Urine is a major source of reclaimed water and fertilizer. Urine treatment involves two key processes: the recovery of nutrients and the rejection of trace organic compounds (TOrCs). In this study, we investigated the rejection of TOrCs and the recovery of nutrients in human urine using a seawater-driven forward osmosis and membrane distillation (FO–MD) hybrid system. Three 24 h experiments were conducted at draw solution temperatures of 30, 40, and 50 °C. The average rejection rates of cations, anions, and dissolved organic carbon were more than 93.7% and 79.5% in the FO–MD system and FO side, respectively. Ten types of TOrCs were detected in the feed solution, whereas none were detected in the product water, indicating that the TOrCs were completely rejected. The precipitates, i.e., the recovered nutrients in the FO side, were extremely close to magnesium ammonium phosphate (struvite, MgNH4PO4·6H2O), according to their electron microscopic images, elemental composition, and X-ray diffraction spectra, and it was estimated that approximately 85% of the nutrients in the feed solution were recovered. The rejection and recovery efficiencies were unaffected by the draw solution temperature. These results indicate the potential for the sustainable use of FO–MD-based treatments for human urine.

  • More than 93.7% of cations and anions in feed solution were rejected.

  • Trace organic compounds were completely rejected in this system.

  • Nutrients in feed solution were recovered as struvite.

Graphical Abstract

Graphical Abstract
Graphical Abstract
     
  • A

    effective membrane area

  •  
  • Cd

    chemical concentration in the draw solution

  •  
  • Cf

    chemical concentration in the feed solution

  •  
  • CTA

    cellulose triacetate

  •  
  • Cw

    chemical concentration in the product water

  •  
  • DOC

    dissolved organic carbon

  •  
  • DOM

    dissolved organic matter

  •  
  • EC

    electrical conductivity

  •  
  • ED

    electrodialysis

  •  
  • EDS

    energy dispersive X-ray spectroscopy

  •  
  • FO

    forward osmosis

  •  
  • JW

    transmembrane water flux

  •  
  • LC-MS/MS

    high-performance liquid chromatography with tandem mass spectrometry

  •  
  • MD

    membrane distillation

  •  
  • SEM

    scanning electron microscope

  •  
  • TICC

    total ion current chromatogram

  •  
  • TOC

    total organic carbon

  •  
  • TOrCs

    trace organic compounds

  •  
  • XRD

    X-ray diffraction

  •  
  • Δt

    time interval

  •  
  • ΔV

    total increased volume of permeate water

As a means of addressing the increasing human population and climate change, reclaimed water is attracting attention as an alternative water resource. Technology using membranes, such as reverse osmosis (RO) and nanofiltration, has been widely applied to various wastewater, including municipal and industrial wastewater. However, this pressure-driven membrane process consumes high energy and tends to form fouling on the membrane.

Compared with the pressure-driven membrane process, forward osmosis (FO), which is an osmotically driven membrane process, has attracted attention because of lower energy consumption and membrane fouling (Zhao et al. 2012; Xue et al. 2015). An FO system has been utilized not only to concentrate water volume but also to recover nutrients, such as nitrogen and phosphorus, from municipal wastewater and urine (Xie et al. 2014; Zhang et al. 2014). Additionally, an FO system shows high rejection for harmful metals, including mercury and arsenic (Ge et al. 2016; Wu et al. 2017), and trace organic compounds (TOrCs), such as pharmaceuticals in wastewater (Hancock et al. 2011; Xie et al. 2012a, 2012b; Coday et al. 2014; Lutchmiah et al. 2014). However, in an FO system, the difference in osmotic pressure between the feed solution and draw solution gradually decreases with operation time because the draw solution is diluted by permeated water from the feed solution. Therefore, the integration of FO with other processes, including membrane distillation (MD) and electrodialysis (ED), has been applied recently. An FO–MD hybrid system can not only reconcentrate a diluted draw solution for sustainable process performance but also complement wastewater nutrient recovery with freshwater production (Xie et al. 2016).

There has been considerable attention to the recycling of urine worldwide (Larsen et al. 2021; Wald 2022). Among nutrient-rich water, including municipal wastewater, human urine is regarded as one of the best available resources for fertilizer because it contains 4.0–14 g L−1 nitrogen, 0.35–2.5 g L−1 phosphorus, and 0.75–2.6 g L−1 potassium (Rose et al. 2015; Patel et al. 2020; Larsen et al. 2021). Particularly, using separated urine in an FO system is cost effective and environmentally friendly for nutrient recovery (Zhang et al. 2014). Nitrogen and phosphorus in wastewater are frequently recovered as magnesium ammonium phosphate (struvite, MgNH4PO4·6H2O) by adding magnesium (Xie et al. 2014).

Several studies have examined recovering nutrients from wastewater and digested sludge using an FO–MD hybrid system (Xie et al. 2013a, 2014). Also, several studies have reported successful rejection of nitrogen and phosphorus using the FO–MD hybrid system for urine (Liu et al. 2016; Volpin et al. 2019). In the FO–RO hybrid system, the rejection of neutral and ionic TOrCs, such as pharmaceuticals in wastewater, has been studied in detail (Hancock et al. 2011; Holloway et al. 2014). However, a comprehensive analysis of TOrCs for human urine treatment using an FO–MD system has not been conducted, and little is known about the comprehensive rejection of TOrCs using an FO–MD system.

This study aims to evaluate the rejection of ions and TOrCs from feed solution to produce water and the production of struvite potential from human urine using a seawater-driven FO–MD hybrid system. This study comprehensively investigated TOrCs in urine using the full-scan mode of high-performance liquid chromatography with tandem mass spectrometry (LC-MS/MS). Seawater is abundant and low cost, and it has frequently been used as a draw solution, as well as a source of NaCl (Xue et al. 2015). Therefore, seawater was chosen in this study as a draw solution from the viewpoint of sustainable and simple operation.

Feed and draw solutions

Human urine (pooled from 13 normal donors, 991-03-P), which is a feed solution, was purchased from Lee Biosolutions, Inc. (MO, USA). The electrical conductivity (EC) and pH of the urine samples were 9.51 mS/cm and 6.14, respectively. Insoluble material in the urine samples after thawing was removed by a precombusted glass fiber filter (0.45 μm pore size, GF/B, Whatman®, UK) at 450 °C for 2 h before the experiments. Synthetic seawater (Sealife, Nihonkaisui Co., Ltd, Tokyo, Japan) was used as a draw solution and adjusted 30% with distilled water. Table 1 lists the details of the content of urine (feed solution) and synthetic seawater (draw solution).

Table 1

Content of the feed and draw solutions

Feed solution (mg L−1)Draw solution (mg L−1)
Ca2+ 65 ± 5.5 420 ± 28 
K+ 1,100 ± 54 350 ± 27 
Mg2+ 33 ± 2.8 1,300 ± 74 
Na+ 14,400 ± 65 10,400 ± 1,030 
NH4+ 230 ± 24 0.46 ± 0.65 
Br 18 ± 3.2 92 ± 21 
Cl 1,800 ± 47 21,000 ± 4,900 
F 36 ± 8.0 N.D. 
PO43− 850 ± 30 N.D. 
NO2 N.D. N.D. 
NO3 38 ± 4.8 N.D. 
SO42− 750 ± 92 4,300 ± 2,300 
DOC 4,600 ± 140 N.A. 
Feed solution (mg L−1)Draw solution (mg L−1)
Ca2+ 65 ± 5.5 420 ± 28 
K+ 1,100 ± 54 350 ± 27 
Mg2+ 33 ± 2.8 1,300 ± 74 
Na+ 14,400 ± 65 10,400 ± 1,030 
NH4+ 230 ± 24 0.46 ± 0.65 
Br 18 ± 3.2 92 ± 21 
Cl 1,800 ± 47 21,000 ± 4,900 
F 36 ± 8.0 N.D. 
PO43− 850 ± 30 N.D. 
NO2 N.D. N.D. 
NO3 38 ± 4.8 N.D. 
SO42− 750 ± 92 4,300 ± 2,300 
DOC 4,600 ± 140 N.A. 

Values show average and standard deviation (n = 3).

N.D., not detected; N.A., not analyzed.

FO–MD system setup

Figure 1 shows a FO–MD system. A labscale FO–MD system is composed of an FO cell, a direct contact MD cell and thermostatic baths, peristaltic pumps, and digital balances. A flat sheet FO membrane of cellulose triacetate (CTA) (FTS H2O™ flat sheet membrane), a flat sheet MD membrane of polytetrafluoroethylene (0.2 μm pore size, CF042), and an FO cell (CF042D-FO) were purchased from Sterlitech Corporation (WA, USA). The FO cell holds an FO membrane with an effective area of 34 cm2. The MD cell is made of acrylic plastic and holds an MD membrane with an effective area of 37 cm2. These membranes were used with an active layer facing the feed solution. Counter current crossflow circulation of each solution was applied by peristaltic pumps at a rate of 0.1 L min−1. Thermostatic baths were used to adjust the water temperature in the draw solution and the distilled water in the MD system. The feed solution in the FO system and the distilled water (product water) in the MD system were weighed, and the changes in their weight were recorded by digital balances at regular time intervals for 24 h.
Figure 1

Schematic diagram of the laboratory scale forward osmosis and membrane distillation (FO–MD) system.

Figure 1

Schematic diagram of the laboratory scale forward osmosis and membrane distillation (FO–MD) system.

Close modal

Experiment procedure

The initial water volume of each solution was as follows: the feed solution reservoir (1 L glass bottle) contained 0.5 L of the urine sample, the draw solution reservoir (2 L Erlenmeyer flask) contained 1 L of 30% synthetic seawater, and the product water reservoir (1 L glass bottle) contained 0.5 L of distilled water. The feed solution and the draw solution reservoirs were connected to the FO cell and the pump, and the draw solution and the product water reservoirs were connected to the MD cell and the pump. The water temperature of the feed solution was not adjusted, whereas that of the draw solution was adjusted at 30, 40, and 50 °C. These water temperatures were set to investigate the appropriate temperature for maintaining the stability and sustainability of the FO–MD system; that is, the temperature where the difference in flux between the feed and draw solutions and the draw solution and distilled water is at the minimum. The distilled water in the MD system was maintained at 20 °C. The experiments were conducted under three different temperatures of the draw solution for 24 h. For the feed solution, the pH and EC were measured at 6 h intervals. From the feed, draw, and product solutions, 5 mL of the samples were taken at 6 h intervals for chemical analysis.

Chemical analysis

Water temperature and pH were measured using a portable multisensor (D-73; Horiba, Ltd, Kyoto, Japan); EC was measured using a portable EC sensor (ES-71; Horiba, Ltd). Anions (Br, Cl, F, NO2, NO3, PO43−, and SO42−) and cations (Ca2+, K+, Li+, Mg2+, Na+, and NH4+) in the water samples were measured using an ion chromatograph (IC-2010, Tosoh Corporation, Tokyo, Japan). After filtration (GF/B, Whatman, UK), dissolved organic carbon (DOC) was analyzed using a total organic carbon (TOC) analyzer (TOC-L, Shimadzu, Kyoto, Japan). TOC in the draw solution could not be measured because of high salinity.

TOrCs were measured using an LC-MS/MS (Ultimate 3000/TSQ Quantum Access Max, Thermo Fisher Scientific) with a full scan mode (m/z 100–500). The water samples were filtered using a disposal filter (pore size 0.2 μm, Autovial® 5, Whatman®, UK), and then 0.5 mL methanol was added to 0.5 mL of the filtered sample. An aliquot of 10 μL of the sample was injected onto a ZORBAX Eclipse Plus C-18 (2.1 mm i.d. × 150 mm length, 3.5 μm; Agilent Technologies, CA, USA) and separated using a mobile phase of 0.01% CH3COOH and methanol at a flow rate of 0.2 mL min−1 at 40 °C.

Recovery of magnesium ammonium phosphate (struvite)

After the experiment, spontaneous precipitation of the feed solution was removed by filtration (GF/B, Whatman) in order not to mix with struvite. The recovery of struvite was according to the method of Xie et al. (2014). Magnesium chloride was added to the feed solution to achieve a mole ratio of 1:1.5 between PO43− and Mg2+. This solution was then adjusted at pH 9.5–9.6 by adding 1 M NaOH and filtered with a cellulose filter (No.7, Advantec Toyo Kaisha, Ltd, Tokyo, Japan). The residual precipitate on the filter was transferred to a glass beaker and dried in a desiccator. Particles of the precipitate were observed using a scanning electron microscope (SEM) (JCM-6000 NeoScope™) (JEOL Ltd, Tokyo, Japan) with 15 kV of acceleration voltage. Elements in the precipitate and in the fouling in the FO membrane were observed using energy-dispersive X-ray spectroscopy (EDS) (JED-2300, JEOL). These samples were sputter-coated with a thin layer of platinum by auto-fine coater (JEC-3000FC, JEOL). X-ray diffraction (XRD) analysis of precipitation was conducted using an XRD spectrometer (SmartLab-9KW, Rigaku, Tokyo). Standard material of struvite (purity >99.997%) for an XRD analysis was purchased from Sigma-Aldrich Co. (MO, USA).

Data analysis

The transmembrane water flux, JW (L m−2 h−1), of the FO and MD processes was calculated as follows:
formula
(1)
where ΔV (L) is the total increased volume of permeate water collected over a predetermined time interval Δt (h), and A (m2) is the effective membrane area.
Rejection of cations, anions, and TOrCs in the FO–MD system was determined from the difference in chemical concentration between the feed solution (Cf) and the product water (Cw) and is defined as follows:
formula
(2)
The precise rejection of chemicals in the FO process in the FO–MD system is difficult to evaluate because the draw solution is continuously distilled in the MD process. Therefore, we defined a rejection of cations, anions, and TOrCs in the FO process as an apparent rejection. Apparent rejection in the FO process was determined from the difference in the chemical concentration between the feed solution (Cf) and the draw solution (Cd) and is defined as follows:
formula
(3)

Water flux

The average water flux from the feed solution to the draw solution (FO side) for 24 h ranged from 3.0 to 3.4 L m−2 h−1 (Figure 2). The water fluxes in this study were within the reported values in the studies that used an FO or FO–MD system (Xue et al. 2015; Liu et al. 2016) and were similar to the reported values in a study that used 1.0 M NaCl as the draw solution and human urine as the feed solution in an FO–MD hybrid system (Liu et al. 2016). Figure 3 shows the time trend of the water flux. Water fluxes for the FO side gradually decreased with operation time, whereas those for the MD side periodically fluctuated. These time trends of the water fluxes were probably affected by fouling on the FO membrane and water sampling for chemical analysis. Regarding the decline in the water flux for the FO side, the suspended matter was spontaneously formed in the feed solution with operation time, and the fouling may be made on the FO membrane surface. The standard deviation (SD) of water flux increased with an increase in the water temperature of the draw solution (Figure 2). The fouling was more severe with increasing water temperatures and probably resulted in the large SD of the water flux in the experiment at 50 °C of the draw solution. The water flux rapidly declined after 12 h due to severe fouling on the membrane in the experiment at 50 °C of the draw solution. An EDS analysis of the FO membrane surface indicated that P, Ca, O, and C were the main elements. This result suggested that the fouling on the FO membrane is caused by the mixture of these elements. To estimate the composition of the mixture, chemical equilibrium was estimated by the Visual MINTEQ (Gustafsson 2018) using the measured concentrations of cations, anions, and DOC in the feed solution for the experiment at 50 °C, which observed an extreme decrease in the water flux. The estimated result suggested that Ca5(PO4)3OH (hydroxyapatite) is oversaturation under the final condition of pH (7.4) and water temperature (37.4 °C) in the feed solution after 24 h. Additionally, forming of Ca-dissolved organic matter (DOM) and Mg-DOM was also suggested.
Figure 2

Water flux in the FO side and MD side. Values of the y-axis indicate the average and SD of water flux. The values of the x-axis indicate the water temperature set in the draw solution. The FO side indicates water flux from the feed solution to the draw solution. The MD side indicates water flux from the draw solution to the product water.

Figure 2

Water flux in the FO side and MD side. Values of the y-axis indicate the average and SD of water flux. The values of the x-axis indicate the water temperature set in the draw solution. The FO side indicates water flux from the feed solution to the draw solution. The MD side indicates water flux from the draw solution to the product water.

Close modal
Figure 3

Time trend of water flux in the FO side and MD side. Each point indicates a 60 min moving average of flux monitored at 10 min intervals. The FO side indicates the water flux from the feed solution to the draw solution. The MD side indicates water flux from the draw solution to the product water. The water flux of the MD side at 30 °C was not obtained because the balance was out of order. The dashed line shows the decreased volume of water (%) in the feed solution.

Figure 3

Time trend of water flux in the FO side and MD side. Each point indicates a 60 min moving average of flux monitored at 10 min intervals. The FO side indicates the water flux from the feed solution to the draw solution. The MD side indicates water flux from the draw solution to the product water. The water flux of the MD side at 30 °C was not obtained because the balance was out of order. The dashed line shows the decreased volume of water (%) in the feed solution.

Close modal

The average water flux from the draw solution to the product water (MD side) increased from 0.5 (30 °C) to 5.4 L m−2 h−1 (50 °C) with an increasing water temperature of the draw solution. The driving force of the MD is the difference in the water vapor pressure between the solutions; therefore, the water flux increased with increasing water temperature. The observed water fluxes at 40 and 50 °C were similar to the reported values in the MD process (Liu et al. 2016).

Comparing the water flux between the FO side and the MD side at each water temperature, the difference in the water flux was large at 30 and 50 °C, with the water flux at the FO side > than that at the MD side at 30 °C and the water flux at the MD side > than that at the FO side at 50 °C (Figure 2). The smallest difference in the water flux between the FO side and the MD side was observed at 40 °C, and this result implies that the suitable temperature of the draw solution was approximately 40 °C to operate stably in this FO–MD system. Although experiment conditions in the FO–MD system differed from those in this study, it has also been reported that a water transfer rate in an FO process with 1 M NaCl may be suitable for balancing a water temperature of 40 °C in a previous study (Liu et al. 2016).

Concentration ratio of nutrients in the feed solution in the FO system

Figure 4 shows the concentration ratios of representative cations and anions. Average concentration ratios of NH4+, NO3, and PO43− for the experiments after 24 h were 2.3, 1.4, and 1.7, respectively. Furthermore, the concentration ratios of Ca2+ and Mg2+ were increased with operation time until 12–18 h but finally decreased to less than 1 (0.7–0.9) after 24 h. It was expected that the concentration ratio of each ion was to be approximately 2 when the water volume in the feed solution reached half. However, the concentration ratio of NH4+ slightly exceeded 2, and that of NO3, PO43−, Ca2+, and Mg2+ was less than 2. For an increase in ammonium, hydrolysis of urea by bacteria and free urease in the feed solution may occur with operation time (Udert et al. 2003a). For a decrease in concentration ratios of PO43−, Ca2+, and Mg2+, spontaneous precipitation in fresh urine has been predicted to be dicalcium phosphate (CaHPO4·2H2O) (Zhang et al. 2014). For hydrolyzed stale urine, a mixture of struvite (MgNH4PO4·6H2O) crystals and hydroxyapatite (Ca5(PO4)3OH) would mainly be the precipitate (Udert et al. 2003b; Zhang et al. 2014). The formation of these precipitations may be supported by decreasing the concentration ratios of Ca2+ and Mg2+. Also, a remarkable decrease in the concentration ratios of Ca2+ and Mg2+ suggested absorption to the membrane surface by internal polarization because the zeta potential of CTA membranes shows a generally negative value (Xue et al. 2015).
Figure 4

Time trend of concentration ratio for representative cations and anions in the feed solution. The dashed line shows the decreased volume of water (%) in the feed solution.

Figure 4

Time trend of concentration ratio for representative cations and anions in the feed solution. The dashed line shows the decreased volume of water (%) in the feed solution.

Close modal

Rejection of cations and anions

On the FO side, average apparent rejections of four ion species and DOC were obtained ranging from 79.5% (ammonium) to 100% (Table 2). Although the rejection of ammonium was lower than that of others, moderate rejection of ammonium in a FO system has been reported, and the rejection in this study was within the reported values in the laboratory- and pilot-scale experiments (Zhang et al. 2014; Xue et al. 2015; Wang et al. 2016; Li et al. 2018). The rejection of anions, such as NO3 and PO43− and DOC (>99.4%), was higher than that of ammonium. This result was possibly due to electrostatic repulsion between membrane and solute molecules (Xue et al. 2015).

Table 2

Rejection of cations, anions, and DOC in the FO and FO–MD hybrid systems (%)

FO sideaFO–MDa
Na+ b 99.8 ± 0.2 
NH4+ 79.5 ± 3.9 93.7 ± 2.2 
Mg2+ b 99.2 ± 0.7 
Ca2+ b 99.4 ± 0.6 
K+ b 100.0 
F 100.0 97.9 ± 2.3 
Cl b 97.9 ± 2.9 
Br b 98.8 ± 1.7 
NO3 100.0 100.0 
PO42− 100.0 100.0 
SO42− b 99.5 ± 0.8 
DOC c 99.1 ± 0.9 
FO sideaFO–MDa
Na+ b 99.8 ± 0.2 
NH4+ 79.5 ± 3.9 93.7 ± 2.2 
Mg2+ b 99.2 ± 0.7 
Ca2+ b 99.4 ± 0.6 
K+ b 100.0 
F 100.0 97.9 ± 2.3 
Cl b 97.9 ± 2.9 
Br b 98.8 ± 1.7 
NO3 100.0 100.0 
PO42− 100.0 100.0 
SO42− b 99.5 ± 0.8 
DOC c 99.1 ± 0.9 

aData show average and standard deviation for all the experiments (n = 3).

bRejection could not be calculated because of abundance in draw solution (seawater).

cDOC in draw solution could not be measured because of high salinity.

For the FO–MD system, rejections of each ion and DOC ranged from 93.7% to 100% (Table 2). The rejection of ammonium improved from 79.5% (on the FO side) to 93.7% (after combining the MD process). High rejection of these was also reported in a 120-h continuous experiment using an FO–MD system (Li et al. 2018). Rejections of bromide, chloride, and fluoride tended to be slightly lower (97.9–98.8%) than those of other ions. These halide anions are probably transferred from the draw solution (seawater) to the product water through the MD membrane.

Rejection of trace organic compounds

Ten peaks were detected in the feed solution by positive and negative ion modes (Figure 5 and Table 3). A mass spectrum of the 10 peaks ranging from 113 to 494 of molecular weight was obtained. However, the compositions and names of the chemicals could not be identified because of the lack of mass resolution of the LC-MS/MS. These 10 peaks were not detected in the draw solution and product water, and they were perfectly rejected in the FO side and FO–MD system. The high rejection of positively and negatively charged TOrCs was consistent with the previous studies using the FO or FO–RO systems (Hancock et al. 2011; Xie et al. 2013a; Coday et al. 2014). The rejection of charged TOrCs, such as pharmaceuticals in domestic wastewater, that are treated at the facility by a demonstration-scale sequencing batch membrane bioreactor system was greater than 80 and 99% in a bench-scale FO experiment and a hybrid FO–RO process, respectively (Hancock et al. 2011). Rejection of negatively charged TOrCs tends to increase because of enhanced electrostatic repulsion between the FO membrane and solutes (Xie et al. 2013a). Additionally, the formation of a larger hydrated layer around the ionic species may also affect the rejection of negatively charged TOrCs (Holloway et al. 2014). The rejection of positively charged compounds follows the general principle of size exclusion (Coday et al. 2014). The high rejection of positively charged TOrCs can be explained by a large hydrated radius of ionic species in an aqueous solution (Holloway et al. 2014). Such high rejection of TOrCs was also observed in a pilot-scale FO–RO system for 40 d (Hancock et al. 2011). In this pilot-scale experiment, TOrC rejection was greater than that observed in bench-scale experiments. This result was possibly due to membrane compaction, the establishment of a fouling layer, and optimized hydrodynamic conditions in the pilot-scale system (Hancock et al. 2011).
Table 3

Rejection of trace organic compounds (TOrCs) by the FO–MD system

Retention time (min)FO sideFO–MDEstimated molecular weight
Positive ion mode   
#1 1.55 100 100 250 
#2 1.93 100 100 113 
#3 7.29 100 100 264 
#4 12.39 100 100 278 
Negative ion mode   
#5 1.93 100 100 182 
#6 7.27 100 100 181 
#7 7.66 100 100 188 
#8 7.99 100 100 188 and 194a 
#9 16.58 100 100 174 
#10 18.58 100 100 494 
Retention time (min)FO sideFO–MDEstimated molecular weight
Positive ion mode   
#1 1.55 100 100 250 
#2 1.93 100 100 113 
#3 7.29 100 100 264 
#4 12.39 100 100 278 
Negative ion mode   
#5 1.93 100 100 182 
#6 7.27 100 100 181 
#7 7.66 100 100 188 
#8 7.99 100 100 188 and 194a 
#9 16.58 100 100 174 
#10 18.58 100 100 494 

aMolecular weight could not be specified because of possible coelution.

Figure 5

Total ion current chromatogram (TICC) of trace organic compounds and mass spectra in each solution. (a) TICC of TOrCs in each solution. (b) Mass spectrum of peak #1. The numbers assigned to the peaks in Figure 5(a) correspond to those in Table 3. The value in parenthesis for each product water indicates the water temperature set in the draw solution.

Figure 5

Total ion current chromatogram (TICC) of trace organic compounds and mass spectra in each solution. (a) TICC of TOrCs in each solution. (b) Mass spectrum of peak #1. The numbers assigned to the peaks in Figure 5(a) correspond to those in Table 3. The value in parenthesis for each product water indicates the water temperature set in the draw solution.

Close modal

High rejection of TOrCs was observed regardless of the temperature of the experiment in this study. The rejection of charged TrOCs was higher than that of neutral TrOCs, and their rejection was insensitive to temperature variation (20–40 °C) in FO (Xie et al. 2013b). The rejection of TOrCs may be more affected by fouling on an FO than water temperature because membrane fouling increases electron repulsion, resulting in high rejection of negatively charged TOrCs (Coday et al. 2014). Since the fouling on the FO membrane was observed in the experiments at 40 and 50 °C in this study, the rejection of the TOrCs might be somewhat affected by the fouling.

Two peaks were detected in the positive ion mode in the feed at retention times of 1.55 and 1.93 min in the draw solution and product water in the experiments at 30 and 40 °C, respectively (Figure 5(a)). As a result of comparing mass spectra among the solutions, these peaks in the product water and draw solution were different from those in the feed solution (Figure 5(b)). This difference in the mass spectra suggested that these peaks originated from the draw solution, not from the feed solution. Therefore, these peaks were excluded from the calculation of the rejection.

The detected TOrCs in the feed solution could not be identified; nevertheless, these are possibly biodegradable chemicals, such as metabolites in urine. If the retained TOrCs in the feed solution are biodegradable chemicals, a biological wastewater treatment process may be applied to treat the feed solution.

Recovery of struvite

Figure 6 shows SEM images and observed elements of the precipitate in the feed solution. The precipitate had an orthorhombic structure (Figure 6(a)). In the EDS analysis, three peaks of oxygen, magnesium, and phosphorus, which are the main elements composing struvite, were detected in the precipitate (Figure 6(b)). Nitrogen, which is another important element in struvite, could not be detected in this study because it is light, as well as in the previous study (Xie et al. 2014). Therefore, to determine the proportion of nitrogen, we examined this by comparing the mass of NH4+ in the feed solution before and after forming the precipitation. The mass of NH4+ in the feed solution decreased to approximately 20% after the experiments. This decrease suggests that approximately 80% of NH4+ had been consumed by forming the precipitate. Regarding the XRD spectrum of the precipitation, the main peaks were observed at 15.9, 21.5, and 32.1 2θ (deg). These peaks corresponded well with that of the standard material and the crystal structure of struvite (Graeser et al. 2008) stored in the American Mineralogist Crystal Structure Database (Downs & Hall-Wallace 2003) (Figure 6(c)). This result demonstrates that the precipitation obtained in this study was a struvite.
Figure 6

Crystal structure analysis for the precipitate. (a) SEM image. (b) Detected elements by EDS. (c) XRD spectraa. aCrystal structure data of struvite (Graeser et al. 2008) were obtained from the American Mineralogist Crystal Structure Database. The standard material of purity is >99.997%.

Figure 6

Crystal structure analysis for the precipitate. (a) SEM image. (b) Detected elements by EDS. (c) XRD spectraa. aCrystal structure data of struvite (Graeser et al. 2008) were obtained from the American Mineralogist Crystal Structure Database. The standard material of purity is >99.997%.

Close modal

The average recovery rate of the struvite was 85% of the theoretical yield at the initial feed solution. Effective struvite precipitation could only be achieved when phosphorus concentration was above 100 mg L−1 (Xie et al. 2016), and in this study, the phosphorus concentration (approximately 830 mg L−1) was enough to recover struvite effectively. The difference between the recovery rate and the theoretical yield of the struvite may imply that phosphorus was consumed by forming the precipitate and fouling on the FO membrane in the FO–MD experiments. To recover more amounts of struvite, using hydrolyzed stale urine may be helpful because ammonium increases by the hydrolysis of urea (Zhang et al. 2014).

In this study, the rejection of ions and TOrCs and the recovery of nutrients as struvite from human urine were investigated using a seawater-driven FO–MD hybrid system. More than 93.7% of the cations and anions in the feed solution were rejected by the FO–MD system. Furthermore, TOrCs in the feed solution were perfectly rejected regardless of the temperature of the experiment. Additionally, struvite was recovered from the concentrated feed solution. Thus, this study indicates the importance and potential of human urine as reclaimed water and fertilizer. Further evaluation of the quality of reclaimed water, nontarget analysis by a high-resolution LC-MS/MS, and gas chromatography/mass spectrometry would be needed.

In this study, the experiments were conducted at 30, 40, and 50 °C of the draw solution. The water flux was sensitive to the temperature, but the efficiencies of the rejections of the ions and TOrCs and the recovery of the struvite were not. From the viewpoint of sustainable operation, i.e., the smallest difference in the water flux between the FO and the MD systems, 40 °C is a favorable temperature for the draw solution in this system.

Thus, this study demonstrated the applicability of an FO–MD hybrid system for recovering nutrients and product water from urine. Furthermore, this hybrid system is useful for producing reclaimed water that minimizes chemicals, such as ammonium and TOrCs.

The authors thank Prof. Tetsuro Agusa and Yuki Imuta of the Prefectural University of Kumamoto for their valuable comments. The authors are grateful to Dr Yoshiro Ohgi of Kumamoto Industrial Research Institute, Japan, for the XRD analysis.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

Coday
B. D.
,
Yaffe
B. G. M.
,
Xu
P.
&
Cath
T. Y.
2014
Rejection of trace organic compounds by forward osmosis membranes: a literature review
.
Environmental Science and Technology
48
(
7
),
3612
3624
.
doi: 10.1021/es4038676
.
Downs
R. T.
,
Hall-Wallace
M.
2003
The American Mineralogist Crystal Structure Database
.
American Mineralogist
88
,
247
250
.
Ge
Q.
,
Han
G.
&
Chung
T. S.
2016
Effective As (III) removal by a multi-charged hydroacid complex draw solute facilitated forward osmosis-membrane distillation (FO-MD) processes
.
Environmental Science and Technology
50
(
5
),
2363
2370
.
doi:10.1021/acs.est.5b05402
.
Graeser
S.
,
Postl
W.
,
Bojar
H. P.
,
Berlepsch
P.
,
Armbruster
T.
,
Raber
T.
,
Ettinger
K.
&
Walter
F.
2008
Struvite-(K), KMgPO4·6H2O, the potassium equivalent of struvite–a new mineral
.
European Journal of Mineralogy
20
(
4
),
629
633
.
doi:10.1127/0935-1221/2008/0020-1810
.
Gustafsson
J. P.
2018
Visual MINTEQ Version 3.1
.
Stockholm
,
Sweden
.
Hancock
N. T.
,
Xu
P.
,
Heil
D. M.
,
Bellona
C.
&
Cath
T. Y.
2011
Comprehensive bench- and pilot-scale investigation of trace organic compounds rejection by forward osmosis
.
Environmental Science and Technology
45
(
19
),
8483
8490
.
doi:10.1021/es201654k
.
Holloway
R. W.
,
Regnery
J.
,
Nghiem
L. D.
&
Cath
T. Y.
2014
Removal of trace organic chemicals and performance of a novel hybrid ultrafiltration-osmotic membrane bioreactor
.
Environmental Science and Technology
48
(
18
),
10859
10868
.
doi:10.1021/es501051b
.
Larsen
T. A.
,
Riechmann
M. E.
&
Udert
K. M.
2021
State of the art of urine treatment technologies: a critical review
.
Water Res X
doi: 10.1016/j.wroa.2021.100114
.
Li
J.
,
Hou
D.
,
Li
K.
,
Zhang
Y.
,
Wang
J.
&
Zhang
X.
2018
Domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system
.
Water Science and Technology
77
(
5–6
),
1514
1523
.
doi:10.2166/wst.2018.031
.
Liu
Q.
,
Liu
C.
,
Zhao
L.
,
Ma
W.
,
Liu
H.
&
Ma
J.
2016
Integrated forward osmosis-membrane distillation process for human urine treatment
.
Water Research
91
,
45
54
.
doi:10.1016/j.watres.2015.12.045
.
Lutchmiah
K.
,
Verliefde
A. R. D.
,
Roest
K.
,
Rietveld
L. C.
&
Cornelissen
E. R.
2014
Forward osmosis for application in wastewater treatment: a review
.
Water Research
58
,
179
197
.
doi:10.1016/j.watres.2014.03.045
.
Patel
A.
,
Mungray
A. A.
&
Mungray
A. K.
2020
Technologies for the recovery of nutrients, water and energy from human urine: a review
.
Chemosphere
259
,
127372
.
doi:10.1016/j.chemosphere.2020.127372
.
Rose
C.
,
Parker
A.
,
Jefferson
B.
&
Cartmell
E.
2015
The characterization of feces and urine: a review of the literature to inform advanced treatment technology
.
Critical Reviews in Environmental Science and Technology
45
,
1827
1879
.
doi: 10.1080/10643389.2014.1000761
.
Udert
K. M.
,
Larsen
T. A.
,
Biebow
M.
&
Gujer
W.
2003a
Urea hydrolysis and precipitation dynamics in a urine-collecting system
.
Water Research
37
(
11
),
2571
2582
.
doi: 10.1016/S0043-1354(03)00065-4
.
Udert
K. M.
,
Larsen
T. A.
&
Gujer
W.
2003b
Estimating the precipitation potential in urine-collecting systems
.
Water Research
37
(
11
),
2667
2677
.
doi:10.1016/S0043-1354(03)00071-X
.
Volpin
F.
,
Chekli
L.
,
Phuntsho
S.
,
Ghaffour
N.
,
Vrouwenvelder
J. S.
&
Shon
H. K.
2019
Optimisation of a forward osmosis and membrane distillation hybrid system for the treatment of source-separated urine
.
Separation and Purification Technology
212
,
368
375
.
doi:10.1016/j.seppur.2018.11.003
.
Wald
C.
2022
The urine revolution: how recycling pee could help to save the world
.
Nature
602
(
7896
),
202
206
.
doi:10.1038/d41586-022-00338-6
.
Wu
C. Y.
,
Chen
S. S.
,
Zhang
D. Z.
&
Kobayashi
J.
2017
Hg removal and the effects of coexisting metals in forward osmosis and membrane distillation
.
Water Science and Technology
75
(
11–12
),
2622
2630
.
doi:10.2166/wst.2017.143
.
Xie
M.
,
Nghiem
L. D.
,
Price
W. E.
&
Elimelech
M.
2012a
Comparison of the removal of hydrophobic trace organic contaminants by forward osmosis and reverse osmosis
.
Water Research
46
(
8
),
2683
2692
.
doi:10.1016/j.watres.2012.02.023
.
Xie
M.
,
Price
W. E.
&
Nghiem
L. D.
2012b
Rejection of pharmaceutically active compounds by forward osmosis: role of solution pH and membrane orientation
.
Separation and Purification Technology
93
,
107
114
.
doi:10.1016/j.seppur.2012.03.030
.
Xie
M.
,
Nghiem
L. D.
,
Price
W. E.
&
Elimelech
M.
2013a
A forward osmosis-membrane distillation hybrid process for direct sewer mining: system performance and limitations
.
Environmental Science and Technology
47
(
23
),
13486
13493
.
doi:10.1021/es404056e
.
Xie
M.
,
Price
W. E.
,
Nghiem
L. D.
&
Elimelech
M.
2013b
Effects of feed and draw solution temperature and transmembrane temperature difference on the rejection of trace organic contaminants by forward osmosis
.
Journal of Membrane Science
438
,
57
64
.
doi: 10.1016/j.memsci.2013.03.031
.
Xie
M.
,
Nghiem
L. D.
,
Price
W. E.
&
Elimelech
M.
2014
Toward resource recovery from wastewater: extraction of phosphorus from digested sludge using a hybrid forward osmosis–membrane distillation process
.
Environmental Science and Technology Letters
1
(
2
),
191
195
.
doi:10.1021/ez400189z
.
Xie
M.
,
Shon
H. K.
,
Gray
S. R.
&
Elimelech
M.
2016
Membrane-based processes for wastewater nutrient recovery: technology, challenges, and future direction
.
Water Research
89
,
210
221
.
doi:10.1016/j.watres.2015.11.045
.
Zhang
J.
,
She
Q.
,
Chang
V. W. C.
,
Tang
C. Y.
&
Webster
R. D.
2014
Mining nutrients (N, K, P) from urban source-separated urine by forward osmosis dewatering
.
Environmental Science and Technology
48
(
6
),
3386
3394
.
doi:10.1021/es405266d
.
Zhao
S. F.
,
Zou
L.
,
Tang
C. Y. Y.
&
Mulcahy
D.
2012
Recent developments in forward osmosis: opportunities and challenges
.
Journal of Membrane Science
396
,
1
21
.
doi: 10.1016/j.memsci.2011.12.023
.
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