Recovering nutrients and rejecting trace organic compounds in human urine by a forward osmosis–membrane distillation (FO–MD) hybrid system

A

effective membrane area

C_d

chemical concentration in the draw solution

C_f

chemical concentration in the feed solution

CTA

cellulose triacetate

C_w

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

J_W

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.

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).

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.

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.

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).

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).

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).

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.

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.

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.