HANOI UNIVERSITY OF SCIENCE TECHNISCHE UNIVERSITÄT DRESDENMaster 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.

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

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


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

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

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,

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

, 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

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

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

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 ,

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.