diplomarbeit_alexandrahug
Phosphate Recovery from Urine by
Electrocoagulation
Master Thesis of Alexandra Hug
July 25, 2010
Supervision: Dr. Kai M. Udert, Prof. Dr. Eberhard Morgenroth
Institute of Environmental Engineering
Chair of Urban Water Management
ETH Zürich
Abstract
Phosphorus is an essential compound for plants. But the major phosphorus source for fertilizers - phosphate rock - is a limited resource. To close the nutrient cycle, phosphorus should
be recovered from waste streams. Urine has a high phosphate content, which can be recovered
as struvite.
Struvite is a magnesium-ammonium-phosphate (NH4 MgPO4 · 6 H2 O) and can be used as
a slow release fertilizer in agriculture. As magnesium is the limited compound in urine, a
magnesium source has to be added for struvite precipitation. Conventional methods consist
in the dosage of a magnesium salt. Electrocoagulation on the other hand dissolves magnesium
electrochemically from a magnesium electrode.
For electrocoagulation magnesium is oxidized to magnesium ions at the magnesium anode,
water is reduced to hydrogen gas at the steel cathode. To enhance the reaction speed a voltage
source can be added. The voltage source can either be applied between the magnesium and
a reference electrode (magnesium potential, three electrode configuration) or between the
magnesium and the steel electrode (cell voltage, two electrode configuration). The advantage
of the three electrode configuration being that the magnesium potential determines directly
the reactions that can take place at the magnesium electrode.
Two sets of experiments have been conducted: voltammetry and batch experiments. For
voltammetry the applied magnesium potential was changed with time to detect electrode
characteristics. For the batch experiments the voltage source (magnesium potential or cell
voltage) was kept constant over time.
Voltammetry and batch experiments showed that the magnesium electrode is sensitive to
layer formation at magnesium potentials below -800 mV. An electrode potential of -600 mV
seemed to be a very appropriate potential concerning reaction speed and energy consumption.
An electrocoagulation reactor can either be operated by applying an electrode potential or
a cell voltage. Applying a cell voltage will be the easier configuration, as no potentiostat is
needed. A cell voltage of at least 1000 mV should be applied to prevent layer formation.
The monitored cell voltage/electrode potential (depending on electrode configuration) showed
a maximum/minimum as phosphate was removed from urine. It can therefore be used as
a parameter to control the magnesium dissolution in order to not waste magnesium. A
continuous reactor could then be operated in sequencing batch mode, using three tanks:
storage tank, electrocoagulation reactor and settling tank.
It has been shown that electrocoagulation is a very functional method to recover struvite from
natural urine. The advantage of electrocoagulation against chemical dosing will be found in
a high magnesium efficiency and in a high automation possibility.
I
Contents
1 Introduction
1.1 Problem definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Aims and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1
1
2 Materials and Methods
2.1 Electrocoagulation and struvite precipitation
2.2 Urine . . . . . . . . . . . . . . . . . . . . . . .
2.3 Reactor and measurement . . . . . . . . . . .
2.4 Calculations . . . . . . . . . . . . . . . . . . .
2.4.1 Basic equations . . . . . . . . . . . . .
2.4.2 Efficiencies and rates . . . . . . . . . .
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3
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3 Experiments
3.1 Voltammetry . . . . . . . . . . . . . . . . . . .
3.2 Batch experiments . . . . . . . . . . . . . . . .
3.2.1 Volatilization of ammonia . . . . . . . .
3.2.2 Magnesium self dissolution . . . . . . .
3.2.3 Applying magnesium electrode potential
3.2.4 Applying constant cell voltage . . . . .
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9
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5 Discussion
5.1 Processes at the magnesium electrode . . . . . . . . .
5.1.1 Voltammetry . . . . . . . . . . . . . . . . . . .
5.1.2 Effect of non-electrochemical dissolution . . . .
5.2 Processes in batch reactor . . . . . . . . . . . . . . . .
5.2.1 Volatilization of ammonia . . . . . . . . . . . .
5.2.2 pH . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.3 Conductivity . . . . . . . . . . . . . . . . . . .
5.3 Optimal electrode potential range . . . . . . . . . . . .
5.3.1 Efficiencies . . . . . . . . . . . . . . . . . . . .
5.4 Possible applications for an electrocoagulation reactor
5.4.1 Magnesium electrode potential or cell voltage .
5.4.2 Reactor operation and process control . . . . .
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6 Conclusion
6.1 Outlook and suggestions for future research . . . . . . . . . . . . . . . . . . .
23
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Appendix
27
A Previous work
27
4 Results
4.1 Voltammetry . . . . . . . . . . . . . . . . . .
4.2 Batch experiments . . . . . . . . . . . . . . .
4.2.1 Volatilization of ammonia . . . . . . .
4.2.2 Experiments with and without voltage
4.2.3 Efficiencies . . . . . . . . . . . . . . .
4.2.4 Process control . . . . . . . . . . . . .
4.2.5 Precipitate analysis . . . . . . . . . .
II
B Methods
27
C Calculations
C.1 Ag/AgCl reference electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C.2 Steel electrode potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
28
28
D Cyclic voltammetry
D.1 Magnetic stirrer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D.2 Theoretical potentials calculated with Nernst equation . . . . . . . . . . . . .
D.3 Electrode shifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
29
29
30
E Batch experiments
E.1 Urine composition . . . . . . . . . . . . .
E.2 Volatilization of ammonia . . . . . . . . .
E.3 Applying magnesium electrode potential .
E.3.1 Efficiencies . . . . . . . . . . . . .
E.3.2 Process Control . . . . . . . . . . .
E.4 Applying cell voltage . . . . . . . . . . . .
E.5 Struvite . . . . . . . . . . . . . . . . . . .
E.6 Magnesium and phosphate concentrations
31
31
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32
34
35
36
36
37
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III
Abbreviations
a
E0
E
∆ E0
∆E
I
Ka
M
P
pKa
Q
R
SHE
T
V
VCell
W
z
chemical activity
Standard electrode potential (tabulated)
Electrode potential
Standard electromotive force (EMF)
Electromotive force (EMF)
Current
Acid dissociation constant
Molar mass
Electric power
pKa = -log10 Ka
Electric charge
Electrical resistance
Standard hydrogen electrode
Temperature
Voltage
Cell voltage (Voltage between magnesium and steel electrode)
Work
number of moles of electrons transferred in the cell reaction
Constants
F
R
Faraday constant = 9.648531 · 104 C mole−1
Gas constant = 8.31451 J mole−1 K−1
(-)
(V)
(V)
(V)
(V)
(A)
(-)
(g/mole)
(W)
(-)
(C)
(Ω)
(K)
(V)
(V)
(J)
V
List of Figures
2.1
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
A.1
B.1
C.1
D.1
D.2
E.1
E.2
E.3
E.4
E.5
E.6
Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cyclic voltammogram for the magnesium electrode in natural urine . . . . . .
Sensitivity analysis for scanning speed, urine composition, starting potential
and stirring on voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drift of the magnesium and the steel electrode potentials . . . . . . . . . . .
Volatilization of ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pH, magnesium and phosphate concentrations, current densities and cell voltages from the batch experiments . . . . . . . . . . . . . . . . . . . . . . . . .
Linear voltammetry before and after batch experiments . . . . . . . . . . . .
Comparison between electrochemical and non-electrochemical magnesium dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Phosphate concentration at different Mg:P dosage ratios . . . . . . . . . . . .
Cell voltage or magnesium potential as parameter for process control . . . . .
Phosphate removal from synthetic urine for different applied cell voltages . .
System for electrocoagulation a with magnesium and a steel electrode . . . .
Comparison of the measured with the calculated steel potential . . . . . . . .
Dependency of the magnesium potential and the magnesium concentration
according to Nernst equation . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drift of the magnesium and the steel electrode potentials (full range) . . . . .
Effect of stirring speed on pH and volatilization of ammonia . . . . . . . . . .
Conductivity measurement for the batch experiments . . . . . . . . . . . . . .
Linear voltammetry before batch experiments . . . . . . . . . . . . . . . . . .
Electrode surfaces after batch experiments . . . . . . . . . . . . . . . . . . . .
Monitored cell voltage as parameter for process control for a magnesium potential of -590 and -190 mV . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison between the batch experiments with applied cell voltage and applied electrode potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
9
10
10
11
12
13
14
14
15
27
27
28
30
30
32
32
33
33
35
36
List of Tables
2.1
2.2
3.1
4.1
4.2
C.1
D.1
E.1
E.2
E.3
E.4
E.5
E.6
E.7
Urine composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Two and three electrode configuration . . . . . . . . . . . . . . . . . . . . . .
Redox reactions at the magnesium electrode . . . . . . . . . . . . . . . . . . .
A comparison of mean current densities, mean cell voltages, mean electrochemical dissolution rates and energy consumptions . . . . . . . . . . . . . . . . .
Precipitation analysis of Mg, P and N . . . . . . . . . . . . . . . . . . . . . .
Temperature dependency of the Ag/AgCl reference electrode . . . . . . . . .
Stirring speed of magnetic stirrer . . . . . . . . . . . . . . . . . . . . . . . . .
Influent urine composition for all batch experiments . . . . . . . . . . . . . .
Effluent urine composition for all batch experiments . . . . . . . . . . . . . .
Coulombic efficiencies for -1190 and -790 mV . . . . . . . . . . . . . . . . . .
Coulombic efficiencies for -590 and -190 mV . . . . . . . . . . . . . . . . . . .
Precipitate analysis for three batch experiments . . . . . . . . . . . . . . . . .
Magnesium and phosphate concentration for the self dissolution experiment
and for -1190 and -790 mV . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Magnesium and phosphate concentration for -590 and -190 mV and for the cell
voltage of 1250 mV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
4
7
15
16
28
29
31
31
34
34
37
37
38
Introduction
1
1
Introduction
1.1
Problem definition
Phosphorus is an essential compound of fertilizers, because plants need phosphorus for growth.
Most of the phosphorus is gained from phosphate rock which is a limited resource. The reserves of rock phosphate could be exhausted in 50-100 years. To conserve rock phosphate
a sustainable phosphate management is needed, which includes the closure of phosphorus
cycle by phosphate recovery [1]. On the other hand phosphorus needs to be eliminated from
wastewater, because high phosphate concentrations lead to eutrophication of rivers, lakes and
coastal areas of seas. Switzerland limits the phosphate concentration since 1976 [2]. Other
states (e.g. European community) also limit the phosphate concentration from municipal
wastewater treatment plants [3].
There are several ways to eliminate pollutants from wastewater streams. The conventional
way in industrialized countries is the centralized treatment of the polluted water in a wastewater treatment plant. A centralized treatment does not need to be the best treatment option
in all cases. Especially in countries with arid climate, in fast growing cities and in areas of
high population density decentralized treatment options might be advantageous. These options can include the separation of urine, the advantage being that urine contains most of
the nutrients but sums up to less than one percent of the wastewater volume [4].
Looking at human excreta, around 67% of phosphorus mass is in urine, 33% in faeces. The
phosphorus in urine is almost completely present as phosphate [5]. Phosphate can be recovered of urine by struvite precipitation. Struvite consists of magnesium, phosphate and
ammonium in equal molar ratios (NH4 MgPO4 · 6 H2 O). It is a low soluble mineral salt and
can therefore be used as a longtime fertilizer in agriculture [6]. Struvite precipitation has
already been investigated with different waste streams containing high phosphate concentrations such as digester liquor, landfill leachate and dewatering liquors [7]. Compared to other
waste streams human urine brings the advantage of high pH, which favors the precipitation
of struvite.
As phosphate and ammonium are present in urine, magnesium is the limiting element for
struvite formation. Therefore a magnesium source needs to be added. Conventional options
are the chemical dosage of MgCl2 , MgO, MgSO4 or bittern. Electrocoagulation on the other
dissolves magnesium electrochemically from a magnesium electrode.
1.2
Aims and objectives
The magnesium dissolution by electrocoagulation for struvite precipitation has been the topic
of several master theses at Eawag (Swiss Federal Institute of Aquatic Science and Technology).
So far batch experiments with synthetic urine have been conducted [8]. The main findings
can be summarized as follows (appendix A):
• Magnesium dissolution and therefore also phosphate precipitation is enhanced by applying a cell voltage
• Low cell voltages are sensitive to layer formation
• High cell voltages reach a maximal magnesium dissolution, which can not be enhanced
anymore
The aim of this thesis is to develop the electrocoagulation technology further by the usage of
natural urine from the Eawag NoMix system. The Eawag operates a NoMix system for its
office buildings where urine and faeces are collected separately.
The main goal of this thesis is the improved understanding of the process, in particular:
2
Introduction
• Understanding processes at the magnesium electrode
• Understanding reactions in the batch reactor
• Determining the optimal potential range for electrocoagulation in natural urine
• Possible application and process control
The processes at magnesium electrode were investigated using linear and cyclic voltammetry.
Interpretation of voltammograms were mainly based on literature.
To confirm the results from voltammetry, several batch experiments were conducted. The
batch experiments were focused on understanding the processes in the reactor, such as ammonia volatilization as well as pH and conductivity dependencies. Additionally rates and
efficiencies were analyzed for different magnesium potentials. This lead to a recommendation
for an optimal potential range to operate the reactor.
With batch experiments two different electrode configurations were tested (applying cell
voltage or magnesium electrode potential). Recommendations for the operation of a reactor
and for reactor design are given.
Materials and Methods
2
2.1
3
Materials and Methods
Electrocoagulation and struvite precipitation
Electrocoagulation uses a metal anode for the dosage of polluted water with a coagulating
agent. At the same time gases are generated at the cathode [9]. In our case a magnesium and
a steel plate electrode are connected. At the magnesium anode magnesium is oxidized and
released into the urine. At the steel cathode the reduction of water to elemental hydrogen
and hydroxide proceeds (appendix B). The predominant reactions can be summarized as
follows:
Mg ⇀
↽ Mg2+ + 2 e−
-E0 = 2.363 V
2H2 O + 2e− ⇀
↽ H2 + 2OH−
E0 = 0.000 V
Anode:
Cathode:
∆ E0 = 2.363 V
The overall reaction has a positive standard electromotive force (EMF), this means that the
reaction will occur spontaneous at standard conditions (25 C; 101.3 kPa; pH=0; Ion activity
= 1). However in practice a voltage is induced to enhance reaction speed.
As ammonium and phosphate are present in urine, the release of magnesium ions leads to
the formation of struvite:
M g 2+ + N H4+ + P O43− + 6H2 O ⇀ M gN H4 P O4 · 6H2 O
(1)
Because the ammonium concentration is much higher in urine, struvite precipitation will stop
as soon as phosphate gets limited.
2.2
Urine
Natural urine is collected separately by means of NoMix toilets at the Eawag building in
Dübendorf. NoMix means that urine is separated without flushing water as a concentrated
stream.
Table 2.1: Comparison of natural urine used for the experiments and fresh natural urine
Parameter
Unit
pH
Ca
Cl
K
Mg
Na
NH4 -N
PO4 -P
SO4
gCa m−3
gCl m−3
gK m−3
gM g m−3
gN a m−3
gN m−3
gP m−3
gSO4 m−3
Measured urine
Mean ± Std.
8.9
16.5 ±
3064 ±
1977 ±
<5
1988 ±
2536 ±
197 ±
721 ±
3.6
143
448
226
327
16
25
Fresh urine
[4], [10]
6.2
233
4970
2737
119
3450
463
450
Figure 2.1 compares fresh human urine with the stored urine used for the experiments (8
measurements, appendix E.1). Differences are especially found for nitrogen, phosphate, magnesium and calcium.
As urine is stored in a tank, urea hydrolysis takes place which converts urea into ammonium.
4
Materials and Methods
This leads to an increase of pH. Ammonium concentrations could even be much higher [4].
Depending on the storage system ammonia is more likely to evaporate.
The existence of magnesium or calcium ions can lead to the uncontrolled precipitation of
struvite (MgNH4 PO4 · 6 H2 O), hydroxyapatite (Ca5 (PO4 )3 OH) and calcite (CaCO3 ). Precipitation is advantaged by the high pH [11], [12].
2.3
Reactor and measurement
All experiments have been conducted in a batch reactor (volume 1l). The magnesium and
the steel electrode have been fixed in a distance of 5.5 cm from each other (figure 2.1, right).
As reference an Ag/AgCl electrode (Metrohm 6.0724.140) was used. It was placed as close to
the electrode surface as possible. The measured voltages have been converted into electrode
potentials against standard hydrogen electrode (SHE, equation 2). Either the magnesium or
the steel electrode potential have been measured. The correlation between the two potentials
can be found in equation 3.
To supply a voltage a potentiostat (PGU 10V-1-IMP-S) has been used. The potentiostat
either provided a constant voltage or was forced to sweep from one voltage to another (chapter
3). Depending on the configuration the cell potential (voltage between steel and magnesium
electrode) or the magnesium electrode potential was applied (table 2.2).
Table 2.2: Differences between two and three electrode configuration
Two electrode configuration
Three electrode configuration
Working electrode
Counter electrode
Reference electrode
Mg electrode
Steel electrode
Steel electrode
Mg electrode
Steel electrode
Ag/AgCl
Applying:
Cell potential (Mg - Steel)
Mg electrode potential
Figure 2.1: Experimental setup with reactor, potentiostat, data logger, pH and conductivity meter
(left), magnesium and steel electrodes (right).
During the experiments several continuous signals were measured and stored in a data logger
(RSG40). The following signals have been recorded: current, voltage (for three electrode
configuration), electrode potential (for two electrode configuration), pH (WTW PH 196),
temperature (WTW PH 196) and conductivity (WTW LF 96-B).
Materials and Methods
5
All samples taken during the experiments have been filtrated (0.45 µm), acidulated with
nitric acid and were measured for Mg2+ and PO3−
4 . Natural urine has been analyzed before
+
+
2+
2+
and after the experiments measuring the following parameters: NH+
4 , Na , K , Mg , Ca ,
2−
3−
Cl− , SO4 , PO4 and COD.
The measurement of the samples was carried out with the following instruments:
3−
• FIA (Flow injection analysis): NH+
4 , PO4
• IC (Ion Chromatography): Cl− , SO2−
4
• ICP (Inductively coupled plasma): Na+ , K+ , Mg2+ , Ca2+
+
• Dr. Lange: COD (LCK 014), PO3−
4 (LCK 350), NH4 (LCK 303)
The precipitate was analyzed after the batch experiments with FIA, IC, ICP and with XRDanalysis. Urine was filtrated, the filtrate was dried and pounded. For FIA, IC, ICP measurement the filtrate was dissolved in concentrated hydrochloric acid (32%) and then analysed
3−
2+
as well as K+ , Ca2+ , Na+ , SO2−
(appendix E.5). In the case of
for NH+
4 , Mg , PO4
4
2+ and PO3− in equal
struvite, the composition of the precipitate should contain NH+
4 , Mg
4
molar ratios.
2.4
2.4.1
Calculations
Basic equations
Converting electrode potentials into standard potentials
The electrode potential was measured against an Ag/AgCl reference electrode. The electrode
potential against standard hydrogen electrode (SHE) has been calculated as follows:
ESHE = EAg/AgCl + 0.210V
(2)
All electrode potentials are given in potentials against standard hydrogen electrode.
The Ag/AgCl reference electrode is slightly temperature dependent (appendix C.1), which
has not been taken into account.
Magnesium and steel electrode dependency
The magnesium electrode potential can be calculated from the steel electrode potential and
the cell voltage or vice versa:
Emagnesium = Vcell + Esteel
(3)
A comparison of the measured and the calculated steel electrode potentials during a batch
experiment can be found in appendix C.2.
Nernst equation
The Nernst equation describes the dependency of the electrode potential on the concentrations in the solution:
2.303 · R · T
aOx
E = E0 +
log
(4)
z·F
aRed
An application of the equation is given in appendix D.2.
Faraday’s laws of electrolysis
Faraday’s laws describe the dependency of electric
electrolysis:
M
mF araday =
z
charge and dissolved mass by means of
·
Q
F
(5)
6
Materials and Methods
with Q as the total electric charge passed through the substance:
Q=
Zt
(6)
Idτ
0
Joule’s law
Electric energy can be calculated from voltage, current and time:
W =
Zt
Vcell · Idτ
(7)
0
2.4.2
Efficiencies and rates
Magnesium dissolution rates
The dissolution rates are given in mgMg/h. The overall dissolution rate can be divided into
an electrochemical and a non-electrochemical fraction:
• Overall dissolution rate: calculated from the measured phosphate and magnesium concentrations. Assumptions: phosphate precipitates with magnesium in a 1:1 molar ratio
• Electrochemical dissolution rate: dissolution of magnesium due to electric current. Calculated by using Faraday’s laws
• Non-electrochemical dissolution rate: the self dissolution of magnesium due to local
processes at the electrode surface. Calculated as the difference of the overall dissolution
and the electrochemical dissolution
Coulombic efficiency
Coulombic efficiency compares directly the overall magnesium dissolution with the electrochemical dissolution. It determines how efficient current is used for magnesium dissolution:
Coulombic ef f iciency =
mdissolved
mF araday
(8)
Phosphate removal and magnesium efficiency
Phosphate removal is dependent on the amount of magnesium added. The phosphate concentrations for different dosage ratios (Mg:P) are calculated. The magnesium dosage is calculated
as the sum of magnesium in urine and the precipitated magnesium in struvite. Assumption:
phosphate precipitates with magnesium in a 1:1 molar ratio.
Energy efficiency
Energy efficiency for the electrocoagulation process is calculated using Joule’s law. It is given
in Wh/gP.
Experiments
3
7
Experiments
Two different sets of experiments have been conducted: voltammetry and batch experiments.
To apply a voltage source, two different configurations are possible (table 2.2):
• Applying magnesium electrode potential (three electrode configuration)
• Applying cell voltage (two electrode configuration)
A magnesium electrode potential was applied for voltammetry and the first set of batch
experiments. The magnesium potential was controlled, because it determines the reactions
that occur at the magnesium electrode. Cell voltage was tested in another batch experiment,
because this would be an easier application, as no potentiostat is needed.
3.1
Voltammetry
Two different types of voltammetry have been used to conduct experiments: linear sweep
voltammetry and cyclic voltammetry.
For linear sweep voltammetry the electrode potential is increased from one potential to
another. For cyclic voltammetry the electrode potential is swept forward and backward
between two potentials. For both methods the scan rate is fixed to a constant value.
Comparing the measured current with the applied electrode potential gives information about
the redox reactions of species enclosed in the system. The higher the current, the faster the
electrons are transferred, the faster the redox reactions.
Theoretically several redox processes could take place at the magnesium electrode. Magnesium oxidation and reduction is one of these processes, the others are listed in table 3.1 with
the calculated redox potentials by Nernst equation (appendix D.2). To observe the oxidation
reactions the potential at the magnesium electrode has to be at least as high as the theoretical
potential. For reduction processes it has to be at least as low. The difference between the
observed redox reaction and the calculated redox potential is known as overpotential.
Table 3.1: Possible redox reactions at the magnesium electrode in urine
Reactions
2H2 O + 2e− ⇀ H2 + 2OH−
O2 + 2H2 O + 4e− ↼ 4OH−
Cl2 + 2e− ↼ 2Cl−
E0
V
E
V
-0.83 (pH 14)
0.401 (pH 14)
1.3595
-0.54
0.69
1.39
Conclusion
Reduction if Mg potential ≤ -0.54 V
Oxidation if Mg potential ≥ 0.69 V
Oxidation if Mg potential ≥ 1.39 V
For the sensitivity analysis of voltammetry the following parameters have been investigated,
using linear voltammetry:
• Scanning speed: 0.5 mV/s, 1 mV/s, 2 mV/s, 5 mV/s
• Urine composition: three reference runs
• Starting potential: -2300 mV, -1800 mV, -1300 mV
• Stirrer: slow stirring (275/min), fast stirring (750/min), without stirring (appendix
D.1)
Measurement
- Current
- Cell voltage
Setup
Mg potential is swept between
two potentials (-1.8 V, 1 V). A
new mg electrode with a surface of 2.5 x 0.5 cm was used.
Goals
- Redox reactions at mg electrode
- Sensitivity analysis of parameters
influencing voltammograms
8
Experiments
3.2
3.2.1
Batch experiments
Volatilization of ammonia
NH3 is a volatile gas. The stirring of urine enhances the volatilization of NH3 . This influences
pH and the total ammonium/ammonia concentration.
Measurement
- pH
- NH4 -N
3.2.2
Setup
Urine is stirred in a batch test
during 8 hours
Goals
- Influence of stirring on pH
- Quantification of ammonia loss
Magnesium self dissolution
Self dissolution is the non-electrochemical dissolution of magnesium. As no current is flowing,
it is due to local processes at the magnesium electrode.
-
Measurement
pH
Conductivity
Mg potential
PO4 -P
3.2.3
Setup
The magnesium electrode is
put into urine (no counter electrode, no current)
Goals
- Observing the self dissolution behavior of the magnesium electrode in
urine
Applying magnesium electrode potential
Batch tests were carried out by applying a constant magnesium potential. The applied potentials were: -1190 mV, -790 mV, -590 mV and -190 mV.
Measurement
- pH
- Conductivity
- Cell voltage
- Current
- PO4 -P
- Mg2+
- Urine analysis
(before/after)
- Struvite anal.
3.2.4
Setup
A constant mg potential is
applied. The experiment is
running until phosphate is removed from urine. The electrode has a surface of 2.2 x
8.8 cm. Before and after experiment linear voltammetry is
performed (electrode surface:
2.2 x 1 cm).
Goals
- Optimal potential range for mg dissolution
- Efficiencies and rates (chapter
2.4.2)
- Evaluation of possible parameters
for process control (cell voltage)
Applying constant cell voltage
As it is unusual to operate a reactor with a potentiostat and constant electrode potential,
one batch experiment with a constant cell voltage of +1250 mV was conducted.
Measurement
- Mg potential
(others as 3.2.4)
Setup
A constant voltage is applied.
(other as 3.2.4)
Goals
- Evaluation of possible parameters
for process control (mg potential)
- Evaluation of possible applications
Results
4
4.1
9
Results
Voltammetry
Figure 4.1 shows one reference run of cyclic voltammetry for natural urine. For the increasing potential two different reactions were observed, one reaction at potentials below -900 mV
reaching a current density of 40 A/m2 and one at potentials above -900 mV. The turning
potential at -900 mV will be referred to as breakdown potential. When the potential was
decreased, current fell. For a reversible redox reaction, the current would decrease more or
less along the same curve as for the increasing electrode potential. The fast fell indicates that
magnesium dissolution is a totally irreversible process.
The corrosion potential for magnesium in natural urine was around -1500 mV. For magnesium potentials lower than the corrosion potential electrons are transferred from steel to
magnesium, for potentials higher than corrosion potential from magnesium to steel.
Current density (A/m2)
200
150
100
Corrosion
50 potential
0
−50
−1500
0.5 mV/s up
Breakdown
0.5
mV/s down
potential
−1000
−500
0
500
1000
Mg Potential (mV)
Figure 4.1: Cyclic voltammogram for the magnesium electrode in natural urine (scanning speed
0.5 mV/s).
Results from the sensitivity analysis are shown in figure 4.2. The variation of scanning speeds
showed very similar voltammograms for the scan rates of 0.5 and 1 mV/s. The curves for 2
and 5 mV/s behaved differently. The breakdown potentials were -900 mV for all curves. For
0.5 and 1 mV/s the increase of current slowed down rapidly after the breakdown potential.
For 2 and 5 mV/s no peak current and not such a slow down in currents was observed.
The change in the urine composition and in the starting potential did not affect the breakdown potentials very much. Differences were obtained in the height of peak currents and in
the currents after the peak. Stirring influenced the layer formation on the electrode surface.
Differences were observed at the point of peak currents.
Figure 4.3 shows the drift of the magnesium and the steel electrode in relation to the cell
voltage. Magnesium electrode potentials have been applied, cell voltages have been measured
and steel electrode potentials were calculated (equation 3). It was observed that the steel
electrode potential drifted very fast for cell voltage ranges smaller than -500 mV and from
200 to 1000 mV. In contrast the magnesium potential shifted faster for cell voltages ranging
from -500 to 200 mV and >1000 mV. The electrode behavior for the whole range from -3400
to 4000 mV cell voltage is shown in appendix D.3.
10
Results
600
0.5 mV/s
1 mV/s
2 mV/s
5 mV/s
Current density (A/m2)
800
400
200
0
Current density (A/m2)
−1500 −1000 −500
0
Mg Potential (mV)
200
100
250
250
200
200
150
100
50
0
−2300 mV
−1800 mV
−1300 mV
−50
−100
−2000−1500−1000 −500
0
Mg Potential (mV)
500
Urine 1
Urine 2
Urine 3
0
−100
−1500 −1000 −500
0
500
Mg Potential (mV)
500
Current density (A/m2)
Current density (A/m2)
1000
150
100
50
0
Fast stirring
Slow stirring
Without stirring
−50
−100
−1500 −1000 −500
0
500
Mg Potential (mV)
Figure 4.2: Different effects for linear voltammetry have been investigated for sensitivity analysis:
scanning speed, urine composition, starting potential and stirring speed. Scanning speed turned out
to be the most sensitive parameter.
500
Electrode potential (mV)
Magnesium potential
Steel potential
0
−500
−1000
−1500
−2000
−1000
0
1000
Cell voltage (mV)
2000
Figure 4.3: Drift of the magnesium and the steel electrode potentials. The magnesium electrode
shifted faster at cell voltages between -500 and 200 mV and for cell voltages higher than 1000 mV.
The steel potential increased faster at cell voltages below -500 mV and between 200 and 1000 mV.
Results
4.2
4.2.1
11
Batch experiments
Volatilization of ammonia
NH4−N (mg/l)
The stirring of urine lead to a small decrease of pH (figure 4.4). The total amount of ammonium/ammonia decreased as well, although the decrease was very low and the uncertainty in
the measurement quite high. The drop in pH was dependent on the stirring speed and the
duration of the experiment (figure E.1).
2000
1000
0
0
2
4
Time (h)
6
8
2
4
Time (h)
6
8
pH (−)
9
8.9
8.8
8.7
0
Figure 4.4: Volatilization of ammonia. As ammonia volatilized, the total concentration of ammonium/ammonia decreased and pH dropped.
4.2.2
Experiments with and without voltage source
Results from batch experiments without applied voltage and with the applied magnesium
potentials of -1190, -790, -590 and -190 mV are shown in figure 4.5.
The self dissolution of magnesium in stored urine was very fast even without any voltage
applied. After 8 hours more than 75% of phosphate was removed . At a magnesium potential
of -1190 mV, the phosphate removal rate was increased slightly. The electrode potentials of
-790, -590 and -190 mV showed a very fast removal of phosphate. When phosphate started to
be limited, the magnesium concentrations increased. For the self dissolution experiment and
the magnesium potential of -1190 mV the point of phosphate limitation was not yet reached.
The current densities were much higher for -790, -590 and -190 mV than for -1190 mV. For
-1190 mV the current density started at 25 A/m2 , but then decreased quite rapidly towards
small currents (5 A/m2 after 8 hours). The currents stayed quite stable for -790 and -590 mV,
whereas high fluctuation was observed for -190 mV. The cell voltage curves showed a very
similar shape to currents. As expected the cell voltage increased with the applied magnesium
potential.
pH increased slightly during the experiments. The rate of increase was faster when phosphate
was removed. The conductivities decreased during the batch experiments. The faster the
phosphate removal rate, the faster was the decrease in conductivity. This trend was obvious,
although the adjustment of the conductivity meter was not very sensitive. The observed
conductivities are shown in figure E.2.
The experiments with constant cell voltage of 1250 mV and constant magnesium potential
of -590 mV showed quite similar phosphate removal rates. The mean current density was at
67 A/m2 for -590 mV, at 60 A/m2 for 1250 mV. The measured cell voltage for the applied
potential of -590 mV was between 1050 and 1200 mV. A comparison between the two experiments can be found in appendix E.4.
Results
200
100
2
4
Time (h)
6
8
150
100
50
0
0
0
Cell voltage (mV)
Current density (A/m2)
1000
200
0
0
2
4
Time (h)
6
9
8.8
Concentration (mgMg/l)
−200
−1500 −1000 −500
0
500
Mg Potential (mV)
pH (−)
Self dissolution
−1190 mV
−790 mV
−590 mV
−190 mV
0
Concentration (mgP/l)
Current density (A/m2)
12
8
2
4
Time (h)
6
8
2
4
Time (h)
6
8
2
4
Time (h)
6
8
200
100
0
0
2000
1000
0
0
Figure 4.5: pH, magnesium and phosphate concentrations, current densities and cell voltages from the
batch experiments. The applied magnesium electrode potentials were kept constant at -1190, -790,
-590 and -190 mV. The self dissolution experiment was carried out without counter electrode and
current.
The electrode surfaces after the batch experiments are shown in figure E.4. A white layer was
formed on the electrode surface for the self dissolution experiment and -1190 mV. Some white
spots were still present for -790 mV. The higher the applied potentials the more homogeneous
the electrode surface became.
To get some more information about the condition of the electrode surface, linear voltammetry before and after the experiments was performed. In order not to destroy the electrode,
the scan was kept as short as possible (scan from -1200 to -500 mV). The scan of the electrode before the experiment showed that the breakdown potentials were between -900 and
-800 mV in all cases (figure E.3). For the self dissolution experiment and for the applied magnesium potential of -590 mV the breakdown potential stayed very constant. For -590 mV and
1250 mV (cell voltage) a little shift of the breakdown potential towards higher magnesium
potentials was observed. -190 and -1190 mV did not show any breakdown in the investigated
potential range. For linear voltammetry after the batch experiments, the current densities
were usually lower than for voltammetry before the experiments. This effect was very clear
for the self dissolution, -1190 and -190 mV as well as 1250 mV (cell voltage), but not for -790
and -590 mV (figure 4.6).
Results
13
Self dissolution
−1190 mV
100
50
150
Scan 1
Scan 2
100
50
Current dens. (A/m2)
Scan 1
Scan 2
0
−1200 −1000 −800 −600
Mg Pot. (mV)
0
−1200−1000 −800 −600
Mg Pot. (mV)
−590 mV
0
−1200−1000 −800 −600
Mg Pot. (mV)
50
+1250 mV
150
Scan 1
Scan 2
100
50
0
−1200−1000 −800 −600
Mg Pot. (mV)
Current dens. (A/m2)
50
Current dens. (A/m2)
100
100
0
−1200 −1000 −800 −600
Mg Pot. (mV)
150
Scan 1
Scan 2
Scan 1
Scan 2
−190 mV
150
Current dens. (A/m2)
−790 mV
150
Current dens. (A/m2)
Current dens. (A/m2)
150
Scan 1
Scan 2
100
50
0
−1200 −1000 −800 −600
Mg Pot. (mV)
Figure 4.6: Linear voltammetry before and after the batch experiments to detect the breakdown
potentials. The breakdown potentials stayed quite constant for the self dissolution experiment, applied
magnesium potentials of -790 and -590 mV and for the applied cell voltage of +1250 mV.
4.2.3
Efficiencies
The magnesium dissolution rate can be divided into an electrochemical and non-electrochemical dissolution (chapter 2.4.2). The electrochemical magnesium dissolution was faster
the higher the applied potential. The non-electrochemical dissolution rate was high at the
beginning and then decreased during all experiments (figure 4.7). Because of the nonelectrochemical effect, the coulombic efficiencies resulted higher than 100%. Coulombic efficiencies at different times are given in appendix E.3.1.
The phosphorus concentration at different Mg:P dosage ratios is shown in figure 4.8. The
higher the magnesium dosage was, the lower were the phosphate concentrations. The reached
phosphate concentrations were almost independent of the applied potentials.
The calculated energy consumption was higher the higher the applied potential. The energy
consumption doubled from the applied potential of -790 mV to the potential at -190 mV
(table 4.1). A survey of energy consumption, the mean current densities, cell voltages and
electrochemical dissolution rates for -790, -590 and -190 mV are given in table 4.1.
14
Results
−1190 mV
−790 mV
150
overall dissolution
Electrochemical
Non−electrochemical
100
Rates (mgMg/h)
Rates (mgMg/h)
150
50
0
0
2
4
6
Time (h)
−590 mV
50
0
0
8
1
2
3
Time (h)
−190 mV
4
5
150
Rates (mgMg/h)
150
Rates (mgMg/h)
100
100
50
0
0
1
2
Time (h)
3
4
100
50
0
0
1
2
Time (h)
3
4
Figure 4.7: Comparison between the electrochemical and the non-electrochemical magnesium dissolution. The electrochemical dissolution was calculated out of currents using Faraday’s laws and stayed
relatively constant during the experiment (exception: -1190 mV). The non-electrochemical dissolution
on the other hand was decreasing.
50
−1190 mV
−790 mV
−590 mV
−190 mV
Concentration (mgP/l)
40
30
20
10
0
0.8
1
1.2
1.4
1.6
Dosage ratio (Mg:P)
1.8
2
Figure 4.8: Phosphate concentration at different Mg:P dosage ratios. The phosphate concentration is
almost independent of the applied magnesium potentials.
Results
15
Table 4.1: A comparison of mean current densities, mean cell voltages, mean electrochemical dissolution rates and energy consumptions for the batch runs with constant magnesium potential of -790,
-590, -190 mV. The current densities, the cell voltages and the dissolution rates were calculated as a
mean value between the start of the experiment and 90% removal of phosphate.
Current density
Cell voltage
A/m2
(mean)
50
67
87
-790 mV
-590 mV
-190 mV
4.2.4
Energy consumption
mV
(mean)
Dissolution rate
(electrochemical)
mgMg/h
(mean)
620
1080
1590
44
59
77
1.0
1.6
2.1
Wh/gP
(at 90% removal)
Process control
For the applied magnesium potential, the cell voltage was monitored. For the applied cell
voltage, the magnesium potential was monitored. Both configurations showed that the monitored magnesium potentials/cell voltages are useful to predict phosphate removal. For a
constant magnesium potential the cell voltage showed a maximum as phosphate was limited.
In contrast, for a constant cell voltage the magnesium potential showed a minimum (figure
4.9, figure E.5).
Cell voltage (mV)
100
200
650
600
100
200
300
0.02
0
−0.02
0
100
200
Time (min)
300
200
100
0
0
300
700
550
0
Conc. (mg/l)
Mg2+
100
0
0
Slope cell volt. (−)
PO4−P
Mg potential (mV)
200
Cell voltage: +1250 mV
−550
Slope mg pot. (−)
Conc. (mg/l)
Mg potential: −790 mV
0.02
50
100
150
200
50
100
150
200
100
150
Time (min)
200
−600
−650
−700
0
0
−0.02
0
50
Figure 4.9: Cell voltage or magnesium potential as parameter for process control. For the applied
magnesium potential of -790 mV, the cell voltage showed a maximum at the time where phosphate was
removed (maximum: the slope of cell voltage gets negative). For the applied cell voltage of 1250 mV
on the other hand, the magnesium potential showed a minimum at phosphate removal (minimum: the
slope of magnesium potential gets positive).
16
4.2.5
Results
Precipitate analysis
The precipitates were analyzed for three different batch runs: -790, -590, -190 mV. The
struvite samples were taken at the end of the experiments. The Mg:P:N ratio is given in
table 4.2. Hardly any other elements such as K+ , Ca2+ , Na+ or SO2−
4 were found (appendix
E.5).
Table 4.2: Precipitation analysis of Mg, P and N for three batch experiments.
-790 SHE
-590 SHE
-190 SHE
mole/moleMg
mole/moleMg
mole/moleMg
XRD-analysis yielded pure struvite (100%).
Mg
P
N
1
1
1
1.01
1.08
1.07
1.04
1.07
1.05
Discussion
5
17
Discussion
5.1
5.1.1
Processes at the magnesium electrode
Voltammetry
The magnesium oxidation is not a reversible process, as no magnesium reduction was observed
for the decreasing magnesium potential. For the increasing magnesium potential two reactions
were observed (figure 4.1).
One reaction already started at cathodic currents (e− from steel to magnesium) and then
continued at anodic currents (e− from magnesium to steel). To induce a cathodic current,
a reduction process needs to proceed at the magnesium electrode, which accepts electrons.
According to table 3.1 hydrogen evolution is the only possible reduction process in these
potential range. The fact that the reaction just continued from anodic to cathodic currents
implies that together with hydrogen evolution also the magnesium oxidation reaction started.
Many other authors found that at the magnesium electrode several processes can take place
simultaneously [13]-[18]. The phenomenon of hydrogen evolution and magnesium dissolution
is known as local cell action (LCA) [17]. The anodic and cathodic processes have been
summarized with equation 9 and 10 [14]. Recent publications suggest an intermediate step
with Mg+ , which will not be discussed in more detail here [15], [18].
2H2 O + 2e− ⇀ H2 + 2OH −
(9)
M g ⇀ M g 2+ + 2e−
(10)
The hydrogen evolution reaction started to slow down at magnesium potentials around
-1200 mV. This was due to the formation of a surface layer, most probably consisting of
Mg(OH)2 . Mg(OH)2 is the product of the overall reaction resulting from equation 9 and 10:
M g 2+ + 2OH − ⇀ M g(OH)2
(11)
The formation of a Mg(OH)2 layer is also known as passivation [15], [19]. The passivation
range started to break at potentials higher than -900 mV. The breakdown lead to a fast oxidation of magnesium (according to equation 10). High concentrations of chloride as present
in urine favor the breakdown of the Mg(OH)2 layer [15].
The dissolution started to stay constant at potentials higher than -500 mV. This is unlike
we would expect form Nernst Equation (figure D.1), as Nernst Equation expects a further
increase of magnesium dissolution. In reality the process starts to get mass transfer limited. The magnesium has not only to be dissolved but also to be transferred into the bulk
solution [20]. For the magnesium dissolution a peak current was observed, which exceeded
the diffusion-limited current clearly. This peak (polarographic maxima) is caused by several
effects such as stirring, nonuniform current densities on the electrode surface and others [21].
As this peak is an effect of voltammetry, the reaching of these peak current densities at constant potentials is not expected.
The theoretical potential for oxygen evolution would be at 690 mV. As no further increase
of the current was observed in these range, it is not very likely that oxygen was produced.
There are several reaction rates that determine and limit the overall electrode reaction:
• Mass transfer from bulk solution to the electrode surface
• Electron transfer rate at the electrode surface
• Chemical reactions following the electron transfer
18
Discussion
• Other surface reactions such as adsorption and desorption
A more detailed description can be found in [20]. A mass transfer limitation has been observed
at high magnesium potentials by reaching a diffusion-limited current, which was not exceeded
anymore. A limitation of the electron transfer processes has been observed for the sensitivity
analysis of the scan rate. At high scan rates, the rates of electron transfer processes at the
electrode surface were limiting and no equilibrium condition was established. This resulted
in a retardation of the peak currents (figure 4.2).
The scan rate was therefore the most influencing parameter on voltammetry. However the
breakdown potential seemed to be very insensitive also concerning the scan rate. Limitations
for voltammetry are rather found for high potentials and diffusion-limited currents, as they
were much more stochastic (figure 4.2).
5.1.2
Effect of non-electrochemical dissolution
The phenomenon of non-electrochemical dissolution has mainly been investigated in the field
of cathodic protection (CP) with magnesium electrodes. Self dissolution is a problem for
cathodic protection, because the aim is to induce a current and not to dissolve magnesium.
Self dissolution therefore lowers efficiency [17]. As cathodic protection is only used within
small current densities, very few literature can be found for higher currents as used for
electrocoagulation. The effect of self dissolution in the field of electrocoagulation does not
need to be negative, as it enhances current efficiency in the case of electrocoagulation.
If only the magnesium electrode is put into urine, it showed a very fast self dissolution rate
(figure 4.5). The same effect was observed when an electrode potential was applied: much
more magnesium was dissolved than expected by Faraday’s law (figure 4.7). There are mainly
two phenomena which might explain this non-electrochemical magnesium dissolution:
• Local cell action (LCA)
• Chunk effect (CE)
The effect of local cell action has been explained earlier (chapter 5.1.1). LCA is due to
the presence of micro galvanic couples on a metal surface. LCA increases with increasing
magnesium potential, which means that magnesium dissolution as well as hydrogen evolution
rate rise. The increase of hydrogen evolution with higher potentials is very uncommon and
not observed for every metal. The phenomenon of enhanced LCA with increasing potential
is known as negative difference effect and is explained in more detail in [15].
The chunk effect describes the mechanical loss of pieces from the electrode surface. The
detached particles are thought to have very small dimensions [17]. The chunk effect is believed
to proceed up to high anodic currents [22].
The non-electrochemical dissolution rate was very high at the beginning of the experiment
and then decreased (figure 4.7). LCA might be the key factor for this fast dissolution at the
beginning. Observing the electrode also showed that a lot of little black spots appeared. Song
et al. [15] visually observed that hydrogen evolution mainly focused on such black spots. The
non-electrochemical dissolution decreased during the experiments which could be explained
by growing layers, either passivation or diffusion layer. LCA will most probably not be very
likely to occur at coated surfaces.
Discussion
5.2
5.2.1
19
Processes in batch reactor
Volatilization of ammonia
With the volatilization of NH3 , NH3 will be produced to reach ammonium/ammonia equilibrium again. At the same time protons are released and pH will drop:
N H4+ ⇀
↽ N H3 + H + , pKa = 9.25
(12)
The more the pH drops the more the equilibrium will get on the side of NH4 + .
At a starting pH of around 8.9, about 34% of total ammonium is in the form of ammonia
(appendix E.2). The observed volatilization of ammonia was therefore quite low. However,
the observed loss of ammonia due to volatilization was around 5% (100 mgN/l) within 4
hours. So, if 200 mgP are precipitated as struvite in a batch cycle of 4 hours, only 90 mgN
will be needed for struvite precipitation, but 100 mgN will volatilize. The part of ammonia
loss due to volatilization will therefore be an essential part of ammonia decrease during batch
experiments. Because of this and because of the high uncertainty in the measurement, the
ammonium concentration is not a good parameter to follow struvite precipitation.
Additionally three points have to be considered which will affect volatilization behavior:
• Temperature
• pH
• Ammonium concentrations
Ammonium/ammonia equilibrium is very sensitive to temperature and pH. An increase of
temperature from 21 to 25◦ C will increase the ratio of ammonia from 34% to 45%. A pH
increase from 8.9 to 9.0 will also enhance the ammonia ratio from 34% to 43% (appendix
E.2). The measured ammonium concentration was low compared to other literature values
(chapter 2.2). If the ammonium concentration was higher, the volatilized ammonia would of
course increase as well.
5.2.2
pH
As long as struvite was precipitated from batch tests, pH stayed more or less constant (figure
4.5). After phosphate removal pH increased. There are several processes which influence pH:
• Stirring
• Struvite precipitation
• Hydrogen production
As shown in figure 4.4 stirring decreased pH due to ammonia volatilization (equation 12).
+
The provision of PO3−
4 and NH4 for struvite precipitation can be summarized with equation
12 and 13:
(13)
HP O42− ⇀
↽ H + + P O43− , pKa = 12
At a pH of around 9 phosphoric acid will almost fully be present as HPO2−
4 . Therefore for
3−
+
providing PO4 pH drops. In contrast the production of NH4 increases pH (same reaction
than stirring, but opposite direction). But as most of the species at pH 9 are already present
as NH+
4 this effect is much lower. The overall effect of providing the ions for struvite precipitation is therefore a pH reduction.
The cathodic reaction of hydrogen production at the steel electrode increases pH. This effect
was very predominant after phosphate removal, because pH reduction from struvite precipitation stopped.
20
Discussion
5.2.3
Conductivity
Conductivity is mainly influenced by ions. pH stayed more or less constant, but phosphate
and ammonium ions were removed. The removal of these ions lead to a decrease in conductivity (figure E.2). The faster the ion removal, the faster the decrease in conductivity.
5.3
Optimal electrode potential range
-800 mV has turned out to be a threshold potential concerning layer formation on the electrode surface:
• Magnesium potential below -800 mV: sensitive to passivation
• Magnesium potential above -800 mV: effective magnesium dissolution
The range of passivation has been observed with voltammetry and batch experiments. During
the self dissolution experiment and at -1190 mV the magnesium dissolution rate was decreasing (figure 4.7). Visually the passivation range consisted in a white layer, very thin for the
self dissolution experiment, but rugged for -1190 mV (figure E.4). Voltammetry after the
batch experiment with the applied potential of -1190 mV did not even show a breakdown
potential any more, illustrating the thickness of this layer (figure 4.6).
A nearly constant magnesium dissolution rate was reached for applied potentials of -790,
-590 and -190 mV. The higher the applied electrode potential, the faster the dissolution rate
(figure 4.7).
The diffusion limitation as observed for voltammetry could not be verified clearly for the
batch experiments, because the current still increased from -590 to -190 mV. However there
are several indicators that a diffusion layer was growing. From -790 to -590 mV the current
density increased from 50 to 67 A/m2 , from -590 to -190 mV the increase was from 67 to 87
A/m2 (table 4.1). The increase in currents was almost similar, although the difference in the
applied potentials is much higher between -590 and -190 mV. Additionally voltammetry after
the batch experiments showed a shift in breakdown potentials for -590 and -190 mV towards
higher potentials (figure 4.6). This could be an indication that diffusion layers of different
thickness have grown.
Although a diffusion layer has not fully be proven and the magnesium potentials are below
the potential ranges where oxygen evolution could start, it is not recommended to apply too
high potentials. -600 mV seemed to be a very suitable potential in terms of reaction speed
and energy consumption (chapter 5.3.1).
5.3.1
Efficiencies
Current was used very efficiently for magnesium dissolution. The high non-electrochemical
dissolution rate additionally increased the overall dissolution and enhanced coulombic efficiency. Coulombic efficiency did not drop below 100% for all experiments (table E.3 and E.4).
For a long term experiment the self dissolution rate will not be as high as observed, which
will decrease coulombic efficiency. A decrease of coulombic efficiency will increase energy
consumption.
Energy consumption was lower for -790 mV than for -590 mV and -190 mV. The shorter
duration for the phosphate removal could not compensate the higher electric power that was
needed. However, the energy consumption for the electrolysis process of magnesium dissolution was very low compared to the energy consumption for the magnesium production
[23]:
• Electrolysis: 2 kWh/kgMg
Discussion
21
• Magnesium production (only electricity): 20 kWh/kgMg
• Magnesium production (life cycle approach): 40 kWh/kgMg
The life cycle approach includes electricity demand as well as extraction, production and
transport of raw materials.
To calculate the magnesium efficiency, magnesium was assumed to precipitate in a Mg:P ratio
of 1 (chapter 2.4.2). According to the struvite analysis this assumption seems appropriate
(table 4.2). The comparison of the observed magnesium efficiencies with literature values of
chemical dosing is very difficult, as not always the same conditions can be found. Wilsenach
et al. [24] tested MgCl2 dosage in synthetic urine in a continuous stirred tank reactor. The
reached effluent concentrations were around 24 mgP/l (Mg:P 1.1) and 10 mgP/l (Mg:P 2.6).
Tilley et al. [25] did not reach effluent concentration below 4 mgP/l for batch experiments
in natural urine (dosage of MgCl6 , Mg:P 1.7). During the electrocoagulation experiments
15 mgP/l (Mg:P 1.1) and concentrations below 3 mgP/l (Mg:P 1.7) were reached. Electrocoagulation seems therefore to be a very competitive process concerning magnesium efficiency.
The phosphate concentrations in the batch reactor were only dependent on Mg:P dosage and
not on the applied potentials (figure 4.8). The longer residence time for lower magnesium
dosage rates could not enhance the struvite precipitation. This shows that the kinetics of
struvite precipitation are very fast. Hence the dissolution rate of magnesium was limiting
for reaction speed. As struvite precipitation is not limiting, there is still the possibility to
increase the reaction speed by dosing more magnesium. This could be done by enlarging the
electrode surface.
5.4
5.4.1
Possible applications for an electrocoagulation reactor
Magnesium electrode potential or cell voltage
Two configurations have been investigated and both showed good results: applying magnesium electrode potential and applying cell voltage. Good magnesium dissolution is expected
for:
• Magnesium electrode potential above -800 mV, recommended -600 mV (chapter 5.3)
• Cell voltage above 1000 mV, recommended 1250 mV
With applying an electrode potential, the magnesium potential can be controlled at any time.
The possibility to control the magnesium potential directly, will most probably lead to an
unproblematic longtime behavior of the magnesium dissolution.
If a cell voltage is applied, it is important that the magnesium electrode potential stays above
-800 mV. A cell voltage of 1000 mV should ensure this criterion, although this has not been
tested experimentally. As the magnesium potential is not controlled, it is not clear how the
system will react on a long term. A longterm reactor might lead to a magnesium potential
shift. Magnesium potential should therefore be monitored for this configuration.
Generally the chosen approach seemed to be adequate. The applied magnesium potential
delivered important informations about the processes at the electrode. With the improved
knowledge the operation with a cell voltage should be possible.
5.4.2
Reactor operation and process control
Depending on which configuration is chosen, the measured cell voltage or the magnesium
electrode potential could be used to predict phosphate removal. The monitored cell voltage
showed a maximum at phosphate removal, the magnesium electrode potential a minimum
(figure 4.9, figure E.5). As this was found for a batch reactor, it can only be used for a
22
Discussion
continuous reactor in sequencing batch mode.
If this minimum/maximum is used for process control, this will lead to a small stoichiometric
overdosage of magnesium. The Mg:P ratio will be around 1.2. To prevent magnesium loss,
the process has to be stopped at the moment of maximum cell voltage/minimum electrode
potential. The easiest way to stop the process, is to empty the reactor. Another possibility
would be to induce a small cathodic current to prevent the self dissolution at the magnesium
electrode. Song et al. [15] found that local cell action and dissolution of magnesium were
very low at current densities below -5 A/m2 in 1 N NaCl.
However, the simplest application of a reactor would include three tanks:
• Storage tank: urea hydrolysis, balancing of load
• Sequencing batch reactor: filling and magnesium dissolution, emptying
• Settling tank
Conclusion
6
23
Conclusion
Voltammetry turned out to be a very appropriate method to detect electrode characteristics
at different magnesium potentials. The main advantage is the fast and insensitive detection of the breakdown potential where active magnesium dissolution starts. Limitations of
voltammetry are found at high magnesium potentials and diffusion limited currents.
Voltammetry and batch experiments showed that the magnesium electrode is sensitive to
layer formation at electrode potentials below -800 mV. The passivation layer leads to a decrease in the current and therefore in the electrochemical magnesium dissolution rate.
To prevent layer formation at the magnesium electrode, the magnesium potential has to be
kept above -800 mV. An electrode potential of -600 mV seemed to be a very appropriate
potential concerning reaction speed and energy consumption. Too high potentials will lead
to diffusion limited conditions, which will increase energy consumption.
The magnesium electrode had a very fast self dissolution behavior in urine. It was mainly
caused by hydrogen evolution at the surface of the magnesium electrode, which is known as
local cell action. The non-electrochemical dissolution is the reason why coulombic efficiencies higher than 100% were reached. An effective use of current for magnesium oxidation is
also expected for a longterm experiment, although the effect of self dissolution will be much
smaller.
An electrocoagulation reactor could either be operated by applying an electrode potential
(three electrode configuration) or a cell voltage (two electrode configuration). Applying a
cell voltage will be the easier configuration, as no potentiostat is needed. A cell voltage of
at least 1000 mV should be applied. Monitoring magnesium potential could help to prevent
layer formation.
The magnesium potential (for two electrode configuration) or the cell voltage (for three electrode configuration) is recommended for monitoring. For a sequencing batch reactor the
magnesium potential/cell voltage could be used as a parameter to control the process in order not to waste magnesium. An electrocoagulation application could exist of three tanks:
storage tank, reactor and settling tank.
6.1
Outlook and suggestions for future research
Chemical dosing of a magnesium source is an already functional and much more tested possibility to recover phosphate from urine. The method of electrocoagulation has only a chance,
if it has some advantages compared to chemical dosing.
One advantage of electrocoagulation is the high magnesium efficiency and the ability to control and stop the magnesium dosage. If wastage of magnesium is prevented, costs and energy
for magnesium production will be saved. The savings with a sustainable magnesium management could also compensate for the energy needed during electrolysis process.
Electrocoagulation uses a highly technical system. High reaction speed, low reactor volume
and low maintenance costs could therefore be another advantages of this method.
On the other hand the need for constant energy supply points out the limitations of electrocoagulation. Electrocoagulation can only be used at places with electric power supply.
Three different areas for future investigations are suggested:
• Operation of a continuous reactor
• Comparing chemical dosing with electrocoagulation
• Potential areas for electrocoagulation
The next step to improve the method of electrocoagulation will be the operation of a continuous reactor. This reactor could for example be operated as sequencing batch reactor with
24
Conclusion
a storage tank, an electrocoagulation reactor and a settling tank. One key point will be to
investigate how efficient the system will react in the long term, especially concerning energy
consumption. Settling speed and struvite removal rate will be an other issue, which has not
been looked at so far. Not only the phosphate removal, but also the recovery of struvite
plays an important role in overall efficiency. Additionally the effort for maintenance could be
estimated for a longterm reactor.
It has been shown that electrocoagulation is a very functional method to recover struvite
from natural urine. However electrocoagulation should be compared to different magnesium
sources in terms of costs and energy consumption. The efficiency of magnesium usage will
be a key issue to look at. The different dosage sources should be tested at very similar conditions. Especially the magnesium content of urine is a very influencing parameter.
Last but not least potential areas for an electrocoagulation reactor should be evaluated. Because of its high automation potential, electrocoagulation will be favored at places where
labor costs are high.
Electrocoagulation as well as chemical dosing of magnesium is a possibility to recover phosphate from urine. However urine is not treated adequately by only eliminating phosphate,
as still a lot of organic substances and nitrogen are present. Therefore urine will still need
some more treatment steps after struvite precipitation. Struvite precipitation is not the only
process to recover nutrients from urine. The best treatment option has therefore to be chosen
from case to case, to optimize overall benefits.
REFERENCES
25
References
[1] Cordell, D., Drangert, J.-O., White, S. (2009). The story of phosphorus: Global food
security and food for thought. Global Environmental Change 19, 292-305.
[2] Gujer, W. (2002). Siedlungswasserwirtschaft, 2nd Edition, Springer-Verlag, Berlin Heidelberg New York.
[3] Richtlinie des Rates vom 21. Mai 1991 über die Behandlung von kommunalem Abwasser,
91/271/EWG.
[4] Maurer, M., Pronk, W., Larsen, T. A. (2006). Treatment processes for source-separated
urine. Water research 40, 3151-3166.
[5] Jönsson, H., Richert Stinzing, A., Vinnerås, B., Salomon, E. (2004). Guidelines on the
use of urine and faeces in crop production. EcoSanRes Publication Series. Stockholm
Environment Institute, Sweden.
[6] Larsen, T. A., Lienert, J. (2007). Novaquatis final report. NoMix - A new approach to
urban water management. Eawag, 8600 Duebendorf, Switzerland.
[7] Doyle, J. D., Parsons, S. A. (2002). Struvite formation, control and recovery. Water
research 36, 3925-3940.
[8] Bourgeois, A., Udert, K. M., Gujer, W. (2010), Electro-coagulation of Phosphate from
Urine. Master thesis. Eawag, 8600 Duebendorf, Switzerland.
[9] Holt, P. K., Barton, G. W., Wark, M., Mitchell, C. A. (2002). A quantitative comparison
between chemical dosing and electrocoagulation. Eng. Aspects 211, 233-248.
[10] Ciba-Geigy (1977). Wissenschaftliche Tabellen Geigy. Teilband Körperflüssigkeiten, 8.
Auflage, Basel.
[11] Udert, K. M., Larsen, T. A., Gujer, W. (2003). Biologically induced precipitation in
urine-collecting systems. Water Science Technology: Water supply, 3, 71-78.
[12] Udert, K. M., Larsen, T. A., Biebow, M., Gujer, W. (2003). Urea hydrolysis and precipitation dynamics in a urine-collecting system. Water Research 37, 2571-2582.
[13] Koch, R. (2002). Einfluss von Korrosion und Temperatur auf das Ermüdungsverhalten
der Magnesium-Druckgusslegierung AZ91 hp. Technische Universität Darmstadt.
[14] Makar, G. L., Kruger, J. (1990). Corrosion Studies of Rapidly Solidified Magnesium.
Journal of the electrochemical society 137, 414-421.
[15] Song, G., Atrens, A., St John, D., Nairn, J., Li, Y. (1997). The electrochemical corrosion
of pure magnesium in 1 N NaCl. Corrosion Science 39, 855-875.
[16] Song, G., Atrens, A., St John, D., Wu, X., Nairn, J. (1997). The anodic dissolution of
magnesium in chloride and sulphate solutions. Corrosion Science 39, 1981-2004.
[17] Andrei, M., di Gabriele, F., Bonora, P. L., Scantlebury, D. (2003). Corrosion behaviour
of magnesium sacrificial anodes in tap water. Materials and Corrosion 54, 5-11.
[18] Liu, L. J., Schlesinger, M. (2009). Corrosion of magnesium and its alloys. Corrosion
Science 51, 1733-1737.
26
REFERENCES
[19] Ciorba, G. A., Radovan, C., Vlaicu, I., Pitulice, L. (2000). Correlation between organic
component and electrode material: consequences on removal of surfactants form wastewater. Electrochimica Acta 46, 297-303.
[20] Bard, A. J., Faulkner, L. R. (1980). Electrochemical Methods: Fundamentals and Applications. John Wiley & Sons, New York Chichester Brisbane Toronto.
[21] Noufi, M., Yarnitzky, C., Ariel, M. (1996). Polarographic Maxima Revisited. Electroanalysis 8, No. 8-9.
[22] Marsh, G. A., Schaschl E. (1960). The Difference Effect and the Chunk Effect. Journal
of the electrochemical society 107, 960-965.
[23] Classen, M., Althaus, H.-J., Blaser, S., Tuchschmid, M., Jungbluth, N., Doka, G., Faist
Emmenegger, M., Scharnhost, W. (2009). Life Cycle Inventories of Metals. Final report
ecoinvent data v2.1, No 10. EMPA Dübendorf, Swiss Centre for Life Cycle Inventories,
Dübendorf, CH
[24] Wilsenach, J. A., Schuurbiers, C.A.H., van Loosdrecht, M.C.M. (2007). Phosphate and
potassium recovery from source separated urine through struvite precipitation. Water
Research 41, 458-466.
[25] Tilley, E., Atwater, J., Mavinic, D. (2008). Recovery of struvite from stored human
urine. Environmental technology 29, 797-806.
[26] Erickson, R. (1985). An evaluation of mathematical models for the effects of pH and
temperature on ammonia toxicity to aquatic organisms. Water Research 19, 1047-1058.
Methods
A
27
Previous work
Previous work with synthetic urine showed that phosphate could be recovered from urine by
electrocoagulation (figure A.1) [8]. Phosphate removal was faster with higher cell voltage.
But there was reached a maximal phosphate removal rate.
600
0.25 V
0.5 V
0.75 V
1V
1.25 V
Concentration (mgP/l)
500
400
300
200
100
0
0
2
4
6
Time (h)
8
10
12
Figure A.1: Phosphate removal from synthetic urine for different applied cell voltages
B
Methods
An illustration of the system is shown in figure B.1. Magnesium is dissolved at the magnesium
anode, electrons are transferred to the steel cathode, where the oxidation of water to hydrogen
gas proceeds.
Figure B.1: System for electrocoagulation with a magnesium and a steel electrode. Magnesium
is oxidized at the magnesium anode, the electrons are transferred to the steel cathode, where the
reduction takes place.
28
C
C.1
Calculations
Calculations
Ag/AgCl reference electrode
The temperature dependence of the Ag/AgCl reference electrode can be found in table
C.1.The temperature has been chosen as constant value of 21◦ C and for the correction to
SHE 210 mV have been used.
Table C.1: Temperature dependency of the Ag/AgCl reference electrode
◦C
E0
c (KCl) = 3 mole/l
mV
10
20
25
217.4
210.5
207.0
Temperature
C.2
Steel electrode potential
During a batch experiment the magnesium electrode potential was kept constant. The cell
voltage and the steel electrode potential were measured. The comparison of the measured
steel potential and the calculated steel potential (equation 3) is shown in figure C.1. The
measured signal reacted slower than the calculated potential.
−1400
Steel potential (mV)
−1600
−1800
−2000
−2200
calculated
measured
−2400
0
50
100
150
Time (min)
200
250
Figure C.1: Comparison of the measured with the calculated steel potential
Cyclic voltammetry
D
D.1
29
Cyclic voltammetry
Magnetic stirrer
The stirring speed can only be estimated knowing the maximal and minimal stirring speed
(table D.1). Two stirring speeds have been used for experiments: 3 and 7. 3 is allocated to
275/min, 7 to 750/min assuming a linear dependency of stirring speed.
Table D.1: Stirring speed of magnetic stirrer
D.2
Adjustment
Revs
1
10
50/min
1100/min
Theoretical potentials calculated with Nernst equation
The following example shows the application of the Nernst equation for hydrogen evolution
in alkaline solution.
Chemical reaction:
2H2 O + 2e− ⇀
↽ H2 + 2OH −
Nernst equation for a half-cell:
E = E0 +
E0
R
T
z
F
aOx
aRed
aOx
2.303 ∗ R ∗ T
log
z∗F
aRed
0
8.31451
294.15
2 e−
9.648531 · 104
1
2
10−(14−pH) (pH = 9.1)
V
J mole−1 K−1
K
C mole−1
-
Assumptions are made for temperature and pH.
For the reaction of chloride, activity of chloride is assumed to be 3500 mg/l.
The calculation of magnesium concentration at different electrode potentials is shown in figure
D.1. The higher the magnesium potential, the higher the expected magnesium concentration.
30
Cyclic voltammetry
60
Concentration (moleMg/l)
10
40
10
20
10
0
10
−2000 −1500 −1000 −500
0
Mg Potential (mV)
500
1000
Figure D.1: Dependency of the magnesium potential and the magnesium concentration according to
Nernst equation. The higher the magnesium potential, the higher the expected concentration.
D.3
Electrode shifting
Drift of the magnesium and the steel electrode potentials are shown in figure D.2. The
magnesium electrode potential was applied, the cell voltage was measured and the steel
electrode potential was calculated.
2000
Magnesium potential
Steel potential
Electrode potential (mV)
1500
1000
500
0
−500
−1000
−1500
−3000 −2000 −1000
0
1000 2000 3000 4000
Cell voltage (mV)
Figure D.2: Drift of the magnesium and the steel electrode potentials. The magnesium electrode
starts to drift faster at cell voltages above -500 mV, whereas the drift of the steel electrode is stronger
at lower cell voltages.
Batch experiments
E
31
Batch experiments
E.1
Urine composition
The composition of urine before and after the experiments can be found in table E.1 and E.2.
Table E.1: Influent urine composition for all batch experiments
Influent
Na
K
mgN a /l mgK /l
Mg
mgM g /l
PO4 -P
mgP /l
NH4 -N
mgN /l
1
2
3
4
5
6
7
8
<5
<5
<5
<5
<5
<5
<5
<5
200
206
186
209
204
218
188
166
2137
2949
2570
2570
2610
3000
2190
2260
1629
1869
2033
1918
1896
2073
2418
2066
Mean
Std
<5
<5
197
16
2536
327
1988
226
Ca
mgCa /l
Cl
mgCl /l
SO4
mgSO4 /l
TIC
mg/l
1958
2975
1795
1679
1582
1695
2204
1930
11.6
11.4
21.9
17.8
18.6
15.0
17.2
18.8
3120
3120
3114
3163
3146
3104
3022
2726
710
760
725
741
733
721
697
680
1260
1230
1310
1420
1290
1320
1080
1050
1977
448
16.5
3.6
3064
143
721
25
1245
124
Table E.2: Effluent urine composition for all batch experiments
1
2
3
4
5
6
7
8
Mean
Std
E.2
Effluent
Na
K
mgN a /l mgK /l
Mg
mgM g /l
PO4 -P
mgP /l
NH4 -N
mgN /l
211.0
75.7
11.5
12.7
<5
149.8
87.6
158.0
2.2
3.1
3.8
10.2
34.6
2.2
3.3
3.1
2575
2669
2390
2460
2444
2734
2060
2140
1920
1854
1932
1883
1910
2975
2221
1861
2434
237
2069
384
Ca
mgCa /l
Cl
mgCl /l
SO4
mgSO4 /l
TIC
mg/l
1632
2981
1656
1664
1607
1622
1970
1746
8.0
7.0
14.9
12.9
15.4
9.5
12.3
13.8
3160
3185
3087
3247
3113
3036
2944
2757
750
787
715
761
727
702
659
667
1320
1130
1290
1370
1240
1320
744
1030
1860
468
11.7
3.2
3066
156
721
45
1181
209
Volatilization of ammonia
The ratio of ammonia compared to ammonium can be calculated using the following equation:
N H3
= 10pH−pKa
N H4+
(14)
pKa is temperature dependent [26]:
pKa = 0.09018 +
2729.92
273.2 + T
(15)
32
Batch experiments
Concentration (mgN/l)
With a temperature of 21◦ C, pKa will be around 9.37. At a pH of 8.9, 34% is present as
NH3 , 66% as NH+
4.
The effect of stirring speed on volatilization of ammonia and on pH can be found in figure
E.1. The faster the stirring the faster the volatilization and the faster the decrease in pH.
2000
1000
0
0
5
10
15
20
25
30
15
20
Time (h)
25
30
pH (−)
9
8.8
8.6
0
750/min
275/min
5
10
Figure E.1: Effect of stirring speed on pH and volatilization of ammonia. The faster the stirring the
faster the decrease in pH.
Applying magnesium electrode potential
Cond.
(mS/cm)
Cond.
Cond.
Cond.
(mS/cm) (mS/cm) (mS/cm)
Cond.
(mS/cm)
E.3
26
24
0
26
24
0
26
24
0
26
24
22
0
26
24
0
Self dissolution
1
2
3
4
5
6
7
8
−1190 mV
1
2
3
4
5
6
7
8
−790 mV
1
2
3
4
5
6
7
8
−590 mV
1
2
3
4
5
6
7
8
−190 mV
1
2
3
4
Time (h)
5
6
7
8
Figure E.2: Conductivity measurement for the batch experiments. Conductivity decreased during the
experiments. The higher the applied potential the faster the decrease.
Batch experiments
33
Current density (A/m2)
150
100
Self dissolution
−1190 mV
−790 mV
−590 mV
−190 mV
+1250 mV (cell voltage)
50
0
−1200−1100−1000 −900 −800 −700 −600 −500
Mg potential (mV)
Figure E.3: Linear voltammetry before the batch experiments. All curves showed a breakdown potential between -900 and -800 mV.
Figure E.4: Electrode surfaces after the batch experiments. The self dissolution experiment and
-1190 mV showed the formation of a white layer on the electrode surface. The electrode got more
homogeneous for higher potentials.
34
E.3.1
Batch experiments
Efficiencies
The coulombic efficiencies for -1190, -790, -590 and -190 mV are listed in table E.3 and E.3.
The efficiencies are given at different times during the experiments, as single efficiency and
cumulated over the experiments. Coulombic efficiencies stayed generally above 100%. For
some experiments the measured concentrations were subject to high fluctuations, indicating
high measuring uncertainty (especially -790 mV). Due to measuring uncertainty, single values
below 100% resulted.
Table E.3: Coulombic efficiencies for -1190 and -790 mV
Time
min
53
113
171
236
296
354
417
466
-1190 mV
Coulombic eff
%
%
single cumulated
312
152
240
175
188
271
161
181
312
238
238
228
224
228
223
221
Time
min
15
30
60
90
120
150
180
210
240
270
300
-790 mV
Coulombic eff
%
%
single cumulated
141
105
149
67
108
137
81
134
110
92
125
141
123
135
115
113
118
112
115
114
112
113
Table E.4: Coulombic efficiencies for -590 and -190 mV
Time
min
30
60
90
120
150
180
201
-590 mV
Coulombic eff
%
%
single
139
97
122
117
120
112
121
cumulated
139
119
120
119
120
118
118
Time
min
43
88
130
181
220
-190 mV
Coulombic eff
%
%
single
135
141
118
107
105
cumulated
135
137
131
125
122
Batch experiments
E.3.2
35
Process Control
PO4−P
Mg2+
100
0
0
1400
100
200
100
200
1200
1000
0
0.1
0.05
0
0
100
Time (min)
200
Conc. (mg/l)
200
Mg potential: −190 mV
Slope cell volt. (−) Cell voltage (mV)
Slope cell volt. (−) Cell voltage (mV)
Conc. (mg/l)
Mg potential: −590 mV
200
100
0
0
2200
2000
1800
1600
1400
1200
0
0.1
100
200
100
200
0.05
0
0
100
Time (min)
200
Figure E.5: Monitored cell voltage as parameter for process control for a magnesium potential of -590
and -190 mV.
36
Batch experiments
E.4
Applying cell voltage
200
−590 mV
+1250 mV (cell voltage)
150
100
50
0
0
1
2
Time (h)
3
Concentration (mgMg/l)
250
250
200
150
100
50
0
0
1
2
Time (h)
3
1
2
Time (h)
3
150
9.1
100
pH (−)
Current density (A/m2)
Concentration (mgP/l)
The applied cell voltage of 1250 mV is very comparable with the applied electrode potential
of -590 mV (figure E.6).
50
9
8.9
8.8
0
0
1
2
Time (h)
3
8.7
0
Figure E.6: Comparison between the batch experiments with applied cell voltage of +1250 mV and
applied electrode potential of -590 mV. Both experiments showed very similar results.
E.5
Struvite
The precipitate has been analysed with the following procedure:
• Filtration of the precipitants after the batch experiment
• Drying of the filtrate
• Pounding the filtrate
• Dissolving the filtrate in concentrated hydrochloric acid (32%)
• Diluting the solution
• Filtration of the solution (45 µm)
3−
2−
2+
+
2+
+
• Analysis of NH+
4 , Mg , PO4 as well as K , Ca , Na , SO4
The results of the precipitate analysis can be found in table E.5.
Batch experiments
37
Table E.5: Precipitate analysis for three batch experiments. The analysis showed that precipitate is
almost fully present as struvite.
-790 SHE
-590 SHE
-190 SHE
E.6
NH4 -N
Na
K
Mg
Ca
SO4
PO4 -P
1.04
1.07
1.05
0.00007
0.00008
0.00007
0.00004
0.00005
0.00004
1.00
1.00
1.00
0.00013
0.00018
0.00016
0.00000
0.00000
0.00000
1.01
1.08
1.07
mole/moleMg
mole/moleMg
mole/moleMg
Magnesium and phosphate concentrations
The magnesium and the phosphate concentrations measured during the experiments are given
in table E.6 and E.7. The number of the experiment correlates with the numbering of urine
composition.
Table E.6: Magnesium and phosphate concentration for the self dissolution experiment and for -1190
and -790 mV
Self dissolution (5)
Time
Mg
PO4 -P
min
mgM g /l mgP /l
0
60
120
171
240
300
300
360
420
480
1.7
4.5
4.7
6.1
6.1
6.3
6.4
7.5
7.8
9.5
199.0
155.0
120.0
101.0
84.9
74.3
74.1
63.2
56.3
46.0
-1190 mV (3)
Time
Mg
PO4 -P
min
mgM g /l mgP /l
0
53
113
171
236
296
296
354
417
466
1.4
4.4
5.2
6.0
8.0
9.8
9.8
12.8
15.4
18.8
220.0
145.0
113.0
79.4
61.2
49.0
43.5
30.4
22.4
18.0
Time
min
0
15
30
60
60
90
120
150
180
210
240
270
300
-790 mV (2)
Mg
PO4 -P
mgM g /l mgP /l
1.2
3.6
2.9
3.1
3.8
3.7
5.5
5.9
9.6
20.1
35.7
48.8
70.6
220.0
199.0
181.0
139.0
137.0
120.0
94.0
58.0
41.0
19.0
11.0
5.2
3.8
38
Batch experiments
Table E.7: Magnesium and phosphate concentration for -590 and -190 mV and for the cell voltage of
1250 mV
-590 mV (7)
Time
Mg
PO4 -P
min
mgM g /l mgP /l
0
30
60
60
90
120
150
180
180
201
2.0
6.0
4.0
4.0
8.0
16.0
36.0
65.9
66.3
87.7
188.1
136.0
99.0
99.0
60.0
27.0
8.6
4.6
4.5
3.3
-190 mV (6)
Time
Mg
PO4 -P
min
mgM g /l mgP /l
0
43
88
88
130
181
220
220
1.2
6.2
14.5
14.2
53.3
113.0
155.8
149.8
218.0
114.0
36.0
35.0
6.5
3.0
2.2
2.2
Cell voltage 1250 mV (8)
Time
Mg
PO4 -P
min
mgM g /l
mgP /l
0.0
30.0
60.0
60.0
90.0
120.0
150.0
180.0
201.0
6.0
6.0
8.0
8.0
16.0
28.0
50.0
70.0
83.0
138.0
99.0
58.0
59.0
26.0
11.0
5.7
3.9
3.1