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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
Table 6 Basic characteristics of wastewater samples after
adsorption/ precipitation column test with slag size
2-4 mm...................................................................................44
Table 7 Basic characteristics of wastewater samples after
adsorption/ precipitation column test with slag size
1-2 mm...................................................................................45
Table 8 Basic characteristics of wastewater samples after
adsorption/ precipitation column test with slag size
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
Figure 8 Orthophosphate and total phosphorus concentration after
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
Figure 13 Kinetics of orthophosphate and total phosphorus
adsorption with slag size 1-2 mm.....................................39
Figure 14 Kinetics of orthophosphate and total phosphorus
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
Figure 17 Basic characteristics of wastewater samples and
phosphorus concentration after adsorption/precipitation
column test with slag size 1-2 mm....................................53
viii
Figure 18 Basic characteristics of wastewater samples and
phosphorus concentration after adsorption/precipitation
column test with slag size 0.5-1 mm................................54
Figure 19 Orthophosphate and total phosphorus concentration after
adsorption/precipitation column test with slag size
2-4 mm................................................................................55
Figure 20 Orthophosphate and total phosphorus concentration after
adsorption/precipitation column test with slag size
1-2 mm................................................................................56
Figure 21 Orthophosphate and total phosphorus concentration after
adsorption/precipitation column test with slag size
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.
Table 2 Basic characteristics of wastewater samples after
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.
Figure 7 Orthophosphate and total phosphorus concentration after
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
Figure 8 Orthophosphate and total phosphorus concentration after
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.
Figure 12 Kinetics of orthophosphate and total phosphorus
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.
Figure 13 Kinetics of orthophosphate and total phosphorus
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.
Figure 14 Kinetics of orthophosphate and total phosphorus
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
Table 6 Basic characteristics of wastewater samples after
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
Table 7 Basic characteristics of wastewater samples after
adsorption/precipitation column test with slag size 1-2 mm
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.
Bed Volume pH Conductivity
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.
Table 8 Basic characteristics of wastewater samples after
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
Bed Volume pH Conductivity
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
Figure 16 Basic characteristics of wastewater samples and
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
Figure 17 Basic characteristics of wastewater samples and
phosphorus concentration after adsorption/precipitation column test
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
Figure 18 Basic characteristics of wastewater samples and
phosphorus concentration after adsorption/precipitation column test
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.
Figure 19 Orthophosphate and total phosphorus concentration after
adsorption/precipitation column test with slag size 2-4 mm
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
Figure 20 Orthophosphate and total phosphorus concentration after
adsorption/precipitation column test with slag size 1-2 mm
Figure 21 Orthophosphate and total phosphorus concentration after
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