DEDICATION
This project is dedicated to my warrior mother who made sure I got where
I am today. Ruth Mkhwananzi the journey continues with many battles to
be fought.
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ACKNOWLEDGEMENTS
I give thanks to the Lord God for being my rock and strength and for the gift of life.
My heartfelt gratitude goes to my lecturer and supervisor Mr L Moyo for his humour, support
and guidance throughout the entire preparation of this project. His guidance helped me set
very high standards and his expertise was instrumental towards the project and included a
number of original and novel ideas.
My thanks must also be extended to a number of others. The first would be to the NUST
library staff for their excellent and immediate help on many occasions on where to find the
textbooks or journals that I needed.
Last but not least I would like to thank my family and friends for keeping me sane throughout
my late hours at the library and offering emotional support.
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ABSRACT
Phosphorus is an essential nutrient to sustain life. Since there are limited phosphorus
resources, recovery and reuse of phosphorus is therefore necessary. Wastewater usually
contains large amount of phosphorus which can cause severe environmental problems such as
eutrophication in water bodies. Thus, recovery of phosphorus from wastewater removes the
excess amounts and prevents environmental pollution. The recovered phosphorus could also
be considered as a rich fertilizer and helps in reaching sustainable use of phosphorus
resources. Struvite precipitation is a new method to remove and recover phosphorus from
wastewater. The precipitated struvite could be reused as slow release fertilizer. On the other
hand, addition of chemicals like Iron and Aluminium in order to remove Phosphorus in
wastewater treatment plants is costly and also affects adversely the plant availability of
phosphorus. Therefore, struvite crystallization as a no chemical method increases the
efficiency in phosphorus removal and reuse capacity. In this study, the main goal is to review
the sources of magnesium for struvite precipitation. Spontaneous struvite formation depletes
the magnesium in wastewater so that additional magnesium source is required to produce
induced struvite for P recovery. The cost of chemicals used as magnesium source has been
the primary factor preventing the process from being widespread. In this study alternative
sources to expensive magnesium salts are reviewed. Thus, waste products can be interesting
alternatives to industrially-produced magnesium salts. The common alternatives are bittern,
brine, seawater and wood-ash. Each of the alternative sources has its merits and de-merits, as
a result the sources where compared using the Kepner-Tregoe (KT) analysis method. The
best alternative source from this analysis is brine. A cost benefit analysis is carried out to
verify the economic viability of using brine as an alternative cheaper source for struvite
precipitation.
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Contents
ABSRACT.................................................................................................................................................. 3
1
CHAPTER ONE ................................................................................................................................. 8
1.1
TOPIC ....................................................................................................................................... 8
1.2
TITLE ........................................................................................................................................ 8
1.3
AIM/GENERAL OBJECTIVE ....................................................................................................... 8
1.4
SPECIFIC OBJECTIVES............................................................................................................... 8
1.5
INTRODUCTION ....................................................................................................................... 8
1.5.1
1.6
2
Challenges Of Phosphorus ............................................................................................. 8
STRUVITE FOR PHOSPHORUS RECOVERY.............................................................................. 10
1.6.1
Background ................................................................................................................... 10
1.6.2
Advantages.................................................................................................................... 10
1.6.3
Magnesium Addition ..................................................................................................... 10
CHAPTER TWO .............................................................................................................................. 10
2.1
PROPERTIES OF STRUVITE ..................................................................................................... 10
2.1.1
Basic Stoichiometry ....................................................................................................... 10
2.1.2
Conditional Solubility Product ...................................................................................... 11
2.1.3
Crystal Morphology ....................................................................................................... 11
2.2
CONDITIONS FOR Efficient STRUVITE CRYSTALLIZATION IN WASTEWATER STREAMS ........ 12
2.2.1
Supersaturation ............................................................................................................ 12
2.2.2
Composition .................................................................................................................. 13
2.2.3
Optimum pH.................................................................................................................. 14
2.2.4
Wastewater ................................................................................................................... 14
2.2.5
Precipitation Reactor .................................................................................................... 14
2.3
FACTORS AFFECTING THE CHOICE OF AN ALTERNATIVE SOURCE OF MAGNESIUM ............ 15
2.3.1
Economic and Environmental Factors .......................................................................... 15
2.3.2
Potential For P recovery and Product quality ............................................................... 15
2.3.3
Drawbacks ..................................................................................................................... 16
2.4
ALTERNATIVE SOURCES OF MAGNESIUM FOR STRUVITE PRECIPITATION ........................... 16
2.4.1 .............................................................................................................................................. 16
3
2.4.2
Bittern ........................................................................................................................... 18
2.4.3
Seawater ....................................................................................................................... 19
2.4.4
Magnesite ..................................................................................................................... 21
CHAPTER THREE (METHODOLOGY) .............................................................................................. 23
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3.1
Introduction .......................................................................................................................... 23
3.2
Factors to be considered....................................................................................................... 24
3.2.1
Total cost of the source in the entire process .............................................................. 24
3.2.2
Product quality and recovery efficiencies .................................................................... 24
3.2.3
Availability and accessibility of the source ................................................................... 25
3.2.4
Safety ............................................................................................................................ 25
3.2.5
Rate of production ........................................................................................................ 25
3.2.6
Environmental impact of using the source ................................................................... 25
3.3
4
Analysis Of Results ................................................................................................................ 26
CHAPTER FOUR (CONCLUSION AND RECOMMENDATIONS) ........................................................ 26
4.1
Conclusion ............................................................................................................................. 26
4.2
Recommendations ................................................................................................................ 26
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List of tables
Table 1 Struvite component ions' set equilibrium equations. ............................................................... 12
Table 2 wood-ash chemical compositions ............................................................................................ 16
Table 3 A typical Bittern chemical composition .................................................................................... 18
Table 4 A typical seawater composition ............................................................................................... 19
Table 5 Struvite purity found from previous experiments..................................................................... 20
Table 6 Magnesite and Magnesia typical compositions ....................................................................... 22
Table 7 Weighted analysis for choosing the best source ....................................................................... 25
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List of figures
Figure 1 A Phosphate production curve showing peak production by the year 2030 ........................... 9
Figure 2 A pilot-scale struvite crystallization reactor.From Matsumiya[45]. ...................................... 15
Figure 3 Graphical representation of the calcination process ............................................................... 22
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1 CHAPTER ONE
1.1 TOPIC
Struvite Precipitation
1.2 TITLE
A review of the sources of magnesium for struvite precipitation
1.3 AIM/GENERAL OBJECTIVE
To evaluate the alternative sources for struvite precipitation
1.4 SPECIFIC OBJECTIVES




To analyse the properties of struvite crystals and the conditions for efficient struvite
precipitation
To analyse the various alternative sources of magnesium sources for struvite
precipitation
To compare the various alternative sources of magnesium using the Kepner-Tregoe
(KT) analysis method.
To determine the most effective alternative source of magnesium for struvite
precipitation
1.5 INTRODUCTION
Phosphorus is an essential nutrient to sustain life. Since there are limited phosphorus
resources, recovery and reuse of phosphorus is therefore necessary. Wastewater usually
contains large amount of phosphorus which can cause severe environmental problems such as
eutrophication in water bodies. Thus, recovery of phosphorus from wastewater removes the
excess amounts and prevents environmental pollution. The recovered phosphorus could also
be considered as a rich fertilizer and helps in reaching sustainable use of phosphorus
resources. Struvite precipitation is a new method to remove and recover phosphorus from
wastewater. In this method, Magnesium, Ammonium and Phosphate are mixed in specific
molar ratios and phosphorus precipitates as struvite. Generally, struvite consists of 13%
Phosphorus, 6% Nitrogen and 10% Magnesium. The precipitated struvite could be reused as
slow release fertilizer. On the other hand, addition of chemicals like Iron and Aluminium in
order to remove Phosphorus in wastewater treatment plants is costly and also affects
adversely the plant availability of phosphorus. Therefore, struvite crystallization as a no
chemical method increases the efficiency in phosphorus removal and reuse capacity.The
biggest hindrance to the widespread application of the struvite crystallization technology has
been the cost magnesium salts like MgCl2, MgO and MgSO4. Their cost therefore makes the
price of struvite for fertilizer application expensive and not economically viable hence the
need to review the alternative sources like seawater and others.
1.5.1
Challenges Of Phosphorus
All living organisms need phosphorus (P) for their growth which is also a critical element in
fresh water ecosystems [1]. Receiving waters such as dams and rivers access P from the
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runoff agricultural areas and effluent from wastewater, mainly as a result of fertilizers being
used [2]. When large quantities of phosphorus are disposed in water bodies, quality of water
is compromised and also eutrophication occurs. Phosphate rock is the main source of
phosphorus containing fertilizers used in agriculture. Over the years over-fertilization of
agricultural lands using phosphorus fertilizer has been a common thing, in the northern
hemisphere, it is estimated that about 25% of the phosphate rock mined since 1950 ended up
in water bodies and landfills [3]. Besides damaging the nutrient balance in the receiving
environment, it is a waste of a limited resource. With the world‟s population continuous
growth and changing consumption patterns there will be demand for intense fertilizing, thus
an increased pressure on the demand for P containing fertilizers [4]. A peak production of P
containing fertilizers is estimated to occur around the year 2030 (Figure 1-1), while in the
next century depletion of the P containing rock reserves is expected [5]. The price of
remaining P rock will also increase because of the processing of low-quality reserves
remaining. as the remaining r making the availability of P for fertiliser production skewed to
countries with high purchasing power [6].
Figure 1 A Phosphate production curve showing peak production by the year 2030
As a result, emphasis is being put into increasing efforts in recycling and reuse of products
for environmental and economic benefits. An example is the European Union‟s action plan to
meet growing concern for P scarcity [7,8]. Waste treatment or handling industries will play a
key role in the measures proposed. Recovery and re-circulation of finite nutrients like
phosphorus and other nutrients is becoming of paramount in importance in this century.
Depletion time of the P rock will be extended if recovered phosphorus replaces the P rock in
fertilizer production.
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1.6 STRUVITE FOR PHOSPHORUS RECOVERY
1.6.1
Background
Struvite is a crystalline mineral with low solubility consisting of equimolar amount (1:1:1) of
magnesium, ammonium and phosphate ions. Its chemical name is magnesium ammonium
phosphate (MgNH4PO36H2O), also abbreviated as MAPS. It is a good source of P and a slow
release fertilizer. Wastewater carries within nutrients from urine and other sources,
spontaneous struvite crystallization in wastewater management utilities has been a common
problem [11]. The occurrence is often found in pipe connections where turbulent flows are
dominant, valves and pumps where pH is elevated due to the release of CO2 [12, 13]. Most of
the research on struvite has therefore focused on mitigation strategies to prevent reduction of
system efficiency from clogging and scaling [14]. However, recently struvite has also become
a part of the solutions in resource recovery and pollution prevention of nutrients.
1.6.2
Advantages
It has been shown that struvite has good capabilities as a product for P recovery. An added
advantage of recovering P as struvite, besides the fact that wastewater treatment plants offer
favourable conditions for precipitation, is that it has good fertilizer properties [15]. When
compared to other phosphorus products, for example hydroxyapatite and other calcium
phosphates (Ca-P), struvite releases nutrients at a slower rate and has essential nutrients
present in the same crystal [16, 17]. Struvite extraction from wastewater streams removes
both nitrogen (N) and P, which both have an impact on the nutrient balance of receiving
waters. Use of struvite can also replace a part of imported P rock used in the fertilizer
industry [9]. Struvite precipitation also prevents the scaling problem of the pipes and
treatment facilities in wastewater treatment plants. This problem is common in most of
treatment plants mainly due to use of chemicals such as Aluminium and Iron.
1.6.3
Magnesium Addition
Most wastewater streams do not contain a high enough amount of Mg for efficient struvite
production. To assure a supersaturation level of struvite that gives continuous crystal growth
in wastewater streams, addition of magnesium ions is needed [18]. High molar ratios of Mg2+
to both Ca2+ and P are important to prevent low purity product and keep P recovery high [19,
20]. Several commercially available Mg salts exist. The most common sources are MgO,
MgCl2 and Mg (OH)2. The phosphorus recovery efficiency has been found to be the highest
with MgO and lowest with Mg(OH)2 [20]. The availability, solubility and reactivity of
magnesium are important parameters when selecting a source [21].
2 CHAPTER TWO
2.1 PROPERTIES OF STRUVITE
2.1.1
Basic Stoichiometry
Struvite forms according to the chemical reaction [14],
Mg2+ + NH4+ + PO43- + 6H2O → MgNH4PO3 ∙ 6(H2O)
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The precipitation process is separated into two stages: nucleation and growth. Nucleation is
the
formation of crystal embryos as the constituent ions combine. This may homogenous
nucleation which occurs spontaneously, heterogenous nucleation which occurs in the
presence of impurities or secondary nucleation due to seed crystals [22]. Growth is the
incorporation and accumulation of constituents into the crystal lattice of the embryos to form
detectable crystals [23]. For these processes occur the product of the concentrations of Mg2+,
NH4+ and PO43- exceed the solubility product, Ksp, meaning the solution is supersaturated
[14]. The solubility product can be expressed as the product of ion concentrations:
Ksp = [Mg2+] ∙ [NH4+] ∙ [PO43-]
2.1.2
Conditional Solubility Product
The struvite precipitation process is largely controlled by temperature, pH and the presence
of other ions like Ca2+ [24]. Thus, for specific reaction conditions, the conditional
solubility product ( Kso), should be used in thermodynamic calculations. It incorporates the
ionic strength and ion activity of the struvite component ions as well as pH. Kso is as
calculated as:
Kso = 𝑎𝑀𝑔2+ ∙ 𝑎𝑁𝐻4+ ∙ 𝑎𝑃𝑂43With 𝑎𝑖 being the activity defined as, 𝑎𝑖 = 𝛾𝑖 ∙ [𝐶𝑖]
Where γi and [Ci] are the activity coefficient and total concentration, respectively, of the
𝑖th ion in solution. This means that for γi =1 we have Kso = Ksp [11]. It was found
Ohlinger [25] that the minimum solubility product of struvite to be 10-13.26 (pKso=13.26)
at pH 10.2 when considering the presence of three magnesium phosphate complexes
(MgPO4)-, MgHPO4 and MgH2PO4+. These complexes reduce the concentrations of Mg2+
and PO43- available for struvite precipitation, as a result decreases the struvite precipitation
potential (SPP) [25]. Thermodynamic models like Visual MINTEQ, use pKso as the pKsp of
struvite widely.
2.1.3
Crystal Morphology
Struvite crystals are orthorhombic in structure, while the morphology depends on
supersaturation and competing ions. When supersaturation is low (pH<9.0), coffin-shaped
crystals are very common. Whilst dendrite and X-shaped crystals indicate higher growth rates
at high supersaturation [12, 26-28]. Literature also has common terms like „rod-like and
needle-like‟, which show similarities in terms of shape and appear at medium supersaturation
levels. The crystal length can range from only few to several hundred micrometres [29].
Competing ions like calcium ions, tend to inhibit crystal growth, resulting in smaller crystals
[30]. It has also been found that the growth of crystals in fluidized bed reactors (FBR) is
mainly a transport-controlled process that is dependent on fluid dynamics, and that growth
rate can be related to the mixing energy [23, 31].
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2.2 CONDITIONS FOR Efficient STRUVITE CRYSTALLIZATION IN
WASTEWATER STREAMS
2.2.1
Supersaturation
Struvite precipitation to occur, the supersaturation ratio (Ω) is greater than 1. Ω is
defined in equation 1 [22, 32]:
(1)
The ion activities in the numerator of the equation above can be calculated from all equilibria
equations for the different species of the ions. An example of a set of equations is given in
Table2-1 for struvite precipitation in a typical animal waste treatment lagoon [14].
Struvite component ions’ set equilibrium equations.The equations are then used to
calculating the concentration of component ion in solution.The result is then used for
activity (a) calculations.
Table 1 Struvite component ions' set equilibrium equations.
To be
calculated
Equilibrium Equation
pK
From [25] to
[30]*
2.56
4.8
Mg2+
2.91
0.45
2.15
7.2
3-
PO4
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12.25
NH4+
9.25*
In calculations of the activity coefficient,γi, and consequently, Kso for ionic strength similar
to that of a wastewater stream, Davies approximation to Debye-Hückels equation can be
used. [14, 33]. When the activities are known, e.g. by using calculation software like Visual
MINTEQ, the saturation index (SI) can be calculated. it is a function of the ion activity
product (IAP) and Ksp. Rahman [34] define these as follows:
(2)
(3)
When 𝐼𝐴𝑃 > Ksp, meaning SI > 0, the solution is supersaturated, and precipitation occurs and
solids start to appear [33]. We can also calculate saturation 𝑆𝑎 from equation 4 where 𝑣 is the
number of ions in the compound. For struvite, 𝑣 = 3 [35].
(4)
The molar ratio of the components of struvite is 1:1:1, Since the relative activities of Mg2+
and NH4+ to PO43- can be lower in impure solutions and at different pH, an excess of these
ions may be necessary to avoid limiting the precipitation and recovery of phosphorus [36].
2.2.2
Composition
Essential factors for struvite crystallization and purity is the molar ratios of struvite
components (Mg:P:N), and the molar ratios of struvite components to competing ions with
the struvite precipitation process. Particularly magnesium-to-calcium (Mg:Ca), magnesium to
phosphorus (Mg:P) and phosphorus-to-calcium (P:Ca) molar ratios are paramount in this
context. Ca2+ can substitute Mg2+ in precipitation with phosphate and form calcium
phosphates (Ca-P). For example, Monenite (CHPO4), tricalcium phosphate (Ca3(PO4)2,
Hydroxyapatite (Ca5(PO4)3OH) etc.). Research shows that the higher Mg:Ca and P:Ca molar
ratio, the higher purity of struvite, and that these should be kept higher than approximately
2:1 to prevent inhibition of formation and growth of struvite crystals. Mg:Ca >1:1 has been
found to avoid crystalline compounds of calcium phosphates, although amorphous forms are
still possible. When Mg2+ and NH4+ are not limiting for precipitation, Ca2+ ions concentration
determines the composition of the deposit. If Mg:P molar ratio is less than 1, both Mg and Ca
concentration impact deposit composition. [16, 19, 30, 37]. Calcium inhibition seems also to
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depend on NH4+ concentration. Crutchik [38] state that calcium inhibition is less likely in
wastewaters with high ammonia concentration (N:P ) is approximately 4. High N
concentration also helps prevent the formation of newberyite (Mg(PO3OH)·3H2O) [39].
2.2.3
Optimum pH
The reaction pH is considered as one of the most important process parameters for struvite
precipitation [18, 40]. Many studies on optimum pH levels have reported that struvite has a
high potential to precipitate in the pH range of 8.0 to 10,7 [14, 16]. Ohlinger [25] suggested
a minimum struvite solubility occurring at pH 10,3-10,7, considering ionic strength and
complexes formed. For wastewater streams, the minimum solubility of struvite is assumed to
occur at a pH between 8,9 and 9,3 [16, 29, 40, 41]. The importance of pH is also shown in
Table 2-1, as all of the equilibrium reactions are pH dependant. As a result, pH control and
adjustments are very essential for struvite reactors. Co-precipitation will also be determined
by pH, as Ca-P generally need higher pH than struvite to be formed. Hao [42] found that at
pH below 8,5 (in tap water with ∼87 mg/L Ca2+) Ca was not detected in the deposit, but
increasing pH above this gave Ca compounds. It concluded that at pH 8.0, it seems unlikely
to form relatively pure struvite in wastewaters containing Ca2+.
2.2.4
Wastewater
Struvite crystallization is first of all a method to remove P from the liquid phase of
wastewater streams. This is most common in wastewater treatment plants (WWTPs) with
enhanced biological P removal (EBPR). These are plants with designs which make
polyphosphate accumulating organisms (PAOs) take up more polyphosphate in the aerobic
zone, than what they release in the anaerobic zone, thus giving a net removal of P. For the
recovery P by struvite crystallization, which possible in the presents of NH4+, the reject
streams from the sludge dewatering step is suitable. At that stage, the water has gone through
anaerobic digestion, thus increasing the concentration of dissolved P and N. Ammonium
(NH4+) is released during protein degradation, at the same time as Mg and P is released from
PAOs and other bacteria through cell lysis [21, 25, 43, 44].
2.2.5
Precipitation Reactor
Fluidized or mixed reactors are the most suitable and widely used for struvite crystallization.
A struvite reactor made for a pilot scale study in Japan is shown in Figure 2-1. Here,
seawater was used for adding Mg and air was used for circulating and mixing the streams
[46, 47]. Sodium hydroxide (NaOH) addition or CO2 stripping can be used to adjust pH [44].
The changing diameters of the reactor helps the mixing, as it causes turbulent eddies [31].
Without further treatment, the struvite precipitate will be in the form of a crystalline powder.
To produce struvite in a form that is more practical to handle, a binder can be added to the
product before drying [48]. Additionally, a filtration step of the wastewater stream prior to
entering the reactor could be necessary to lower TSS concentration, as this may lower the
quality of the end product [49].
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Figure 2 A pilot-scale struvite crystallization reactor.From Matsumiya[45].
2.3 FACTORS AFFECTING THE CHOICE OF AN ALTERNATIVE SOURCE OF
MAGNESIUM
2.3.1
Economic and Environmental Factors
The main purpose of investigating the alternative sources of magnesium is to lower the costs
of the whole process of struvite precipitation. It has been discovered that when using pure
magnesium salts like MgCl2 and MgSO4, Mg addition can contribute to about 75% cost of the
entire production process [50,51]. Alternative sources are like bittern a waste product from
the crystallization of table salt (NaCl), when used contribute to waste management objectives
of recycling and re-use whenever it is possible. For WWTPs located in coastal areas,
seawater can be used as a free source of Mg2+ ions with very little transport costs as distances
travelled will be shorter.
2.3.2
Potential For P recovery and Product quality
The amount of P recovered in wastewater when alternative magnesium sources are added
during struvite precipitation has been investigated through different studies. Seawater, bittern,
NF brine, magnesite and wood-ash have been studied and show appreciable recoveries that
are acceptable with room for improvements. Most of these alternative sources undergo pretreatment before application to increase their recovery potential. For example, seawater and
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brine are treated by passing them through NF membranes to improve Mg2+ concentrations.
Magnesite is treated by being with an acid to improve its solubility and suitability for struvite
precipitation processes.
2.3.3
I.
II.
III.
IV.
Drawbacks
Since Mg2+ is much lower in alternative Mg sources than in pure Mg sources, like for
example when using brine and seawater where the result may be the blending and
dilution of the wastewater stream. This causes TAN and total inorganic
orthophosphates at the entrance of the precipitation reactor to decrease. It also
reduces the struvite precipitation potential (SPP).
Alternative sources of Mg2+ tend to add impurities into the wastewater as they
contain other salts like sulphates and chloride and also other ions present at lower
concentrations like Na+. This can form ion pairs which lower the SPP, resulting in the
need to keep the pH at higher level to get a specific P recovery.
Presence of Ca2+ ions may cause the formation of Ca-P like Ca3(PO4)2, Ca5(PO4)3OH
etc. This results in reduced purity of struvite.
Sources like magnesite (MgCO3) are partially soluble when compared to pure Mg
sources leading to increased dosage of Mg2+ being added and thus increase the cost.
2.4 ALTERNATIVE SOURCES OF MAGNESIUM FOR STRUVITE
PRECIPITATION
2.4.1
Wood-ash is a waste product of burning wood and industrial coal. It commonly used by
gardeners as a source of potash. Woo-ash is an interesting low-cost source of magnesium in
struvite precipitation.
2.4.1.1 Chemical Composition
Wood-ash is a waste product of burning wood and industrial coal. It commonly used by
gardeners as a source of potash. Woo-ash is an interesting low-cost source of magnesium in
struvite precipitation
1
Coniferous wood, 60% bark/40% wood chips, burnt at 500°C.
2
Lodgepole pine sawdust burnt at 538°C.
3
Wood chips, wood residue, bark. .
Table 2 wood-ash chemical compositions
Calcium
Potassium
Magnesium
Manganese
Phosphate
Sulphate-S
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Olanders and
Steenari (1995)1
226
47.4
15.8
-
Etiegni and
Campbell (1991)2
187
111
59.7
10.5
17.0
-
Etiegni (1991a)3
303
40.9
22.6
6.68
14.4
4.56
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Aluminium
Iron
Sodium
Zinc
Chromium
Copper
Lead
Nickel
Cadmium
Nitrogen
20.8
23.7
6.2
2.7
0.06
-
10.4
8.80
2.96
2.68
0.052
0.345
0.051
0.059
<0.002
-
23.0
19.9
3.8
0709
0.0805
0.148
0.133
0.0430
0.0153
0.6
2.4.1.2 Economic and Environmental Factors
Alternative sources are studied a means to reduce costs. For all magnesium sources except
magnesium sulphate, the chemical input costs to produce struvite using wood-ash are lower
than the estimated financial value of the struvite produced. Wood-ash was found to be four
times cheaper than industrially produced magnesium salts However, this calculation does not
include all of the costs for reactor construction and operation, wastewater pre-treatments and
struvite transport and marketing. and bittern in areas that are far away from the sea [50].
Wood ash could be a well-suited magnesium source for use in rural areas where industrially
produced chemicals are either not available or too expensive, due to transport and
intermediary costs. It is also an environmentally friendly substance which has less toxic to the
environment, helping in the global efforts of recycling and re-use of resources for sustainable
development.
2.4.1.3 Potential For P Recovery and Product Quality
It can be used to precipitate struvite phosphate from wastewaters in the form of struvite. The
precipitate produced when wood-ash is used is of poor crystal morphology and is
contaminated by other precipitates like calcium phosphates (Ca-P). Also, it may contain
heavy metals in levels above fertilizer accepted stipulations. Presence of Ca2+ which compete
with Mg2+ and substitute magnesium to form calcium phosphates (Ca-P) is a big hindrance to
precipitate quality and concentration of Mg2+ increased by increasing furnace temperatures
for burning wood. The precipitate produced using wood ash is a mixture of various minerals,
with calcite a phosphorus containing mineral as a main compound. Wood-ash forms a
precipitate that is rather a soil conditioner enhanced with phosphorus than a phosphorus
fertilizer.
2.4.1.4 Drawbacks



Direct comparison with other magnesium sources that produce pure struvite is not
possible since the precipitate produced is a phosphorus rich soil conditioner not a
fertilizer.
To increase Mg2+ concentration in wood-ash temperatures to produce it should be
very high, way above 600 °C, which therefore increases its cost significantly.
The precipitate produced may contain micro-pollutants, though most of them can be
killed by drying at ambient temperatures.
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

2.4.2
Wood-ash is not a very suitable precipitant for struvite production because of the high
heavy metal content and the low nutrient content of the precipitate.
Wood-ash is applicable as a precipitant for struvite precipitation only in limited types
of wastewater streams. Thus far, it is only applicable in source separated urine.
Bittern
Bittern is a residue of minerals in a highly concentrated salt brine which remains behind after
the industrial extraction of NaCl and/or halogen compounds from seawater. It can be used as
a magnesium source for precipitating struvite as a means to remove phosphorus and nitrogen
from wastewater.
2.4.2.1 Chemical Composition
Bittern as residue of NaCl and/ or halogen compounds extraction from seawater contains a
number of dissolved ions. Typical composition of a 1.2 g/ml density consists of 30 mg/l
magnesium, which is approximately 30 times the concentrations in sea water, 10 mg/l
potassium, and significant concentrations of sodium, sulphate and chlorine ions and traces of
bromine and boron.
The table below shows a typical bittern chemical composition used for struvite precipitation
for treatment of fertilizer manufacturing effluent.
Table 3 A typical Bittern chemical composition
Element
TDS
Calcium
Magnesium
Sodium
Chlorides
Sulphates
Carbonates
Potassium
Bicarbonate
Bromine
Boron
Iodine
Lithium
Conductivity
Value (mg/L)
292
1600
73.84
21.76
218.63
3.2
0.5
9.81
1.73
12
70
5
0.31
583mS/cm
2.4.2.2 Economic and environmental Benefits
Bittern is a waste product and is very cheap for areas near coastlands where there is no need
for transportation over long distances. As a source of magnesium in struvite precipitation, it is
cheaper than the industrially manufactured Mg salts, yet with phosphate recoveries of about
99% in certain instances. In coastlands, it lowers the of transport as well as it replaces the use
of manufactured chemical, thus introducing a high level of sustainability to the process of
struvite precipitation. Bittern of magnesium content 9220mg/L-3200mg/L can be
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commercialized. From an ecological point of view the combination of two waste products
bittern and wastewaters to a fertilizer, seems to be an ideal process.
2.4.2.3 Potential For P Recovery and Product Quality
Bittern has been studied and found to be suitable source for phosphorus recovery wastewater
effluent through struvite precipitation [51,52]. It is applicable in various types of conditions
and wastewaters like landfill leachate, animal slurry, coke manufacturing wastewater,
fertilizer manufacturing wastewater and urine. The quality of struvite produced is high and
comparable to the model struvite standards. The crystal morphology is sometimes
comparable to the one expected when Mg salts are used. Shapes like elongated needle crystal
shape and the elongated prismatic crystal shape.
2.4.2.4 Drawbacks



2.4.3
Storage tanks for bittern should be lined with a highly rust resistant material in their
design to avert rusting and corrosion, since it is a residue of seawater highly
concentrated with ions.
Some of the Mg in bittern is insoluble leading to the addition of higher dosages of Mg
being added to offset the insoluble concentrations for efficient phosphorus recovery.
The ratio of magnesium ions to phosphate ion is increased to (Mg2+:PO43-is 1.5:1) not
the usual 1:1.
For areas far away from coastlands it becomes expensive as a result of the
transportation costs and difficulty in handling it since it is a liquid. To transport it
easier and quickly, it has been dried and transported in a powder form though the
drying process becomes an added expense.
Seawater
Magnesium is the second most abundant cation in seawater, it enters the sea from the
weathering of Mg rich minerals. It is an abundant and cheap resource in coastlands which can
be utilised for P recovery via struvite precipitation.
2.4.3.1 Chemical Composition
Seawater contains a number of anions and cations and solids suspended in it that can be
removed to a large extent by simple filtration. Table 4 contains a typical seawater
composition and some of its properties.
Table 4 A typical seawater composition
Element
Na+
Mg2+
Ca2+
K+
ClSO42NO3N0139581H
Ion concentration [mgL-1]
10570
1276
447
393
16085
2740
160
Page 19
HCO32BrpH
Conductivity
140*
80
31600µS/cm
7.9-8.3
2.4.3.2 Economic and Environmental Benefits
Seawater as a hugely abundant resource with a substantial presence of Mg2+ has the potential
to lower the cost of chemicals used for struvite precipitation in coastlands. Use of seawater
reduces the amount of manufactured chemicals needed thus increasing sustainability. Also
has the potential to increase and diversify revenue streams for coastlands through fertilizer
sales. With huge emphasis being put on re-use and recycling of resources recovery of
phosphorus through the use of seawater becomes an ideal standard.
2.4.3.3 Potential For P Recovery and Product Quality
Studies have shown that high quality could be recovered through the use of seawater as a
source of magnesium. The precipitate produced consists of impurities like Ca-P complexes
from because of the interference of Ca2+ and Na+ ions. Also, magnesium calcite is another coprecipitate is produced when using seawater. The particle size and crystal morphology of the
products has a slight deviation from products formed from Mg salts. The morphology of
crystals produced is feather like shaped crystals. The median particle size is also reduced
when using seawater.
Table 5 Struvite purity found from previous experiments. Chemical composition of pure
struvite and composition of precipitates obtained using different Mg sources, given in total
weight %
Table 5 Struvite purity found from previous experiments
Theoretical value of
struvite
Liu [28]
Liu [28]
Liu[28]
Matsumiya [46]
P
N
Mg
12.63 5.71 9.9
Pure struvite
Magnesium chloride
struvite
Seawater struvite
Seawater Struvite
Ca
-
K
-
Al
-
Na
-
5.71
5.42
5.71 9.79 5.42 9.52 0.71 0.13 <0.01 0.31
4.19
5.5
4.19 7.56 1.39 0.05 0.09
5.5 9.6 0.06 -
0.18
-
2.4.3.4 Drawbacks


Since Mg2+ concentration is much lower seawater than when using pure sources, it
can result in blending and dilution of the wastewater stream, causing TAN and total
inorganic orthophosphate concentrations at the entrance of the precipitation reactor to
decrease. This reduces the struvite precipitation potential (SPP).
Seawater contains other ions than magnesium, like sulphates, chlorides and sodium
and other ions present in lower concentrations. This can form ion pairs that reduce the
N0139581H
Page 20

2.4.4
struvite precipitation potential (SPP), resulting in a need to keep pH at a higher level
to get a given P removal efficiency.
Presence of Ca2+ions in seawater may cause the formation of Ca-P, like Ca3(PO4)2 and
Ca5(PO4)3(OH) which reduce the purity and value of the struvite product
Magnesite
Magnesite (MgCO3) is a natural mineral mined as a source of MgO production which is 94%
MgCO3 by mass. It is largely insoluble in water requiring higher doses for struvite
precipitation. There are two ways that have been employed to increase the solubility of
magnesite namely acid dissolution and thermal-decomposition (calcination)
2.4.4.1 Acid Dissolution
Magnesite is treated with an acid which dissolves 99% of the Mg into a soluble form. This
increases the struvite precipitated by 50% as compared to the achieved with untreated
magnesite [53]. Market prices for such magnesite have been found to be 10% of the prices of
pure MgCl2 [54]. Overall production costs are reduced by the use of acid dissolved
magnesite, however the alkali consumption is higher so as to achieve acid neutralization of
the acid used for decomposition and the optimum pH for struvite precipitation. Thus, the
overall benefit from cost reduction is somewhat curtailed.
2.4.4.2 Thermal Decomposition (Calcination)
Calcination is the process of heating magnesite (MgCO3) in a furnace or high temperature
kiln to form MgO. During this process CO2 is released from MgCO3 forming MgO. When
magnesite is thermally decomposed it produces magnesia (MgO) which has a higher
solubility and reactivity that yields recovery of about 99.7% and 90.2% for NH4+ and PO43respectively [54]. Calcination temperatures and time have an influence on the recovery
performance and quality of struvite precipitated. An optimum temperature and time have to
be determined for the kiln which produce the best MgO for the best possible struvite
production (700ºC and 1.5hrs in the case of rare earth wastewater ) [55]. Struvite production
using magnesite that is not pre-treated reduces process cost up to 18% [53], with further
reduction up to 34% when thermally calcinated magnesite is used as Mg source compared to
MgCl2 [54].
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Page 21
Figure 3
Graphical representation of the calcination process
2.4.4.3 Chemical Composition
Table 6 contains the typical magnesite (MgCO3) and magnesia (MgO) Chemical
Compositional values from literature.
Table 6 Magnesite and Magnesia typical compositions
Magnesite (ppm) [53,54]
Mg
9400002
Ca
10000-1500003
Al2O3
2000
Fe2O3
3000-8000
SO3
SiO2
7000-38000
1
2
3
as MgO, as MgCO3, as CaO
Magnesia (ppm) [56]
898000-6340001
15000-870003
240000
38000
32000
2.4.4.4 Economic and Environmental benefits
It is a cheaper alternative to areas that are far away from the sea and with local reserves for
magnesite mining. Studies have found phosphorus recovery as high 90.2% at costs that are
N0139581H
Page 22
lower that than those of MgCl2, about 34% reductions [54]. It is relatively easier to handle
compared to fluids like brine, bittern and seawater. Thermal decomposition process has been
reported to be 37% more cost effective compared to struvite precipitation using pure
chemicals. While adding Mg source, it is to be noted that, there might be addit Magnesite is a
by-product during MgO production making its use a sustainable to environmentalists.
2.4.4.5 Potential For P Recovery and Product Quality
Magnesite has a huge recovery potential when it is pre-treated. Both acid dissolution and
thermal decomposition produce a product of high quality with high recoveries. Pre-treated
mgnesite can recover be as high as 90.2% of P from wastewaters [54]. It has been confirmed
that the struvite produced can be granulated for marketing purposes and easier handling. This
therefore proves that the product is of a high quality.
2.4.4.6 Drawbacks





Magnesite is largely insoluble in most wastewaters, thus requiring pre-treatment
before being used for P recovery
When the pre-treatment is acid dissolution, there is an increased need for the alkali
needed to neutralize the acid and also maintain alkaline conditions for struvite
precipitation. As a result the cost reductions are curtailed by increased alkali
consumption.
When pre-treatment is calcination, the soluble product produced requires optimum
conditions for efficient and cost effective P recovery. These optimum conditions
require specialized kilns that can maintain specific temperatures like 700ºC for about
1 hour to 1.5 hours. Normal kilns are not able to do so.
Calcination has the disadvantage of emitting CO2, meaning its emissions have to be
kept in check and maintained below levels stipulated by environmental standards and
regulations.
Presence of heavy metals like cadmium which is toxic and mobile mean that the
struvite produced has to be tested for heavy metal presence and it is below stipulated
levels.
3 CHAPTER THREE (METHODOLOGY)
3.1 Introduction
The different sources outlined in Chapter 2 have different advantages and disadvantages and
therefore there exists the best source among them. From the many methodologies used to
select the best source, in this project the Kepner-Tregoe (K-T) analysis is going to be used.
The K-T analysis is a structured methodology for identifying and ranking all factors critical
to decision making and evaluation. This analysis provides assistance with unbiased decision
making. It is a very detailed and complex method applicable in many areas, which is much
broader than just idea selection. It is called also a root cause analysis and decision-making
method. It is a step-by-step approach for systematically solving problems, making decisions,
and analyzing potential risks. It comprehensively evaluates all alternative courses of action to
optimize the ultimate results based on explicit objectives. It is a conscious, step-by-step
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approach for systematically solving problems, making good decisions and analyzing potential
risks and opportunities. It helps to maximize critical thinking skills, systematically organize
and prioritise information, set objectives, evaluate alternatives and analyze impact. The idea
is not to find a perfect solution but rather the best possible choice.
The procedure for carrying out the K-T analysis is as follows:






Identify all factors to be considered for selection of the best alternative source.
Assign values for level of importance (I) of each factor. These values are out of 10.
For each alternative source, rate each factor out of 10 by assigning values which stand
for factor rating (FR).
For each alternative source, multiply corresponding values of I and FR to obtain a
weighted score (WS) for each factor.
Add the weighted scores of each alternative source together to get a total weighted
score.
The alternative source with the highest weighted score is the best alternative source.
3.2 Factors to be considered
During the process of choosing a technology, there are certain factors that must be put into
consideration. The best alternative source must be able to satisfy most of these factors. The
factors to be considered when selecting the best alternative source of magnesium for struvite
precipitation are:






3.2.1
Total cost of the source in the entire process.
Product quality
Accessibility and availability of the source
Rate of precipitation
Safety
Environmental impact of using the source
Total cost of the source in the entire process
The main reason for slow uptake of the struvite precipitation process has been the cost the
entire process which is largely driven by the cost of acquiring Mg sources. Use of industrially
produced magnesium salts makes struvite precipitation to be expensive, making struvite
fertilizers hard to market because of their cost. It was found that when using pure magnesium
salts, 75% of total cost is the cost of purchasing magnesium sources. For example wood-ash
is the cheapest since it is readily available and a waste. Magnesite is on the other hand a bit
expensive since it has to be mined and pre-treated to improve its solubility. Other costs are
operational like for example seawater and bittern cause a lot of rusting to steel, thus requiring
higher maintenance costs.
3.2.2
Product quality and recovery efficiencies
The quality of struvite is very important as it determines the usefulness of that Mg source in
struvite precipitation. The crystal produced should meet all the fertilizer standards or even
better them so as to compete in the fertilizer markets. The heterogenous nature, of the sources
of magnesium causes difficulty in P recovery. Most of these alternative sources contain
N0139581H
Page 24
competing ions like Ca2+ ions and contaminants like heavy metals. This may reduce the
quality of struvite produced or a cause the production of precipitate that does not have
fertilizer qualities as in the case of wood-ash. The product produced should be able to
crystallize with ease for easy handling and application like all other fertilizers.
3.2.3
Availability and accessibility of the source
Availability and accessibility of the source determines its usefulness for production. A source
may be of good quality but if it is not readily available in the vicinities it becomes costly or
may cause the project to be aborted. Seawater and bittern are readily are available in
coastlands and salt producing areas which are largely coastlands. However, to inland areas
like Zimbabwe it would be costly due to transportation costs. Also magnesite as a mineral is
not readily available everywhere, however in Zimbabwe it is mined in kadoma thus readily
available.
3.2.4
Safety
Now more than ever, safety on the jobsite has become the number one priority of plant
management. With this focus on safety, the impact of the alternative sources to plant designs
should not result in a compromised working environment in the plant. Generally, all these
sources would use similar reactor and plant designs such that safety precautions done are the
same. Plant managers would have to plan for the rusting of pipes caused by bittern and
seawater and thus ensure the safety of its workforce in the long run.
3.2.5
Rate of production
The phrase “Time is money,” is very much applicable to chemical process industries. The
faster the rate of precipitation and crystallization, the more useful is the alternative source.
Presence of other ions like Na+ and SO42- has been found to reduce induction time [56]. Also
formation of co-precipitates result in smaller quantities per unit kg resulting in longer time
frames to produce set targets of product. An example is seawater and bittern where the
concentration of Mg is lower in seawater than in bittern, thus larger volumes are pumped for
a similar quantity of Mg as in bittern hence taking a longer time.
3.2.6
Environmental impact of using the source
When choosing a source its environmental impact must weigh on the selection process. The
presence of heavy metals has to be monitored in sources like wood-ash and magnesite.
Cadmium levels have to be maintained below the specified levels for the fertilizer industry
due to its mobility and toxicity in the soil-plant-water system. Also calcination of magnesite
emits CO2 which is currently under the spotlight due to the impacts of global warming.
Table 7 Weighted analysis for choosing the best source
a)
b)
N0139581H
Importance
I
9
9
Wood-ash
FR
WS
7
63
Seawater
FR
WS
5
45
Bittern
FR
WS
6
54
Magnesite
FR
WS
7
63
4
7
8
8
36
63
72
72
Page 25
c)
9
9
81
4
36
4
36
8
72
d)
7
8
56
7
49
7
49
5
35
e)
9
4
36
6
54
8
72
6
63
f)
6
5
30
8
48
9
54
7
36
Total WS
302
295
337
342
3.3 Analysis Of Results
The analysis is based on the value of the Total Weighted Score obtained from the K-T
Analysis. From Table 3-1, magnesite has the highest Total Weighted Score hence it is the
best alternative source in our country for recovery of phosphorus from wastewaters by
struvite precipitation. Bittern would be a better substitute to magnesite as compared to other
remaining alternative sources of magnesium. Wood-ash has the lowest Total Weighted Score
hence it is a less advisable alternative.
4 CHAPTER FOUR (CONCLUSION AND RECOMMENDATIONS)
4.1 Conclusion
The K-T analysis done in chapter 3 established that magnesite is most suitable magnesium
source in Zimbabwe for struvite precipitation. In other developing countries like Nepal it has
been shown to be an economically viable way of producing phosphorus rich fertilizers.
However, magnesite in its raw form insoluble in many wastewaters or in collected urine.
Therefore it has to undergo pre-treatment that is calcination which is the most suitable for our
country since we also produce a lot of coal. The amount of CO2 produced during the
calcination of magnesite has to be managed, though the amounts produced in furnaces
globally have shown to be below stipulated levels by environmental legislations. Struvite
precipitatioon as an interesting new technology for P rich fertilizer production is an attractive
and beneficial project to engage in a developing economy like Zimbabwe.
4.2 Recommendations
Since this project was a comparative study mainly based on literature review, there is need to
carry out experiments on the applicability of magnesite for the recovery of phosphorus via
struvite precipitation. The experiments should determine;
 The solubility of magnesite and the calcined magnesite in various wastewaters. To
determine the precipitation rates and their viability.
 The energy consumptions and costs for calcinating the magnesite to properly
determine the cost benefit analysis of using magnesite as a magnesium source for
struvite precipitation.
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 The actual presence of heavy metals in the struvite produced using magnesite,
especially the presence of cadmium which can make the struvite produced to be
illegal to apply if its concentrations are not controlled
 CO2 emissions and their impact on the environment.
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