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Characterization of induced struvite formation from source-separated

urine using seawater and brine as magnesium sources

Bianxia Liu a, Apostolos Giannis a, Jiefeng Zhang b, Victor W.-C. Chang a,b,⇑, Jing-Yuan Wang a,b

a Residues and Resource Reclamation Centre (R3C), Nanyang Environment and Water Research Institute, Nanyang Technological University, 1 Cleantech Loop,

CleanTech One, Singapore 637141, Singaporeb Division of Environmental and Water Resources, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue,

Singapore 639798, Singapore

h i g h l i g h t s

Seawater and brine enable struvite formation from source-separated hydrolyzed urine.

 Modified chemical equilibrium model agrees with experimental results.

 Struvite from synthetic and real urine have feather and coffin shape, respectively.

 Co-precipitates magnesium-calcite and calcite compromise induced struvite purity.

a r t i c l e i n f o

 Article history:

Received 7 March 2013

Received in revised form 5 September 2013

Accepted 7 September 2013

Available online 14 October 2013

Keywords:

Urine

Struvite

Phosphorus recovery

Seawater

Brine

Equilibrium model

a b s t r a c t

Struvite (MgNH4PO46H2O) precipitation is widely used for nutrient recovery from source-separated

urine in view of limited natural resources. Spontaneous struvite formation depletes the magnesium in

hydrolyzed urine so that additional magnesium source is required to produce induced struvite for P-

recovery. The present study investigated the morphology and purity of induced struvite crystals obtained

from hydrolyzed urine by using seawater and desalination brine as low cost magnesium sources. The

results demonstrated that both seawater and brine were effective magnesium sources to recover phos-phorus from hydrolyzed urine. Crystals obtained from synthetic and real urine were revealed that the

morphology was feather and coffin shape, respectively. Structural characterization of the precipitates

confirmed that crystallized struvite was the main product. However, co-precipitates magnesium calcite

and calcite were observed when seawater was added into synthetic and real urine, respectively. It was

found that the presence of calcium in the magnesium sources could compromise struvite purity. Higher

struvite purity could be obtained with higherMg/Ca ratio in the magnesium source. Comparativeanalysis

indicated that seawater and brine had similar effect on the crystallized struvite purity.

 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Phosphorus recovery from wastewater streams is getting more

and more interests globally because of the depletion of phosphorusresources (Smil, 2000). It is also shown that the demand of phos-

phorus fertilizer is expected to peak around 2030 (Cordell et al.,

2009). Moreover, macronutrients in the wastewater are often dis-

charged to the natural water bodies without control causing eutro-

phication and other issues in water cycle. It is of great interests to

develop proper technologies that facilitate nutrient recovery and

improve the quality of wastewater effluent.

Human urine has been characterized as the main contributor of 

nutrients in domestic wastewater with 85% of total N, 50% of thetotal P, and 55% of total K, but it consists only 1% of raw wastewa-

ter volume (Larsen and Gujer, 1996). The separate collection of hu-

man urine reduces nutrients loading to domestic wastewater and

facilitates nutrient recycling as well ( Jönsson et al., 2005; Larsen

et al., 2009). A number of physico-chemical and biological tech-

niques have been developed to harvest nutrients (mainly N, P)

from the urine stream. Phosphorus recovery can be readily

achieved by crystallization as struvite (magnesium ammonium

phosphate hexahydrate), which can be further utilized as a valu-

able slow-release fertilizer ( Johnston and Richards, 2003; Maurer

et al., 2006; Römer, 2006).

0045-6535/$ - see front matter    2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2013.09.025

⇑ Corresponding author at: Division of Environmental and Water Resources,

School of Civil and Environmental Engineering, Nanyang Technological University,

50 Nanyang Avenue, Singapore 639798, Singapore. Tel.: +65 67904773; fax: +65

67921650.

E-mail address: wcchang@ntu.edu.sg (V.W.-C. Chang).

Chemosphere 93 (2013) 2738–2747

Contents lists available at  ScienceDirect

Chemosphere

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / c h e m o s p h e r e

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After urinary excretion, urea is quickly hydrolyzed and con-

verted into ammonia and CO2. The elevated pH-value causes spon-

taneous precipitation of struvite in the hydrolyzed urine,

exhausting most of the magnesium and about 30% of phosphorus

(Zhang et al., 2013). Magnesium is the major limiting factor to har-

vest the remaining phosphorus. Although the direct usage of com-

mercial magnesium salt is effective, the additional cost is likely to

restrict further P-recovery from urine, which indicates the impor-

tance of exploring cost-effective magnesium sources.   Kumashiro

et al. (2001) used seawater as magnesium source for struvite crys-

tallization at pilot scale wastewater treatment. It was found that

the use of seawater was superior because it could simplify facilities

and operation without affecting subsequent processes. Bittern, a

by-product from salt manufacture, was investigated as low-cost

magnesium source to recover phosphorus and nitrogen from

wastewater (Li and Zhao, 2002; Lee et al., 2003; El Diwani et al.,

2007). Results showed that application of bittern at treated waste-

water could achieve high phosphate and ammonia removal effi-

ciencies.   Etter et al. (2011)   investigated different magnesium

sources for phosphorus recovery from human urine in Nepal. The

result showed that bittern was a practical magnesium source, but

it had to be imported from India. Also, magnesium oxide produced

from locally available magnesite was an alternative option. In a re-

cent study, wood ash was also tested as magnesium source (Sakthi-

vel et al., 2012). The cost-benefit analysis revealed that wood ash

was considerably cheaper than industrially-produced magnesium

sources and even cheaper than bittern. However, due to high con-

tent of heavy metals, wood ash was concluded as an unsafe mag-

nesium source for struvite production. It is obvious that among

the magnesium sources, some of them are limited with high cost,

while some others produce low quality products.

Chemical equilibrium models have frequently been applied to

study the struvite precipitation process. These models were devel-

oped to optimize and control the struvite reaction in order to make

it practical for application. Gadekar and Pullammanappallil (2010)

developed a mathematical model to predict struvite precipitation

in the solution of ammonium, magnesium and phosphate. Themodel was validated against experimental data and exhibited sat-

isfactory prediction efficiency. Harada et al. (2006)  predicted the

struvite formation from urine by a chemical equilibrium model

having good agreement with experimental results. Several

researchers have employed Visual Minteq to model struvite forma-

tion. Lee et al. (2003) used Visual Minteq to model struvite forma-

tion for nitrogen and phosphorus removal from wastewater by

bittern addition. Celen et al. (2007) amended Visual Minteq to pre-

dict struvite precipitation from liquid swine manure. However,

chemical equilibrium modeling and experimental validation were

mainly focused on the precipitation process overlooking struvite

purity and co-precipitates identification. Therefore, it is of great

importance to ensure the efficiency of inexhaustible magnesium

sources like seawater or brine on struvite quality in terms of reac-tion conditions and products obtained.

The above mentioned challenges prompt us to investigate the

feasibility of using seawater, brine and MgCl2   as magnesium

sources for phosphorus recovery from source-separated urine.

Visual Minteq was modified in order to predict the struvite forma-

tion reactions under different conditions. Effects of pH and Mg/P

molar ratio on the phosphorus recovery efficiency were simulated

and outcomes were validated against experimental data. In addi-

tion, the morphology and purity of the struvite obtained were

characterized using various techniques in order to help us to eval-

uate the potential for using low cost magnesium alternatives for

large scale application.

2. Materials and methods

 2.1. Urine samples and magnesium sources

Synthetic and real urine samples were used after complete urea

hydrolysis in bench-scale batch experiments. Synthetic urine was

prepared according to Griffith et al. (1976), and it was free from

any unspecified substances that could affect the struvite precipita-

tion process. Fresh human urine was collected from 25 healthy

adults on a voluntary basis. Small amount (1 mg) of urease en-

zyme (Merck) was added to accelerate urea hydrolysis.

Three types of magnesium were investigated as additional mag-

nesium sources, namely, seawater, brine and MgCl2

. Seawater was

collected from west coast of Singapore and filtered through

0.45 lm pore size membrane filter to remove suspended particles

before usage. Brine solution was prepared by dissolving 70 g com-

mercial sea salt (Sigma–Aldrich) in 1 L of deionized water (Ahmed

et al., 2003; Zhou et al., 2013). In addition, commercial MgCl2 (Sig-

ma–Aldrich) was also tested for comparison purposes. The major

characteristics of synthetic and human hydrolyzed urine, seawater

and brine are presented in Table 1.

 2.2. Crystallizations experiments

During hydrolysis in both synthetic and real urine, almost all

magnesium and calcium ions and 30% of initial phosphates were

naturally precipitated. The hydrolyzed urine still contained

1054mg L 1 phosphate with strong buffering capacity. Magne-

sium-to-phosphate (Mg/P) molar ratio and pH-value were consid-

ered the most essential factors during struvite precipitation.

Hence, batch experiments were carried out to investigate the opti-

mum Mg/P molar ratio and pH for P-recovery from hydrolyzed ur-

ine. Ronteltap et al. (2007) have mentioned that thermodynamics

and kinetics of precipitation in synthetic urine is almost identical

to real urine.

Previously to precipitation experiments, the hydrolyzed urine

was filtered through 0.45 lm pore size membrane filter to remove

the spontaneous struvite crystals. To test the effect of Mg/P molar

ratio, standard amount of the magnesium sources was added to

50 mL urine sample in order to reach stoichiometric Mg/P molar

ratios 0.7, 1, 1.1, 1.5 and 2. To study the effect of pH, the optimum

Mg/P molar ratio was used. The pH-values were adjusted to 6.3,7.0, 8.0, 9.3 (no pH adjustment), 10.2 and 11.6 with 1 M sulfuric

acid or 1 M sodium hydroxide. Furthermore, estimations of nutri-

ents recovery were conducted for fresh and hydrolyzed urine at

different dilution ratios (1:2, 1:5), which represent the typical

 Table 1

Chemical composition of hydrolyzed urine, seawater, and brine.

Component Na+ (mg L 1) K+ (mg L 1) Mg2+ (mg L 1) Ca2+ (mg L 1)   PO34   –P (mg L 1)   NHþ

4 –N (mg L 1)   SO24   (mg L 1) CO2

3   (mg L 1)   Cl– (mg L 1)

Human urine 2600 2000 – – 344 4250 1500 5448 3800

Synthetic urine 2600 2000 – – 565 3515 1500 4989 4694

Seawater 8850 565 1248 410 – – 900 – 13467

Brine 21600 916 2374 656 – – 1800 – 32869

–: Denotes the concentration below detection limits

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dilution for low flushing toilets. In all the experiments, magnetic

stirring was applied for 15 min to enhance the reactions. The mix-

ture was then centrifuged at 5000 rpm for 10 min to separate the

precipitates. The obtained solids were rinsed twice with 99.9% eth-anol and three times with deionized water to remove any organic

impurities. The precipitates were dried overnight at 80 C for var-

ious characterization analyses. According to   Bhuiyan et al.

(2008), during drying process there was 10% struvite mass loss at

80

C which was considered into the purity calculations. Further-more, the supernatants were filtered through 0.45lm pore size

 Table 2

Estimated precipitates by using chemical equilibrium model (modified Visual Minteq).

Precipitate Chemical formula Database

consideration

Remark Reference

Hydroxyapatite Ca5(PO4)3OH Not included Slow precipitation kinetics   Musvoto et al., 2000

Tricalcium

phosphate

Ca3(PO4)2   Not included Slow precipitation kinetics   Ferguson and McCarty, 1971

Dolomite CaMg(CO3)2   Not included Slow precipitation kinetics   Mamais et al., 1994

Magnesite MgCO3   Not included Prior formation of its metastable phase – hydromagnesite   Case et al., 2011

Hydromagnesite Mg5(CO3)4(OH)24H2O Not included High formation temperature (>40C)   Cheng and Li, 2010

Hunitite CaMg3(CO3)4   Not included Complex mineral with hydromagnesite, magnesite,

aragonite

and dolomite

Kangal and Güney, 2006

Aritinite Mg2CO3(OH)23H2O Not included Formation on the surface of existing magnesite   Muchi and Matsumoto, 1979

Struvite MgNH4PO46H2O Included Precipitation at pH (7–11); Mg2+/Ca2+ ratio >0.6   Song et al., 2007

Monenite CaHPO4   Included Thermodynamically stable   Musvoto et al., 2000

Calcite CaCO3   Included Thermodynamically stable   Harada et al., 2006

Brucite Mg(OH)2   Included Precipitation at high pH   Musvoto et al., 2000

Synthetic urine + MgCl2 Real urine + MgCl2

(a-1) (b-1)

Synthetic urine + seawater Real urine + seawater

(a-2) (b-2)

Fig. 1. Effect of Mg/P molar ratio on therecoveryrates of phosphate, ammonium,calcium andmagnesiumfrom hydrolyzed syntheticurine (pH= 9.3), ––: phosphate(model),

d: phosphate (experimental), ––––: ammonium (model),j: ammonium (experimental), - - - -: magnesium (model),N: magnesium (experimental), : calcium (model), and

s: calcium (experimental).

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membrane filter before analysis of the remaining ammonium,

phosphate, calcium and magnesium. All batch experiments were

carried out in duplicates at room temperature (23 ± 1 C).

 2.3. Analytical methods

After struvite precipitation, total ammonium concentration was

measured colorimetrically based on salicylate method using spec-trophotometer (Hach DR2800, USA). Orthophosphate and chloride

concentration in the supernatant was measured by ion chromatog-

raphy (ICS-1100, Dionex). The concentrations of magnesium and

calcium were analyzed with an Inductively Coupled Plasma – Opti-

cal Emission Spectrometer (Spectro Analytical Instruments). Car-

bonate concentration was measured by TOC analyzer (Shimadzu,

 Japan). Electrical conductivity and pH-values were measured with

a portable D-54 conductivity/pH meter (Horiba), respectively. In all

experiments, phosphate, ammonium, magnesium and calcium

recovery efficiencies (Ri) were calculated by the following

equation.

Ri ¼  1  C i;supC i;ini

ð1Þ

where C i,sup  and C i,ini  are ion concentrations in the supernatant and

initial urine sample, respectively.

Regarding the precipitates production, it can be estimated by

the following equation.

K sp;struv ite ¼ ½Mg2þ½NHþ

4 ½PO34   ð2Þ

where   K sp,struvite   is the solubility product constant of struvite;

[Mg2+], [NHþ

4 ] and [PO34   ] are magnesium, ammonium and phos-

phate concentrations in the balanced solution after struvite

precipitation.

X-ray diffraction (XRD, Scintag, USA) analysis and Fourier Trans-

form Infrared Spectroscopy (FTIR, Perkin Elmer PE 1600, USA) were

applied to characterize the precipitates. Energy Dispersive X-ray

Detector (EDX, Oxford Instrument, UK) and X-ray Fluorescence(XRF, Bruker AXS, Germany) were used to quantify the composi-

tions of the harvested precipitates. Furthermore, Field Emission

Scanning Electron Microscopy (FESEM, JEOL Asia Pte, Japan) images

were obtained to characterize the morphology of the precipitates.

 2.4. Chemical equilibrium modeling 

The chemical equilibrium model Visual Minteq 3.0 has been

widely used for calculating speciation, solubility and sorption equi-

librium of solids in an aqueous solution (Gustafsson, 2011). Since

struvite was not included in the current solid database, the solubil-

ity constant pK sp = 13.15 was manually incorporated into the mod-

el database based on Buchanan’s study (Buchanan et al., 1994).

Visual Minteq involves a management tool to add or delete mineralspecies. The activity coefficient computation of electrolyte solution

at relatively high concentrations was based on the Davies approx-

imation on Debye–Huckle equation. Default values were used for

all other equilibrium constants.

It has been reported that struvite precipitation can be com-

pleted within fewminutes (Lee et al., 2003). Since the precipitation

kinetics was not considered in the current model, those minerals

with low formation rates (in the scales of hours or days) were

intentionally omitted from the modeling calculation. Real data

based in Table 1 were loaded in the equilibrium model to estimate

the probable precipitates (Celen et al., 2007). Theoretically, eleven

minerals could be formed as listed in Table 2. A further literature

review was conducted on each mineral to determine the possibility

of its formation under the defined conditions. At last, the mineralsstruvite, monenite, calcite, and brucite were included in the

database because they are most probably precipitated under the

specific experimental conditions.

3. Results and discussion

 3.1. Optimization of struvite recovery process

 3.1.1. Effect of Mg/P molar ratioFig. 1a presents the effect of Mg/P molar ratio on the recovery

efficiencies of phosphate, ammonium, calcium, and magnesium

with the addition of MgCl2  and seawater in synthetic urine. From

the modeling results, phosphate recovery efficiency increased from

50% to 98% when Mg/P molar ratio increased from 0.5:1 to 1.1:1,

and reached almost 100% after that. Likewise at Mg/P molar ratio

1.1:1, magnesium recovery was nearly 100%, while increasing the

ratio made magnesium surpass the demand for phosphate recov-

ery and resulted in residual magnesium in the supernatant. Under

the same ratio, the recovery efficiency of ammonium was low (only

7%), because of the high initial ammonium concentration in the

solution. Furthermore, calcium was precipitated almost 100%

when seawater was added to hydrolyzed urine without any influ-

ence of Mg/P molar ratio.Laboratory experiments were done for further validation of Vi-

sual Minteq.   Fig. 1a demonstrates that experimental results on

phosphate and ammonium recovery efficiencies using MgCl2  were

in accordance with the model. However, when seawater was

Synthetic urine + MgCl2

Synthetic urine + seawater

(a)

(b)

Fig. 2.   Effect of pH on the recovery rates of phosphate, ammonium and the

magnesium from hydrolyzed synthetic urine (Mg/P molar ratio of 1.1:1), ––:

phosphate (model),  d: phosphate (experimental), ––––: ammonium (model),  j:

ammonium (experimental), - - -: magnesium (model), and   N: magnesium(experimental).

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added to synthetic urine, the experimental data on magnesium

and calcium precipitation efficiencies were different from the

model. Compared to the model, magnesium had higher precipita-

tion efficiency, while calcium had lower efficiency. The reason

involving the struvite purity will be discussed in the following

sections.

Fig. 1b shows the results using real hydrolyzed urine. Visual

Minteq predicted similar results to those of synthetic urine. A good

agreement was observed between experimental and modeling data

on phosphate, ammonium, and magnesium recovery. However, the

calcium recovery efficiency in the model was higher than that in

the experiment (100 vs 90% in this case). This discrepancy might

be attributed to: (1) model assumed that all reactions were in equi-

librium state, whereas the actual conditions might yet not reach

the equilibrium; and (2) the reaction efficiency of calcium might

be lower than the theoritical value due to low actual concentration

as indicated by Harada et al. (2006).

 3.1.2. Effect of pH 

Fig. 2   shows the effect of pH on the recovery efficiencies of 

phosphate, ammonium, and magnesium at optimum Mg/P molar

ratio 1.1:1. The experimental data showed that phosphate, ammo-nium and magnesium recovery were well matched with the pre-

dicted results in the modified model. It indicated that pH range

8–10.5 is favorable for phosphate recovery. Similar trends were re-

ported by Buchanan et al. (1994); Ali et al. (2003). As a matter of 

interest, hydrolyzed urine has strong buffer capacity with pH-value

around 9.3 based on our previous study (Zhang et al., 2013). Phos-

phate recovery from hydrolyzed urine could be effectively

achieved without any pH adjustment, which carries favorable

implication for the engineering application.

 3.1.3. Effect of urine hydrolysis and dilution

Maximum nutrients recovery from fresh and hydrolyzed urine

with the addition of seawater was first estimated by modeling

(Fig. 3). For fresh urine, phosphate recovery efficiency was 33%while the calcium recovery efficiency was high (87%) due to its

lower concentration. There was no ammonium recovery because

urea was not hydrolyzed yet. As a result, there was no struvite for-

mation, and the main precipitate was CaHPO4. For hydrolyzed ur-

ine, struvite formation could proceed because of the ammonium

release from the hydrolysis process. The maximum amount of pre-

cipitates that could be formed was 2.95 kg m3 of hydrolyzed

urine.

The harvested amount of precipitates at experimental condi-

tions was relatively lower (about 2–2.5 kg m3), probably due to

mass loss during filtration, drying and transferring procedures.

When hydrolyzed urine was diluted at 1:2 and 1:5, the estimated

amount of precipitated mass was 1.46 and 0.55 kg m3, respec-

tively. That was slightly less than half and fifth of estimatedamount of precipitates for undiluted hydrolyzed urine.

 3.1.4. Comparisons of magnesium sources

Three types of magnesium were investigated as additional mag-

nesium sources, namely, seawater, brine and MgCl2   (Table 3).

Based on modeling and experimental results, brine was an efficient

additive for struvite formation. According to the data in Table 1, re-

verse osmosis brine and sea water have the same chemical compo-

sitions, except that brine has higher concentrations (almost twice).

At optimum Mg/P molar ratio 1.1:1, 99.5% of phosphate could be

recovered through struvite precipitation.

As summarized in   Table 3,   a cost-effective analysis was also

conducted to compare different low-cost magnesium sources,

including bittern and wood ashes. The primary cost of the magne-sium sources come from transportation cost. For coastal cities like

Singapore, seawater is readily available due to short transport dis-

tance (<20 km). Desalination brine can be obtained from local

desalination plants. Compared with seawater, less volume of brine

is needed to produce the same amount of struvite because of high-

er Mg2+concentration in brine. This can dramatically reduce the

transportation cost, making brine a cheaper magnesium source.

Besides, the reuse of brine is eco-friendly, as brine is generally dis-

charged back to the ocean, which might affect the oceanic eco-sys-

tem. The required amount of bittern was much lower than

seawater or brine, but this source is not easily available in local

context. The transport cost will make it more expensive than

Fig. 3.  Maximum nutrient recovery from fresh urine (FU) (pH 6.5) and hydrolyzed

urine (HU) with no dilution (pH9.3), 1:2 dilution (pH9.2) and 1:5 dilution (pH9.1).

Others include carbon, hydrogen and oxygen in the solid products.

 Table 3

Comparison of different magnesium sources for phosphorus recovery from urine.

Magnesium source Seawater Brine Bittern Wood ash

Optimal Mg/P molar ratio 1.1 1.1 1.1 1.9c

Required input (kg m3 urine) 247a or 0.24 m3 146 or 0.14 m3 3.2b 16.2c

Struvite purity, % 95.6 96.6 – 23c

P recovery efficiency, % 99.0 99.0 >99.0 99.0

N recovery efficiency, % 5.2 7.4 – –

Product price (dollar kg1) – – – –

Transport distance (km) 0–20 0–20 50–2000c 0–50c

Transport cost (dollar kg1 km1) 0.0002c 0.0002c 0.0002c 0.0002c

Total input cost (dollar m3 urine) 0–0.99 0–0.58 0.03–1.27 0–0.16

Issues Further treatment Further treatment Limited source Heavy metal contamination, low purity

a The density of seawater is 1.03 kg L 1.b

Data calculated based on Lee et al. (2003).c Data calculated based on Sakthivel et al. (2012). Product prices for raw materials are considered negligible.

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seawater and brine. Wood ash is also cheap and easy to transport,

but the precipitates might suffer from low purity due to the con-

tamination of heavy metals.

 3.2. Precipitate characterization

In order to confirm the presence of struvite, extensive analyses

were conducted on the precipitates that obtained by adding sea-

water, brine, and MgCl2  to synthetic and real urine at Mg/P molar

ratio 1.1:1 and under pH-value 9.3.

 3.2.1. Struvite formation

The FT-IR spectra of pure struvite and the precipitates pro-

duced from MgCl2   and seawater using synthetic urine are pre-

sented in Fig. 4. The FT-IR spectra are assigned into four regionsdepicting the main identification peaks due to the presence of 

water in the crystallized struvite. According to identification

peaks, intensities between 1630–1750 cm1 was attributed to

N–H bending vabrition in ammonium group (Chauhan and Joshi,

2013). Intensities of bands between 1017–1163 and 894.2 cm1

were associated with phosphate stretching vibration, while inten-

sities between 400–650 cm1 was associated to metal–oxygen

bonds. It can be confirmed that the precipitates obtained in the

experiments were struvite crystals as the FT-IR spectra were

identical.

Fig. 5 shows the XRD patterns of precipitates after the addition

of MgCl2 and seawater to synthetic and real urine. For synthetic ur-

ine, the precipitate derived from MgCl2 addition was determined to

be struvite (Fig. 5a). This was in accordance to the FT-IR results.When seawater was added to synthetic urine, Mg-calcite

(Mg,Ca)CO3   was also identified by the XRD in the precipitates

(Fig. 5b). From the chemical equilibrium model, the predicted

product was the mixture of struvite and CaCO3. The formation of 

Mg-calcite in the experiment was unexpected. This result was con-

sistant with the observation of higher magnesium recovery effi-

ciency in the experiment than that in the model.

For real urine, the precipitate produced by MgCl2  addition was

also struvite as in synthetic urine (data not shown). When sea-

water was added to real urine, an additional precipitate, CaCO3,

was also observed (Fig. 5c). The precipitates were a mixture of 

struvite and CaCO3. In this case, the mineral species found from

experiments and predicted by Visual Minteq were in agreement.

The XRD results from brine addition were similar to those of seawater.

Comparing synthetic and real urine, different precipitates were

formed after the addition of magnesium sources. The formation of 

Mg-calcite in synthetic urine might be related to the high Mg/Ca

ratio and supersaturation effect as proposed by  Berner (1975). Un-

der high magnesium and calcium concentrations, the magnesium

could be incorporated within the calcite lattice forming Mg-calcite.

Loste et al. (2003)  have briefly indicated that the increase of oc-

cluded Mg was observed along with the increasing Mg/Ca ratio

in the solution. Actually, the formation of Mg-calcite was compli-

cated because the Mg/Ca ratio was continuously changing during

the precipitation process. This was most likely the main reasonfor the incorrect precipitates predicted by the Visual Minteq.

Fig. 4.  FT-IR spectrum of pure struvite and precipitates produced adding seawater

and MgCl2   to synthetic urine.

Fig. 5.   XRD patterns of precipitates after the addition of magnesium sources to

synthetic and real urine: (a) – struvite peaks; (b) – struvite, Mg-calcite; and (c) –struvite,    calcite.

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No other crystals were abundant enough to be recognize by the

XRD measurements. However, it is still possible that they were

formed in small amounts as transient compounds due to the effectof other ions such as Na+, K+, and SO

24   (Sakthivel et al., 2012).

Although seawater or brine contained potassium, the potassium

struvite could precipitate only in the case of low ammonium con-

centrations. The specific compound was not found in any crystal

analysis nor in the model simulations.

 3.2.2. Crystal morphologies

Fig. 6 shows the SEM images of precipitates from the matrix of 

different magnesium sources and type of urines. For synthetic ur-

ine, the morphology of the precipitates was mainly feather shape.

When MgCl2 was added, the surface of the precipitates was smooth

(Fig. 6a-1), whereas there were some fine crystals on the surface of 

the precipitates with the addition of seawater and brine (Fig. 6a-

2,3). For real urine, the morphology of the precipitates was coffinshape (Fig. 6b-1,2,3). The precipitates with addition of seawater

and brine had some fine crystals on their surfaces as well

(Fig. 6b-2,3). This observation has also been reported in other stud-

ies (Le Corre et al., 2005). Regarding the crystal morphology, Chau-han and Joshi (2013)   carried out struvite growth studies and

reported that struvite can exist in different morphologies (pris-

matic type, pyramid type, coffin shaped, feather shaped, needle

type, etc.) which are strongly dependent on growth parameters.

In order to determine the composition of the fine crystals ap-

peared on the surface of the precipitates, scanning electron micros-

copy coupled with EDX has been used on a specific area as labelled

on the SEM images (Fig. 7). The results are presented in Table 4. For

synthetic urine, the first analyzed area was at the smooth part of 

the feather shaped crystal (Fig. 7a-1); the main compositions were

Mg, P, O and C while calcium concentration was very low at 0.4%.

The second analyzed area was at the fine crystal (Fig. 7a-2). It was

found that the relative calcium concentration was more than 5%.

The difference at the elemental composition confirmed thepresence of two distinct entities. For further verification, the

(a-1) Synthetic urine + MgCl2 (b-1) Real urine + MgCl2

(a-2) Synthetic urine + seawater (b-2) Real urine + seawater

(a-3) Synthetic urine + brine (b-3) Real urine + brine

Fig. 6.   SEM images of precipitates after the addition of magnesium sources to synthetic and real urine.

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Mg/P molar ratio at the first analyzed area was nearly 1:1 that is

consisted with the stoichiometric proportion of struvite. According

to previous FT-IR and XRD results, it can be concluded that the

feather shaped crystal was struvite. At the second analyzed area,

the Mg/P molar ratio reveals that Mg was in excess compared to

P, which partially incorporated with calcite crystals. This suggests

that the fine crystal was Mg-calcite.

For real urine, the first analyzed area was at the coffin-shaped

crystal (Fig. 7b-1). The main elements were Mg, P, C and O and

the relative concentration of calcium was low at 0.4%. The second

analyzed area was an inter-growth rhombohedral crystal with

rounded morthologies indicating calcite formation (Fig. 7b-2).The main elements were Ca, C and O and the relative concentration

of calcium was 11.4%. Combined with FT-IR and XRD, the coffin-

shaped was struvite and the fine crystal was CaCO3. In each case

the formation of fine crystals on the surface of struvite could ex-

plain the multitude of background noise observed previously on

the XRD patterns.

 3.2.3. Struvite quantification

The struvite content in the precipitates (purity) is very impor-

tant for the successful and economical implementationof precipita-

tion process. Precipitates obtained from MgCl2, seawater and brine

addition to syntheticand real urine were analyzedby XRF. The main

elements of these precipitates were Mg, P and O with an Mg/P/Oratio of about 1:1:4, which complied with the stoichiometric

(a-1) Synthetic urine + brine (a-2) Synthetic urine +brine

(b-1) Realurine + brine (b-2) Realurine + brine

Fig. 7.  EDX images of precipitates after the addition of brine to synthetic and real urine.

 Table 4

Compositions of the analyzed areas by EDX.

Element atomic (%) Brine addition to synthetic urine Brine addition to real urine

a-1 image a-2 image b-1 image b-2 image

Mg 10.6 5.0 7.0 1.5

P 11.3 3.5 11.3 0.7

Ca 0.4 5.5 0.4 11.1

O 59.1 65.1 39.7 63.1

C 18.5 20.4 41.3 22.9

 Table 5

Struvite purity of the precipitates obtained from synthetic and real urine using different magnesium sources.

Mg2+ sources MgCl2   Synthetic urine Real urine

Seawater Brine MgCl2   Seawater Brine

Crystal: Mg/Ca ratio – 4.7/1 6.8/1 – 8.0/1 10.4/1

Struvite purity, % 99.7 92.8 94.3 99.9 95.6 96.6

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proportion of struvite. When MgCl2 was added to synthetic andreal

urine, struvite purity was 99.7% and 99.9%, respectively (Table 5).

When seawater and brine were added, the precipitates contained

calcium crystals.It seemsthat thehigher Mg/Caratioin thecrystals,

the higher struvite purity was derived. According to this, precipi-

tates with Mg/Ca molar ratio of 4.7:1 and 6.8:1 from seawater

and brine addition in synthetic urine had 92.8% and 94.3% struvite

purity, respectively. For real urine, precipitates with Mg/Ca molar

ratio of 8.0:1 and 10.4:1 from seawater and brine addition had

95.6% and 96.6% struvite purity, respectively. This study confirmed

that the initial calcium concentration affected struvite purity. Sim-

ilar findings were observed in previous studies (Song et al., 2007).

As indicated in Table 1, the Mg/Ca ratio in seawater and brine

was 3:1 and 3.3:1, respectively. Using seawater or brine as addi-

tional magnesium sources, calcium is inevitably added into the

system. Due to the high carbonate concentration in urine solution,

the addition of calcium could lead to the formation of calcite or

Mg-calcite. Besides, calcium precipitation was affected by Ca2+

concentration in the solution. The lower the Ca2+ concentration

was, the lesser the precipitate formed. Since phosphate concentra-

tion in real urine was relatively lower compared to synthetic urine,

the required amount of Mg2+ was equally lower. Therefore, the

amount of Ca2+ inevitably added was accordingly lower, which

led to less formation of calcite. This might explain the higher stru-

vite purity in real urine. In this view, bittern could be another

promising magnesium source since the remaining calcium in bit-

tern was very limited. Hence, the formation of calcite or Mg-calcite

impurities would not significantly compromise the struvite purity.

However, bittern is not available in local context, therefore, it was

not considered as a cheap alternative magnesium source.

It is proven that both seawater and brine are low-cost magne-

sium sources in the local context. The struvite comprised more

than 95% of the total precipitates under all conditions in this study,

which still make the precipitate mixture beneficial in the actual

fertilizer application. This indicated the great potential of engaging

seawater or desalination brine for large scale struvite harvesting.

The impurities, mainly the calcite or Mg-calcite, might affect thefertilizing efficiency of the precipitates comparing to the struvite

harvested by adding MgCl2. It is still considered economical and

beneficial given the lower cost of the raw materials. In addition,

Sakthivel et al. (2012) have observed high calcite production when

wood ash was used as magnesium source. This kind of impurity

can be used as soil conditioner in acidic soils.

4. Conclusions

The present study demonstrated that both seawater and desali-

nation brine can be considered as low-cost magnesium source to

recover phosphorus from hydrolyzed urine. High phosphate recov-

ery efficiency (>99%) was achieved at Mg/P molar ration of 1.1without further pH adjustment (pH 9.3). The induced precipitate

was mainly composed of coffin-shaped struvite in real urine and

feather-shaped crystal in synthetic urine. For real urine, fine calcite

crystals were formed on the surface of struvite and the final prod-

uct contained roughly 90% of struvite and 10% of CaCO3. For syn-

thetic urine, Mg-rich calcite was formed along with the struvite.

It indicated the calcium in the magnesium sources could compro-

mise the purity of struvite product. Although the presence of cal-

cite or Mg-calcite in the fertilizer might make it slightly

ineffective, the final products can be used as phosphorus enhanced

conditioner in acidic soils.

The outcomes of this project can be used for the development of 

P-recovery process in industrial-scale. Both seawater and brine are

low-cost magnesium sources to be incorporated into source-separating sanitation systems. However, there are implications of 

the P-recovery process such as the high ionic concentrations in

the effluent. Among these ions, ammonium can be further recov-

ered by air stripping to form of ammonium sulphate as proposed

by other studies (Saracco and Genon, 1994). The rest can be dis-

charged to the sewer system or further treated. Considering the

small volume of urine stream compared to the total domestic

wastewater, the effluent from nutrient recovery process might

not impose restriction for it application.

 Acknowledgements

This study is supported by the National Research Foundation,

Singapore; program number NRF-CRP5-2009-02, for the School of 

Civil and Environmental Engineering/Residues and Resource Recla-

mation Centre, Nanyang Technological University, Singapore. We

appreciate Mr. Bernard Ng and Ms. Peijun Zhao for their assistance

in the experimental work.

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