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Master’s Thesis of Engineering

Chemical behavior of

orthophosphate and total

phosphorus in phosphorus removal

process

인 거 공 에서 르토 인과 총

인의 화학 거동 연구

August 2015

Graduate School of Engineering

Seoul National University

Energy System Engineering

Vanvimol Ampunan

Chemical behavior of

orthophosphate and total

phosphorus in phosphorus removal

process

Advisor Chung Eunhyea

Submitting a master’s thesis of Engineering

July 2015

Graduate School of Engineering

Seoul National University Energy System Engineering

Vanvimol Ampunan

Confirming the master’s thesis written by

Vanvimol Ampunan

June 2015

Chair (Seal)

Vice Chair (Seal)

Examiner (Seal)

i

Abstract

Removal of phosphorus in wastewater was performed in lab

scale to determine the chemical behaviors of orthophosphate and

total phosphorus. Firstly, coagulation/precipitation test was

conducted for wastewater which was collected after a secondary

treatment process at a wastewater treatment plant. Alum and Ferric

chloride were used as coagulants in different mole ratio

concentrations which are 1:1, 2:1 and 3:1 (Al or Fe : P).

Orthophosphate and total phosphorus concentration were analyzed

to identify their chemical behaviors. Results indicate that the

removal of phosphorus species is influenced by the concentration of

the coagulants and the reactivity of the phosphorus species. It is

believed that the reactivity differences between orthophosphate and

polyphosphate does not play a significant role in chemical reaction

in low concentration of coagulant (1:1). Polyphosphate has greater

molecular weight and larger particle size than orthophosphate,

therefore, the polyphosphate has higher possibility to participate in

chemical reaction with coagulants than the orthophosphate. On the

other hand, the reactivity differences might play a significant role in

high concentration of coagulants. The orthophosphate which is the

most reactive species of phosphorus was dominant in chemical

reaction in high mole ratio concentration (2:1 and 3:1).

Adsorption/precipitation column test was conducted by

feeding wastewater upward through the column with various sizes

ii

of slag (0.5-1 mm, 1-2 mm and 2-4 mm) as absorbent. The

effluent samples were collected after up to 100 bed volume for the

analysis of orthophosphate and total phosphorus concentrations.

According to the results, phosphorus was removed rapidly in the

initial phases of the adsorption/precipitation column test and the

slag of which size is between 0.5 and 1 mm showed the highest

phosphorus removal efficiency. With the coarser sized slag samples,

adsorption and precipitation tends to occur less than with the finer

sized slag. In addition, during the initial phases, the conditions of the

wastewater were appropriate for precipitation in terms of a high pH

and a high concentration of cations, such that both orthophosphate

and polyphosphate were removed completely by precipitation.

However, the proportion of orthophosphate was high in the initial

phases and it indicates that the polyphosphate was dominant in

adsorption because it has larger particle size than the

orthophosphate and, therefore, the polyphosphate was adsorbed on

the surface of the slag in initial phases of the experiment more than

the orthophosphate. In later phases, the proportion of

orthophosphate becomes lower because orthophosphate participated

in the adsorption reaction more than in the initial phases, and both

orthophosphate and polyphosphate were adsorbed on the surface of

slag in later phases.

Keyword : Phosphorus removal, orthophosphate, coagulation,

adsorption, precipitation, slag

Student number : 2013-23864

iii

Contents

Chapter 1. Introduction .......................................1

Chapter 2. Theoretical background....................6

2.1 Classification of phosphorus species....................................6

2.2 Phosphorus removal from wastewater.................................7

2.3 Use of steel slag in phosphorus removal...........................12

2.3.1 Steel slag.............................................................................12

2.3.2 Phosphorus removal mechanism........................................13

Chapter 3. Materials and Methods................... 15

3.1 Coagulation/precipitation test.............................................15

3.1.1 Sample preparation.............................................................15

3.1.2 Apparatus and chemicals....................................................17

3.1.3 Analytical procedure...........................................................18

3.2 Adsorption/precipitation column test.................................19

3.2.1 Sample preparation..............................................................19

3.2.2 Kinetic batch test................................................................22

3.2.3 Column tests experiment....................................................22

3.2.4 Apparatus and chemicals....................................................24

3.2.5 Analytical procedure...........................................................24

Chapter 4. Result and Discussion ....................25

4.1 Coagulation/precipitation test.............................................25

4.1.1 Basic characteristics of wastewater...................................25

iv

4.1.2 Phosphorus removal efficiency...........................................28

4.1.3 Orthophosphate and total phosphorus analysis.................29

4.1.4 The comparison between lab scale experiment and real

wastewater treatment plant process.................................35

4.2 Adsorption/precipitation column test.................................37

4.2.1 Phosphorus adsorption/precipitation kinetics....................37

4.2.2 Basic characteristics of wastewater...................................42

4.2.3 Phosphorus removal efficiency...........................................49

4.2.4 Orthophosphate and total phosphorus analysis.................51

Chapter 5. Conclusion.......................................60

References.........................................................63

.....................................................................69

v

List of Tables

Table 1 Basic characteristics of wastewater samples collected in

different depths (upper part (U), middle part (M) and lower

part (L)) after secondary treatment process.......................26

Table 2 Basic characteristics of wastewater samples after

coagulation/precipiration test in different mole ratio

concentrations of ferric chloride and alum (Fe or Al : P)....27

Table 3 Proportion of orthophosphate before and after

coagulation/precipitation test in different mole ratio

concentrations of coagulants (Fe or Al : P) (unit : %)........32

Table 4 Orthophosphate and total phosphorus concentration in input

(I) and output (O) samples....................................................35

Table 5 Basic characteristics of wastewater samples used for

adsorption/precipiration column test.....................................43


adsorption/ precipitation column test with slag size

2-4 mm...................................................................................44



1-2 mm...................................................................................45



0.5-1 mm................................................................................46

vi

List of Figures

Figure 1 Chemical structure of orthophosphate and polyphosphate.7

Figure 2 Wastewater treatment plant system flow diagram with

sample collecting point for coagulation/precipitation

test........................................................................................16

Figure 3 Wastewater treatment plant system flow diagram with

sample collecting point for adsorption/precipitation column

test....................................................................................... 20

Figure 4 Slag samples in different by particle sizes (0.5-1 mm, 1-

2 mm and 2-4 mm).............................................................21

Figure 5 Schematics of experimental setup for adsorption/

precipitation column test.....................................................23

Figure 6 Phosphorus removal efficiency of coagulation/precipitation

test in different mole ratio concentrations of coagulants

(Fe or Al : P).......................................................................29

Figure 7 Orthophosphate and total phosphorus concentration after

coagulation/precipitation test with different concentrations

of ferric chloride..................................................................30


coagulation/precipitation test with different concentrations

of alum..................................................................................31

vii

Figure 9 Proportion of orthophosphate after

coagulation/precipitation test in different concentrations of

ferric chloride......................................................................33

Figure 10 Proportion of orthophosphate after

coagulation/precipitation test in different concentrations of

alum......................................................................................33

Figure 11 Proportion of orthophosphate in input (I) and output (O)

samples.................................................................................36

Figure 12 Kinetics of orthophosphate and total phosphorus

adsorption with slag size 2-4 mm.....................................38


adsorption with slag size 1-2 mm.....................................39


adsorption with slag size 0.5-1 mm..................................40

Figure 15 Efficiency of phosphorus removal after

adsorption/precipitation column test with slag size 0.5-1

mm, 1-2 mm and 2-4 mm.................................................50

Figure 16 Basic characteristics of wastewater samples and

phosphorus concentration after adsorption/precipitation

column test with slag size 2-4 mm....................................52



column test with slag size 1-2 mm....................................53

viii



column test with slag size 0.5-1 mm................................54


adsorption/precipitation column test with slag size

2-4 mm................................................................................55



1-2 mm................................................................................56



0.5-1 mm.............................................................................56

1

1. Introduction

Excessive concentrations of phosphorus in a body of water

can have a significant environmental impact by causing algal blooms

that lead to the phenomenon of eutrophication (Smith, 2003).

Orthophosphate, which is one species of phosphorus, has been

determined to be a major cause of eutrophication. Orthophosphate is

a chemically or enzymatically hydrolyzed form of total phosphorus

and the only form that can be assimilated by algae, bacteria and

plants. Therefore, when the concentration of phosphorus in a water

body is high, the autotroph population will increase. As a result of

the high respiration rate of autotrophs, the oxygen level in the

water body becomes deficient, leading to the death of fish and

greatly reduced biodiversity (Correll, 1998).

The main sources of phosphorus in natural waters are the

drainage of agricultural land, domestic and industrial wastewater

and diffuse urban drainage (Lee et al., 1978). Phosphorus inputs

from point sources, such as industrial effluents, are easier to

control than phosphorous from non-point sources (Yeoman et al.,

1987). Thus, reducing the phosphorus concentrations in wastewater

from point sources is an effective strategy for controlling

phosphorus.

A number of technologies for removing phosphorus from

wastewater, both established and under development, have been

2

proposed. The technology that is most applicable in wastewater

treatment plants is chemical precipitation, which is accomplished by

adding metal salts into the wastewater, causing the transformation

of dissolved inorganic phosphate to a particulate form of phosphate.

The suspended solid is subsequently removed. In the biological

process, the phosphorus is removed by activated sludge. However,

this process requires more complex plant configurations and

operation regimes. Therefore, this process can be difficult to apply

in wastewater treatment plants (Morse et al., 1998).

Moreover, adsorption and crystallization technologies are

based on the adsorption or crystallization of phosphate on seeding

materials that contain essential elements for phosphate adsorption

and crystallization, such as calcium, magnesium and aluminum. The

widely used seed materials are steel slag, fly-ash and red mud.

These materials are low-cost and easily available; thus, this

approach has been widely investigated during recent years (Oguz,

2004).

Based on its frequent application in wastewater treatment

plants, the chemical precipitation process was selected for further

studies. Moreover, the alternative method of phosphorus removal

using seed materials as an absorbent is also of interest for many

researchers; therefore, this thesis will cover coagulation, adsorption

and precipitation in the context of phosphorus removal.

For many years, studies of the coagulation process were

3

primarily related to coagulant types and the concentration of

coagulants, which affect the phosphorus removal efficiency.

Aluminum sulfate octadecahydrate (alum) and anhydrous iron (III)

chloride (ferric chloride) are widely used coagulants (Rybicki,

1997). The efficiency of removal by each of these coagulants was

reviewed previously (Sedlak, 1991; Omoike and Vanloon, 1999;

Galaneau and Gehr, 1997; Aguilar et al., 2002; Hano et al., 1997;

Caravelli et al., 2010; Mamais et al., 1994). Moreover, the

conditions of the wastewater before and after the coagulation

process, such as pH, coagulant concentration, temperature,

biochemical oxygen demand (BOD), chemical oxygen demand

(COD), total kjehldahl nitrogen (TKN) and suspended solid

concentration, were also studied (Zhou et al., 2008; Georgantas and

Grigoropoulou, 2007).

There are limited studies on the interaction mechanisms of

chemicals with orthophosphate and polyphosphate. In previous

studies, alum and aluminum hydroxide were used for

orthophosphate and metaphosphate (one form of polyphosphate)

removal from aqueous solution. In similar conditions,

orthophosphate and metaphosphate appeared to have the same

behavior affected by pH variations. On the other hand, these forms

of phosphorus have different affinities for surface sites of aluminum

hydroxide. Orthophosphate was found to be removed more

efficiently than metaphosphate due to orientation effects and the

4

charge per phosphorus atom (Zhou et al., 2008; Clark et al., 1997;

Razali et al., 2007; Ormaza-Gonzalez and Statham, 1996; Loijklema,

1980; Gao et al., 2013).

Phosphorus removal using slag, which is a by-product from

the steel industry, is becoming a well-known alternative method of

phosphorus removal. Slag is enriched in calcium and contains

various components, such as aluminum, magnesium, iron,

manganese and titanium, on its surface. Yamada (1986) found that

adsorption depends on the pH, the temperature, the concentration of

coexisting salts and the porosity of the materials. Phosphate is

adsorbed well onto materials with a large porosity. Moreover, based

on observations of slag surfaces that adsorbed orthophosphate, the

adsorption site of phosphate was coincident with the site of calcium,

magnesium, aluminum and silicon compounds on the surface of the

slag.

Although a number of studies investigated the phosphorus

removal process, most of those studies focused on the removal of

total phosphorus. To date, few studies have addressed the

mechanism of orthophosphate removal during the phosphorus

removal process. However, because orthophosphate is the main

species of phosphorus, this molecule can cause significant

environmental impacts. Therefore, orthophosphate species should

be investigated more in future studies.

Hence, the purpose of this thesis is to understand the

5

chemical behaviors of orthophosphate and total phosphorus during

coagulation, adsorption and precipitation in the phosphorus removal

process. Moreover, optimal conditions for orthophosphate removal

will be suggested to enhance the phosphorus removal efficiency and

improve the design of wastewater treatment systems.

6

2. Theoretical background

2.1 Classification of phosphorus species

Phosphorus or phosphates are defined as compounds that

contain P-O linkages. Phosphorus can be classified into three types:

orthophosphate, polyphosphate and organic phosphate.

Phosphorus exists in nature almost exclusively in the

oxidized state as orthophosphate. Therefore, among the various

types of phosphorus, orthophosphate is the most abundant because

it is the most stable (Averbuch-Pouchot and Durif., 1996). The

crystal structures of orthophosphates represent the tetrahedral

distribution of four oxygen atoms attached to a central phosphorus

atom (PO43-), as shown in Figure 1. Most orthophosphate is

insoluble, and the melting point is often more than 1,000˚C

(Corbridge, 1995).

Polyphosphate is formed by the repeated condensation

(polymerization) of tetrahedral (PO4) units; therefore, this form of

phosphate exists as chains of tetrahedrals, each sharing the oxygen

atom at one or two corners of the adjacent tetrahedral (Figure 1).

Polyphosphate is stable in neutral or alkaline solutions at room

temperature, but hydrolysis occurs in acidic solutions. These

compounds form soluble complexes with metal ions. Thus,

7

polyphosphate is used to control heavy metal ions in wastewater

before discharge into the environment (Rashchi et al., 2000).

Figure 1 Chemical structure of orthophosphate and polyphosphate

2.2 Phosphorus removal from wastewater

The general purpose of phosphorus removal is to eliminate

excess phosphorus from effluent prior to discharge into natural

water and to utilize the excluded phosphorus. Phosphorus is

typically present in wastewater in soluble form. Only 15% of the

total phosphorus can be settled and removed by primary

sedimentation without the addition of metal salts. Therefore,

phosphorus removal processes are based on transforming soluble

phosphorus to a solid phase and are complemented by solid-liquid

separation (Rybicki, 1997).

Based on the employed principle of phosphorus removal

from wastewater, the removal processes can be classified as

8

follows:

i. Chemical precipitation

In this process, phosphorus precipitation is achieved by

the addition of divalent or trivalent metal salts to wastewater,

causing the precipitation of an insoluble phosphate. During this

process, phosphate ions are transformed into the solid state,

which occurs in three stages: core formation of solid matter,

the storage of a precipitate and the start of crystal growth and

maturation. Therefore, in this process, precipitation dominates,

while coagulation and adsorption play minor roles. The most

commonly employed chemicals for precipitation in municipal

wastewater are as follows:

a. Lime (Ca(OH)2) - This chemical has been used

frequently recently due to low costs and less problems

with sludge dewatering but has a disadvantage due to its

low solubility in water and requirement for a high pH

(≥10) to achieve phosphate precipitation and concurrent

biological growth. The product of lime precipitation is

mainly calcium phosphate, such as hydroxyapatite

(Ca5(OH)(PO4)3), as follows in the equation:

9

5 + 3 + → ( )( ) ↓

Moreover, to achieve poorly soluble orthophosphate

residuals, the pH of the wastewater must be adjusted to

a high value (pH≥10). When the pH value is ≤10, the

bicarbonate alkalinity of the wastewater will react with

lime (Sedlak, 1991), as follows:

( ) + → + ↓

Therefore, the lime dose for calcium phosphate

precipitation is determined by the total alkalinity of the

wastewater (equal to approximately 1.5 times the total

alkalinity) (Sedlak, 1991).

b. Alum (aluminum sulfate, (Al2(SO4)3)) – This chemical is

used primarily for treated wastewater because it is an

efficient precipitant and because phosphorus is not

released during sludge recycling, storage or digestion.

Moreover, low sludge volumes are generated, no pH

adjustment is required, the point of addition is flexible

and clarifier performance is improved.

10

c. Ferric sulfates and ferric chloride (Fe(SO4)3 and FeCl3)

- These options have advantages due to their low cost

and the fact that the produced sludge has excellent

dewatering properties. In contrast, the disadvantages

are that the chemical properties of the salts can cause

corrosion, staining and colored effluents.

For the addition of ferric iron or aluminum, the two

possible participates are ferric or aluminum phosphate and

ferric or aluminum hydroxide. The formation of these

precipitates is dominated by the equilibrium constant that

governs their solubility and the initial pH, alkalinity and soluble

orthophosphate concentration of the wastewater. The following

equations represent examples of the ferric iron and aluminum

precipitation reactions:

Aluminum ions combine with phosphate ions:

+ ↔ ↓

Ferric ions combine with phosphate ions:

+ ↔ ↓

11

ii. Biological phosphorus removal

The biological phosphorus removal process relies on

enhancing the ability of microorganisms to bring phosphorus

into the cell. Thus, these processes are often referred to as

enhanced biological phosphorus removal (EBPR). The EBPR

process basically consists of anaerobic and aerobic zones. The

major mechanism is that organic matter uptake and phosphorus

release occur under anaerobic conditions and phosphorus

uptake occurs during the subsequent sludge process. This

process has the advantages of avoiding the use of chemicals

and excess sludge production. However, this process requires

more complex plant systems and operations. In addition, in

practice, the removal is variable; thus, the effluent may need

complementary chemical precipitation.

iii. Crystallization and adsorption processes

The mechanism of this process is the deposition of

phosphorus particles on the surface of seed materials (Kaneko

et al., 1988) such that phosphorus can be removed from the

wastewater. A number of potential materials from industrial

by-products, such as fly ash and burnt oil shale, especially

slag, have been used for this phosphorus removal process.

12

These materials contain high contents of essential elements for

phosphorus binding, such as calcium, aluminum and ferric. The

crystallization and adsorption process has become an

innovative and alternative method for removing phosphorus

from wastewater not only because it does not have sludge

production problems and uses a comparatively small amount of

chemicals but also because it is cost effective as by-products

or wastes from industrial plants can be used as seed materials.

Moreover, through this process, phosphorus can be recovered

and reused in the future.

2.3 Use of steel slag in phosphorus removal

2.3.1 Steel slag

Steel slag is a by-product from steel industry plants during

the steel manufacturing process. Slag is separated into many types,

which are named for the processes from which they are generated

(for example, blast furnace slag (BF), basic oxygen furnace (BOF),

furnace acid slag (EAF) and ladle slag (LF)) (Navarro et al., 2010).

Most slags consist primarily of CaO, MgO, SiO2 and FeO. However,

the proportions of these oxides and the concentrations of other

components are highly variable depending on the raw material, the

type of steel made, the furnace condition, etc. The mineral

13

composition of slag also varies significantly among sources. The

common minerals in slag are olivine, merwinite, C3S, C2F, etc. Slag

generally has a high concentration of lime (CaO), which has the

ability to increase the pH of an aqueous solution. Lime in slag

originates from two sources: residual lime from the raw material

when the total free lime content in slag is more than 4% and

precipitated lime from molten slag when the total lime content in

slag is less than 4% (Shi, 2004).

2.3.2 Phosphorus removal mechanism

A number of studies have been conducted in the lab or on-

site to study the mechanisms of phosphorus removal using slag.

However, the causes of phosphorus removal remain uncertain. The

mechanism was reported to be either adsorption or precipitation.

Many studies inferred that adsorption onto metal

oxides/oxyhydroxides throughout the pores of the slag was the

major mechanism. Other studies suggested that precipitation was

the mechanism of phosphorus removal. Lu (2008) performed a lab-

scale experiment using BFS and steel furnace slag (SFS) and an

aqueous phosphate solution. The results demonstrated that the

majority of adsorption was completed in 5-10 min. The adsorption

capacity was reduced dramatically by acid treatment. The pH and

Ca2+ concentration decreased with the addition of a phosphate

14

solution, suggesting the formation of a calcium phosphate

precipitate. Pratt (2007) applied a melter slag filter to treated pond

effluent on-site for a decade and discovered that phosphorus was

adsorbed onto metal oxides/oxyhydroxides throughout the pores of

the slag and that phosphorus precipitation occurred primarily in the

form of Fe-phosphates on the surface of the slag.

Therefore, in this study, phosphorus removal by the

coagulation process is called the coagulation/precipitation test.

Moreover, phosphorus removal using slag is called the

adsorption/precipitation test.

15

3. Materials and Methods

In this study, a coagulation/precipitation test was performed

at the lab scale using a batch test with real wastewater and

different molar ratios of coagulants. Moreover, an

adsorption/precipitation column test was performed using slag as a

seed material with real wastewater and different sizes of slag. The

chemical mechanisms associated with both orthophosphate and total

phosphorus during these two phosphorus removal processes were

observed and analyzed.

3.1 Coagulation/precipitation test

3.1.1 Sample preparation

A lab-scale coagulation/precipitation test was conducted

using wastewater samples collected from the wastewater treatment

plant in Suwon, Korea. The wastewater effluent samples were

collected after the secondary treatment process and after the

advanced treatment process (phosphorus controlling process) to

identify the chemical mechanisms associated with the differentiation

of phosphorus during coagulation process. The collection points of

the samples are indicated below, according to the circle in Figure 2.

16

Fig

ure

2 W

aste

wate

r tr

eatm

ent

pla

nt

syste

m flow

dia

gra

m w

ith sam

ple

collecti

ng

poin

t fo

r coagula

tion/p

recip

itati

on t

est

17

The wastewater samples were prepared by filtering using a

0.45 µm filter to analyze the orthophosphate and total phosphorus

concentrations before and after the coagulation/precipitation test.

Wastewater samples after advanced treatment process were used

as representatives of the coagulation process in the real field.

Wastewater after the secondary treatment process was used to

perform the lab-scale coagulation/precipitation test. Therefore, the

chemical mechanisms associated with orthophosphate and total

phosphorus will be compared between the lab test and the real

wastewater treatment plant.

For the lab-scale coagulation process, aluminum sulfate

octadecahydrate (alum) and anhydrous iron (III) chloride (ferric

chloride) were prepared at molar ratios of 1:1, 2:1 and 3:1 for

aluminum:phosphorus and ferric ion:phosphorus. The coagulants

were added and shaken at a mixing speed of 150 rpm for 1 minute,

followed by shaking at 30 rpm for 10 minutes. The samples were

then allowed 30 minutes for sedimentation and settling before

supernatant samples were collected for the subsequent phosphorus

analysis.

3.1.2 Apparatus and chemicals

The coagulation/precipitation test was carried out using a

DR900 colorimeter apparatus (model number 9385160 by HACH).

18

Reactive phosphorus (orthophosphate) was measured using the

PhosVer 3 (ascorbic acid) method (Method 8048), and total

phosphorus was measured using the PhosVer 3 with acid persulfate

digestion method (Method 8190). Shaking incubator model SH-

BSI16R from Samheung Instrument was used to control the

temperature and shake the samples. An Orion STAR A329 multi-

meter from Thermo Scientific and a LaMotte 2020 turbidimeter

were used to check the basic characteristics of the samples.

All chemicals were reagent grade, and the coagulants were

purchased from Sigma-Aldrich for aluminum sulfate

octadecahydrate (alum) and Kanto chemical Co., Inc. for anhydrous

iron (III) chloride (ferric chloride).

3.1.3 Analytical procedure

The basic conditions (pH, conductivity and turbidity) of the

wastewater were measured using an Orion STAR A329 multi-

meter and a LaMotte 2020 turbidimeter.

For orthophosphate concentration analysis, the PhosVer 3

(ascorbic acid) method (Method 8048) was used with a DR900

colorimeter (HACH, Model number 9385160). The sample was

filled in a sample cell with 10 ml then PhosVer 3 phosphate powder

pillow (21060-69) was added. The sample was shaken for 30

seconds, left to react for 2 minutes and measured orthophosphate

19

concentration using a colorimeter at wavelength 610 nm.

The total phosphorus concentration was measured with a

colorimeter using the PhosVer 3 with acid persulfate digestion

method (Method 8190). 5 ml of sample was added to a Total

phosphorus test vial. A potassium persulfate powder pillow for

phosphonate (20847-66) was added to the sample, and after

shaking the vial, the samples were placed in a DRB200 heating

block reactor (Digital Reactor Block 200, HACH) at 150°C for a

30-minute digestion. Then, sodium hydroxide 2 ml and a PhosVer 3

powder pillow (21060-64) were added to the samples. Finally, the

samples were shaken and left for 2 minutes, and the total

phosphorus concentration was measured using a colorimeter at

wavelength 610 nm.

3.2 Adsorption/precipitation column test

3.2.1 Sample preparation

The wastewater samples in the sludge treatment process

were collected from the wastewater treatment plant in Suwon,

Korea. During the sludge treatment process, exhausted sludge is

extracted by pressure and removed from the process. Therefore,

wastewater from this process contains a high concentration of many

elements. The samples were collected at this point to obtain highly

20

concentrated wastewater. The collection points of the samples are

shown below, according to the circle in Figure 3.

Fig

ure

3 W

aste

wate

r tr

eatm

ent

pla

nt

syste

m flow

dia

gra

m w

ith sam

ple

collecting

poin

t fo

r adsorp

tion/p

recip

itation c

olu

mn t

est

21

Ladle furnace slag (LF) samples were obtained from Dongbu

Steel in Chungcheongnam-do, South Korea. Before the experiment,

the slag was crushed by a jaw crusher and a rod mill and sieved

with a sieving machine to achieve sizes 0.5-1 mm, 1-2 mm and 2-

4 mm to observe the chemical mechanisms associated with

orthophosphate and total phosphorus in wastewater before and after

treatment with different sizes of slag in the column test.

Figure 4 Slag samples in different by particle sizes (0.5-1 mm, 1-2

mm and 2-4 mm)

22

3.2.2 Kinetic batch test

5 ml of slag was placed into a 45 ml of wastewater sample in

a 50 ml centrifuge tube. The wastewater samples were taken for

analysis of orthophosphate and total phosphorus concentration after

5, 10, 15, 30, 45 min, 1, 2, 4, 8, 16, 24, 36 and 48 hr. Before the

phosphorus analysis, the wastewater was separated from the slag

by filtration (0.45 µm filter). The quantity of phosphorus adsorbed

on the surface of slag was calculated by the changes of

orthophosphate and total phosphorus concentration.

3.2.3 Column test experiment

Before the adsorption/precipitation column test, the basic

characteristics of the wastewater samples and orthophosphate and

total phosphorus concentrations were measured. For the column

test, 200 ml slag samples were packed inside the bottom of the

column. Then, a pump fed the wastewater upward from the bottom

of the column to the top of the column through the slag at a rate of

25 rpm with a linear velocity of 150 m/hr. The output wastewater

samples were collected at the top of the column for analyzing the

basic characteristics of the wastewater and the orthophosphate and

total phosphorus concentrations after 0, 8, 16, 25, 37.5, 50, 75, 100

bed volumes (200 ml of slag as a bed). The schematic of column

23

that used in the experiment are shown below in Figure 5.

Figure 5 Schematics of experiment setup for adsorption/precipitation

column test

24

3.2.4 Apparatus and chemicals

The apparatus and chemicals that were used to measure the

orthophosphate and total phosphorus concentrations are similar to

those described in section 3.1.2 (except for the coagulant

chemicals).

3.2.5 Analytical procedure

The analytical procedures used to measure the

orthophosphate and total phosphorus concentrations are similar to

those described in section 3.1.3.

25

4. Result and Discussion

4.1 Coagulation/precipitation test

4.1.1 Basic characteristics of wastewater

Wastewater samples were obtained after the secondary

treatment process from three depths: upper (U), middle (M), and

lower (L). Moreover, input (I) and output (O) samples were

obtained from different wastewater treatment plants. The I samples

were collected after the secondary treatment process, and the O

samples were collected after the advanced treatment process

(phosphorus controlling process). The basic characteristics of the

wastewater were measured in triplicate before and after the lab-

scale coagulation/precipitation test. All results are presented in

Table 1.

26

Table 1 Basic characteristics of wastewater samples collected in

different depths (upper part (U), middle part (M) and lower part (L))

after secondary treatment process

The results presented in Table 1 indicate that the collected

samples were in the neutral pH range (approximately pH 7). The

conductivity was also in the normal range, according to the

conductivity of potable waters in the United States, which typically

ranges from 50-1,500 μS/cm (Rice et al., 2012). The turbidity

increased from 0.26 to 1.91 NTU as the depth of the collection

point increased. Sample L was chosen for the following experiments

because no differences in basic characteristics of the samples were

observed, except the differences in turbidity. Because the turbidity

of the L sample was the highest value among the three samples, the

Sample

Parameters

U M L

pH 6.85 6.95 6.70

Conductivity

(μS/cm) 490.9 493.0 493.7

Turbidity

(NTU) 0.26 1.14 1.91

27

L sample was expected to contain a large amount of suspended

materials. Accordingly, suspended materials would be helpful to

analyze the extent of coagulation and precipitation after treatment.

After the lab-scale coagulation/precipitation test with

different coagulants (alum and ferric chloride) and molar

concentration ratios of 1:1, 2:1 and 3:1 (Fe: P or Al: P), the basic

characteristics of the wastewater samples were analyzed again. The

results are presented in Table 2.


coagulation/precipitation test in different mole ratio concentrations of

ferric chloride and alum (Fe or Al : P)

Parameters

Fe Al

1:1 2:1 3:1 1;1 2:1 3:1

pH 7.36 6.90 6.64 7.02 6.90 6.64

Conductivity

(μS/cm) 501.3 507.1 510.8 501.7 507.1 510.8

Turbidity

(NTU) 0.31 0.46 0.36 0.35 0.46 0.36

28

The results indicate that both samples from different

coagulants exhibited similar basic characteristics after the

coagulation/precipitation test. The pH of the samples did not change

significantly and remained in the neutral range. The conductivity

increased as the molar ratio of the coagulants in the samples

increased. This increase occurs because when the coagulants are

added to the wastewater samples, ions from the coagulants dissolve

in the solution. However, all conductivity values remained in the

normal range, as mentioned above. The turbidity decreased

significantly after the coagulation/precipitation test due to the

removal of phosphorus from the samples. However, the turbidity did

not vary appreciably with different molar ratios.

4.1.2 Phosphorus removal efficiency

The initial total phosphorus concentration in the wastewater

before the coagulation/precipitation test was measured. The results

indicated that the initial total phosphorus concentration in the L

sample was 5.79 mg/L. The total phosphorus concentration of the

samples after treatment with coagulants was analyzed using a

colorimeter. As shown in Figure 6, the percentages of removal are

34.6%, 60.0% and 82.7% using ferric chloride and 59.2%, 81.8%

and 94% using alum at molar ratios of 1:1, 2:1 and 3:1 (Fe:P or

Al:P), respectively. Thus, increasing the molar ratio of coagulants

29

enhances the phosphorus removal efficiency. Moreover, alum shows

a higher efficiency than ferric chloride during phosphorus removal

at the same molar ratios.

Figure 6 Phosphorus removal efficiency of coagulation/precipitation

test in different mole ratio concentrations of coagulants (Fe or Al : P)

4.1.3 Orthophosphate and total phosphorus analysis

Overall, the orthophosphate and total phosphorus

concentrations tended to decrease with both ferric chloride and

alum coagulants as the molar ratio increased from 1:1 to 3:1. As

shown in Figure 7-8, the concentration of total phosphorus

0

10

20

30

40

50

60

70

80

90

100

1:1 2:1 3:1

Phosphoru

s r

em

oval (%

)

Alum

Ferric chloride

30

decreased from 5.79 to 3.79, 2.4 and 1.08 mg/L and the

concentration of orthophosphate decreased from 2.56 to 1.64, 0.87

and 0.35 mg/L when ferric chloride was used as a coagulant. The

use of alum as a coagulant revealed a similar tendency, with the

total phosphorus decreasing from 5.79 to 2.4, 1.13 and 0.44 mg/L

and the orthophosphate decreasing from 2.56 to 1.04, 0.22 and

0.03mg/L.


coagulation/precipitation test with different concentrations of ferric

chloride

0

1

2

3

4

5

6

7

L 1:1 2:1 3:1

Concentr

ation (

mg/L

)

Total-P

Ortho-P

31


coagulation/precipitation test with different concentrations of alum

The proportions of orthophosphate to total phosphorus were

calculated and are shown in Table 3 to reveal the chemical

behaviors of orthophosphate and total phosphorus in the

coagulation/precipitation test.

0

1

2

3

4

5

6

7

L 1:1 2:1 3:1

Concentr

ation (

mg/L

)

Total-P

Ortho-P

32

Table 3 Proportion of orthophosphate before and after

coagulation/precipitation test in different mole ratio concentrations of

coagulants (Fe or Al : P) (unit : %)

Coagulants L 1:1 2:1 3:1

Ferric ions 45.3 44.5 38.4 35.4

Alum 45.3 45.3 21.5 9.8

The proportion of orthophosphate to total phosphorus in

sample L before the coagulation/precipitation test was 45.3%. After

the coagulation/precipitation test, the proportion did not change

significantly when a 1:1 molar ratio of ferric chloride or alum was

applied (44.5% and 45.3%, respectively). However, after increasing

the molar ratio to 2:1 or 3:1, the proportion of orthophosphate

decreased significantly. The proportions decreased to 38.5% and

35.7% with ferric chloride and 21.4% and 10.0% with alum at molar

ratios of 2:1 and 3:1, respectively, as shown in Figure 9-10.

33

Figure 9 Proportion of orthophosphate after coagulation/precipitation

test in different concentrations of ferric chloride

Figure10 Proportion of orthophosphate after coagulation/precipitation

test in different concentrations of alum

0

10

20

30

40

50

60

70

80

90

100

L 1:1 2:1 3:1

Pro

port

ion o

f ort

hophophate

(%

)

Total-P

Ortho-P

0

10

20

30

40

50

60

70

80

90

100

L 1:1 2:1 3:1

Pro

port

ion o

f ort

hophosphate

(%

)

Total-P

Ortho-P

34

The results indicated that when the molar ratios are 2:1 and

3:1, the proportions of orthophosphate decrease significantly with

both coagulants due to the reactivity of orthophosphate, which is

greater than the reactivity of other species of phosphorus. For this

reason, orthophosphate was expected to be the most coagulated

species of phosphorus, regardless of the molar ratio.

However, at a 1:1 molar ratio, orthophosphate and other

species of phosphorus, such as polyphosphate, react with the

coagulants at the same rate. Thus, the phenomenon that was

observed at a low molar ratio of coagulants was unexpected.

This phenomenon could be explained by the reactivity

difference between orthophosphate and polyphosphate. At a low

molar ratio, the probability that phosphorus will react with

coagulants is lower. Polyphosphate has a higher molecular weight

and a larger particle size than orthophosphate (Corbridge, 1995;

Altundogan and Tument, 2001). Thus, polyphosphate will have a

higher probability of coagulating with coagulants than

orthophosphate, which has a low molecular weight and a small

particle size. Therefore, at a low molar ratio, the reactivity

difference between orthophosphate and polyphosphate does not

play an important role in the coagulation process. In contrast, the

reactivity of orthophosphate plays a significant role at high molar

ratios of coagulants.

35

4.1.4 The comparison between ab scale experiment and real

wastewater treatment plant process

Orthophosphate and total phosphorus concentrations were

analyzed in the input (I) and output (O) samples, which were

collected from the wastewater treatment plant before and after the

advanced phosphorus controlling process, respectively. The results

were compared with the L samples to clarify the chemical

mechanisms behind the coagulation process in the lab (L samples)

and at the real wastewater treatment plant (I and O samples).

The results in Table 4 showed that the orthophosphate

concentration decreased from 0.06 to 0.03 mg/L. Meanwhile, the

total phosphorus concentration decreased from 0.33 to 0.23 mg/L

after the advanced phosphorus controlling process.

Table 4 Orthophosphate and total phosphorus concentration in input

(I) and output (O) samples

(mg/L) I O

Orthophosphate 0.06 0.03

Total phosphorus 0.33 0.23

36

As shown in Figure 11, the proportion of orthophosphate to

total phosphorus decreased from 19.3% (I) to 14.6% (O) after the

coagulation process in the advanced wastewater treatment plant.

According to the lab-scale coagulation/precipitation test

with the L sample, the proportion of orthophosphate also decreased

after the wastewater was treated with coagulants (Figure 9-10).

Therefore, the tendency of the orthophosphate proportion to

decrease after the coagulation process in the real wastewater

treatment plant is similar to the decreasing tendency that was

observed in the lab-scale coagulation/precipitation test.

Figure 11 Proportion of orthophosphate in input (I) and output (O)

samples

0

10

20

30

40

50

60

70

80

90

100

I O

Pro

port

ion o

f ort

hophosphate

(%

)

Total-P

Ortho-P

37

Although the concentration of total phosphorus differed

between the L samples and the I and O samples, the phenomena

derived from the L samples could be regarded as an input sample

phenomena because the same treatment process was applied to

those samples. The different results obtained for those samples

were caused by differences in the season of acquisition between the

samples (Aigars, 2001), differences in the processes that were

used to treat the wastewater, and differences in the composition of

wastewater from the two wastewater treatment plants.

4.2 Adsorption/precipitation column test

The kinetics of phosphorus adsorption/precipitation with a

batch test was investigated to understand the removal mechanisms

of orthophosphate, polyphosphate and total phosphorus in batch

experiments before the adsorption/precipitation column test was

performed. These results demonstrate the adsorption/precipitation

mechanisms of orthophosphate, polyphosphate and total phosphorus

using slag in the column tests. This will be performed afterward.

4.2.1 Phosphorus adsorption/precipitation kinetics

The kinetics of orthophosphate and total phosphorus

38

removal using wastewater and slag are shown in Figure 12-14.

With a slag size 2-4 mm (Figure 12), the total phosphorus

concentration slightly eliminated after slag was contacted with

wastewater for 30 min. The total phosphorus concentration slightly

decreased and significantly decreased after 4 h of contact time with

slag. Equilibrium was reached after 36 h. The orthophosphate

concentration showed a similar decreasing trend with total

phosphorus. It was removed after 30 min. However, the remarkable

reduction of orthophosphate was observed after 8 h of the batch

experiment. Equilibrium was achieved after 36 h.


adsorption with slag size 2-4 mm

0

50

100

150

200

250

300

0.25 0.5 1 2 4 8 16 32 64

Concentr

ation (

mg/L

)

Time (hour)

2-4 mm

Ortho-P

Total-P

0

39

The removal of orthophosphate and total phosphorus using

slag size 1-2 mm (Figure 13) showed that total phosphorus was

removed from the wastewater after 30 min of contact time with the

slag. The decrease with 1-2 mm slag was lower than that with 2-4

mm slag after 2 h. The initial orthophosphate removal occurred

after 30 min of contact time with the slag. A noticeable point of

reduction occurred at 4 h. Both the orthophosphate and the total

phosphorus concentration reached equilibrium after 36 h.


adsorption with slag size 1-2 mm

0

50

100

150

200

250

300

0.25 0.5 1 2 4 8 16 32 64

Concentr

ation (

mg/L

)

Time (hour)

1-2 mm

Ortho-P

Total-P

0

40

The total phosphorus concentration decreased after 5 min;

wastewater with 0.5 - 1 mm slag (Figure 14) continuously

decreased. The dramatically removal rate was observed after 1 h of

the batch experiment. This was the most rapid rate of total

phosphorus removal among the different slag sizes studied here.

Moreover, equilibrium was reached after 24 h, which is much more

rapid than for 2-4 mm and 1-2 mm slag at 12 h. The

orthophosphate concentration was removed after 5 min of contact

time. The remarkable decrease occurred 1 h into the batch

experiment. Equilibrium was reached after 24 h.


adsorption with slag size 0.5-1 mm

0

50

100

150

200

250

300

0.25 0.5 1 2 4 8 16 32 64

Concentr

ation (

mg/L

)

Time (hour)

0.5-1 mm

Ortho-P

Total-P

0

41

According to the phosphorus kinetics of

adsorption/precipitation, the orthophosphate and total phosphorus

were not removed in the initial phases of the removal because the

sample was not agitated. However, the removal of orthophosphate

and total phosphorus occurred later. The total phosphorus

concentrations that were eliminated were dependent on the surface

area of the slag. Slag size of 0.5-1 mm showed the most rapid total

phosphorus removal followed by slag sizes of 1-2 mm and 2-4 mm,

respectively. This is because the finer slag size will have greater

surface area availability for the adsorption of phosphorus on the

surface of slag. Besides, the surface area availability also

contributes to the precipitation of calcium phosphate because a

greater amount of cations can be leached out from the surface of

the slag.

The orthophosphate concentrations are reduced, which

correlates to the total phosphorus concentration. However, the

orthophosphate removal with all slag sizes illustrate that decreasing

trends of orthophosphate were observed with decreasing total

phosphorus concentrations. This demonstrates that polyphosphate

was removed from the wastewater during the beginning of the

decline at a higher rate than orthophosphate. The orthophosphate

then participated in the removal mechanisms—more significantly

with reduced total phosphorus concentration. This is seen in the

noticeably decreased orthophosphate concentration in the later

42

phases of the batch experiment.

4.2.2 Basic characteristics of wastewater

The basic characteristics of the wastewater, such as pH and

conductivity, were measured before and after the

adsorption/precipitation column test. The samples before the

column test will be called the original samples. The column test was

performed separately 3 times for each slag size; therefore, the

basic characteristics of the wastewater samples were also

measured individually for each slag size.

The results presented in Table 5 indicate that all pH values

were in the neutral pH range (pH 7), with values of 6.26, 6.29 and

6.35, and the conductivity values were also in the normal range

(Rice et al., 2012), with values of 2, 1.963 and 1.999 mS/cm, in the

wastewater that was used for the column test with slag sizes 2-4

mm, 1-2 mm and 0.5-1 mm, respectively.

43

Table 5 Basic characteristics of wastewater samples used for

adsorption/precipitation column test

Basic

characteristics

Slag size 2-4

mm

Slag size 1-2

mm

Slag size 0.5-1

mm

pH 6.26 6.29 6.35

Conductivity 2 1.963 1.999

The adsorption/precipitation column test was performed by

feeding wastewater upward from the bottom of the column through

the column packed with slag. The wastewater samples were then

collected at the top of the column for basic characteristic

measurement.

The results from the adsorption/precipitation column test

with a slag size of 2-4 mm (Table 6) showed that the pH value

increased significantly from 6.26 in the original sample to 9.41 after

0 bed volumes. The pH then decreased to 7.28 after 8 bed volumes

and decreased to 6.97, 6.84, 6.82 and 6.76 after 16, 25, 37.5 and 50

bed volumes, respectively. The conductivity values decreased

slightly from 2 mS/cm in the original sample before the column test

to 1.787 mS/cm after the column test and remained constant near 2

mS/cm until after 50 bed volumes.

44


adsorption/precipitation column test with slag size 2-4 mm

When a slag size of 1-2 mm was used for the experiment,

the results from Table 7 indicated that after 0 bed volumes, the pH

increased from 6.29 in the original sample to 11.82, which is higher

than the value observed for a slag size of 2-4 mm. The pH then

continuously decreased from 9.69 after 8 bed volumes to 6.78 after

100 bed volumes. However, a slag size of 1-2 mm can prolong high

pH conditions in the wastewater in comparison to a slag size of 2-4

mm. The conductivity increased from 1.963 mS/cm in the original

sample to 2.759 mS/cm after 0 bed volumes and decreased to 1.742

and 1.938 mS/cm after 8 and 16 bed volumes, respectively. Then,

from 25-100 bed volumes, the conductivity was maintained at

approximately 2 mS/cm.

Bed Volume pH Conductivity

Original 6.26 2

0 9.41 1.787

8 7.28 2.075

16 6.97 2.076

25 6.84 2.074

37.5 6.82 2.059

50 6.76 2.062

45



The results of the adsorption/precipitation column test with

a slag size of 0.5-1 mm, which are presented in Table 8, showed

that the pH increased from 6.35 in the original sample to 12.05,

which was the highest pH obtained among all of the sizes of slag

that were used in the experiment. After 8 and 16 bed volumes, the

pH decreased to 9.99 and 8.88, respectively, followed by 7.57, 7.21,

7.13 7.07 and 7.01 after 25, 37.5, 50, 75 and 100 bed volumes,

respectively. Therefore, a slag size of 0.5-1 mm provides the

highest pH and the longest period of high pH conditions in

wastewater in comparison to slag sizes of 1-2 mm and 2-4 mm.


Original 6.29 1.963

0 11.82 2.759

8 9.69 1.742

16 8.51 1.938

25 7.37 2.073

37.5 7.05 2.07

50 6.93 2.069

75 6.81 2.06

100 6.78 2.059

46

The conductivity values increased from 1.999 mS/cm to 3.591

mS/cm after 0 bed volumes and decreased to 1.759 mS/cm after 8

volumes; the conductivity was then maintained at approximately 2

mS/cm until after 100 bed volumes.


adsorption/precipitation column test with slag size 0.5-1 mm

Overall, the results obtained for each slag size showed

similar tendencies with respect to both pH and conductivity. The pH

values increased from the neutral pH of the original wastewater

samples before the column test to a high pH after the wastewater


Original 6.35 1.999

0 12.05 3.561

8 9.99 1.759

16 8.88 1.976

25 7.57 2.135

37.5 7.21 2.147

50 7.13 2.161

75 7.07 2.184

100 7.01 2.17

47

was treated with slag. The observed pH increase could be due to

the main component of slag, which is lime (CaO). When slag comes

into contact with water, lime leaches out from the slag and tends to

form calcium hydroxide (Ca(OH)2), which increases the alkalinity of

the wastewater.

The other mechanism occurs due to the surface charge of

the slag surface. Positive charges on the slag surface, which are

calcium, magnesium and aluminum, tend to weaken the forces that

hold the proton (-H) to the oxygen in water molecules; therefore,

the hydroxyl (-OH) groups are relatively easy to release, causing

the pH to increase (Xue et al., 2009).

Moreover, the particle size inversely affects the pH.

According to the results, a slag size of 0.5-1 mm generates the

highest pH at 12.05, followed by slag sizes of 1-2 mm and 2-4 mm

at 11.82 and 9.41, respectively, after 0 bed volumes. A slag size of

0.5-1 mm is the finest slag in comparison to the other sizes used in

the adsorption/precipitation column test. Therefore, 0.5-1 mm slag

has the largest surface area and leaches out the most lime and

dissolved cations, which increase the pH of the solution. Then, the

pH of all wastewater samples from each column test has a tendency

to decrease significantly after 8 bed volumes with a slag size of 2-

4 mm and after 8 and 16 bed volumes with slag sizes of 0.5-1 mm

and 1-2 mm.

48

After that, the pH continuously decreased until after 50 bed

volumes with a slag size of 2-4 mm and until after 100 bed volumes

with slag sizes of 0.5-1 mm and 1-2 mm. The reduction trend

could be due to the deficiency of lime and cations to form the

calcium hydroxide which is the main factor of pH increment in the

wastewater sample. The circumstances of the lime and cations

shortage were determined to be the high rate of lime and cation

dissolution during the initial phases. The calcium phosphate

precipitation was consumed calcium ions from the wastewater

sample. In addition, the adsorption of phosphorus on the surface of

slag limits lime and cation leaching from slag in later phases.

The conductivity results obtained for every size of slag

indicate similar tendencies after the slag came into contact with the

wastewater. The conductivity value increased after 0 bed volumes

due to the dissolution of cations that are released from the slag

surface, such as calcium, magnesium and aluminum. From 8 bed

volumes until 50 bed volumes with a slag size of 2-4 mm and until

100 bed volumes with slag sizes of 1-2 mm and 0.5-1 mm, the

conductivity was maintained at a slightly higher level than the

conductivity of the wastewater samples before the column test due

to the remaining ions in the wastewater samples.

49

4.2.3 Phosphorus removal efficiency

The phosphorus removal efficiency using the different slag

sizes are showed in Figure 15. The phosphorus removal using slag

size 2-4 mm revealed a high phosphorus removal efficiency of 96.6%

after 0 bed volumes. However, from 8 bed volumes, the removal

efficiency dropped dramatically to 22.1% and decreased

continuously to a poor removal efficiency of 5.3% after 50 bed

volumes. With slag size 1-2 mm, the removal efficiency was also

high after 0 bed volumes at 96.9%. Unlike the 2-4 mm slag, slag

sizes of 1-2 mm maintained high removal efficiency till 16 bed

volumes as 96.9%, 97.7% and 82.5% in 0, 8 and 16 bed volumes,

respectively. The removal efficiency was decreased noticeably to

31.5% after 25 bed volumes and decreased continuously to 4.3%

after 100 bed volumes. However, a slag size of 1-2 mm can

prolong the time for phosphorus removal to 100 bed volumes, even

with a poor removal rate.

The similar tendency of the removal with the slag size 1-2

mm was observed with slag size 0.5-1 mm but with the highest

removal efficiency values of 98.1%, 97.8% and 91.0% were

observed after 0, 8 and 16 bed volumes, respectively. Then, the

removal efficiency decreased to 41.1% after 25 bed volumes and

decreased continuously decreasing to 6.2% after 100 bed volumes.

Overall, slag size 0.5-1 mm illustrated the highest

50

phosphorus removal efficiency follow be slag size 1-2 mm and 2-4

mm, respectively.

Figure 15 Efficiency of phosphorus removal after

adsorption/precipitation column test with slag size 0.5-1 mm, 1-2

mm and 2-4 mm

The mechanism of phosphorus removal could be the

adsorption of phosphorus because slag contains dissolved cations,

such as calcium, magnesium, and aluminum on the surface. These

cations have a strong ability to bind phosphate which is anions by

the ion exchange mechanism. Therefore, phosphorus was removed

from the wastewater by the chemisorption on the surface of the

slag.

0

20

40

60

80

100

0 25 50 75 100

Phosphoru

s r

em

oval eff

icie

ncy

(%

)

Bed volume

0.5-1 mm

1-2 mm

2-4 mm

51

Moreover, the other potential mechanism is the precipitation

of calcium phosphate because the above-mentioned cations can

form a solid precipitate of phosphorus, such as calcium phosphate

(Ca3(PO4)2) and hydroxyapatite (Ca5(OH)(PO4)3).

4.2.4 Orthophosphate and total phosphorus analysis

The total phosphorus concentration in the wastewater after

treatment with a slag size of 2-4 mm (Figure 16) decreased from

263 mg/L in the original wastewater samples to 9 mg/L after 0 bed

volumes. However, the total phosphorus concentration was

significantly increased to 205 mg/L after 8 bed volumes and

increased continuously till 50 bed volumes indicated the poor

removal efficiency.

52


phosphorus concentration after adsorption/precipitation column test

with slag size 2-4 mm

In the column test with a slag size of 1-2 mm (Figure 17),

the total phosphorus concentration after 0 bed volumes was low as

a reduction of total phosphorus concentration from 257 mg/L in

original sample to 8 mg/L after 0 bed volumes. The concentration

was then slightly increased to 45 mg/L after 16 bed volumes. The

remarkable increasing of the concentration was pointed after 25 bed

volumes. The increasing of total phosphorus concentration was

observed till after 100 bed volumes.

0

1

2

3

4

5

6

7

8

9

10

0

50

100

150

200

250

300

0 25 50 75 100

pH

& C

onductivit

y (

mS/c

m)

Tota

l phosphoru

s c

oncentr

ation

(m

g/L

)

Bed volume

2-4 mm

Total-P

pH

Cond

53



with slag size 1-2 mm

Wastewater samples treated with a slag size of 0.5-1 mm

(Figure 18) showed tendencies similar to those observed for a slag

size of 1-2 mm. The total phosphorus concentration was nominal

from after 0-16 bed volumes at 5, 6 and 24 mg/L, respectively.

After 25 bed volumes, the concentration was increased significantly

to 155 mg/L and continuously increasing till after 100 bed volume at

251 mg/L.

0

2

4

6

8

10

12

14

0

50

100

150

200

250

300

0 25 50 75 100

pH

& C

onductivit

y (

mS/c

m)

Tota

l phosphoru

s c

oncentr

ation

(m

g/L

)

Bed volume

1-2 mm

Total-P

pH

Cond

54



with slag size 0.5-1 mm

The orthophosphate and total phosphorus concentrations of

the wastewater were measured after the adsorption/precipitation

column test using different sizes of slag, and the proportion of

orthophosphate to total phosphorus was calculated to determine the

chemical behaviors of orthophosphate and total phosphorus.

The orthophosphate concentration exhibited tendencies

similar to those observed for total phosphorus (Figure 19-21);

orthophosphate was removed significantly after 0 bed volumes with

a slag size of 2-4 mm and after 0, 8 and 16 bed volumes with slag

sizes of 1-2 mm and 0.5-1 mm. Then, the removal decreased

0

2

4

6

8

10

12

14

0

50

100

150

200

250

300

0 25 50 75 100

pH

& C

onductivit

y (

mS/c

m)

Tota

l phosphoru

s c

oncentr

ation

(m

g/L

)

Bed volume

0.5-1 mm

Total-P

pH

Cond

55

remarkably from 8 bed volumes for a slag size of 2-4 mm and 25

bed volumes for slag sizes of 1-2 mm and 0.5-1 mm, followed by

continuously decreasing until the end of the column test.



0

50

100

150

200

250

300

0 25 50 75 100

Concentr

ation (

mg/L

)

Bed volume

2-4 mm

Total-P

Ortho-P

Ori Total-P

Ori Ortho-P

56




adsorption/precipitation column test with slag size 0.5-1 mm

0

50

100

150

200

250

300

0 25 50 75 100

Concentr

ation (

mg/L

)

Bed volume

1-2 mm

Total-P

Ortho-P

Ori Total-P

Ori Ortho-P

0

50

100

150

200

250

300

0 25 50 75 100

Concentr

ation (

mg/L

)

Bed volume

0.5-1 mm

Total-P

Ortho-P

Ori Total-P

Ori Ortho-P

57

According to the results obtained from the

adsorption/precipitation column test, the removal rates of

orthophosphate and total phosphorus are strongly dependent on the

size of the slag that correlates to the results obtained from kinetic

adsorption/precipitation batch test. The tendency of adsorption and

precipitation was higher with a finer slag size than with a larger slag

size because the slag with a finer size has a greater surface area for

phosphorus adsorption and for leaching cations to form calcium

phosphate. Therefore, the phosphorus removal efficiency was

highest with a slag size of 0.5-1 mm, followed by sizes of 1-2 mm

and 2-4 mm.

Furthermore, phosphorus tends to be adsorbed onto the slag

surface during the initial phases of the column test due to the

availability on the slag surface. Moreover, precipitation also tends

to occur during the initial phases. During the initial phases, cations

can be leached out from the slag surface at a high rate because the

slag still has availability on the surface. Thus, the adsorption and

precipitation rate was high after 0 bed volumes for a slag size of 2-

4 mm and after 0, 8 and 16 bed volumes for slag sizes of 1-2 mm

and 0.5-1 mm. On the other hand, the removal efficiency decreases

significantly when the surface of the slag is limited by the

adsorption of phosphorus on the surface of the slag due to the

exhaustion of the slag and the restricted cation dissolution from 8

bed volumes with a slag size of 2-4 mm and 25 bed volumes with

58

slag sizes of 1-2 mm and 0.5-1 mm.

The proportion of orthophosphate to total phosphorus in

each size of slag was evaluated, indicating that the proportion is

high in comparison to the total phosphorus concentration during the

initial phases of the column test. In other words, orthophosphate

remained in the solution more than polyphosphate. The proportion

of orthophosphate in the initial phases is different from the

proportion of orthophosphate in the original samples before the

column test. This phenomenon was unexpected due to the binding

ability of orthophosphate, which is the most reactive species of

phosphorus. Therefore, orthophosphate was expected to be

adsorbed onto the surface of slag or precipitated during the initial

phases before other species of phosphorus.

This phenomenon could be explained by the different

characteristics of orthophosphate and polyphosphate.

Polyphosphate has a higher molecular weight and a larger particle

size than orthophosphate (Corbridge, 1995; Altundogan and Tument,

2001); thus, polyphosphate tends to have a greater chance than

orthophosphate of adsorbing onto the surface of the slag during the

initial phases, regardless of the reactivity of orthophosphate. For

this reason, the proportion of orthophosphate is higher during the

initial phases than in the original samples or during the later phases.

However, in the system that utilizes slag, a certain amount

of removal occurred by precipitation as well as by adsorption (for

59

instance, 70% precipitation and 30% adsorption) (Lu et al., 2008).

During the initial phases, the conditions of the wastewater were

appropriate for precipitation in terms of a high concentration of

cations and a high pH, such that both orthophosphate and

polyphosphate were removed completely by precipitation,

regardless of reactivity or differences in characteristics.

During later phases, the proportion of orthophosphate

decreased, which was similar to the proportion of orthophosphate in

the original samples. This result indicated that orthophosphate was

adsorbed onto the surface of slag at these times more than was

observed during the initial phases of the column test. Because the

surface of the slag became packed by phosphorus and various

substances in the wastewater, in later phases, a larger particle size

no longer plays a significant role. Nevertheless, polyphosphate still

adsorbs onto the surface of the slag during later phases, leading to

a proportion of orthophosphate that is similar to that of the original

samples before the column test.

The assumptions derived from the batch test support the

results obtained from the column test. Both experiments

demonstrated that polyphosphate tends to be removed from the

wastewater before orthophosphate during the initial phases. During

the later phases, more orthophosphate was removed than the initial

phases.

60

5. Conclusions

The chemical behaviors of orthophosphate and total

phosphorus associated with coagulation/precipitation and

adsorption/precipitation using slag were determined and established

in this study.

i. Coagulation/precipitation

Based on the coagulation/precipitation test with different

molar ratios of coagulants (1:1, 2:1 and 3:1 (Al: P or Fe: P)), at a

low molar ratio of coagulant (1:1), reactivity differences between

orthophosphate and polyphosphate do not play a significant role in

the chemical reaction. Polyphosphate has a higher molecular weight

and a larger particle size than orthophosphate; thus, polyphosphate

has the opportunity to participate in the chemical reaction and

coagulate more than orthophosphate at a low molar ratio. On the

other hand, orthophosphate, which is the most reactive species of

phosphorus, was dominant in chemical reactions at high molar ratios

(2:1 and 3:1). Therefore, the reactivity differences between

orthophosphate and polyphosphate play a significant role at high

molar ratios.

61

ii. Adsorption/precipitation

The major mechanisms of adsorption/precipitation column

test with various sizes of slag are adsorption and precipitation. The

size of the slag mainly affects phosphorus removal. Phosphorus

tends to adsorb onto the surface of fine slag particles more than

larger slag particles due to surface area availability. Moreover, with

fine slag particles, cations are more likely to leach out from the

surface of the slag and increase the pH of wastewater than with

larger slag particles, leading to appropriate conditions for

phosphorus precipitation.

Polyphosphate adsorption was dominant during the initial

phases of the column test because polyphosphate has a larger

particle size than orthophosphate. Thus, polyphosphate was

adsorbed onto the surface of the slag during the initial phases more

than orthophosphate, which has a smaller particle size. During later

phases, surface of the slag became packed by phosphorus and

various substances in the wastewater. Hence, larger particles size

of polyphosphate no longer plays a significant role leading to the

adsorption of orthophosphate more than in initial phases. However,

polyphosphate was still adsorbing during later phases of adsorption.

62

In summary, the reactivity differences between

orthophosphate and polyphosphate play a significant role only

during coagulation/precipitation with a high molar ratio of coagulant.

On the other hand, differences in size and molecular weight between

orthophosphate and polyphosphate play a significant role at a low

molar ratio of coagulant during both coagulation/precipitation and

adsorption/precipitation.

63

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69

르토 인과 인 학 거동 규명하 해 랩 스 일

하수 내 인 거 실험 진행하 다. , 하수처리장 내 2차 처리공

이후 류수를 상 집/침 실험 수행하 다. Alum과

염 철이 1:1, 2:1, 3:1 ( 집 : 인) 집 사용 었고,

인 학 거동 인하 해 르토 인과 인 분 하 다.

실험에 인 거 집 농도 인 종류에 른

차이에 향 는 것 인 었다. 집 농도가 낮

조건에 는 (1:1) 르토 인과 폴리 인 간 차이가 큰

역할 하지 못했다. 폴리 인 르토 인보다 입자 분자량이 크

에 폴리 인이 상 집 학 에 참여할 수 있는

률이 높 것 추 다. 면, 고농도 집 를 여한

조건에 는, 차이가 인 거 에 뚜 한 향 주었다. 가장

이 좋 인 종류인 르토 인 고농도 집 를 여한

조건에 우 한 거 보 다. (2:1, 3:1)

착/침 컬럼 실험 다양한 입자 크 (0.5-1 mm, 1-2 mm,

2-4 mm) 분류 슬래그를 착 사용하여 수행 었다. 하수는

컬럼 하부에 주입 어 상부 동 었고 컬럼에 처리 샘플

르토 인과 인 분 해 100 bed volume 지 컬럼 상부에

채수 었다.

컬럼 실험 에 인 르게 거 었고, 0.5에 1mm

크 슬래그가 가장 좋 인 거 보 다. 상 큰 입자

70

크 슬래그는 작 입자 크 슬래그보다 착과 침 이 게

생하는 것 찰 었다. 또한, 컬럼 실험 단계에 , 고농도

양이 과 높 pH를 갖는 조건 침 에 한 조건

조 하 고, 르토 인과 인 높 거 거 었지만 인

르토 인 이 에 높게 분 었다. 이는 큰 크 폴리

인이 우 하게 착 어 거 미한다. 그러므 르토

인보다는 폴리 인이 에 슬래그 면에 착 것 추 할 수

있다. 컬럼 실험 진행할수 르토 인 이 어 들었고, 이는

르토 인이 착 에 보다 많이 참여함 미한다. 라

후 에는 르토 인과 폴리 인이 슷한 슬래그 면에 착 는

것 보인다.

주요어 : 인 거, 르토 인, 집, 착, 침 , 슬래그

학 번 : 2013-23864

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