CET Volume 86
DOI: 10.3303/CET2186245
Paper Received: 6 September 2020; Revised: 2 February 2021; Accepted: 19 April 2021
Please cite this article as: Seodigeng R., Tshilenge J., Rutto H., 2021, Struvite Crystallisation of Synthetic Urine Using Magnesium Nitrate:
Effect of Parameters on Crystal Size Distribution, Chemical Engineering Transactions, 86, 1465-1470 DOI:10.3303/CET2186245
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 86, 2021
A publication of
The Italian Association
of Chemical Engineering
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ISBN 978-88-95608-84-6; ISSN 2283-9216
Struvite Crystallisation of Synthetic Urine Using Magnesium
Nitrate: Effect of Parameters on Crystal Size Distribution
Reneiloe C F Seodigeng*, John K. Tshilenge, Hilary L Rutto
Department of Chemical Engineering, Vaal University of Technology, Private Bag X021, South Africa.
reneiloe@seodigeng.co.za
Urine diversion toilets have become popular as a means of solving the challenges in sanitation. As a result,
the source separated urine must be adequately treated so that it can be disposed of safely and valuable
struvite can be extracted for use as fertiliser. Struvite crystallization has been investigated for this purpose,
however, for the crystallization process to be viable and economical, a cost effective yet optimal magnesium
source is required. Crystallisation must have optimal yield as well as crystal size and morphology (CSD).
Crystal size is an important factor in crystallisation as it affects further processing steps such as washing and
filtering, therefore CSD must be controlled in crystallisation processes. In this study, synthetic urine was
prepared, and struvite crystallisation experiments carried out using Magnesium Nitrate (MgNO3). The effect of
parameters on CSD were investigated. Residence time was found to be the parameter with the most effect on
CSD. At residence time of 10, 30 and 60 minutes, mean particle sizes were 17, 34 and 53 µm showing that
with higher residence times, larger crystal sizes can be achieved. SEM analysis of the crystal showed that the
resultant crystals had the typical morphology of struvite crystals.
1. Introduction
Struvite or magnesium ammonium phosphate (MAP) is a mineral which is made up of Magnesium (Mg),
Ammonium (NH4) and Phosphate (PO4) as the name suggests. It occurs naturally in stored urine (Udert et al.,
2006). Struvite has also been found in wastewater treatment systems, where it causes scaling problems
(Ariyanto et al., 2014). Struvite has large potential for use as fertiliser and it may be the preferred fertiliser for
several reasons some of which are stated below:
• It is slow releasing which enables the plants to take up the nutrients before the fertiliser is leached and
therefore less frequent application is required (Munch and Barr, 2001)
• Heavy metals are much lower in struvite fertilisers than other fertilisers (Doino et al., 2011)
• Essential nutrients (N,P, Mg) can be applied to the plants simultaneously (Taiz and Zeiger 1991).
A number of studies have been conducted on struvite precipitation from source separated urine. The rationale
behind the studies is because urine naturally contains nutrients present in synthetic fertilisers. On average
urine contains 1g/l of P, 9g/l of N and 10g/l of chemical oxygen demand (COD). Human urine makes up
approximately 1% of municipal wastewater but it contributes approximately 50% of the phosphorus load
(Wilsenach et al., 2007). The abovementioned factors therefore make urine a valuable potential source of
nutrients for use as fertiliser (Maurer et al., 2006)
Struvite crystallization is also the subject of various studies aiming at removing phosphates from wastewaters.
Discharging of wastewaters containing nitrogen and phosphorus to the environment can lead to
eutrophication. This has led to limits being imposed on the amount of P and N that can be discharged to
waters in order to prevent this (Doyle and Parsons, 2002). This has encouraged investigation into various
methods of extracting these nutrients from wastewaters before discharge to rivers, oceans, lakes etc.
Struvite crystallization occurs through two processes, namely nucleation and crystal growth (Ariyanto et al.,
2014). Nucleation is the process whereby ions combine to form crystal embryos (Doyle and Parsons, 2002).
Crystal growth then occurs after nucleation, where growth continues until equilibrium is reached. The process
occurs when parameters are such that the concentration of Mg2+, NH4
+ and PO4
3- in the solution exceed the
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solubility product of struvite. The parameters which have an effect on struvite precipitation include pH, Mg:P
ratio, supersaturation, temperature, mixing intensity and presence of other ions in the solution ( Matynia et al.,
2006).
Bhuyian et al. (2007) and Ariyanto et al. (2014) found that supersaturation and pH had the most influence on
crystallisation while Barbosa et al. (2016) found that in urine, the factors that had the most effect were stirring
speed and Mg:P ratio. Harrison et al. (2011) also found that growth rates increased with an increase in pH, up
to 8.5.
An important objective of crystallisation is to produce crystals of the appropriate size or crystal size distribution
(CSD) and crystal morphology. Crystal size is an important factor in crystallisation as it affects further
processing steps such as washing and filtering, therefore CSD must be controlled in crystallisation processes.
In the study by Doino et al., 2011, crystal sizes of 170 µm were obtained in a pilot fluidised reactor, which
made the crystals easily filterable. The crystal morphology is affected by pH. Abbona and Boistelle, (1982)
found that at pH greater than 8, twinned dendritic structure was formed, between pH 7 and 8, a tabular
structure was supported. At pH lower than 6, elongated, needle like crystals formed. Tabular form is the most
preferred form for industrial equipment. (Abbona and Boistelle, 1982). Struvite crystals typically exhibit an
orthorhombic structure (Doyle and Parson, 2002). Some crystals can exhibit a needle like structure, an x –
shape structure or rod-shaped structure. (Wilsenach et al., 2007). Barbosa et al. (2016) found that different Mg
sources resulted in different crystal structures. MgO resulted in a combination of irregular shaped and x-
shaped crystals while MgCl 2 and Mg(OH)2 resulted in an orthorhombic structure, which is a typical pattern for
struvite. Similar shapes were reported by Munch and Barr (2001) and Korchef et al.(2011).
The objective of this study is to study struvite crystallization using Mg(NO3)2 as a Mg source and observe the
CSD and morphology of the resultant struvite crystals. This will determine whether Mg(NO 3)2 as a Mg source
is suitable for struvite crystallization. In addition, the effect of parameter such as Mg:P ratio, pH, residence
time, stirring speed on CSD were also investigated.
2. Materials
A solution of synthetic urine was prepared using a standard recipe from literature. MgCl 2.6H2O, CaCl 2.2H2O
NaCl , Na2SO4, Na 3(C6H5O7).2H20 (Sodium citrate), KCl, C4H7N3O (Creatinine), (NH2)2CO (Urea), NH4Cl,
K2HPO4, Na 2(COO)2 (Sodium oxalate), C6H8O6 (Ascorbic acid), C5H4N4O3 (Uric acid), Na 2CO3
Na 3PO4 were used to prepare the synthetic urine in proportions such that the concentrations mimicked the
typical concentration of urine as it leaves the human body. H2SO4 was used to preserve the synthetic urine.
NaOH was used for pH adjustment. Mg(NO3)2 was used as the Mg source.
2.1 Design of experiments
The key parameters of the struvite crystallisation were investigated are Mg:P ratio, pH, residence time and
stirring speed. The studied key parameters were varied within prescribed range of discrete values as shown in
Table 1. Initial experiments were conducted to determine the combined effects of pH, residence time, Mg:P
ratio and stirring speed on CSD. Using these preliminary results, Stat-Ease Design Expert® was used to
design an optimised set of experiments for studying factor interaction. A set of 20 experiments were designed
and carried out at varying conditions.
Table 1: Discretised parameter ranges
Factor Name Units Minimum Maximum
A pH - 9 10
B Mg:P ratio - 1 2
C Stirring speed RPM 60 120
D Residence time Minutes 5 60
2.2 Crystallisation experiments
Batch crystallisation experiments were conducted using 2L of synthetic urine per experimental run. The
experiments were carried out in a struvite crystallization vessel equipped with a stirrer. pH was adjusted using
NaOH. The stirrer speed was then adjusted as per the predetermined run conditions. A supersaturated
solution of Mg(NO3)2 was prepared and added to the synthetic urine as an Mg source. The amount of
Mg(NO3)2 added was determined based on the targeted final Mg:P ratio. The experiment was done until the
required residence time was reached as per run conditions. The residence time was determined from the time
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the Mg source solution was added. At the required run residence time, the stirrer was stopped, and the
crystals were filtered from the solution using a vacuum pump manifold. The solution was filtered through a
filter paper.
2.3 Analysis techniques
The filtered crystals were dried in an oven overnight at 35°C and then weighed to determine the yield. The
mass of filter paper was subtracted from the total weight. Samples were prepared for particle size distribution
(PSD) analysis by suspending them in distilled water. Crystal size distribution was analysed using a Malvern
PSD analyser. The precipitate was analysed using SEM to determine the crystal morphology and composition
analysed by EDS.
3. Results and Discussion
3.1 Effect of residence time on crystal growth
Figure 1 to Figure 3 show the results for the experiments. The results show crystal size distribution at
residence times of 10, 30 and 60 minutes. The mean particles sizes at 10, 30 and 60 minutes were 17, 34 and
53 µm, respectively as shown in Figure 3. The graph shows that at higher residence times larger crystal sizes
were achieved. This effect was also observed by Hutnik et al. (2013).
Figure 1: Crystal size distribution at 10 min, 30 min and 60 min
Figure 2: Cumulative crystal size distribution at residence time 10 min, 30 min and 60 min
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Figure 3: Mean crystal size distribution at residence time 10 min, 30 min and 60 min
3.2 Effect of Mg:P ratio and residence time on crystal growth
Figure 4 shows a graph of the effect of Mg:P ratio on crystal size distribution showing interactions with
residence time. At higher Mg:P ratio, the supersaturation of struvite is high which supports faster spontaneous
nucleation. When nucleation rates are higher than growth rate, then the average crystal sizes decrease. At low
Mg:P ratio, crystal growth is favoured more than nucleation (Bhuiyan et al., 2008). The interaction is analysed
at an optimum pH of 9.5 and stirring speed of 90 rpm. The two graphs are plotted for a minimum residence
time of 5 min and a maximum residence time of 60 minutes. The graph shows very clear influence of
residence time on crystal size. An increase in mean residence time resulted in larger mean crystal size (Hutnik
et al., 2013).
Figure 4: Effect of Mg:P ratio on CSD (d50) showing interaction with residence time, while stirring speed and
pH are kept constant
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3.3 Struvite elemental analysis
Samples of the collected precipitate obtained after P removal with Mg:P of 1.5 and residence time of 60 was
selected for crystals characterization. The content of struvite in the precipitates was confirmed by SEM, SEM–
EDS. Figure 5 shows the SEM images of the precipitate which shows the crystal morphology of the
precipitate. The elemental analysis of the crystals was also determined by EDS. The SEM images show that
the resultant crystals were similar in morphology and displayed the typical rod-shaped morphology of the
struvite crystal.
SEM–EDS analyses of the crystals are shown in Table 2. The chemical element composition showed the
presence of Mg, P, N and O on the crystals in quantities that are similar to that of theoretical struvite. The EDS
elemental analysis also showed similar composition to theoretical struvite with the exception of elements such
as Na, K and Ca, which are present in synthetic urine but not in pure struvite. These elements are however
present in small quantities (<2%) in the precipitate.
Figure 5: SEM analysis of precipitate at 5000X magnification
Table 2: Precipitate elemental analysis
Element N K O K Na K Mg K P K K K Ca K Totals
Weight% 8.70 52.20 0.21 13.74 21.21 2.48 1.36 100
Atomic % 11.85 62.21 0.26 10.78 13.06 1.21 0.65 100
4. Conclusions
It can be concluded that residence time is an influential parameter on struvite crystallization in terms of crystal
size distribution. Crystals need to be allowed sufficient time to grow to a suitable size. This means that in
crystalliser design and process optimisation, the residence time will be the most important factor as whether
all other parameters are at an optimum is immaterial if there is insufficient time given for nucleation and crystal
growth to occur. Also, sufficient time must be allowed for the solution to reach equilibrium to ensure
maximum yield of crystals. The elemental analysis using SEM-EDS showed that the resultant crystals were
similar in composition to theoretic struvite elemental analysis. Elements such as Na, K and Ca which are
expected to be present in urine were also detected in trace quantities. Crystal morphology was also analysed
with SEM and showed the typical rod-shaped morphology of struvite crystals.
References
Abbona F., Boistelle R.,1979, Growth morphology and crystal habit of struvite crystals (MgNH4PO4 · 6H2O).
Journal of Crystal Growth, 46(3), 339-354.
Ariyanto E., Sen T., Ang H., 2014, The influence of various physico-chemical process parameters on kinetics
and growth mechanism of struvite crystallisation, Advanced Powder Technology, 25(2), 682-694.
Barbosa S., Peixoto L., Meulman B., Alves M., Pereira M., 2016, A design of experiments to assess
phosphorous removal and crystal properties in struvite precipitation of source separated urine using
different Mg sources, Chemical Engineering Journal, 298,146-153.
1469
Bhuiyan M., Mavinic D., Beckie R., 2007, A solubility and thermodynamic study of struvite, Environmental
Technology, 28(9),1015-1026.
Bhuiyan M., Mavinic D., Beckie R., 2008, Nucleation and growth kinetics of struvite in a fluidized bed reactor,
Journal of Crystal Growth, 310(6), 1187-1194.
Doino V., Mozet K., Muhr H., Plasari E., 2011, Study on Struvite precipitation in a Mechanically Stirred
Fluidised Bed Reactor, Chemical Engineering Transactions, 24, 679-684.
Doyle J., Parsons S., 2002, Struvite formation, control and recovery, Water Research, 36(16), 3925-3940.
Harrison M., Johns M., White E., Mehta C., 2011, Growth Rate Kinetics for Struvite Crystallisation, Chemical
Engineering Transactions, 25, 309-314.
Hutnik N., Kozik A., Mazienczuk A., Piotrowski K., Wierzbowska B., Matynia A., 2013, Phosphates (V)
recovery from phosphorus mineral fertilizers industry wastewater by continuous struvite reaction
crystallization process, Water Research, 47(11), 3635-3643.
Korchef A., Saidou H., Amor M., 2011, Phosphate recovery through struvite precipitation by CO2 removal:
Effect of magnesium, phosphate and ammonium concentrations, Journal of Hazardous Materials, 186(1),
602-613.
Matynia A., Koralewska J., Wierzbowska B., Piotrowski K., 2006, The influence of process parameters on
struvite continuous crystallization kinetics, Chemical Engineering Communications, 193(2), 160-176.
Maurer M., Pronk W., Larsen T., 2006, Treatment processes for source-separated urine, Water Research,
40(17), 3151-3166.
Münch E., Barr K., 2001, Controlled struvite crystallisation for removing phosphorus from anaerobic digester
sidestreams, Water Research, 35(1), 151-159.
Taiz L., Zeiger E., Plant physiology, 3rd edn, Annals of Botany, 91(6), 750-751.
Wilsenach J., Schuurbiers C., van Loosdrecht M., 2007, Phosphate and potassium recovery from source
separated urine through struvite precipitation, Water Research, 41(2), 458-466.
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