Microbial Electrochemical Current Accelerates Urea Hydrolysis for

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Microbial Electrochemical Current Accelerates Urea Hydrolysis for Nutrient Recovery from Source-separated Urine Xi Chen, Yifan Gao, Dianxun Hou, He Ma, Lu Lu, Dongya Sun, Xiaoyuan Zhang, Peng Liang, Xia Huang, and Zhiyong Jason Ren Environ. Sci. Technol. Lett., Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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Microbial Electrochemical Current Accelerates Urea Hydrolysis for Nutrient

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Recovery from Source-separated Urine

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Xi Chen† ‡1, Yifan Gao† 1, Dianxun Hou‡, He Ma†, Lu Lu‡, Dongya Sun†, Xiaoyuan Zhang†, Peng Liang†,

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Xia Huang†*, Zhiyong Jason Ren‡*

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†

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Tsinghua University, Beijing, 100084, P. R. China

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‡

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment,

Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder,

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Boulder, CO 80309, USA

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1

These authors are equal contribution

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* Corresponding author: E-mail: [email protected];

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* Corresponding author: E-mail: [email protected]

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Phone: 303-492-4137; fax: 303-492-7317

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ABSTRACT

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This study demonstrates that wastewater-driven microbial electrochemical process greatly facilitates

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traditional rate limiting urea hydrolysis and efficiently recovers ammonium and phosphate nutrient from

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source-separated urine. Using both synthetic and diluted actual urine and wastewater, 76-87% of

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nitrogen and 72-93% of phosphorus were continuously removed from source-separated urine and

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collected in recovery solutions. The acceleration of hydrolysis and nutrient recovery were driven by the

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electrical potential generated during wastewater treatment. The efficient nutrient recovery is attributed to

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the rate increase in hydrolysis induced by continuous ammonium migration and removal, which

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alleviates storage, health, and operational issues associated urine utilization. Further investigations on

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removal behaviors of micropollutants under electrochemical conditions will be performed.

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INTRODUCTION

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Wastewaters are increasingly being viewed as a renewable resource for nutrients recovery, but the

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concentrations of nitrogen (N, 10-50 mg N/L) and phosphorus (P, 1-10 mg P/L) in centralized municipal

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wastewater resource recovery facilities are low, and N and P must first be concentrated before recovery,

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which can be very expensive and energy intensive.1-3 Approximately 80% of nitrogen and 50% of

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phosphate in wastewater are from urine, but it makes up only 1% of total wastewater volume due to

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dilution. One solution to avoid the high cost dilution/re-concentration approach is directly recovering the

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nutrients from source-separated urine, which also reduces the large transportation and energy costs of

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conveying treated wastewater back to catchment areas for reuse. Urine source separation also reduces

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nutrient loads to existing wastewater resource recovery facilities and downstream eﬄuent-receiving

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water bodies.4, 5

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Traditional urine nutrient recovery systems include ammonia stripping, struvite precipitation, ion

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exchange, membrane processes, and biological nitrogen conversion.6-8 Many of these processes require

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substantial amount of energy or chemical inputs, and odor control has been a big issue due to urea

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hydrolysis to ammonium (NH4+) during storage. In addition, the slow urea hydrolysis generally takes

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several days in traditional urine storage tanks which may lead to precipitation of struvite, hydroxyapatite,

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and calcite that blocks collection and treatment systems.9 In this context, microbial electrochemical

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methods may bring advantages in nutrient recovery from urine, as it can directly converts organics

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embedded in urine and wastewater into electricity thus doesn’t consume energy,10-15 and the internal

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electrical potential can drive the separation and recovery of charged ions such as NH4+ and PO43- during

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urine and wastewater treatment.16-20 Taking advantage of the high pH in the cathode, ammonia stripping

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or struvite precipitation can also be realized for N and P recovery from urine.13, 21-25 More importantly,

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the self-generated electrical field can drive reversely charged ammonium and phosphate ions out of

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urine into concentration chambers for targeted nutrient recovery. The microbial electrical nutrient ion

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recovery pathways were derived from microbial desalination cells (MDCs) invented by our group a few

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years ago.26 MDCs use pairs of ion exchange membranes to directionally drive salt ion migration from

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saline water to achieve energy neutral desalination powered by wastewater treatment in a single

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device.26-31 Most researches to date utilizing microbial electrochemistry treating urine used either stored

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real urine which had completely hydrolyzed or synthetic hydrolyzed urine to investigate ammonium

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transformation. These studies neglected the consideration of time and space needed for urea hydrolysis

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and its associated health and environmental problems. Because urea hydrolysis is a rate limiting process

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it requires extra urine storage time, and it causes odor and health issues and leads to N and P loss due to

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evaporation and precipitation, respectively.32, 33 For technology development and implementation, urea

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hydrolysis cannot be ignored and has to be accelerated for timely treatment and nutrient recovery.

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This study investigates for the first time how microbial electrochemical process works as an

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independent process to facilitate urea hydrolysis and recover nitrogen and phosphate from diluted

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actual source-separated urine. A new stacked microbial nutrient recovery cell (SMNRC) was

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developed for analyzing system performance, and real wastewater was used as the anolyte and

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catholyte to generate electric potential and drive the recovery of nutrient ions. Different from many

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previous studies that used hydrolyzed urine, raw urine was used in this study to investigate whether

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bioelectrochemical current can accelerate urea hydrolysis and minimize storage. Also in contrast with

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previous works that used biological process to treat urine, this study employs mainly an abiotic process

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that takes advantage of the self-generated potential during wastewater treatment to continuously

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remove hydrolysis products such as NH4+ from urine thus shifts the reaction equilibrium toward

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continued hydrolysis. This enables a low-energy separation and recovery of nutrients from urine

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without toxicity concerns. With such process, urine storage and associated nutrient loss and odor issues

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can be avoided, and ammonium and phosphate ions can be recovered separately into a clean recovery

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solution for further nutrient recovery via ammonia stripping, struvite production or distillation. In

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addition, different from traditional approach, the ion exchange membranes block microorganisms and

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charge neutral micropollutants in wastewater from entering the nutrient solution and therefore reduce

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further treatment and health and environmental risks associated with nutrient applications.

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MATERIALS AND METHODS

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Reactor Fabrication and Construction. The SMNRC reactor was consisted of one anode chamber,

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one cathode chamber and stacked nutrient removal and recovery chambers (Figure S1). Each cubic

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chamber was 30 mm in diameter. Granular activated carbon was packed in the 30-mm wide anode

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chamber to serve as the anode (Beijing Chunqiudingsheng, China),34 and carbon cloth coated with 0.5

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mg/cm2 platinum catalyst and 4 polytetrafluoroethylene diffusion layers was used as the air cathode in

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the 10-mm wide cathode chamber.35 A 10 Ω external resistance was used to connecting the electrodes of

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SMNRC. The stacked nutrient removal/recovery chambers were 0.5 mm wide36 and insulated by three

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pairs of ion exchange membranes (cation exchange membranes, CEM, Ultrex CMI7000, Membrane

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International Inc.; anion exchange membranes, AEM, Ultrex AMI-7001, Membrane International Inc.)

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to form 3 nutrient removal chambers and 2 nutrient recovery chambers. Each of the 2 nutrient recovery

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chambers was sandwiched by nutrient removal chambers, and 2 of the nutrient removal chambers were

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placed adjacent to anode and cathode chambers respectively.

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Experiment Operation. Synthetic or actual domestic wastewater (50 mL) was used as the electrolyte

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and circulated between the anode and cathode chambers. Electrical current was generated between the

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anode and cathode during anaerobic and aerobic organic oxidization by microorganisms. 50 mL of

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synthetic or actual human urine was circulated in three nutrient removal chambers for nutrient recovery.

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NaCl solution (50 mL, 5 g/L) was circulated in two nutrient recovery chambers to facilitate ion transfer

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and collect ionic substances migrated from the urine. The component of synthetic domestic wastewater

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was as previously described (per liter): 0.4 g glucose, 0.020 g NaH2PO4·2H2O, 0.021 g Na2HPO4·12H2O,

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0.089 g NH4Cl, 0.016 g NaCl, 0.041 g Na2SO4, and 12.5 mL of trace mineral metals solution.37 The

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synthetic source-separated fresh urine contained (per liter): 1,595 mg urea, 390 mg NH4Cl, 338 mg

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NaH2PO4·2H2O, 340 mg KCl, 300 mg NaCl, 38 mg CaCl2, 65 mg MgCl2·6H2O, and 210 mg Na2SO4.38

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Actual wastewater samples was collected from the inlet of a local wastewater treatment plant (Beixiaohe

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treatment plant, Beijing, China). Raw urine was donated by 6 healthy male volunteers aged 20-25 and

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lived on ordinary diet. Samples were collected in sterile containers before mixing and testing. The

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synthetic urine and real urine were diluted by 10 times to simulate the flushing condition in most of the

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actual situations.19

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When the concept of nutrient removal and recovery from synthetic urine was proved feasible with

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synthetic wastewater, the electrolytes and urine were replaced with actual wastewater and authentic

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urine, respectively. A nutrient removal and recovery cycle, referred to as a “cycle”, started from the

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replacement of wastewater, urine and recovery solution in the SMNRC and ended at 36 h when the

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removals of total nitrogen (TN) and total phosphorus (TP) both exceeded 80%. Concentrations of TN,

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ammonium nitrogen (NH4+-N) and TP were measured at 0, 12, 24 and 36 h. To investigate the influence

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of electrical field on urine hydrolysis and nutrient recovery, control experiments were performed using

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the same substrate and procedure except under open-circuit conditions. Experiments were all conducted

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in repeated in duplicate SMNRCs at room temperature (~25 °C). Analyses and Calculations. The output voltages of the SMNRCs were automatically recorded using

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a data acquisition system. Reading of conductivity and pH were measured using conductivity meter

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(SG3-ELK, Mettler Toledo, USA) and pH meter (Inlab 731, Mettler Toledo, USA), respectively.39

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Concentrations of TN, NH4+-N and TP were tested using standard methods.40 Removal (%) of each

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substance from urine was calculated based on the concentration of each parameter measured at different

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time interval compared with the initial concentration. Recovery (%) was the ratio between the

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concentration of a substance increased in the recovery solution and the concentration removed from

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urine.

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RESULTS AND DISCUSSION

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Nutrient Removal from Source-separated Urine. Figure 1 shows concentration changes, removals

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and removal rates of nitrogen and phosphorus in synthetic urine in a 36-h batch cycle. Both nutrients

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were removed significantly. The TN concentration (Figure 1A, red + green in each column) in urine

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decreased from an average 899 mg/L to 129 mg/L within a cycle, representing 85.6% of removal in 36 h.

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Interestingly, the ratio of different nitrogen forms in TN changed during the cycle. The initial

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concentration of NH4+-N was 99.2 mg/L in fresh synthetic urine, which accounted to 11.0% of TN.

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Within 12 hours, NH4+-N concentration more than tripled to 354.4 mg/L while the concentration of TN

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decreased in half to 424 mg/L. This results in more than 83.6% of TN is NH4+-N. This change indicates

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a transformation of other nitrogen forms into NH4+-N during nitrogen removal, and urea hydrolysis is

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believed to be the reason. The increases in urine pH and conductivity also support this hypothesis

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(Figure S2).

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Urea hydrolysis that generates ionized ammonium was considered a major rate limiting step for

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efficient nutrient ions recovery, because this process generally takes a few days in natural condition

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when induced by microorganisms without the application of external enzymes or high temperature and

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pressure.9 The long retention time and storage of urine leads to high operational cost and hygienic

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concerns associated with odor, cross contamination, and salt precipitation.8,

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concentration of NH4+-N increased during the first 12 hours till reaching a peak of 354 mg/L, and then it

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dropped continuously to 74.8 mg/L at 36 h. In the meantime, the reduction of other nitrogen forms

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except NH4+-N mainly occurred in the first 12 hours, decreasing from 800 mg/L to 69.4 mg/L. This

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indicates urea hydrolysis occurred within 12 hours as other nitrogen transformation process was minimal.

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In contrast, in the open circuit control experiment, the concentration of NH4+-N and pH in urine barely

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changed within the 36 h operational cycle (Figure 1A and S2), which confirms that the self-generated

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electrical field indeed accelerated urea hydrolysis. The continuous removal of charged urea hydrolysis

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products such as NH4+-N from urine chamber greatly shifted reaction equilibrium and facilitated the

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forward reaction, which mitigated the aforementioned challenge of urine storage. There is still

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accumulation of NH4+-N during the first 12 hours, suggesting that the system can be further improved

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for faster NH4+-N removal from the urine chamber. A small concentration of nitrogen (54.6 mg/L)

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Figure 1 shows the

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except NH4+-N maintained in urine, which may include unhydrolyzed organic nitrogen or back

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migration of other forms of nitrogen from the electrolyte across the ion exchange membranes.

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The concentration of TP contained in synthetic urine decreased continuously during operation, and

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within 36 hours an average of 94.7% of TP was removed from the urine solution (Figure 1B). The

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removal rate of TP gradually decreased from 2.8 mg/L/hr during the first 12 hours to an average of 1.1

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mg/L/hr from 12-hour to 36-hour. The decrease in removal rate is a combination of reduced current due

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to organic removal in wastewater, the reduced concentration gradient across the membrane, and the

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increased osmosis with back diffusion from the recovery solution toward urine (Figure S3).

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Nutrient Recovery in SMNRC. Nutrient ions removed from urine were effectively collected into the

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recovery solution (Figure 2). TN increased by 446.1 mg/L during a typical 36-h cycle, among which 338

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mg/L was gained within the first 12 h (Figure 2A). The recovered nitrogen was mostly in the form of

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NH4+-N, as the proportions of NH4+-N in TN were 84.0, 92.8, and 89.0% at 12, 24, and 36 h,

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respectively. This finding is in agreement with the removal data and indicates that ammonium as a urea

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hydrolysis product has an outstanding cross-membrane migrating property under an electrical field and

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therefore can be efficiently recovered. Similar as the rate change in the removal chamber, the

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accumulation of NH4+-N mainly occurred in the first 12 hours, during which the concentration increased

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from zero to 308 mg/L, representing a rate of 25.6 mg/L/hr. The rate slowed down gradually, with an

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average recovery rate of 6.4 mg/L/hr and 3.2 mg/L/hr observed during 12-24 h and 24-36 h, respectively,

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which corresponded with another 76.2 and 38.4 mg/L more NH4+-N migrated into the recovery solution.

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Compared with the amount of TN removed from urine, the recovery ratios of TN were 72.0% (0~12 h),

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26.0 % (12~24 h), and 51.4 % (24~36 h) during the three 12 hours’ section within a typical cycle.

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Comparing Figure 1 and 2, one can find the TN removal in removal chambers and recovery in

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concentration chambers was not fully synchronized, and this is likely due to multiple factors that

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affected cross-membrane migration such as variation of electrical strength during wastewater treatment

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in the anode and cathode chamber and competitions of ionic migration. If considering the overall TN

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recovery during a 36-h batch cycle, the recovery ratio is 58.0%, meaning 58.0% of the TN removed

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from the urine was recovered in the recovery solution. Because one of the three nutrient removal

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chambers was located adjacent to the cathode chamber, the removed NH4+ from this chamber were not

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collected by the recovery solution. Therefore, the theoretical maximum recovery ratio is only 66.7%. If

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this theoretical maximum ratio is used in recovery calculation, the actual TN recovery efficiency in the

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SMNRC becomes 87.0%. The recovery efficiency can be further improved by optimizing stack

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configuration and operation.

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The concentration of TP in the recovery solution increased to 45.3 mg/L after the SMNRC operated

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for 36 hours, among which 26.9 mg/L was gained within the first 12 h (Figure 2B). Similar as the rate

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change of TN, the TP recovery rate declined during operation, with a recovery rate of 2.2 mg/L/hr

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during the first 12 hours and then dropped to an average recovery rate of 0.6 mg/L/hr and 0.8 mg/L/hr

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observed during 12-24 h and 24-36 h, respectively, which corresponded with another 7.0 and 10.2 mg/L

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more TP migrated into the recovery solution. The overall recovery ratio of TP was 72.0% during the

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whole cycle, while the recovery ratio within each period varied. The recovery was 79.2% in the first 12

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hours then decreased to 37.7% in the section between 12 to 24 h, and finally climbed back up to 118%

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during the last 12 hours. Similarly as mentioned above, one of the nutrient recovery chambers was

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adjacent to the anode chamber via an AEM, and therefore anions in the anolyte and urine were able

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migrate to another chamber driven by the diffusion pressure or electrical field. Because phosphorus is in

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anion form, the phosphorus contained in wastewater could also migrate into the urine chamber and

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further enter into the recovery chamber driven by the electrical force, leading to a potential recovery of

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TP exceeding 100% at the end of the cycle. A similar situation was also reported in another recent

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research.

This suggests that diffusion pressure also plays a role in ion migration cross the different

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chambers, especially when electrical potential between the electrodes is weak, and this may lead to

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either enhanced or reduced ion migration though not in a significant manner. 42

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Nutrient Removal and Recovery Using Actual Wastewater and Urine. Actual wastewater and

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urine were used after synthetic solution experiments to investigate system performance under real-world

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related conditions. Overall a similar removal and recovery trend was observed. Figure 3 shows the

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average removals of TN and TP were 75.5% and 85.4%, respectively (Figure 3A and 3B). An average of

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55.7% of the TN was removed during the first 12 hours’ operation while 19.8% of the TN was removed

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within the rest 24 hours (Figure 3A). Similar as synthetic urine experiments, the ratio of nitrogen forms

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in TN changed in the actual urine solution and an increase of NH4+-N concentration was found during

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the first 12 hours. Compared with the open circuit real wastewater and urine experiment (Figure 3A), in

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which the concentration of NH4+-N hardly changed during an operational period of over 48 h, the

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acceleration of urea hydrolysis of real urine induced by electrical field in SMNRC could be proved.

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From 12-h to the end of cycle, TN removal rate reduced from 30.3 mg/L/hr to 7.4 mg/L/hr. In

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correspondence, the concentration of TN in the recovery solution increased gradually (Figure 3C), with

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NH4+-N as the primary species of recovered nitrogen. The concentration of NH4+-N increased from

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nearly zero to 392 mg/L by the end of cycle (Figure 3C). The overall recovery ratio of TN was 84.6%.

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In the case of TP removal, an amount of 26.6 mg/L was removed from the authentic urine (Figure 3B).

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Different from synthetic urine TP removal, the removal rate of TP in this case was similar from the

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beginning to the end. It revealed that during the present operation, migration of phosphate might be

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influenced by other factors like charge balancing effect even when the electrical driving force decreased

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in the latter part of the cycle. The concentration of in the recovery solution increased gradually, with an

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average of 24.7 mg/L phosphorus was collected in recovery solution (Figure 3D), leading an overall TP

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recovery of 92.9%.

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OUTLOOK

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The microbial electrochemical reactor demonstrates excellent performance in removing and

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recovering ammonium and phosphate from source-separated urine. The self-generated potential during

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wastewater treatment enables low-energy operation, and the continuous removal NH4+-N from urine

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chamber greatly accelerates urea hydrolysis and alleviates storage, health, and operational issues

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associated with this rate limiting step. Compared with traditional treatment-focused approach, this

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integrated process takes advantage of the internal potential and current generated during wastewater

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treatment, so it avoids complex energy harvesting system required by MFCs and demonstrated higher

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efficiency.43 It also enables simultaneous organic removal and nutrient recovery, which are desired goals

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of decentralized treatment.

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electrochemical process, the ion exchange membranes block microorganisms from entering into the

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recovery solution and presumably some charge neutral micropollutants as well.44 However, further

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studies will be needed to understand the migration behavior of micropollutants during this process, and

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system improvement can be made to minimize these compounds from entering recovery solution thus

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prevent health and environmental concerns of applying the nutrients in agriculture.

Because the remove and recovery of N and P in SMNRC is an abiotic

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Nutrient removal and recovery from urine in SMNRC can be affected by several factors such as urea

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hydrolysis, current production and dynamic ion distribution. Current production directly influences the

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speed of ion migration, and it can be improved via reactor optimization by reducing internal resistance,

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improving mass transfer, and optimizing reactor configuration.45 The optimization of membrane stacks

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and reactor configuration can further improve nutrient recovery efficiency, and detailed mass balance

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characterization can help identify improvement opportunities. The coordination and competition of ion

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migration during nutrient recovery needs further investigation with the aim of improving ion selectivity

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and transfer efficiency.46

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ACKNOWLEDGEMENTS

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This work was partially supported by the Key Program of the National Natural Science Foundation of

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China (No.51238004). DXH and ZJR were supported by the US Office of Naval Research (Award

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N000141612210).

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SUPPORTING INFORMATION AVAILABLE

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Additional tables and figures are included in the Supporting Information. This information is available

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free of charge via the Internet at http://pubs.acs.org/ .

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REFERENCES

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10. Logan, B. E.; Rabaey, K., Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies. Science 2012, 337, (6095), 686-690. 11. Wang, H.; Ren, Z. J., A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnol. Adv. 2013, 31, (8), 1796-1807. 12. Ieropoulos, I.; Greenman, J.; Melhuish, C., Urine utilisation by microbial fuel cells; energy fuel for the future. PCCP 2012, 14, (1), 94-98. 13. Kuntke, P.; Śmiech, K. M.; Bruning, H.; Zeeman, G.; Saakes, M.; Sleutels, T. H. J. A.; Hamelers, H. V. M.; Buisman, C. J. N., Ammonium recovery and energy production from urine by a microbial fuel cell. Water Res. 2012, 46, (8), 2627-2636. 14. Santoro, C.; Babanova, S.; Artyushkova, K.; Atanassov, P.; Greenman, J.; Cristiani, P.; Trasatti, S.; Schuler, A. J.; Li, B.; Ieropoulos, I., The effects of wastewater types on power generation and phosphorus removal of microbial fuel cells (MFCs) with activated carbon (AC) cathodes. Int. J. Hydrogen Energy 2014, 39, (36), 21796-21802. 15. Ledezma, P.; Jermakka, J.; Keller, J.; Freguia, S., Recovering Nitrogen as a Solid without Chemical Dosing: Bio-Electroconcentration for Recovery of Nutrients from Urine. Environ. Sci. Technol. Lett 2017, 4, (3), 119-124. 16. Arredondo, M. R.; Kuntke, P.; Jeremiasse, A. W.; Sleutels, T. H. J. A.; Buisman, C. J. N.; Heijne, A. T., Bioelectrochemical systems for nitrogen removal and recovery from wastewater. Environ. Sci.: Water Res. Technol. 2015, 1, (1), 22-33. 17. Kelly, P. T.; He, Z., Nutrients removal and recovery in bioelectrochemical systems: A review. Bioresour. Technol. 2014, 153, (0), 351-360. 18. Chen, X.; Liang, P.; Zhang, X.; Huang, X., Bioelectrochemical systems-driven directional ion transport enables low-energy water desalination, pollutant removal, and resource recovery. Bioresour. Technol. 2016, 215, 274-284. 19. Tice, R. C.; Kim, Y., Energy efficient reconcentration of diluted human urine using ion exchange membranes in bioelectrochemical systems. Water Res. 2014, 64, (0), 61-72. 20. Hou, D.; Lu, L.; Sun, D.; Ge, Z.; Huang, X.; Cath, T. Y.; Ren, Z. J., Microbial electrochemical nutrient recovery in anaerobic osmotic membrane bioreactors. Water Res. 2017, 114, 181-188. 21. Cusick, R. D.; Logan, B. E., Phosphate recovery as struvite within a single chamber microbial electrolysis cell. Bioresour. Technol. 2012, 107, 110-115. 22. Santoro, C.; Ieropoulos, I.; Greenman, J.; Cristiani, P.; Vadas, T.; Mackay, A.; Li, B., Power generation and contaminant removal in single chamber microbial fuel cells (SCMFCs) treating human urine. Int. J. Hydrogen Energy 2013, 38, (26), 11543-11551. 23. Cord-Ruwisch, R.; Law, Y.; Cheng, K. Y., Ammonium as a sustainable proton shuttle in bioelectrochemical systems. Bioresour. Technol. 2011, 102, (20), 9691-9696. 24. Kuntke, P.; Sleutels, T. H. J. A.; Saakes, M.; Buisman, C. J. N., Hydrogen production and ammonium recovery from urine by a Microbial Electrolysis Cell. Int. J. Hydrogen Energy 2014, 39, (10), 4771-4778. 25. Merino-Jimenez, I.; Celorrio, V.; Fermin, D. J.; Greenman, J.; Ieropoulos, I., Enhanced MFC power production and struvite recovery by the addition of sea salts to urine. Water Res. 2017, 109, 46-53. 26. Cao, X. X.; Huang, X.; Liang, P.; Xiao, K.; Zhou, Y. J.; Zhang, X. Y.; Logan, B. E., A new method for water desalination using microbial desalination cells. Environ. Sci. Technol. 2009, 43, (18), 7148-7152. 27. Mehanna, M.; Saito, T.; Yan, J. L.; Hickner, M.; Cao, X. X.; Huang, X.; Logan, B. E., Using microbial desalination cells to reduce water salinity prior to reverse osmosis. Energy Environ. Sci. 2010, 3, (8), 1114-1120.

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80 Control TN minus NH4+-N Control NH4+-N

600 400

40

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20

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Concentration (mg/L)

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00h

12h 12

24h 24

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B

60 TP TP Removal TP Removal rate

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0 00h

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371 372 373 374 375 376 377

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3 2 1 0

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TN Removal A TN Removal rate

TN Removal (%)

TN minus NH4+-N NH4+-N

1000

TP Removal (%)


370

36h 36

Figure 1. Changes in concentration, percentage of removal, and removal rate of (A) nitrogen and (B) phosphorus in synthetic urine during operation. The fraction of NH4+ in TN is shown in (A) as well. The TN minus NH4+-N, NH4+-N and TP concentration data are corresponding to the y-axis on the left, while removal and removal rate data are corresponding to y-axes on the right.

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600 500

80

25

400 40

300 200

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12h 12 TP

B

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3

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TP Recovery TP Recovery rate

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378 379 380 381 382 383

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2.5 2 1.5 1

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TN minus NH4+-N NH4+-N TN Recovery NH4+-N Recovery rate

A

TP Recovery (%)


800

NH4+-N Recovery rate (mg/L/hr)


TN Recovery (%)

Page 17 of 18

36h 36

Figure 2. Changes in concentration, percentage of removal, and removal rate of (A) nitrogen and (B) phosphorus in recovery solution. The fraction of NH4+ in TN is shown in (A) as well. The column data are corresponding to the y-axis on the left, while curves are corresponding to y-axes on the right.

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Page 18 of 18

384 385

600

60 Control TN minus NH4+-N Control NH4+-N

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20

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0 00h

700

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24 h 24

TN minus NH4+-N NH4+-N TN Recovery

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386 387 388 389 390 391

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24 h 24

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20 40

15 10

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5 0

35 30

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D

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C

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B

36h 36



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TP Removal (%)

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800

40

NH4+-N

TP Recovery (%)

100

TN minus NH4+-N TN Removal

TN Removal (%)

A

TN Recovery (%)


1000

12h 12

24 h 24

36h 36

Figure 3. Concentration change and removal of (A) nitrogen and (B) phosphorus from actual urine, and the recovery of (C) nitrogen and (D) phosphorus in recovery solution. The TN minus NH4+-N, NH4+-N and TP concentration data are corresponding to the y-axis on the left, while removal and recovery data are corresponding to y-axis on the right.

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