Study on phosphorus recovery from digested piggery wastewater by struvite precipitation Nghiên cứu xử lý thu hồi phốt pho trong nước thải chăn nuôi bằng phương

HANOI UNIVERSITY OF SCIENCE TECHNISCHE UNIVERSITÄT DRESDEN


Master Thesis

STUDY ON PHOSPHORUS RECOVERY FROM
DIGESTED PIGGERY WASTEWATER BY
STRUVITE PRECIPITATION



Vu Phuong Thuy


Supervisor: Professor. Dr. Cao The Ha


Hanoi, November 2011
ACKNOWLEDGEMENTS
I would like to thank Professor Doctor Cao The Ha, my supervisor, and greatly
appreciate his supervision, advice, and guidance from the early stage of this research as
well as giving me experiences throughout the work. During the time I was doing the
research, he also gave the great opportunity, the best support and conditions in many
ways for me to go to National Institute for Environment Studies, Japan to participate in
the training course “Sustainable Landfill Management”.
Many thanks to all the staffs in the Center for Environmental Technology and
Sustainable Development and the Faculty of Chemistry of Hanoi University of Science
for helping me with analytical techniques.

I would like to thank to Dr. Kazuyoshi Suzuki, Dr. Anton Perera, Prof. Phil Westerman
for your helps with answering my questions.
Finally, I would like to thank to my family, my friends who are always supportive and
encourage me during difficult times, especially my mom who helped me to take care of
the experiment that was performed in my house.
ABSTRACT
This research studied on struvite precipitation as a method to remove and recovery
phosphorus in the real digested piggery wastewater to form a product as fertilizer.
There are many factors that affect to the precipitation of struvite such as pH, ion molar
ratios, aeration, mixing energy, temperature… In this study, series of jar-test
experiments were conducted to investigate the influence of pH, Mg:P molar ratio, Ca
ions and mixing speed to the phosphorus removal efficiency and the crystallization of
struvite. Experiments with lab-scale reactor in batch and continuous mode were also
conducted to investigate the kinetic of phosphorus removal, the controlled struvite
crystallization and the quality of the generated product. The results showed that, a) At
pH at 9.25, the phosphorus removal efficiency was the highest. b) Increasing Mg:P
molar ratio (from 1 to 2.5) and stirring speed (from 50 rpm to 100 rpm) led to enhanced
phosphorus removal efficiency. c) Ca ions play important role in both phosphorus
removal efficiency and crystallization of struvite. A presence of Ca ions at Mg:Ca of
4:1 to 2:1 will increase the phosphorus removal efficiency and affect the morphology
of struvite crystals compared to Mg:Ca of 1:1, 2:1 or 1:0. d) The rate constant of
phosphorus removal in the pilot-scale experiment with digested piggery wastewater
when Mg ions added estimated as 0.050 min
-1
or 3.00 hr
-1
. e) It is possible to attach and
grow struvite crystals on stainless steel mesh in order to recover it as fertilizer.
KEYWORDS
Struvite, stirring speed, digested piggery wastewater, phosphorus removal,

crystallization.
1

TABLE OF CONTENTS
Page
List of figures 3
List of tables 4
INTRODUCTION 5
1 REVIEW OF LITERATURE 7
1.1 Introduction to phosphorus and its applications 7
1.2 Impacts of excessive phosphorus in water streams 8
1.3 Recovery of phosphorus 8
1.4 Phosphorus removal and recovery techniques 9
1.5 Struvite precipitation 13
1.5.1 Background 13
1.5.2 Struvite chemistry in wastewater 14
2 GOAL AND OBJECTIVES 17
2.1 Goal 17
2.2 Objectives 17
3 MATERIALS AND METHODS 19
3.1 Wastewater collection and analysis 19
3.2 Jar-test experiment procedure 20
3.2.1 Impact of pH 20
3.1.2 Impact of magnesium addition 20
3.1.3 Impact of Calcium ions 21
2

3.2.4 Impact of stirring speed 22
3.3 Bench-scale experiments 22
3.3.1 Reactor design and batch mode experiment 22

3.3.2 Accumulation device design and continuous mode experiment 25
3.4 Analytical and assessment methods 28
3.4.1 Analytical techniques 28
3.4.2 Kinetic model design for phosphorus removal 28
4 RESULTS AND DISCUSSIONS 30
4.1 Characteristics of the digested piggery wastewater 30
4.2 Jar-test experiments - Impact of factors 30
4.2.1 pH 30
4.2.2 Magnesium addition 32
4.2.3 Calcium ions… 34
4.2.4 Stirring speed 41
4.3 Bench-scale experiments 43
4.3.1 Batch mode experiment 43
4.3.2 Continuous mode experiment 46
5 CONCLUSIONS AND RECOMMENDATIONS 53
5.1 Conclusions… 53
5.2 Recommendations 54
REFERENCES 55

3

LIST OF FIGURES
Figure
3-1 Schematic diagram of batch mode experiment 23
3-2 Photo of batch mode experiment 24
3-3 Schematic diagram of continuous mode experiment 26
4-1 Effect of pH to %P-Removal 32
4-2 Effect of magnesium addition to %P-removal 33
4-3 Effect of Mg:Ca ratio to %P-removal 36
4-4 (a), (b), (c) Photos of struvite crystals by light microscope 39

4-4 (d), (e), (f). Photos of struvite crystals by light microscope 40
4-5 Residual P-PO
4
concentration at different stirring speed versus time 42
4-6 Residual P-PO
4
concentration and pH value versus time 44
4-7 Linear of kinetic model of phosphate removal 45
4-8 (a), (b), (c) Photos of struvite crystal growth on stainless steel mesh 48
4-9 Photos of struvite crystals on inner and outer mesh 49
4-10 (a), (b) Photos of struvite crystals taken out of the mesh 50
4-11 X-RAY diffraction of recovered struvite crystals 52
4

LIST OF TABLES
Table
1-1 Summary of phosphorus removal technologies, Morse et al. (1998) 10
1-2 Summary of phosphorus recovery technologies, Morse et al. (1998) 11
4-1 Characteristic of the digested piggery wastewater (19/07/2011) 30
4-2 Effect of pH to %P-Removal 31
4-3 Effect of magnesium addition to %P-removal 33
4-4 Solid species examined and selected as primary precipitation 34
4-5 Effect of Mg:Ca ratio to %P-removal 35
4-6 Residual P-PO
4
concentration at different stirring speed versus time 42
4-7 Residual P-PO
4
concentration and pH value versus time 43
4-8 Characteristic of wastewater before and after the experiment 45

5

INTRODUCTION
Livestock industry is a very important sector of Vietnam which has the strong growth
especially in pork production in recent years. However, this can lead to many serious
environmental issues as well. According to Department of Livestock Husbandry,
Ministry of Agriculture and Rural Development, until October 2009, the number of
pigs in over Vietnam was 27.6 mil and predicted to be 36.9 mil by 2015. In 2006,
there were 18,000 castle farms in Vietnam in which 6,000 were piggery farms. Each
year, 20-30 mil m
3
liquid waste is discharged from pig farms and 80% of that is
discharged directly into the environment without any pretreatment and caused many
pollutions.
Livestock wastewater contains high content of nutrients. In Vietnam, the most common
treatment method for livestock wastewater is biogas tank and lagoon. Biogas tanks can
treat COD and lagoons can treat N, P but with long retention time. The wastewater
quality after treatment by these methods is not good enough for discharging into the
environment yet. Especially, there are lack of management of phosphorus and
ammonium which are elements that can cause pollutions of surface water bodies by
extra eutrophication. Besides, phosphorus is an irreplaceable element essential for
every living thing on earth which global reserve is going to run out in 60-100 years.
Removal of phosphorous by precipitation of struvite has been studied and used
somewhere in Europe and Asia for both animal manures and municipal wastewater.
Struvite is a mineral formed from three specific components: magnesium, ammonium
and phosphate. Struvite (magnesium ammonium phosphate) precipitates as a compact
crystal. Therefore, a small amount of easily settleable solids is generated in a struvite
precipitation process. Unlike other chemical phosphorus precipitation methods (e.g.
using aluminum or ion salt etc.) where only phosphorus alone is removed, in struvite
6

precipitation both phosphorus and ammonium are removed, this this important in the
case of piggery wastewater containing large amount of both phosphorus and
ammonium. In addition, struvite is easily transported and can be used as a slow-release
fertilizer. (NYSERDA, 2006)
This report presents the study on struvite precipitation from digested piggery
wastewater. The study includes discussion of the relevant background information and
a literature review of struvite (Section 1), Goals and objectives of the study (Section 2);
Material and methods with series of jar-test experiments and lab-scale reactor
experiments (Section 3); Results and discussions (Section 4); and Conclusions and
recommendation (Section 5).
7

1 REVIEW OF LITERATURE
1.1 Introduction to phosphorus and its applications
Together with Nitrogen and Potassium, Phosphorus is an important element for living
organisms.
The vast majority of phosphorus sources are consumed as fertilizers in agriculture in
over the world. Phosphorus is necessary for the promotion of plant growth and hence,
improves crop yield, seed formation and quality of fruit. To balance the nutrients in
soils, phosphorus can be added periodically under the form of inorganic fertilizers or
manures.
Phosphorus is also one of the vital elements needed for livestock as well. It is
consumed through the diet of animals. Phosphorus has the function in preventing
health problems, improve bone strength and muscle production in animal bodies.
Besides, phosphorus is also used in other applications such as detergent production,
food processing, chemical production,…
However, phosphorus can be lost from the source and discharged into the environment
by different ways naturally or due to the lack of proper management. For example, it
can be lost from soils by some ways such as crop uptake and removal, runoff and

erosion, or leaching. Or it can also be lost from animal feeding source through the
waste streams because not 100% of intake phosphorus will remain in animal bodies.
According to ICM, 2000 ( />2000/pbasics.html), 14% of phosphorus in corn and 31% of phosphorus in soybean can
be digested by swine, hence, a large percentage of intake phosphorus is excreted into
8

waste streams. Due to the mismanagement of this mineral element, there are many
issues related to environmental problems, especially water environment.
1.2 Impacts of excessive phosphorus in water bodies
Phosphorus is not toxic, but it can affect the biological activity in clean water bodies.
Once phosphorus level in surface water exceeds the critical value for aquatic plant
growth, it can cause eutrophication to happen and degrade the water quality. Advanced
eutrophication can also lower dissolved oxygen and increasing the BOD (biological
oxygen demand) and hence, reduce the aquatic wildlife populations and species
diversity in the water body.
The USEPA (United States Environmental Protection Agency) recommends that total
phosphorus should not exceed 0.1 mg/L in streams and that should not exceed 0.05
mg/L in streams where they enter a lake or reservoir.
1.3 Recovery of phosphorus
Phosphorus in phosphate rocks is an irreplaceable resource. In order to meet the
increasing demand of fertilizers for agriculture and other purposes, phosphate resources
are mined with an increasing rate.
In order to contribute to the security of phosphate resource on earth as well as to
prevent the potential environmental issues, phosphate should be removed and
recovered from waste streams before discharged into the surface water. This includes
extracting phosphates from the sources in the forms that can be used in industry as a
renewable resource or as fertilizers in agriculture.
9

Numerous of studies and applications of phosphorus recovery have been conducted

with effort of recovery this resource from waste streams of which precipitation is the
most common concerned method. Phosphorus can be extracted from the waste streams
(i.e., sewage treatment plant, livestock waste…) in the forms of precipitated
compounds such as calcium phosphate, magnesium phosphate, magnesium ammonium
phosphate or struvite.
Recovery of phosphorus in waste streams will help to minimize environmental
damages, contribute to balance phosphate rock resource on earth and offer economic
returns of the recovered products.
Phosphorus removal and recovery has been the concerned topic all over the world for
several decades. Many methods and technologies have been researched and introduced
until now, however its applications in reality are still limited. In the world, the
industrial use of recovered phosphorus from waste streams has been successfully
implied in several plants in developed countries (Japan, Netherland).
Phosphorus removal and recovery techniques still continue to be an attractive subject
all over the world for national and international authorities, scientific institutions,
industry and other interested parties… In the next part of this report, an overview of
existing techniques on phosphorus removal and recovery will be described in details.
1.4 Phosphorus removal and recovery techniques
According to Morse et al. 1998, a wide range of technologies for phosphorus removal
and recovery were developed including chemical precipitation, biological phosphorus
removal, crystallization, novel chemical precipitation approaches and a number of
wastewater and sludge-based methods.
10

Morse et al, 1998 reported treatment technologies for phosphorus in wastewater
streams into two main groups which are removal technologies and recovery
technologies, as shown in summary in Table 1-1 and Table 1-2 below.
Table 1-1. Summary of phosphorus removal technologies, Morse et al. (1998)

11

Table 1-2. Summary of phosphorus recovery technologies, Morse et al. (1998)

Chemical precipitation, biological phosphorus removal, crystallization, advanced
chemical precipitation, and ion exchange technologies are the most common for
phosphorus removal and recovery from wastewater. Chemical methods are flexible
and can be applied in any stage of wastewater treatment process because it can extract
phosphorus from wastewater and sludge in form of metal salts (Morse et al., 1998).
Meanwhile in biological methods, phosphorus can be taken up from wastewater by
activated sludge and without any chemical addition (Morse et at., 1998).
Crystallization for phosphorus removal produces a marketable end-product in the form
of calcium phosphate, and the crystallization occurs by adding either caustic soda or
milk of lime (Morse et al., 1998). Advanced chemical precipitation is referred to as
12

HYPRO and occurs by the crystallization of P, organic matter and hydrolysis providing
carbon and energy in an available form (Morse et al., 1998). Morse et al. (1998) report
the ion exchange technology produces struvite. During the ion-exchange precipitation
process, P and NH
3
ions produce struvite when removed from the wastewater (Morse
et al., 1998).
Other less common technologies for phosphorus removal include magnetic technology,
adsorbents, and tertiary filtration technologies. For magnetic technology, calcium
phosphate is precipitated in the form of magnetite by the use of lime and separated
using a magnetic field (Morse et al., 1998). Adsorbents also have the ability to remove
P from wastewater without additional sludge being produced (Morse et al., 1998).
Tertiary filtration for phosphorus removal is incidental leaving the recovered sludge
unsuitable for recycling (Morse et al., 1998).
Recovery of phosphorus means to precipitate or crystallize it from wastewater in some

form of products that can be used for other purposes. Calcium phosphate and struvite
are most common forms of recovered phosphorus from wastewater. Compared to
calcium phosphate and other recovered phosphorus, struvite has many benefits. One of
the main advantages of struvite is that it can be utilized as slow released fertilizer in
agriculture. Numerous of researches and applications of this method have been done all
over the world, especially in some developed countries. However, in order to introduce
this technology into real application, it requires the study on many aspects such as
process kinetic and control, reactor design, economic… The next part of the thesis will
introduce and discuss in more details on this method.
13

1.5 Struvite precipitation
1.5.1 Background
Struvite or magnesium ammonium phosphate hexahydrate (MAP) MgNH
4
PO
4
.6H
2
O
was found in 1939 because of the deposition in pipes in a wastewater treatment plant
and it became well known since then. In wastewater treatment plant, it is mostly found
in some special places such as areas of high turbulence (fouling pumps, aerators,
screens,…) (Ohlinger et al., 1998). When it deposits, it can block the pipes that makes
an increasing cost for pumping, maintenance or replacement of equipments. Therefore,
it is necessary to remove phosphorus in wastewater to prevent such problems in
treatment plants Much of the literature and concern with struvite have been in how to
avoid struvite formation in wastewater treatment plants. As the concern developed
further in nutrient management (nitrogen and phosphorus) wastewater, researches and
practical application in controlled struvite formation has increased over the world,

especially in some developed countries. Holland, Australia, Japan are among the first
countries that have concerned struvite formation from waste streams, and hence,
numerous of researches and applications have been done on this field. In Holland and
Japan, there are proprietary struvite recovery from domestic wastewater and industrial
wastewater. Also in Holland, there is non-proprietary struvite recovery process in
treating veal manure at a full-scale. (NYSERDA, 2006)
Struvite recovered from real wastewater has the advantages as following: (1)
application of struvite to the plants in ratios beneficial to plant growth and it is a
product of commercial value as it may be used as a fertilizer and soil conditioner
(Bishop, 2006); (2) Reducing the phosphorus and nitrogen load of side stream and
sludge liquors recirculated to the head of wastewater treatment works (Etter, et al.,
2010), (Quintana, et al., 2005); (3) Struvite precipitation as the pretreatment was
14

proven to be satisfactory in enhancing biological performance in activated sludge
system in aspects of the removals of organic matter, nitrogen and phosphorus, in
addition, the proposed process was found to be advantageous in treating swine
wastewater (Hong-Duck, et al., 2010); (4) The recovery of phosphorus as struvite was
reported to reduce sludge volumes under specific conditions by up to 49% when
compared to chemical phosphorus removal processes (Quintana, et al., 2005); (5)
Struvite precipitation processes reduced the heavy metal content (As, Cr, Ni, Fe, Zn,
Hg, Zn, Cr and Ni), the high reduction efficiencies of these heavy metals indicated
their precipitation together with struvite (Uysal, et al., 2010).
1.5.2 Struvite chemistry in wastewater
Composition of struvite includes nitrogen (N), phosphorus (P) and magnesium (Mg).
Struvite usually precipitates as a stable white orthorhombic crystals in a (1:1:1) molar
ratio (El Diwani, et al., 2006):
OHPOMgNHOHPONHMg
2442
3

44
2
6.6 


Formation of struvite can occur when certain conditions are met. At elevated pH
condition and when the concentrations of magnesium, ammonium, phosphate ions
exceed the solubility product for struvite, K
sp
, struvite precipitation can occur.
K
sp
=[Mg
2+
][NH
4+
][PO
4
3-
] pK
sp
=13.26 (Ohlinger, 1998)
There have been many studies focus on kinetics of struvite formation in order to
determine the factors affect to the process including pH, molar ratios, stirring speed,
temperature, impurities, induction time, etc while other researches focused on different
controlled process such as aeration, chemical addition, seeding, or reactor design in
order to make best conditions for struvite precipitation and recovery.
15

There are many different ions in wastewater which can influence to the kinetic of

struvite formation. Phosphate in wastewater can be in the forms of PO
4
3-
, HPO
4
2-
,
H
2
PO
4
-
, or MgHPO
4
-
, ammonium can be in forms of NH
3
, NH
4
+
, magnesium can be
MgOH
+
, Mg
2+
, etc. Besides, there are Ca
2+
and CO
3
2-

, HCO
3
-
… as well. If pH in
wastewater changes, it can influence to the concentration of the mentioned ions.
Previous studies showed that optimal pH for struvite formation is around 9 or 9.5. Burn
et al. (2003) studied the influence of Mg:P molar ratio to the process and found that
with Mg:P of 1.6:1, phosphate removal efficiency is 91% at pH 9, meanwhile Beal et
al. (1999) showed the rate of 88% at Mg:P of 2:1.
The struvite crystallization process also has been investigated. Researches focused on
factors such as ions in solution, molar ratio, suspended solids, reactor design, which
affect to the nucleation and growth of crystals. Impact of Ca ions or molar ratio Mg:Ca
on struvite crystallization has been investigated and reported in several reports. It has
been shown that the presence of Ca ions in solution has a significant impact on struvite
crystallisation in terms of size, shape, and purity of the product recovered (Kristell et
al., 2004).
Several different types of struvite precipitation reactors have been studied and designed
for removing and recovering phosphorus in wastewater including sophisticated ones to
produce high quality struvite (in Holland and Japan) and simple reactors for industrial
wastewater and animal waste industry (NYSERDA, 2006). In application for treatment
of livestock waste, there are several types of reactors of which most common are
fluidized-bed reactor, air agitated column, stirred reactor. The most known works that
have been reported are Battistoni (1998), Ueno and Fujii (2001), Münch et al. (2001),
Kumashiro et al.(2001), Piekema and Giesen (2001), Mitani et al. (2001), Ohlinger et
al. (1999), Suzuki et al. (2005).
16

Fluidized bed can use a seed material that allows the struvite to form as a pellet within
the reactor and to be removed periodically. Magnesium salt is added just upstream of
the reactor or directly into the reactor. The influent flow can be introduced into either

the top or bottom of the reactor. (NYSERDA, 2006) Mixing is provided by sparging
air into the base of the reactor or using the influent flow to fluidize the bed. The
heavier, larger pellets move to the bottom of the reactor where they are removed
periodically. The operating conditions of the fluidized bed can be set to remove
crystals of a uniform size. Pellets removed from the reactor freely drain to a low
moisture content. (NYSERDA, 2006)
The simpler systems do not use a seed material and result in much variability in the
precipitate. The magnesium salt is typically added at the beginning of a rectangular
reactor with a rapid mixing section (using either a nozzle or air) followed by a
quiescent section to allow the material to settle out in the downstream end of the tank.
Thus the same tank is used as a reactor and as a rectangular clarifier. Crystals and
precipitations are then taken out from the bottom of reactors. (NYSERDA, 2006)
Another significant research on reactor design is from Suzuki et al (2005). The reactor
was designed with dual function crystallization through aeration, and separation of
formed struvite by accumulation on a stainless steel mesh which was thought to be
simple and easy to construct and handle reactor. easy to construct and handle. The
recovered struvite needed only air drying, but no dehydration or composting before use
since it was approximately 95% pure even without washing and was ready for
immediate application to farmland. (Suzuki et al., 2005).
17

2 GOAL AND OBJECTIVES
2.1 Goal
The overall goal of this study is to evaluate the phosphate removal efficiency by
struvite precipitation and to evaluate the recovered struvite from digested piggery
wastewater.
2.2 Objectives
To achieve this goal, specific objectives were developed as following:
1 Analyses of physical and chemical characteristics of the digested piggery
wastewater (i.e. the wastewater after biogas tanks). The analyses include pH,

COD, TSS, TP, P-PO
4
, N-NH
3
, Ca
2+
, Mg
2+
, K
+
, Alkality.
2 Evaluate the effects of factors to the phosphate removal efficiency. A series of
jar test scale experiments were performed for analyzing the effect of four
factors: pH; Mg:P ratio; Calcium ions and stirring speed to phosphorus removal
efficiency and struvite crystallization.
3 Investigate the kinetics of phosphate removal by struvite precipitation in
digested piggery wastewater. A 6 liters column reactor was designed for
phosphate removing and struvite precipitation with aeration and precipitation
settling. Batch mode experiment with 5 liters of wastewater was performed to
examine the process.
4 Examine the accumulation of struvite crystals on stainless steel mesh for
separation of recovered struvite. A continuous mode experiment with a device
(which was made of stainless steel and put inside the reactor) was performed to
test the accumulation and the quality of recovered struvite.
18

The experiment with Calcium influence, synthetic wastewater was created with
analytical chemicals. For other experiments, real digested piggery wastewater (i.e., the
wastewater after biogas tanks) was used.
Achieving all the above objectives will lead to a comprehensive knowledge on

phosphate removal and recovery as struvite precipitation from wastewater. This is the
very important step in order to apply struvite method into reality as full scale for
livestock production facilities.
19

The Thuy Phuong Pig Research Center
raises around 1000 pigs and discharges
about 200 m
3
wastewater per day. The
wastewater is treated with the common
methods as the same as other pig farms
do, treated with biogas tanks first and
then discharged to lagoons. The biogas
tanks are constructed and managed
very well and the effluent is quite
stabilized. During the time doing this
research, the phosphate concentration
in the effluent of biogas tanks was
ranging from about 45 mg/l to 60 mg/l.
3 MATERIALS AND METHODS
3.1 Wastewater collection and analysis
The wastewater source used was digested piggery wastewater (i.e., output of biogas
tanks) at the Pig Research Center, National Instiute of Animal Husbandry, Thuy
Phuong village, Tu Liem district , Ha Noi city.
Wastewater was collected in 20 liter plastic cans. In order to make it homogeneous
sample, wastewater was stirred in the drain before collected into cans. Cans were then
transported to the laboratory and stored at 4
o
C. After 24 hours, the solids settled down

and the wastewater was siphoned off from the top of the cans into 1 liter plastic bottles
and stored at 4
o
C for using in the later experiments.
The first experiment was to analyze
necessary parameters of the collected
wastewater. This includes pH, COD,
TSS, TP, P-PO
4
, N-NH
3
, Ca
2+
, Mg
2+
,
K
+
, Alkality.
Initial pH and values of the above
parameters of the wastewater are
displayed in the Table 4-1 (p.30).
20

3.2 Jar-test experiment procedure
In order to determine the influence of above mentioned factors on the phosphate
removal efficiency and struvite crystallization process, series of jar test experiment
were carried out.
3.2.1 Impact of pH
This experiment was aimed to investigate the influence of pH to the phosphate removal

efficiency. 9 glasses were used and each contained 200 ml digested wastewater (which
was analyzed in the previous experiment). 3 ml MgCl
2
.6H
2
O 0.1M was added in all
glasses (in order to make Mg:P molar ratio equal to 1.5:1 to ensure super
stoichiometric proportion of Mg). pH value of wastewater in each glass was adjusted
by NaOH 3M solution. 9 operational pH values were 8.0, 8.3, 8.6, 8.85, 9.0, 9.25, 9.5,
9.75, 10.0, 10.5. All glasses were then stirred for 60 minutes by the magnetic stirrer.
After 60 minutes, pH value and P-PO
4
concentration of liquors in glasses were
measured and compared with the initial values of the wastewater.
The result of this experiment is displayed in the Table 4-2 and Figure 4-1 (p.31, 32).
3.2.2 Impact of magnesium addition
To investigate the influence of magnesium to phosphate removal efficiency, a set of
tests were carried out. 6 glasses of 200 ml of wastewater were used. Mg
2+

concentration in the wastewater was adjusted by adding MgCl
2
.6H
2
O 0.1M to make the
Mg:P molar ratio in all 6 glasses 1:1, 1.25:1, 1.5:1, 1.82:1, 2:1, 2.5:1. And then the pH
value of wastewater in all glasses was adjusted to 9.0 by using NaOH 3M solution. All
glasses were then stirred for 60 minutes by the magnetic stirrer.
21

After 60 minutes, pH value and P-PO
4
concentration of liquors in glasses were
measured and compared with the initial values of the wastewater.
The result of this experiment is displayed in the Table 4-3 and Figure 4-2 (p.33).
3.2.3 Impact of calcium ions
Synthetic wastewater was used to investigate the impact of Ca
2+
to the P-removal
efficiency as well as the struvite crystal morphology. The contents of synthetic
wastewater were:
 100 mg/l P-PO
4
or 3.23 mmol/l
 92.9 mg/l Mg
2+
or 3.87 mmol/l
 139 mg/l N-NH
4
+
or 9.93 mmol/l
The solutions used to create synthetic wastewater were NH
4
H
2
PO
4
0.2M, NH
4
Cl 1M ,

MgCl
2
.6H
2
O 0.1M.
6 glasses were added with 400 ml of synthetic wastewater. CaCl
2
0.4M was used to
adjusted the amount of Ca
2+
in each glass. The first glass was not added with Ca
2+.
The
5 others were added with different Ca
2+
amount to meet the Mg:Ca molar ratio of 4:1,
3:1, 2:1, 1:1, 1:2.
NaOH 3M solution was added to all glasses to adjust pH value to 9.0, the glasses were
stirred by metal stirrer at 70 rpm for 2 hours.
After the reaction of 2 hours, pH value and P-PO
4
concentration were measured and the
precipitations of 6 glasses were stored for analyzing by optical microscope.
22

3.2.4 Impact of stirring speed
This experiment was to investigate the influence of stirring speed to phosphate removal
efficiency. A set of 3 glasses of 400 ml of wastewater was used. Mg
2+
concentration in

the wastewater was adjusted by adding MgCl
2
.6H
2
O 0.1M to make the Mg:P molar
ratio in all 3 glasses 1.2:1. And then the pH value of wastewater in all glasses was
adjusted to 8.80 by using NaOH 3M solution. All glasses were then stirred for 120
minutes by the magnetic stirrer at 3 different speeds of 50 rpm, 80 rpm, and 100 rpm.
After each 20 minutes, P-PO
4
concentration of liquors in glasses were measured and
compared with the initial values of the wastewater.
The result of this experiment is displayed in the Table 4-6 and Figure 4-5 (p.42).
3. Bench-scale experiments
3.3.1 Reactor design and batch mode experiment
A schematic diagram of struvite precipitation in batch mode is shown in the Figure 1.
The struvite precipitation reactor is a clear Plexiglass 1 meter tall with effective volume
of 6.35 liters. The inner diameter is 90 mm.