concentrations

• The bubbles generated during electrolysis result in the flotation of the pollutants, and consequently their separation is facilitated

• EC produces effluent with less total dissolved solids (TDS) content as compared with chemical treatments If this water is reused, the low TDS level contributes to a lower water recovery cost

• Even the smallest colloidal particles are removed by EC since the applied electric current makes collision faster and facilitates coagulation

Drawbacks of EC

• Gelatinous hydroxides may solubilize

• Sacrificial anodes, which are oxidized should be replaced regularly

• An impermeable oxide film may be formed on the cathode leading to loss of efficiency of the EC unit (Yousuf et al., 2001)

1.2.3 The principle of electrocoagulation

The EC theory has been discussed by several authors and it has been agreed that the EC process consists of three consecutive stages: (1) formation of flocculation by electrolytic oxidation of sacrificial electrodes; (2) destabilize contaminants, suspension particles and break emulsions; (3) synthesize destabilized phases to form flocs (Yousuf et al., 2001) The mechanism of emulsification and instability of pollutants has been described as follows:

• Double-layer compression diffuses around charged particles, achieved by the interaction of ions generated by the dissolution of the sacrificial electrode due to the current flowing through the solution

• Neutralizing the charge of various ions in solution These reactions will reduce the repulsive force between electric particles enough for Van der Waals to prevail, thus causing precipitation

• Floc formation and floc are formed because of the coagulation process creating a layer of sludge

The mechanism of the EC is highly dependent on the chemistry of the water environment, especially the conductivity In addition, other characteristics such as pH, particle size, and metal that make up the electrode also affect the EC process The mechanisms for removing ions by EC will be explained in detail by the example regarding the removal of phosphorus by EC using an iron electrode

Figure 1.6 Iron electrolysis principle

At the Anode

Fe → Fe2+ → Fe3+ Fe3+ + PO43- → FePO4

1.3.4 Application of EC

EC has been widely used in the field of wastewater treatment in recent years EC can eliminate non-metallic inorganic, heavy metals, organic substances, and actual industrial wastewater (Garcia-Segura et al., 2017) Therefore, EC is applied for wastewater treatment of almost industries such as tannery and textile industry wastewater, food processing industry wastewater, paper industry wastewater, etc However, the EC is still limited in removing certain compounds such as ammonium ions and it has not been able to remove dissolved substances such as glucose and volatile fatty acid Therefore, in many cases optimization is needed to improve the removal efficiency of pollutants In some cases, it is necessary to combine with one or two other methods to increase the efficiency of wastewater treatment, i.e hybrid process, to ensure the effluent quality (Yousuf et al., 2001) EC can be designed in combination with membrane separation, reverse osmosis, electro filtration, sludge dewatering, and other conventional technologies in wastewater treatment systems to improve the efficiency of pollutant removal In addition, studies on the removal of color-induced dye materials have been reported by (Lin et al., 1996) In addition, the combination of EC with activated sludge and dissolved air flotation has also been applied in textile and municipal wastewater treatment (Yousuf et al., 2001)

1.3 Iron electrolysis application for phosphorus removal in Johkasou systems

1.3.1 Johkasou systems for decentralized domestic wastewater treatment

treatment plants (SWTP), called Johkasou, are widely used in decentralized domestic wastewater treatment for sparsely populated areas in Japan (Kumokawa, n.d.)

Johkasou systems are designed to be suitable for the treatment of domestic wastewater of individual households or a population cluster of fewer than 10 households Depending on the scale, Johkasou can be classified into the small scale and medium/large scale Johkasou In terms of small Johkasou, they can be mass-produced, easily installed with little topography restriction and the treated water can be discharged directly into the environment (Ogawa, n.d.)

Because small-scale Johkasou can be installed at the household level and locally discharged, they have outstanding advantages in terms of environmental protection and cost effectiveness:

• Advanced technology, high processing efficiency, long-term stability

• The quality of treated water can be used for other purposes such as watering plants, washing cars

• Does not cause unpleasant odors

• Long service life can withstand seismic tremors, easy to install, and space-free

• There are many options suitable for all processing power and easy to move without affecting the equipment inside

• Reasonable investment and operating costs Operation, maintenance, dredging is easy

The Johkasou system plays an important role in reducing pollution from domestic wastewater The conventional Johkasou system possesses anaerobic, anoxic, and aerobic tanks The microorganisms attached to the material are used to remove organic matter The removal rate of organic pollutant discharge load in wastewater of this model is about 95% and the total nitrogen is in the range of 65 to 80% Meanwhile, phosphorus removal efficiency is still low (Fujimura et al., 2019)

1.3.2 Phosphorus removal in Johkasou and application of iron electrolysis

and microbiological methods are rarely applied in phosphorus removal by Johkasou The chemical precipitation method requires a large amount of precipitate, sludge disposal is difficult and requires a strict experimental operation The process of removing phosphorus by microbial activity takes a long time, the amount of sludge generated should accumulate about year inside the tank and require strict biochemical environment, and human activities (Jing et al., 2020) Phosphorus treatment results are usually 30% lower Recently, functions to remove both nitrogen and phosphorus have been developed Johkasou that can remove both phosphorus and nitrogen is called an "advanced treatment type" (Fujimura et al., 2019) Many methods have been studied to improve the removal efficiency of pollutants Methods developed and introduced into Johkasou for phosphorus treatment are Iron electrolysis, Adsorption/desorption by zirconium, using pellet to remove phosphorus The effect of phosphorus removal pellets in wastewater was examined in Johkasou's operation by (Fujimura et al., 2019) This phosphorus removal tablet was developed by Sugawara and is manufactured by Nikka Maintenance Co., Ltd., Japan The main component of the tablet is Potassium aluminum sulfate and a small amount of auxiliary The tablets are cylindrical in shape, weighing 200g and having a diameter of 6.0 cm and a height of 4.5 cm Unlike conventional aluminum sulfate potassium powder, which dissolves immediately in water and flows out of the reaction tank in Johkasou, the pellets dissolve slowly but completely and effectively-being maintained over a long period of time The pellets will be placed in a mesh bag and fixed in the tank Phosphorus removal tablets will be put into Johkasou's aerobic or raw water compartment Phosphorus in wastewater will combine with aluminum ions released from the pellets Removing this precipitate in the form of sludge can remove phosphorus from wastewater Yoko Fujimura reported the results of a phosphorus elimination efficiency survey conducted in Sakura City (Chiba, Japan) showing that most of the phosphorus in the outlet effluent decreased after week when the pellets were removed was placed in Johkasou

phosphorus from domestic wastewater The phosphorus adsorbent used is zirconium

(provided by Japan EnviroChemicals, Ltd) The principle of adsorption is ion

exchange, although it is possible to adsorb different anions the ability to adsorb phosphorus is quite high The phosphorus adsorption process is designed as the next stage of BOD and nitrogen removal Adsorption columns are installed in the reaction tank and part of the wastewater is used for backwashing, as shown Figure 2.7 (Ebie et al., 2008) After the concentration of phosphorus in the output exceeds mg/L, the material will be released The collected adsorbent will be soaked in an alkaline solution (sodium hydroxide) to remove the phosphorus The adsorbent will be reactivated by soaking in acid solution (sulfuric acid) The phosphorus released from the desorption process is recovered with a high crystallinity equal to trisodium phosphate by crystallizing at low temperatures in a vacuum This method is a new method for decentralized wastewater treatment, but it shows good treatment results The phosphorus concentration in the outlet effluent is less than mg/L and the recovered phosphorus is also of high purity This recovered phosphorus can be used in agricultural production as a fertilizer

Figure 1.7 Combination process of BOD and nitrogen removal type Johkasou and phosphorus adsorption column (Ebie et al., 2008)

released from the electrode are oxidized by dissolved water in water to become ferric ions These iron ions combine with the phosphate in the wastewater and easily settle to the bottom of the tank from which the sludge needs to be removed (Morrizumi et al., 1999) In addition, during electrolysis, it does not affect the respiration process of microorganisms in the tank In this method, the iron released from the electrode acts as a precipitant, so the molar ratio between iron and phosphorus determines the removal efficiency of phosphorus Iron supplementation by adjusting the amperage or reaction time ensures that the molar ratio of Fe/P is 2.0 to ensure that the phosphorus concentration after treatment is less than 1.0mg/L This technology is very suitable for SWTP because the entire device is compactly designed and fully automatic and connected to the controller

Figure 1.8 Johkasou for phosphorus – BOD – Nitrogen removal (Kumokawa, n.d.)

1.3.3 Interference of phosphorus removal using iron electrolysis

In the process of electrolysis of iron, iron ions from the anode will dissolve into the solution and combine with the phosphate in the solution to precipitate and settle to the bottom of the tank However, this process also produces iron oxides, which will stick to the anode surface and prevent iron from dissolving into the solution This leads to a reduction in phosphorus removal efficiency But this problem has been solved by swapping the cathode and anode after a period of operation In addition, there have been several cases that have been reported as reducing phosphorus removal performance due to calcium deficiency in water And to overcome this situation, a recommendation has been made by (Mishima et al., 2017) to maintain calcium levels in the water between 20mg / L and 25mg/L

However, in some schools, despite the above situation, the reduction of phosphorus was observed Mishima et al., 2017 conducted a long-term investigation of phosphorus removal by iron electrolysis in Johkasou tanks Based on the data obtained, Mishima et al showed that DOC can inhibit phosphorus removal by iron electrolysis (Mishima et al., 2017) Specifically, phosphorus was removed almost completely when the DOC was low In this study, the average DOC was 8.2 mg/L, but there were times when high DOC values were reached, up to a maximum of 20 mg/L Therefore, DOC may interfere with phosphorus removal Iron can form a complex with HS, which are the components of DOC in wastewater This competition reduces the amount of iron in combination with the phosphates in the water resulting in reduced phosphorus removal efficiency Another evidence that DOC has inhibited phosphorus removal is that despite the posting of molar Fe/P, no improvement was observed in the high DOC concentration (Mishima et al., 2017) Accordingly, the inhibition of DOC on the phosphorus removal process by iron electrolysis is completely clear

1.4 Humic substance

1.4.1 General description

molecular weight, Fulvic acids 800 A and < 3,500 Da) (Becket Tet al, 1987 and Muscolo et al, 2007)

HS in soil and sediment can be divided into three main groups: Humic acids (HA), Fulvic acids (FA), and Humin HS are highly chemically reactions but are considered difficult to biodegrade Aquatic HS contains only HA and FA and these components are generally removed from water by lower the pH to and adsorbing both components on a suitable resin column HA is the main component of HS It can form complexes with metal ions commonly found in the environment, Fulvic acids are like HA However, there are some differences in carbon and oxygen content, acidity of the degree of polymerization, molecular weight, and color

Humic acid (HA) is the main component of humic substances, the main organic component of soil (humus), peat, and coal It is produced by biodegradation of dead organic matter It is not a single acid; rather, it is a complex mixture of different acids containing carboxyl and phenolate groups so that the mixture acts as a dibasic acid or, sometimes, as a tribasic acid HA can form complexes with ions commonly found in humic colloidal media HA is insoluble in water at acidic pH, while fulvic acid is also derived from humic substances but soluble in water throughout the pH range HA and FA are often used as a soil supplement in agriculture

Fulvic acid (FA) is a family of organic acids, natural compounds, and humus components (part of soil organic matter) They are like HA, with differences in carbon and oxygen content, acidity, degree of polymerization, molecular weight, and color Fulvic acid remains in the solution after removing HA from humin by acidification Fulvic acid has a relatively low molecular weight and is more bioactive than HA

Figure 2.9 Hypothetical humic acid structure according to Stevenson (1982)

Figure 2.10 The hypothetical model structure of fulvic acid (Buffle's model)

1.4.2 Chemical characteristic

Humic acids and fulvic acids are aromatic polymer molecules that vary in size and molecular weight The size of humic acid allows these macromolecules to be rolled up to form particles or rings that at a certain pH (acid) will precipitate while the fulvic acids are too small to go through a processed analog and therefore still in the same process solution

The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+, and Fe3+ Many humic

Figure 2.11 Chelation of Cu and Zn in top examples with simple complexation of Zn by an amino acid

The HA and FA are extracted from soil and other solid phase sources using a strong base (NaOH or KOH) (Weber et al., 2018); they are:

Fulvic acid: soluble at all pH values

Humic acid: insoluble under acidic conditions (pH<2) but soluble in solution of higher pH values

CHAPTER 2: MATERIALS AND METHODOLOGY

2.1 Materials

2.1.1 Synthetic test liquor (phosphate solution)

Synthetic test liquor is prepared to simulate the real domestic effluent input of the Johkasou system For iron electrolyte experiments, the test effluent consists of phosphorus (phosphate solution), Chloride, and Calcium The concentration of phosphorus and calcium in the experimental wastewater is nearly the average value of the real wastewater in Johkasou Chloride is added to ensure electrical conductivity for the solution Phosphorus was prepared by completely dissolving Potassium dihydrogen phosphate (KH2PO4) salt in pure water Chloride and Calcium solutions

were also prepared by dissolving sodium chloride (NaCl) and Calcium chloride (CaCl2.2H2O) salts, respectively The concentration of stock solutions and the

concentration of components in the synthetic wastewater are presented in Table 2.1 Table 2.1 Preparation of synthetic test liquor

No Solution Chemicals Mass (g) Volume (ml)

Stock solution (mg/L)

Working solution (mg/L)

1 Cl- NaCl 1.50 1000 900 ≈ 91

2 Ca2+ CaCl

2.2H2O 2.93 1000 800 20

3 PO4-P KH2PO4 2.20 1000 500

Stock solutions were stored in glass bottle and stored for month while working solutions are prepared daily

2.1.2 Humic substance sample liquor

sampling landfill is 21 years old and is in the stabilized phase of methanogenic Leachate has been filtered through filter paper (pore size: 10µm-20µm) to remove suspended solids and store in the refrigerator

Figure 2.1 The map of Hanoi and Nam Son landfill

2.1.3 Humic acid sample liquor

The Humic acid solution is prepared from commercial humic acid (powder form; Nacalai Tesque, Japan) Humic acid is exceedingly difficult to dissolve under normal conditions so to be able to completely dissolve humic acid we must prepare alkaline (pH ≥ 10) And to make sure humic acids are completely dissolved they should be kept for 12 hours The solution is then filtered through a 1µm filter paper The concentration of the stock solution is 1000mg/L

Figure 2.2 Humic acid, Nacalai Tesque, Japan

2.2 Iron electrolysis experiment set-up

The iron electrolysis experiments are set - up according to the batch reactor model The combined wastewater (prepared daily in 3.1.1) is placed in a 200ml high beaker and two iron electrodes (120mm × 20mm × 2mm) are installed above the beaker The two electrodes are connected to direct current (PMC35-1A; Kikusui Electronics Corp) 0.014A Aeration machine is installed and performs air blowing from the bottom of the beaker Electric current is controlled by multimeter, as shown in Figure 2.3 The duration of the experiment was calculated based on Faraday's law, this calculation to ensure the molar ratio between Fe / P = or

Figure 2.3 Schematic diagram of the laboratory-scale experiment

Figure 2.4 The types of equipment used to set-up experiments Aeration machine Power supply

(PMC35-1A; Kikusui Electronics Corp)

Iron electrode

Multimeter

+

+ -

2.3 Operational condition of experiment

2.3.1 Iron electrolysis with or without oxygen supply

To determine the effect of oxygen on iron electrolysis experiments under different conditions were arranged

• The first test is performed under aerobic conditions (using aeration machine) • The second experiment is to allow the process of electrolysis to occur naturally

• The third experiment was performed under anaerobic conditions, an inert gas supply device N2 was used to eliminate oxygen in the water to ensure anaerobic

conditions completely

In addition to the current was measured in multimeter, DO (HACH, HQ11d) and pH (S220-Kit, Metter Toledo) were also measured to ensure that the experiment takes place under the right conditions The synthetic wastewater of all three experiments was prepared in the same way as in Figure 2.5 The concentration of components in the simulated wastewater is the same as the concentration of these components measured and reported in the Johkasou system The specific arrangement of the experiments is shown in Figure 2.6 and Table 2.2

Table 2.2 Operational experiment condition

Figure 2.6 Set – up experiments

Under these three different conditions, the DO is tightly controlled using HACH's sensors (HACH, HQ11d) DO is measured at the following every 15 minutes In addition, parameters such as amperage or electrolysis time are calculated according to Faraday's law to estimate the molar ratio Fe/P as and

Aeration Without

aeration Supply N2 gas Fe/P molar ratio & & &

Current (A) 0.014 0.014 0.014

Electrolysis time

(mins) 15& 30 15& 30 15& 30

pH 5.5 ~ 6.5 5.5 ~ 6.5 5.5 ~ 6.5

DO (mgO2/L) ~ Not control < 0.1

2.3.2 Iron electrolysis with HS addition

The iron electrolysis with HS is performed under aerobic conditions and the electrolysis time is 15 minutes The whole experiment set-up is like the experimental set-up with aeration in section 2.3.1 The HS is added to the synthetic wastewater before starting the experiment in the following order: 0ml 20ml 50ml 100ml -150ml, details were shown in Figure 2.7

Figure 2.7 Humic substance experiment set-up.

2.3.3 Iron electrolysis with humic acid addition

Figure 2.8 Humic acids addition experiment set-up 2.4 Chemical analysis

2.4.1 Suspended solid (SS)

The method chosen for SS analysis is the Determination suspended solids by filtration through glass-fiber filters (ISO 11923: 1997) – TCVN 6625:200

Principle of this method:

• Washing filter paper (3 times) by pure water • Drying at 105oC with oven (3 hours)

• Cooling to normal temperature with desiccator (1 hour) • Weigh the filter paper (W1 - mg)

• Carry out sample filtering (Vsample = 50ml)

• Drying at 105oC with oven (3 hours)

• Cooling to normal temperature with desiccator (1 hour) • Weigh the filter paper (W2 - mg)

Calculate the results:

SS (mg/L) = (𝑊1 −𝑊2)×1000

2.4.2 Iron analysis

In this iron electrolysis experiment, total iron (TFe); Dissolved inorganic iron (D-Fe), and D-Fe2+ were measured The method used is the Determination of iron -

Spectrometric method using 1.10- phenanthroline (ISO 6332: 1988) – TCVN 6177:1996 The principle of this method is that when adding the 1,10 - phenanthroline

reagent solution to a sample containing iron will produce an orange-red complex and measure the absorbance of this complex at a wavelength of 510 nm If you want to determine TFe and S-Fe, add hydroxyl ammonium chloride to reduce Fe2+ to Fe3+

Calibration curves are based on known iron content and their absorbance is measured by UV-VIS Diode Array Spectrophotome (S2100, Unico) By using the calibration curve, the unknown iron concentrations were calculated

Preparation chemicals

Table 2.3 Preparation chemicals to iron analysis

No Regents Preparation

1 Hydroxyl ammonium Chloride (HClNH2OH) – 10% (w/v)

Dissolve 10g HClNH2OH in 100ml

DW

2 1,10 phenanthrolinium Dissolve 0.13g in 100ml DW

3

Ammonium acetate – acetate buffer

Dissolve 250g CH3COONH4 in

120ml DW + 700ml CH3COOH and

make up to 1000ml by DW

Procedure iron calculate

TFe = D-Fe + P-Fe D-Fe = D-Fe2+ + D-Fe3+

Total inorganic iron (TFe)

Dissolved iron (D-Fe)

Insoluble iron (P-Fe)

D-Fe2+

2.4.3 Phosphorus analysis (PO4-P)

The concentration of orthophosphate is determined by the spectrometric method of ammonium molybdate reagent (Ammonium molybdate spectrometric

method - ISO 6878: 2004) – TCVN 6202:2008 The principle of this method is based

on the reaction between orthophosphate ions and the acid solution of molybdate and antimony ions to form an antimony phosphomolybdate complex The reduction of this compound with ascorbic acid forms a dark green molybdenum complex Measure the adsorption of this complex at the most sensitive wavelength of 880 nm (if lower sensitivity is accepted, it can be measured at 700 nm wavelength) Calibration curves are based on known phosphorus content and their absorbance is measured by UV-VIS Diode Array Spectrophotome (S2100, Unico) By using the calibration curve, the unknown phosphorus concentrations were calculated

Preparation chemicals

Table 2.4 Preparation chemicals to phosphorus analysis

No Regents Preparation

1 Ammonium molybdate

Dissolve 6g (NH4) Mo7O24.4H2O + 0.24g

Potasium antimony (III) tartrate in 300ml DW Add 120 ml H2SO4: H2O (v/v: 2/1)

Add 5g ammonium amidosulfate Make up to 500ml by DW

2 L – ascorbic Dissolve 7.2g L-ascorbic in 100ml DW

2.5 Fluorescence spectroscopy analyses by three-dimensional excitation-emission matrix

fluorescence detection are scanned from 200nm to 600nm with 30000nm / A 1cm cuvette was used

CHAPTER 3: RESULTS AND DISCUSSION

3.1 Iron electrolysis without oxygen supply

3.1.1 Iron electrolysis with aeration

Table 3.1 Effluent parameters after electrolysis performed in aeration experiment (n=3)

Parameter (mg)

Fe/P molar ratio

2

D-Fe2+ 0 ± 0.13 0 ± 0.13

D-Fe3+ 0.23 <0.01

D-Fe 0.23 ± 0.14 ± 0.14

P-Fe 2.5 4.52

TFe 2.73 ± 0.06 4.52 ± 0.06

PO4-P 0.01 ± 0.0048 ± 0.004

SS 8.4 ± 0.2 13.7 ± 0.7

The experiments with aeration were carried out times to determine the mean Table 3.1 shows the average value of the effluent quality after the electrolysis of iron The remaining dissolved iron in wastewater is mainly in the form of iron ions III Phosphorus is well removed under aerobic conditions because the average value of residual phosphorus was low, about 0.01mg A trend of decreased PO4-P has been

observed with an increasing molar Fe/P ratio Along with the decrease in PO4-P, an

Figure 3.2 illustrates the result obtained after calculating the ratio between the existing forms of Fe in the solution after electrolysis At a molar ratio of 2, we observed that over 90% of iron exists in an insoluble form And this also corresponds to the removal efficiency of phosphorus proving that the amount of iron added from the electrode is sufficient to reduce the phosphorus concentration below 1.0mg/L

3.1.2 Iron electrolysis without aeration

As mentioned above, for without aeration experiment I set up two experiments: no aeration and N2 blowing

0% 20% 40% 60% 80% 100% 1 2 Pho sp ho rus i ns ol ub il iz at ion (% ) insoluble soluble

Fe / P 2 4

0% 20% 40% 60% 80% 100% 1 2 Ir on coa g ul at ion (% ) insoluble

Fe / P 2 4

0% 20% 40% 60% 80% 100% 1 2 Ir on coa g ul at ion (% ) insoluble soluble

Fe / P 2 4

0% 20% 40% 60% 80% 100% 1 2 Ir on coa g ul at ion (% ) insoluble soluble

Fe / P 2

Figure 3.2 Iron coagulation Figure 3.1 Phosphorus insolubilization

Figures 3.3 and 3.4 show the percent of insoluble iron and iron in solution It is clear that the difference between the two existing forms of iron is quite obvious This proves that in the absence of aeration, the precipitation process still occurs but the difference in the color of the precipitate has been noted While the color of precipitate in aeration medium is orange - brown, the precipitate, in this case, is green This suggests that the difference in the composition of flocculation and the form of iron ions in the precipitate The color of the sediment was greenish showing ferrous salts Therefore, it can be concluded that although oxygen is not supplied, the iron coagulation and sedimentation proceeded

(a) (b)

3.1.3 Discussion

It has always been known that the process of removing phosphorus by iron electrolysis takes place in the order that iron ions are supplied from the anode and then oxidized by dissolved oxygen in the wastewater and becoming ferric ions These ferric ions combine with phosphate in wastewater to precipitate and settle to the bottom of the tank, as shown in Figure 3.8

Figure 3.8 The existing pathway models 0% 20% 40% 60% 80% 100% 1 2 Pho sp ho rus i ns ol ub il iz at ion ( % ) insoluble soluble 0% 20% 40% 60% 80% 100% 1 2 Phosphor us insol ubi li zat ion (% ) insoluble soluble

Fe / P 2 4

Figure 3.6 Phosphorus insolubilization (without aeration)

Figure 3.7 Phosphorus insolubilization (N2 gas bubbling)

However, another way of removing phosphorus by iron electrolysis has been found Based on the results presented above, we can clearly see that the precipitation and deposition process still take place in a different way This pathway is summarized as follows: ferrous ions liberate anodes that can precipitate Fe2(PO4)3 _ iron (II)

phosphate and can settle to the bottom of the tank in the anaerobic condition In addition, one hypothesis is also given that the released ferrous ion will precipitate, and this precipitate continues to be oxidized by dissolved oxygen in the wastewater and takes place at the same time as ferrous oxidation into ferric ions

3.2 The effect of humic substance on iron electrolysis

3.2.1 Iron coagulation decrease by humic substance addition

Table 3.2 Effluent parameters after electrolysis performed in humic substance addition experiment (n=3)

Parameter (mg)

Humic substance addition (ml)

0 20 50 100 150

D-Fe2+ 0.064 ± 0.001 0.07 ±0.003 0.41 ± 0.03 0.66 ± 0.03 0.94 ± 0.03

D-Fe 0.072 ± 0.01 0.08 ± 0.07 0.43 ± 0.003 0.64 ± 0.08 0.93 ± 0.01

D-Fe3+ <0.01 <0.01 <0.01 <0.01 <0.01

P-Fe 3.088 2.55 1.61 2.05 1.64

TFe 3.16 ± 0.17 2.62 ± 0.18 2.02 ± 0.03 2.69 ± 0.42 2.57 ± 0.21

PO4-P 0.01 ± 0.001 0.3 ± 0.002 0.52 ± 0.006 0.64 ± 0.003 0.74 ± 0.014

Figure 3.10 Iron coagulation (Humic substance addition)

3.2.2 Decrease of phosphorus insolubilization by iron coagulation decrease

Compared with the absence of HS, it is clear that HS interfered with the reaction that produced precipitation during iron electrolysis It interferes with the process of iron ions combined with phosphate in the wastewater, resulting in less precipitation Therefore, the amount of free phosphate in the wastewater remains and increases gradually according to the concentration of the HS in the wastewater, as shown in Figure 4.11 It can be easily seen from Figure 4.11 that the ratio of insoluble phosphorus accounts for the majority in the case High HS concentrations mean that the process of phosphorus removal by iron electrolysis has been inhibited by HS Thus, it can be considered that the inhibition of HS elimination by reducing the amount of iron combined with phosphorus

0% 20% 40% 60% 80% 100%

0 20 50 100 150

Iron

coagulation(

%)

HS addition (ml)

Figure 3.11 Phosphorus insolubilization (HS addition)

3.2.3 Discussion

HS supplementation has significantly inhibited the elimination of phosphorus by iron electrolysis After adding HS, the remaining phosphorus increases and is proportional to the amount of HS added These results show that the presence of HS prevents iron ions from being released from the anodes and phosphate in the wastewater due to the formation of complexes between iron and HS (Figure 3.11) HS has also been shown to compete with PO4-P in combination with iron-on goethite,

thereby reducing the adsorption of PO4-P (Sibanda and Young, 1986; Hawke et al.,

1989; Antelo et al., 2007) 0%

20% 40% 60% 80% 100%

0 20 50 100 150

Phosphor

us

insolubil

ization

(%)

HS addition (ml)

Figure 3.12 Molar ratio of ΔFe / ΔP

The molar ratio of ΔFe / ΔP line incline was almost 1.0 and MP/MFe = 31/56 ≈

0.554 nearly incline: 0.536 This suggests that the same mole of P as Fe combined in the complex was soluble (Figure 3.13) This means that the number of moles of P in the soluble form is equal to the number of moles of Fe combined in the complex and soluble phosphorus was kept to be PO4-P because it cannot be combined with Fe The

significance of the same number of moles in P and Fe would be that Fe and P are combined using a monodentate or bidentate ligand without humic substances This phenomenon can be explained by the fact that PO4-P can form complexes with

hydroxyls in water in a pH range of 4.5 to This can occur in the environment around the anode where Fe and P will be start reaction

There is also a hypothesis that the addition of HS creates a negative electrostatic field around the surface of the colloidal particles that will lead to a significant reduction of the Zeta electrodynamic potential Thereby reducing the possibility of collisions of charged particles in the solution resulting in the reduction of phosphorus In addition, in the high pH environment PO43- subjected to the high

competition of OH- as well as electrostatic repulsion (Fu et al., 2013)

y = 0.536x + 0.280 R² = 0.988

0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

Δ Fe (mg/200mL)

Δ P

(mg/

20

Figure 3.13 Soluble complex formation of ferrous ion and HS 3.3 The effect of fulvic acid to iron electrolysis

3.3.1 Iron electrolysis with humic acid addition

Table 3.3 Effluent parameters after electrolysis performed in humic acid addition experiment (n=3)

Parameter (mg)

Humic acid addition (ml)

0 20 40 50

D-Fe2+ 0.064 ± 0.001 0.13 ±0.06 ± 0.03 ± 0.03 D-Fe 0.072 ± 0.01 0.12 ± 0.016 0.07 ± 0.01 0.03 ± 0.003

D-Fe3+ <0.01 <0.01 0.07 0.03

P-Fe 3.09 3.1 3.54 3.57

TFe 3.16 ± 0.17 3.22 ± 0.08 3.61 ± 0.03 3.60 ± 0.06 PO4-P 0.01 ± 0.001 0.01 ± 0.002 0.01 ± 0.002 0.02 ± 0.002

Fe2+

Fe

insoluble coagulated Fe

(decrease)

soluble complex of Fe–humic substance

(increase)

P

insoluble P (decrease)

soluble P (increase)

ΔFe

ΔP

Figure 3.14 Iron coagulation (Humic acid addition)

Figure 3.14 and Table 3.3 illustrates the result obtained after calculating the ratio between the existing forms of Fe in the solution after electrolysis It is clearly visible from Table 3.3 and Figure 3.14 that most of the iron supplied from the anode has precipitated Although, under conditions of humic acid addition, the precipitation process is still the same as in the absence of humic acid No obstruction to the process is observed This suggests that humic acid may not inhibit precipitation This is also evident in Figure 3.15, where one can clearly see that most of the dissolved phosphorus in PO43- form is almost removal Instead, phosphorus is insoluble in the

form of precipitation with iron ions released from the anode This proves that the removal of phosphorus by iron electrolysis has occurred under this condition

0% 20% 40% 60% 80% 100%

0 20 40 50

Ir

on

coagulation(

%)

HA addition (ml)

Figure 3.15 Phosphorus insolubilization (HA addition)

3.3.2 EEM of the used humic substance and humic acid sample

Figure 3.16 EEMs Fluorescence spectra of humic substance sample (leachate sample)

0% 20% 40% 60% 80% 100%

0 20 40 50

Phosphorus

insolubl

e

(%

)

HA addition (ml)

insoluble soluble

fulvic acid like peak

Figure 3.17 EEMs Fluorescence spectra of humic acid sample

In Figure 3.16, it is clear that the leachate shows the presence of humic and fulvic acids Specifically, the fluorescence peak is considered to be fulvic acid λEx around 300nm and λEm from 400 - 450 nm This result is also reported by previous studies, Baker et al (2004) reported with λEx = 320 - 360 and λEm = 400 - 470nm, it came from the fulvic acid-like peak Another peak is also found at λEx around 250nm and λEm between 400-500nm, which was considered humic acid Whereas for humic acid samples, the observed results were λEx around 250nm and λEm between 400-500nm

3.3.3 Discussion

The two peaks of humic acid and fulvic acid were detected in the fluorescence excitation-emission matrix (EEM) of the used humic substance sample While only one peak of humic acid was detected in the EEM of the used humic acid sample This led to a new hypothesis that fulvic acid in humic substance samples appears to reduce iron coagulation during iron electrolysis This theory is described in detail in Figure 3.18, in this new hypothesis, fulvic acid will complex with iron and reduce the amount of iron combined with phosphate in water, thereby inhibiting the process of removing phosphorus by iron electrolysis This may account for the fact that although adding HA to the wastewater does not impede the phosphorus removal process by binding competition This contrasts with the previously reported assumptions about the

adsorption of iron oxides of HA by complexing with iron (Tipping, 1981; Gerke, 1993)

It may also explain that HA does not interfere with phosphorus removal as follows: Humic acid is known to be insolubilized in low pH conditions Therefore, it seems that humic acid will be insolubilized in high H+ conditions near anode in

electrolysis and then would be coagulated together with FeOOH as well as insoluble P On the contrary, fulvic acid will be kept soluble even near anode and cathode in electrolysis It would then form a soluble complex with Fe

Figure 3.18 The effect of fulvic acid on iron electrolysis fulvic acid

humic acid

Fe2+

Fe

insoluble coagulated Fe

soluble complex of

Fe and fulvic acid insoluble P Fe

fulvic acid

interference

CONCLUSION

The current research has identified specific factor of DOC that interferes the phosphorus removal by iron electrolysis process in Johkasou The main findings from this study are summarized as follows:

1) Iron coagulation proceeds not only under aerobic condition by forming ferric floc but also under anaerobic conditions by forming ferrous floc, showing a new pathway of phosphorus insolubilization using ferrous floc in iron electrolysis process

REFERENCES

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