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Electrochemical stripping to recover nitrogen from source-separated urine William A. Tarpeh, James M Barazesh, Tzahi Y. Cath, and Kara L. Nelson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05488 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Environmental Science & Technology

Electrochemical stripping to recover nitrogen from source-separated urine

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William A. Tarpeh†,‡, James M. Barazesh†1, Tzahi Y. Cath #,‡, and Kara L. Nelson*†,‡ †

Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720, United States ‡ Engineering Research Center for Re-inventing the Nation’s Urban Water Infrastructure (ReNUWIt), 410 O’Brien Hall, Berkeley, California 94720, United States #Department of Civil & Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States

Submitted to: Environmental Science & Technology October 2017 Revised: December 2017

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Present address: Carollo Engineers., Inc. 3150 Bristol St #500, Costa Mesa, CA 92626 *Corresponding author: [email protected]

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ABSTRACT

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Recovering nitrogen from separately collected urine can potentially reduce costs and energy of

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wastewater nitrogen removal and fertilizer production. Through benchtop experiments, we

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demonstrate the recovery of nitrogen from urine as ammonium sulfate using electrochemical

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stripping, a combination of electrodialysis and membrane stripping. Nitrogen was selectively

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recovered with 93% efficiency in batch experiments with real urine and required 30.6 MJ kg N–1

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in continuous-flow experiments (slightly less than conventional ammonia stripping). The effects

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of solution chemistry on nitrogen flux, electrolytic reactions, and reactions with electro-

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generated oxidants were evaluated using synthetic urine solutions. Fates of urine-relevant trace

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organic contaminants, including electrochemical oxidation and reaction with electro-generated

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chlorine, were investigated with a suite of common pharmaceuticals. Trace organics (1000 users). Pharmaceuticals were quantified by high-performance

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liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) in the multiple reaction

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monitoring (MRM) mode using an Agilent 1200 series HPLC system with a Hydro-RP column

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(150 × 3 mm, 4uM; Phenomenex, Aschaffenburg, Germany) coupled to a 6460 triple quadrupole

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tandem mass spectrometer, as described previously.31

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2.5 Statistical Analysis

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Batch nitrogen recovery efficiencies for each influent (including synthetic urine with phenol)

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were compared using a one-way ANOVA and paired t-tests. In continuous-flow experiments,

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nitrogen fluxes and energy demands were also compared using a one-way ANOVA and paired t-

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tests.

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3. RESULTS AND DISCUSSION

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3.1 Nitrogen recovery from real urine in batch experiments

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Batch experiments with real urine demonstrated that nitrogen was efficiently and selectively

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recovered via electrochemical stripping. More than 90% (92.7 ± 4.12%) of total ammonia in real

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urine was recovered in the trap chamber after 24 h (Figure 2). The recovery efficiency observed

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for electrochemical stripping was similar to that reported for conventional air stripping

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performed on hydrolyzed urine with half the total ammonia concentration (92%, 1960 mg N L–

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detected in the ammonium sulfate trap solution, indicating selective ammonia recovery (Figure

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S1); anions present in urine (e.g., acetate, phosphate) were not detected in the cathode or trap

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chambers, showing negligible mixing between chambers. Using the same operating parameters

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(e.g., current, recirculation rate, reactor geometry) as in batch experiments, the optimal HRT for

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continuous electrochemical stripping appears to be between 12 and 24 hours. For comparison, a

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previous study using electrodialysis for ammonia migration required 6 hours HRT but also an

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additional treatment step to separate ammonia from other cations.19 For electrodialysis, increased

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hydraulic residence time led to higher removal efficiencies but lower fluxes for ammonia,

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particularly at high current densities (≥50 A m–2).19 For electrochemical stripping, changes to

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HRT by changing reactor geometry or flow rate could optimize ammonia recovery efficiency

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and flux.

). Competitor cations (e.g., Na+, K+) present in urine accumulated in the cathode but were not

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3.2 Continuous electrochemical stripping with real urine

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Continuous-flow experiments were used to further characterize electrochemical stripping in

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terms of transmembrane fluxes, recovery efficiencies, and energy demand. With an influent 11



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concentration of 7490 mg N L–1, average concentrations after 3-5 HRTs (3.66-6.10 hr) were

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2960 mg N L–1 (anode), 1950 mg N L–1 (cathode), and 2250 mg N L–1 (trap). Nitrogen flux

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across the cation exchange membrane (1710 g N m–2 d–1) was higher than the flux across the

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hydrophobic gas permeable membrane (1010 g N m–2 d–1, Figure S2, Equations 2-3), indicating

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that the latter is the rate-limiting step of the electrochemical stripping process and a high priority

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for future work to reduce energy demand or increase recovery efficiency. For example, an

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asymmetric cathode chamber could be used to create a larger membrane surface area for the gas

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permeable membrane than the cation exchange membrane. Other gas permeable membranes

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could also be tested to optimize ammonia flux to the trap chamber. Based on open circuit

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experiments (no current), only 9% of flux across the cation exchange membrane was attributed

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to diffusion; electro-migration accounted for the balance. This finding was similar to previous

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research on electrodialysis in urine, in which diffusion accounted for 4-11% of ammonia flux

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and the contribution of diffusion to flux was larger for longer HRTs.19 Flux across the cation

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exchange membrane measured in this study was significantly higher than reported electrodialysis

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values, likely due to the addition of the gas permeable membrane that increased cathodic

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volatilization of NH3. This change reduced back-diffusion of NH3 to the anode chamber, which

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has been documented to limit NH4+ transport from anode to cathode.21 Operation at a higher

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current density (100 A m–2 in this study, maximum 50 A m–2 in literature, Table S5) also

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contributed.

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Overall, 61% of influent nitrogen was removed from the anode (Equation 4). Nitrogen removal

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efficiencies (across cation exchange membrane, 60.6%, Equation 4) were higher than nitrogen

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recovery efficiencies (across gas permeable membrane, 49.6%, Equation 5, Figure S2). Based on

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26% of TAN remaining in the cathode at steady state, increasing cathodic HRT (currently 1.22

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hr) could increase recovery efficiencies for both the gas permeable membrane and the overall

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electrochemical stripping process. Longer HRTs have been demonstrated to reduce flux but

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increase removal efficiency.19 Lower influent ammonia concentrations are expected to decrease

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flux (Equation 2); the degree of this effect is expected to vary based on the relative contribution

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of diffusion, which depends on a concentration gradient and varies directly with HRT.

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Transmembrane nitrogen recovery efficiencies for electrochemical stripping were higher than

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electrodialysis and microbial fuel cells, but lower than electrodialysis with external ammonia

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stripping (Table S7). TAN concentration in the fertilizer product could be increased in

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continuous flow by pumping additional urine through the anode for the same batch of sulfuric

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acid.

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Continuous-flow electrochemical stripping with real urine required 30.6 MJ kg N–1, the lowest

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energy input reported for physicochemical electrochemical treatment of urine to date (Table S6).

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Applying current accounted for 96% of energy required; the remaining 4% was for pumping.

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The energy demand for electrochemical stripping was 5% less than conventional ammonia

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stripping but still 38% higher than centralized nitrification-denitrification, which benefits from

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economies of scale (Figure S3). Current efficiency (mol N (mol e–)–1, Equation S5) for

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ammonium in real urine was greater than that previously reported for electrodialysis (61%)19 and

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even surpassed 100%, indicating contributions from diffusion (Figure S4). The higher current

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efficiency is likely due to the addition of the gas permeable membrane that enhanced transport of

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gaseous NH3 to the trap chamber and reduced accumulation of aqueous NH3 in the cathode.

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Lower current efficiencies were observed in synthetic urine solutions due to differences in

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electrical conductivity and competing oxidation reactions (section 3.3).

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3.3 Electrochemical oxidation reactions

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At the potentials employed for electrochemical stripping, several competing electrochemical

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reactions could potentially take place at the anode:

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2&' ( → (' + 4& , + 4-

E0= 1.229 V

(6)

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2./ → ./' + 2-

E0= 1.358 V

(7)

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&0 ..((& + 2&' ( → 2.(' + 8& , + 8-

E0= -0.29 V

(8)

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2&0 + 2&' ( → 2(' + 7& , + 6-

E0= -0.58 V

(9)

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Cyclic voltammetry and measurements of anticipated products were used to elucidate the

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contributions of each potential oxidation reaction. Acetate is an organic compound abundant in

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urine that could be oxidized (complete mineralization shown in Equation 8); ammonia could be

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oxidized to several nitrogen species, including nitrite (Equation 9).

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As expected, voltammograms performed in aqueous electrolytes confirmed that water oxidation

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was the primary anodic reaction (Figure S5). To isolate the remaining reactions, cyclic

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voltammetry was performed in anhydrous dimethyl sulfoxide (DMSO), an aprotic solvent.

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Comparable voltammograms for a synthetic urine matrix containing LiCl, NaC2H3O2, and

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(NH4)2CO3) with a single-salt solution of LiCl revealed that chloride oxidation was the second

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most predominant reaction (Figure S6). Chloride oxidation leads to disproportionation of Cl2 to

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hypochlorous acid (Equation 10), which was detected in the anode chamber in both synthetic and

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real urine treated via electrochemical stripping.

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./' + &' ( → & , + ./ + &(./

(10)

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Acetate and ammonia oxidation were confirmed to be insignificant based on constant anodic

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acetate concentrations and 0.05).

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Over 24 hours, both TAN and anodic pH (Figure S7) decreased, affecting the kinetics of

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chlorination of ammonia and organics. The pH effects on speciation of HOCl/OCl–, NH3/NH4+,

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and phenol/phenolate were included in kinetic calculations (see Section S1.2). In synthetic urine,

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ammonia chlorination was always faster than acetate chlorination; in synthetic urine with phenol,

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ammonia chlorination was slower than phenol chlorination for at least half of the experimental

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period (Figure S10). Chlorination of phenol preserved ammonia for migration and led to a higher

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nitrogen recovery in synthetic urine with phenol compared to synthetic urine alone. Thus, adding

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phenol to synthetic urine made it a more accurate model solution for studying the

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electrochemical treatment of real urine.

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3.5 Fate of trace organic contaminants

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Trace organic contaminants have been detected at significant concentrations in urine, especially

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because 64% of pharmaceuticals are excreted in urine.28 Most organics in urine are hydrophilic

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(e.g., phenolic, deprotonated amine functional groups) because lipophilic compounds tend to

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partition into feces.36 We measured a suite of pharmaceuticals, including beta blockers,

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antibiotics, and antivirals, in urine from Berkeley, California, USA; Nairobi, Kenya; and Zurich,

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Switzerland. Compounds with notably high concentrations included sulfamethoxazole (up to 10

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mg L–1), abacavir (up to 1 mg L–1), atenolol (up to 200 µg L–1), and carbamazepine (up to 10 µg

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L–1; Figure 4a). Most concentrations were highest in Nairobi urine, with acetaminophen in

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Berkeley and carbamazepine in Zurich being notable exceptions.

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Four primary fates of pharmaceuticals were evaluated: reaction with electro-generated oxidants

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in the anode chamber, electro-migration to cathode and trap chambers, transformation in the

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cathode chamber, and transformation in the trap chamber. The latter two fates were neglected

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based on no observed transformation in control experiments, with the exceptions of cathodic

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transformation of emitricitabine, zidovudine, and acetaminophen (Figure S11). Based on a mass

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balance that neglected cathodic and trap transformation, the fate of each compound in real urine

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is presented in Figure 4b. Most importantly, pharmaceuticals were not detected in the ammonium

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sulfate product (trap chamber). Two compounds were noticeably transported through the cation

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exchange membrane: atenolol (4% in cathode, pKa 9.6) and metoprolol (2% in cathode, pKa 9.5).

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Compounds containing electron-rich moieties (i.e., deprotonated amines) were removed faster in

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the anode due to rapid reactions with electrophilic chlorine species (e.g., HOCl and

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chloramines).37 Key exceptions included zidovudine and acyclovir, which may have back-

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diffused from the cathode to the anode due to their hydrophilicity (log Kow 94% transformed in synthetic urine, Figure S12). For all compounds, observed first-order rate

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constants for anodic transformation were higher in synthetic urine than in real urine (Figure S13).

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This difference was further evidence of less chlorine quenching in synthetic urine due to the

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presence of additional organic compounds in real urine. Higher residual concentrations of free

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chlorine in the anode with synthetic urine could explain higher observed rate constants for trace

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organics and lower nitrogen recovery efficiencies. While nitrogen recovery remains the primary

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goal of electrochemical stripping, further investigations into the mechanisms contributing to

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anodic transformation of pharmaceuticals in urine could enhance understanding of effluent

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treatment required after electrochemical stripping.

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3.6 Implications for urine treatment

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The required energy input was calculated for each influent in continuous experiments. Real urine

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required 30.6 MJ kg N–1, which was 55% less than ammonium sulfate, less energy than synthetic

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urine, and 5% less than conventional ammonia stripping (Figure S2). The high conductivity of

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real urine lowers ohmic resistance for charge transport through the anode, explaining the higher

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energy efficiency compared to ammonium sulfate (Figure S14). Additionally, higher nitrogen

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recovery efficiency in real urine over synthetic urine was responsible for higher energy

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efficiency in real urine. More generally, real urine composition varies, and conductivity could be

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an indicator for nitrogen recovery efficiency by electrochemical stripping (Figure S14).

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In addition to nitrogen, potassium could also be recovered by electrochemical stripping.

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Potassium was effectively recovered in the cathode chamber (Figure S1); no measurable amount

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was detected in the anode nor in the trap after batch experiments. In most regions potassium is an

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inexpensive fertilizer (between 0.50 and 1 USD kg–1);38,39 however, recovering it could enhance

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the value of urine-derived fertilizers. Recovering potassium and ammonium separately, along

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with phosphorus recovery before electrochemical stripping, could allow for production of

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fertilizers with customized macronutrient ratios.

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Urine-derived products should be safe for use, especially in the case of fertilizers. Processes for

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nitrogen recovery benefit from selective concentration of ammonium relative to other urine

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constituents, including other cations, pharmaceuticals, and trace elements. Besides ammonium

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and sulfate, no other ions were observed at measurable concentrations in the ammonium

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concentrate product (detection limits and compounds in Table S10). Similarly, pharmaceuticals

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were not detected in the trap chamber (see section 3.5). Influent urine has already been

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documented to contain low levels of heavy metals because most are excreted in feces.40 Trace

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elements were not detected in the ammonium sulfate product above 30 µg L–, with two

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exceptions: iron and zinc were detected at 400 µg L– due to impurities in sulfuric acid. Similar to

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urine-derived fertilizers, biosolids produced from wastewater sludge are a byproduct of waste

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treatment applied to land as fertilizer. Measured trace element concentrations were all less than

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U.S. Environmental Protection Agency pollutant limits for land application of biosolids (Figure

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S15a).41

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The major effluent produced from electrochemical stripping is electrolyzed urine from the anode

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chamber. Disinfectant residuals were at most 1.6 mg Cl2 equivalents L–1 for combined chlorine

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(Figure 3b) and 0.89 mg Cl2 equivalents L–1 for free chlorine. If treated urine goes to a

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wastewater treatment plant, the disinfectant residual will be consumed quickly when mixed with

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sewage. If treated urine is discharged to the environment, free chlorine will likely be consumed

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by ammonia in surface waters; however, combined chlorine at 1-2 mg L–1 could negatively

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impact aquatic life.42 Disinfection byproducts (DBPs) were not measured in this study; future

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work could lead to further characterization of DBPs in fertilizer and effluent produced from urine

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by electrochemical stripping. Anode material strongly influences production of DBPs (e.g.,

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trihalomethanes, ClO3–, and ClO4–) during electrochemical oxidation of ammonia, and is likely

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to have similar effects during electrochemical stripping.33 Similar to free chlorine, DBPs in

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treated urine discharged to a wastewater treatment plant will undergo further transformation; in

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on-site electrochemical stripping systems that discharge directly to the environment, the fate of

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DBPs is an important consideration. While DBPs may be of concern, a potential benefit of the

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electrochemically produced chlorine is inactivation of pathogens in urine at the anode, which is

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particularly attractive in decentralized settings that lack wastewater treatment facilities to receive

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electrolyzed urine effluent. Trace element concentrations for all three chambers, influent urine,

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and effluent urine from electrochemical were less than pollutant limits for land application of

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biosolids (Figure S15a).

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If catholyte is not recirculated and reused, it could also be a process effluent. Catholyte

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concentrations of pharmaceuticals were less than 0.4 µg L–1, with the exception of atenolol (0.74

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µg L–1) and acetaminophen (1.24 µg L–1). Trace elements were not detected above 30 µg L–1 in

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catholyte, with the exception of iron (200 µg L–1) and zinc (400 µg L–1, Figure S15b-c). Further

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optimization (e.g., of HRT, operating potential, temperature) could be investigated for

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pharmaceuticals and element removal from effluent urine and catholyte.

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In conclusion, electrochemical stripping was demonstrated at lab-scale to be an efficient process

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for selectively recovering nitrogen from urine. Ammonium sulfate fertilizer containing

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acceptably low levels of pharmaceuticals and trace elements was produced, and the role of

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organics such as phenols in scavenging electro-generated chlorine was elucidated. Moving

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toward implementation will require further characterization of the formation and fate of

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disinfection byproducts, pathogen inactivation, and process evaluation at the pilot-scale. With

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additional optimization, electrochemical stripping from urine could be implemented as an

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alternative to conventional nitrogen removal from wastewater that has the potential to reduce

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energy demand, greenhouse gas emissions, and nitrogen loads to receiving waters.

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ASSOCIATED CONTENT

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The Supporting Information (SI) contains equations for flux, energy demand, and efficiencies;

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tables describing influent streams, membranes, and trace organic compounds; and figures

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showing cyclic voltammetry, performance comparison of synthetic and urine solutions, and fate

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of trace organics and elements.

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AUTHOR INFORMATION

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Corresponding author (K.L.N.):

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Phone: 510-643-5023; e-mail: [email protected]

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Notes

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The authors declare no competing financial interests.

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ACKNOWLEDGEMENTS

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W.A.T., K.L.N., and T.Y.C. acknowledge funding provided by the National Science Foundation

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(NSF) through the Reinventing the Nation’s Urban Water Infrastructure (ReNUWIt) Engineering

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Research Center (http://renuwit.org; NSF Grant No. CBET-0853512). W.A.T. was also

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supported by an NSF Graduate Research Fellowship (NSF Grant No. DGE 1106400), a Ford

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Foundation Fellowship, and a Harvey Fellowship. Preliminary work began with a Big Ideas at

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Berkeley Grant (2014). J.M.B. was supported by the U.S. National Institute for Environmental

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Health Sciences (NIEHS) Superfund Research Program (Grant P42 ES004705) and the

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Superfund Research Center at University of California, Berkeley.

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FIGURES

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Figure 1. Schematic of the electrochemical stripping setup. For batch experiments, each

660

solution was recirculated in separate 1-L bottles. For continuous experiments the anode was fed

661

continuously and the cathode and trap were recirculated only. 6

Mass N (g)

5 Anode

4

Cathode

3

Trap

2 1 0 0

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5

10

15

20

25

Time (hr)

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Figure 2. Mass of total ammonia nitrogen in each chamber over time in batch experiment.

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Dotted horizontal line is initial mass of ammonia nitrogen in anode. Error bars represent ± one

665

standard deviation. Bars not shown are smaller than symbol, except for 21 hours (n=1).

27



666 667

668 669

Figure 3. Results for varied influents from 24 hr batch experiments: (a) Nitrogen recovery

670

efficiency measured. Error bars indicate ± one standard deviation. AS= ammonium sulfate,

671

phenol = synthetic urine with phenol addition. (b) Combined chlorine measured in synthetic

672

urine, synthetic urine with phenol addition (10 mM), and real urine.

673 674

675 676 677

Figure 4. (a) Concentrations of 11 pharmaceuticals in urine from Berkeley, Nairobi, and Zurich.

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(b) Fate of pharmaceuticals in batch experiments with real urine. Error bars represent ± one

679

standard deviation. See Table S4 for abbreviations and compound names.

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Electrochemical Stripping to Recover Nitrogen ... - ACS Publications

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