Reimagining liquid waste streams as resources can lead to recovery of valuable products and more efficient, less costly approaches to reducing harmful discharges to the environment. Pollutants in effluent streams can be captured and used as valuable inputs to other processes. For example, municipal wastewater contains resources like energy, water, nutrients, and metals. The Tarpeh Lab develops and evaluates novel approaches to resource recovery from “waste” waters at several synergistic scales: molecular mechanisms of chemical transport and transformation; novel unit processes that increase resource efficiency; and systems-level assessments that identify optimization opportunities. We employ understanding of electrochemistry, separations, thermodynamics, kinetics, and reactor design to preferentially recover resources from waste. We leverage these molecular-scale insights to increase the sustainability of engineered processes in terms of energy, environmental impact, and cost.

Academic Appointments

Professional Education

  • PhD, University of California, Berkeley, Environmental Engineering (2017)
  • MS, University of California, Berkeley, Environmental Engineering (2013)
  • BS, Stanford University, Chemical Engineering (2012)

2023-24 Courses

Stanford Advisees

All Publications

  • Flexible Electrochemical Stripping for Wastewater Ammonia Recovery with On-Demand Product Tunability ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS Kogler, A., Gong, M., Williams, K. S., Tarpeh, W. A. 2024
  • A US perspective on closing the carbon cycle to defossilize difficult-to-electrify segments of our economy. Nature reviews. Chemistry Shaw, W. J., Kidder, M. K., Bare, S. R., Delferro, M., Morris, J. R., Toma, F. M., Senanayake, S. D., Autrey, T., Biddinger, E. J., Boettcher, S., Bowden, M. E., Britt, P. F., Brown, R. C., Bullock, R. M., Chen, J. G., Daniel, C., Dorhout, P. K., Efroymson, R. A., Gaffney, K. J., Gagliardi, L., Harper, A. S., Heldebrant, D. J., Luca, O. R., Lyubovsky, M., Male, J. L., Miller, D. J., Prozorov, T., Rallo, R., Rana, R., Rioux, R. M., Sadow, A. D., Schaidle, J. A., Schulte, L. A., Tarpeh, W. A., Vlachos, D. G., Vogt, B. D., Weber, R. S., Yang, J. Y., Arenholz, E., Helms, B. A., Huang, W., Jordahl, J. L., Karakaya, C., Kian, K. C., Kothandaraman, J., Lercher, J., Liu, P., Malhotra, D., Mueller, K. T., O'Brien, C. P., Palomino, R. M., Qi, L., Rodriguez, J. A., Rousseau, R., Russell, J. C., Sarazen, M. L., Sholl, D. S., Smith, E. A., Stevens, M. B., Surendranath, Y., Tassone, C. J., Tran, B., Tumas, W., Walton, K. S. 2024


    Electrification to reduce or eliminate greenhouse gas emissions is essential to mitigate climate change. However, a substantial portion of our manufacturing and transportation infrastructure will be difficult to electrify and/or will continue to use carbon as a key component, including areas in aviation, heavy-duty and marine transportation, and the chemical industry. In this Roadmap, we explore how multidisciplinary approaches will enable us to close the carbon cycle and create a circular economy by defossilizing these difficult-to-electrify areas and those that will continue to need carbon. We discuss two approaches for this: developing carbon alternatives and improving our ability to reuse carbon, enabled by separations. Furthermore, we posit that co-design and use-driven fundamental science are essential to reach aggressive greenhouse gas reduction targets.

    View details for DOI 10.1038/s41570-024-00587-1

    View details for PubMedID 38693313

    View details for PubMedCentralID 9652356

  • Atomically Dispersed Ru-doped Ti4O7 Electrocatalysts for Chlorine Evolution Reaction with a Universal Activity. Small (Weinheim an der Bergstrasse, Germany) Lee, W., Choung, S., Kim, S., Hong, J., Kim, D., Tarpeh, W. A., Han, J. W., Cho, K. 2024: e2401248


    Chlorine has been supplied by the chlor-alkali process that deploys dimensionally stable anodes (DSAs) for the electrochemical chlorine evolution reaction (ClER). The paramount bottlenecks have been ascribed to an intensive usage of precious elements and inevitable competition with the oxygen evolution reaction. Herein, a unique case of Ru2+-O4 active motifs anchored on Magnéli Ti4O7 (Ru-Ti4O7) via a straightforward wet impregnation and mild annealing is reported. The Ru-Ti4O7 performs radically active ClER with minimal deployment of Ru (0.13 wt%), both in 5 m NaCl (pH 2.3) and 0.1 m NaCl (pH 6.5) electrolytes. Scanning electrochemical microscopy demonstrates superior ClER selectivity on Ru-Ti4O7 compared to the DSA. Operando X-ray absorption spectroscopy and density functional theory calculations reveal a universally active ClER (over a wide range of pH and [Cl-]), through a direct adsorption of Cl- on Ru2+-O4 sites as the most plausible pathway, together with stabilized ClO* at low [Cl-] and high pH.

    View details for DOI 10.1002/smll.202401248

    View details for PubMedID 38639029

  • ATR-SEIRAS Method to Measure Interfacial pH during Electrocatalytic Nitrate Reduction on Cu JOURNAL OF THE ELECTROCHEMICAL SOCIETY Corson, E. R., Guo, J., Tarpeh, W. A. 2024; 171 (4)
  • Long-Term Robustness and Failure Mechanisms of Electrochemical Stripping for Wastewater Ammonia Recovery. ACS environmental Au Kogler, A., Sharma, N., Tiburcio, D., Gong, M., Miller, D. M., Williams, K. S., Chen, X., Tarpeh, W. A. 2024; 4 (2): 89-105


    Nitrogen in wastewater has negative environmental, human health, and economic impacts but can be recovered to reduce the costs and environmental impacts of wastewater treatment and chemical production. To recover ammonia/ammonium (total ammonia nitrogen, TAN) from urine, we operated electrochemical stripping (ECS) for over a month, achieving 83.4 ± 1.5% TAN removal and 73.0 ± 2.9% TAN recovery. With two reactors, we recovered sixteen 500-mL batches (8 L total) of ammonium sulfate (20.9 g/L TAN) approaching commercial fertilizer concentrations (28.4 g/L TAN) and often having >95% purity. While evaluating the operation and maintenance needs, we identified pH, full-cell voltage, product volume, and water flux into the product as informative process monitoring parameters that can be inexpensively and rapidly measured. Characterization of fouled cation exchange and omniphobic membranes informs cleaning and reactor modifications to reduce fouling with organics and calcium/magnesium salts. To evaluate the impact of urine collection and storage on ECS, we conducted experiments with urine at different levels of dilution with flush water, extents of divalent cation precipitation, and degrees of hydrolysis. ECS effectively treated urine under all conditions, but minimizing flush water and ensuring storage until complete hydrolysis would enable energy-efficient TAN recovery. Our experimental results and cost analysis motivate a multifaceted approach to improving ECS's technical and economic viability by extending component lifetimes, decreasing component costs, and reducing energy consumption through material, reactor, and process engineering. In summary, we demonstrated urine treatment as a foothold for electrochemical nutrient recovery from wastewater while supporting the applicability of ECS to seven other wastewaters with widely varying characteristics. Our findings will facilitate the scale-up and deployment of electrochemical nutrient recovery technologies, enabling a circular nitrogen economy that fosters sanitation provision, efficient chemical production, and water resource protection.

    View details for DOI 10.1021/acsenvironau.3c00058

    View details for PubMedID 38525023

    View details for PubMedCentralID PMC10958661

  • Ligand Exchange Adsorbents for Selective Phosphate and Total Ammonia Nitrogen Recovery from Wastewaters ACCOUNTS OF MATERIALS RESEARCH Clark, B., Sharma, N., Apraku, E., Dong, H., Tarpeh, W. A. 2024
  • Electrochemical sulfate production from sulfide-containing wastewaters and integration with electrochemical nitrogen recovery. Journal of hazardous materials Shao, X., Huang, Y., Wood, R. M., Tarpeh, W. A. 2024; 466: 133527


    Electrochemical methods can help manage sulfide in wastewater, which poses environmental and health concerns due to its toxicity, malodor, and corrosiveness. In addition, sulfur could be recovered as fertilizer and commodity chemicals from sulfide-containing wastewaters. Wastewater characteristics vary widely among wastewaters; however, it remains unclear how these characteristics affect electrochemical sulfate production. In this study, we evaluated how four characteristics of influent wastewaters (electrolyte pH, composition, sulfide concentration, and buffer strength) affect sulfide removal (sulfide removal rate, sulfide removal efficiency) and sulfate production metrics (sulfate production rate, sulfate production selectivity). We identified that electrolyte pH (3 × difference, i.e., 25.1 to 84.9 μM h-1 in average removal rate within the studied pH range) and sulfide concentration (16 × difference, i.e., 82.1 to 1347.2 μM h-1 in average removal rate) were the most influential factors for electrochemical sulfide removal. Sulfate production was most sensitive to buffer strength (6 × difference, i.e., 4.4 to 27.4 μM h-1 in average production rate) and insensitive to electrolyte composition. Together, these results provide recommendations for the design of wastewater treatment trains and the feasibility of applying electrochemical methods to varying sulfide-containing wastewaters. In addition, we investigated a simultaneous multi-nutrient (sulfur and nitrogen) process that leverages electrochemical stripping to further enhance the versatility and compatibility of electrochemical nutrient recovery.

    View details for DOI 10.1016/j.jhazmat.2024.133527

    View details for PubMedID 38241833

  • Long-Term Robustness and Failure Mechanisms of Electrochemical Stripping for Wastewater Ammonia Recovery ACS ENVIRONMENTAL AU Kogler, A., Sharma, N., Tiburcio, D., Gong, M., Miller, D. M., Williams, K. S., Chen, X., Tarpeh, W. A. 2024
  • Meta-omic profiling reveals ubiquity of genes encoding for the nitrogen-rich biopolymer cyanophycin in activated sludge microbiomes. Frontiers in microbiology Farmer, M., Rajasabhai, R., Tarpeh, W., Tyo, K., Wells, G. 2023; 14: 1287491


    Recovering nitrogen (N) from municipal wastewater is a promising approach to prevent nutrient pollution, reduce energy use, and transition toward a circular N bioeconomy, but remains a technologically challenging endeavor. Existing N recovery techniques are optimized for high-strength, low-volume wastewater. Therefore, developing methods to concentrate dilute N from mainstream wastewater will bridge the gap between existing technologies and practical implementation. The N-rich biopolymer cyanophycin is a promising candidate for N bioconcentration due to its pH-tunable solubility characteristics and potential for high levels of accumulation. However, the cyanophycin synthesis pathway is poorly explored in engineered microbiomes. In this study, we analyzed over 3,700 publicly available metagenome assembled genomes (MAGs) and found that the cyanophycin synthesis gene cphA was ubiquitous across common activated sludge bacteria. We found that cphA was present in common phosphorus accumulating organisms (PAO) Ca. 'Accumulibacter' and Tetrasphaera, suggesting potential for simultaneous N and P bioconcentration in the same organisms. Using metatranscriptomic data, we confirmed the expression of cphA in lab-scale bioreactors enriched with PAO. Our findings suggest that cyanophycin synthesis is a ubiquitous metabolic activity in activated sludge microbiomes. The possibility of combined N and P bioconcentration could lower barriers to entry for N recovery, since P concentration by PAO is already a widespread biotechnology in municipal wastewater treatment. We anticipate this work to be a starting point for future evaluations of combined N and P bioaccumulation, with the ultimate goal of advancing widespread adoption of N recovery from municipal wastewater.

    View details for DOI 10.3389/fmicb.2023.1287491

    View details for PubMedID 38033562

    View details for PubMedCentralID PMC10687191

  • Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials. ACS applied materials & interfaces Ostervold, L., Smerigan, A., Liu, M. J., Filardi, L. R., Vila, F. D., Perez-Aguilar, J. E., Hong, J., Tarpeh, W. A., Hoffman, A. S., Greenlee, L. F., Clark, E. L., Janik, M. J., Bare, S. R. 2023


    Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performance─direct incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba(OH)2 electrolyte, we find both Cu-O bonds and Cu-Ba scattering persist at potentials as low as -400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts.

    View details for DOI 10.1021/acsami.3c10296

    View details for PubMedID 37756387

  • Electrochemical Wastewater Refining: A Vision for Circular Chemical Manufacturing. Journal of the American Chemical Society Miller, D. M., Abels, K., Guo, J., Williams, K. S., Liu, M. J., Tarpeh, W. A. 2023


    Wastewater is an underleveraged resource; it contains pollutants that can be transformed into valuable high-purity products. Innovations in chemistry and chemical engineering will play critical roles in valorizing wastewater to remediate environmental pollution, provide equitable access to chemical resources and services, and secure critical materials from diminishing feedstock availability. This perspective envisions electrochemical wastewater refining─the use of electrochemical processes to tune and recover specific products from wastewaters─as the necessary framework to accelerate wastewater-based electrochemistry to widespread practice. We define and prescribe a use-informed approach that simultaneously serves specific wastewater-pollutant-product triads and uncovers a mechanistic understanding generalizable to broad use cases. We use this approach to evaluate research needs in specific case studies of electrocatalysis, stoichiometric electrochemical conversions, and electrochemical separations. Finally, we provide rationale and guidance for intentionally expanding the electrochemical wastewater refining product portfolio. Wastewater refining will require a coordinated effort from multiple expertise areas to meet the urgent need of extracting maximal value from complex, variable, diverse, and abundant wastewater resources.

    View details for DOI 10.1021/jacs.3c01142

    View details for PubMedID 37642501

  • Understanding the Catalytic Active Sites of Crystalline CoSbxOy for Electrochemical Chlorine Evolution. ACS applied materials & interfaces Dong, H., Shao, X., Hancox, S., McBeath, S. T., Tarpeh, W. A., Hoffmann, M. R. 2023


    The chlorine evolution reaction (CER) is a key reaction in electrochemical oxidation (EO) of water treatment. Conventional anodes based on platinum group metals can be prohibitively expensive, which hinders further application of EO systems. Crystalline cobalt antimonate (CoSbxOy) was recently identified as a promising alternative to conventional anodes due to its high catalytic activity and stability in acidic media. However, its catalytic sites and reaction mechanism have not yet been elucidated. This study sheds light on the catalytically active sites in crystalline CoSbxOy anodes by using scanning electrochemical microscopy to compare the CER catalytic activities of a series of anode samples with different bulk Sb/Co ratios (from 1.43 to 2.80). The results showed that Sb sites served as more active catalytic sites than the Co sites. The varied Sb/Co ratios were also linked with slightly different electronic states of each element, leading to different CER selectivities in 30 mM chloride solutions under 10 mA cm-2 current density. The high activity of Sb sites toward the CER highlighted the significance of the electronic polarization that changed the oxidation states of Co and Sb.

    View details for DOI 10.1021/acsami.3c05016

    View details for PubMedID 37594304

  • X-ray Absorption Spectroscopy Reveals Mechanisms of Calcium and Silicon Fouling on Reverse Osmosis Membranes Used in Wastewater Reclamation ACS ES&T WATER Niemann, V. A., Huck, M., Steinrueck, H., Toney, M. F., Tarpeh, W. A., Bone, S. E. 2023
  • Reports from the Frontier: Electrifying Chemical Transformations and Separations to Valorize Wastewater Nitrogen ELECTROCHEMICAL SOCIETY INTERFACE Liu, M. J., Tarpeh, W. A. 2023; 32 (2): 29-31

    View details for DOI 10.1149/2.F04232IF

    View details for Web of Science ID 001018522500001

  • Reactive Separation of Ammonia from Wastewater Nitrate via Molecular Electrocatalysis ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS Liu, M. J., Miller, D. M., Tarpeh, W. A. 2023; 10 (5): 458-463
  • Mass Transport Modifies the Interfacial Electrolyte to Influence Electrochemical Nitrate Reduction ACS SUSTAINABLE CHEMISTRY & ENGINEERING Guo, J., Brimley, P., Liu, M. J., Corson, E., Munoz, C., Smith, W., Tarpeh, W. A. 2023
  • Co-designing Electrocatalytic Systems with Separations To Improve the Sustainability of Reactive Nitrogen Management ACS CATALYSIS Niemann, V. A., Benedek, P., Guo, J., Xu, Y., Blair, S. J., Corson, E. R., Nielander, A. C., Jaramillo, T. F., Tarpeh, W. A. 2023; 13 (9): 6268-6279
  • A Unit Process Approach to Nontarget Screening of Organic Contaminants during Urine Treatment ACS ES&T ENGINEERING Tarpeh, W. A., Du, Y., Carpenter, C. G., Rodriguez, E. E., Helbling, D. E., Aga, D. S., Love, N. G., Wigginton, K. R. 2023
  • Electrifying climate change mitigation NATURE ENERGY Tarpeh, W. A. 2022
  • Electrified Ion Exchange Enabled by Water Dissociation in Bipolar Membranes for Nitrogen Recovery from Source-Separated Urine. Environmental science & technology Dong, H., Laguna, C. M., Liu, M. J., Guo, J., Tarpeh, W. A. 2022


    Ion exchange (IX) is a promising technology for selective nitrogen recovery from urine; however, IX requires chemical-intensive regeneration that escalates energy consumption and carbon emissions. To overcome this barrier, we demonstrated and investigated a novel electrified IX stripping process (EXS) enabling electrochemical in situ IX regeneration with simultaneous ammonia stripping. EXS combines a weak acid cation exchange resin (WAC) to concentrate ammonia, a bipolar membrane to produce protons for WAC regeneration, and membrane stripping to recover the eluted ammonium from WAC. We observed over 80% regeneration (elution from resin) and recovery (membrane stripping) efficiencies during multiple adsorption-recovery cycles with synthetic and real urine. Comparing WAC with a strong acid cation exchange resin illustrated the critical role of the proton affinity of resin moieties in regulating resin regenerability and conductivity in EXS, which we distinguished from the rationale for material choice in electrodeionization. Compared to other electrochemical recovery methods using unamended wastewater as an electrolyte, EXS enabled control of electrolyte composition during recovery by separating and equalizing influent ammonium via WAC-mediated removal. This electrolyte engineering facilitated tunable EXS energy efficiency (100-300 MJ/kg N). This study informs the design of electrified, intensified systems that enable decentralized nitrogen recovery from urine.

    View details for DOI 10.1021/acs.est.2c03771

    View details for PubMedID 36223185

  • QSDsan: an integrated platform for quantitative sustainable design of sanitation and resource recovery systems ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY Li, Y., Zhang, X., Morgan, V. L., Lohman, H. C., Rowles, L. S., Mittal, S., Kogler, A., Cusick, R. D., Tarpeh, W. A., Guest, J. S. 2022

    View details for DOI 10.1039/d2ew00455k

    View details for Web of Science ID 000843629200001

  • Advanced ion transfer materials in electro-driven membrane processes for sustainable ion-resource extraction and recovery PROGRESS IN MATERIALS SCIENCE Zhao, Y., Mamrol, N., Tarpeh, W. A., Yang, X., Gao, C., van der Bruggen, B. 2022; 128
  • Taking Earth's Pulse with Low-Cost Sensors ACS SENSORS Bakker, E., Ward, C. P., Tarpeh, W., Wang, Z. 2022; 7 (6): 1613

    View details for DOI 10.1021/acssensors.2c01015

    View details for Web of Science ID 000818543700001

    View details for PubMedID 35609120

  • Recovery of Clean Water and Ammonia from Domestic Wastewater: Impacts on Embodied Energy and Greenhouse Gas Emissions. Environmental science & technology Shin, C., Szczuka, A., Liu, M. J., Mendoza, L., Jiang, R., Tilmans, S. H., Tarpeh, W. A., Mitch, W. A., Criddle, C. S. 2022


    Treatment of domestic wastewater can recover valuable resources, including clean water, energy, and ammonia. Important metrics for these systems are greenhouse gas (GHG) emissions and embodied energy, both of which are location- and technology-dependent. Here, we determine the embodied energy and GHG emissions resulting from a conventional process train, and we compare them to a nonconventional process train. The conventional train assumes freshwater conveyance from a pristine source that requires energy for pumping (US average of 0.29 kWh/m3), aerobic secondary treatment with N removal as N2, and Haber-Bosch synthesis of ammonia. Overall, we find that this process train has an embodied energy of 1.02 kWh/m3 and a GHG emission of 0.77 kg-CO2eq/m3. We compare these metrics to those of a nonconventional process train that features anaerobic secondary treatment technology followed by further purification of the effluent by reverse osmosis and air stripping for ammonia recovery. This "short-cut" process train reduces embodied energy to 0.88 kWh/m3 and GHG emissions to 0.42 kg-CO2eq/m3, while offsetting demand for ammonia from the Haber-Bosch process and decreasing reliance upon water transported over long distances. Finally, to assess the potential impacts of nonconventional nitrogen removal technology, we compared the embodied energy and GHG emissions resulting from partial nitritation/anammox coupled to anaerobic secondary treatment. The resulting process train enabled a lower embodied energy but increased GHG emissions, largely due to emissions of N2O, a potent greenhouse gas.

    View details for DOI 10.1021/acs.est.1c07992

    View details for PubMedID 35656915

  • Diurnal Variability of SARS-CoV-2 RNA Concentrations in Hourly Grab Samples of Wastewater Influent during Low COVID-19 Incidence. ACS ES&T water Mendoza Grijalva, L., Brown, B., Cauble, A., Tarpeh, W. A. 2022; 2 (11): 2125-33


    Wastewater-based epidemiology (WBE) has been widely deployed during the COVID-19 pandemic, but with limited evaluation of the utility of discrete sampling for large sewersheds and low COVID-19 incidence. In this study, SARS-CoV-2 RNA was measured in 72 consecutive hourly influent grab samples collected at a wastewater treatment plant serving nearly 500 000 residents when incidence was low (approximately 20 cases per 100 000). We characterized diurnal variability and relationships between SARS-CoV-2 RNA detection and physicochemical covariates [flow rate, total ammonia nitrogen (TAN), and total solids (TS)]. The highest detection rate observed was 82% during the first peak flow, which occurred in the early afternoon (14:00). Higher detection rates were also observed when sampling above median TAN concentrations (71%; p < 0.01; median = 40.26 mg of NH4/L). SARS-CoV-2 RNA concentrations were weakly correlated with flow rate (Kendall's τ = 0.16; p < 0.01), TAN (τ = 0.19; p < 0.05), and TS (τ = 0.18; p < 0.01), suggesting generally low RNA sewer discharges as expected at low incidence. Our results elucidated sensible adjustments to maximize detection rates, including using multiple gene targets, collecting duplicate samples, and sampling during higher flow and TAN discharges. Optimizing the lower-incidence bounds of WBE can help assess its suitability for verifying COVID-19 reemergence or eradication.

    View details for DOI 10.1021/acsestwater.2c00061

    View details for PubMedID 37552729

    View details for PubMedCentralID PMC9063989

  • Catalytic Performance and Near-Surface X-ray Characterization of Titanium Hydride Electrodes for the Electrochemical Nitrate Reduction Reaction. Journal of the American Chemical Society Liu, M. J., Guo, J., Hoffman, A. S., Stenlid, J. H., Tang, M. T., Corson, E. R., Stone, K. H., Abild-Pedersen, F., Bare, S. R., Tarpeh, W. A. 2022


    The electrochemical nitrate reduction reaction (NO3RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiHx, 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH2 enrichment with increasing NO3RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH2/Ti electrodes for use in NO3RR, thereby decoupling hydride formation from NO3RR performance. A wide range of NO3RR activity and selectivity on TiH2/Ti electrodes between -0.4 and -1.0 VRHE was observed and analyzed with density functional theory (DFT) calculations on TiH2(111). This work underscores the importance of relating NO3RR performance with near-surface electrode structure to advance catalyst design and operation.

    View details for DOI 10.1021/jacs.2c01274

    View details for PubMedID 35315649

  • Resin-Mediated pH Control of Metal-Loaded Ligand Exchangers for Selective Nitrogen Recovery from Wastewaters. ACS applied materials & interfaces Clark, B., Gilles, G., Tarpeh, W. A. 2022


    Highly selective separation materials that recover total ammonia nitrogen (i.e., ammonia plus ammonium, or TAN) from wastewaters as a pure product can supplement energy-intensive ammonia production and incentivize pollution mitigation. We recently demonstrated that commercial acrylate cation exchange polymer resins loaded with transition metal cations, or metal-loaded ligand exchangers, can recover TAN from wastewater with high selectivity (TAN/K+ equilibrium selectivity of 10.1) via metal-ammine bond formation. However, the TAN adsorption efficiency required further improvement (35%), and the optimal concentration and pH ranges were limited by both low ammonia fractions and an insufficiently strong resin carboxylate-metal bond that caused metal elution. To overcome these deficiencies, we used a zinc-acrylate ligand exchange resin and a tertiary amine acrylic weak base resin (pH buffer resin) together to achieve resin-mediated pH control for optimal adsorption conditions. The high buffer capacity around pH 9 facilitated gains in the adsorbed TAN per ligand resin mass that enhanced the TAN adsorption efficiency to greater than 90%, and constrained zinc elution (below 0.01% up to 1 M TAN) because of decreased ammonia competition for zinc-carboxylate bonds. During TAN recovery, resin-mediated pH buffering facilitated recovery of greater than 99% of adsorbed TAN with 0.2% zinc elution, holding the pH low enough to favor ammonium but high enough to prevent carboxylate protonation. For selective ion separation, solid phase buffers outperform aqueous buffers because the initial solution pH, the buffering capacity, and the ion purity can be independently controlled. Finally, because preserving the resin-zinc bond is crucial to sustained ligand exchange performance, the properties of an ideal ligand resin functional group were investigated to improve the properties beyond those of carboxylate. Ultimately, ligand exchange adsorbents combined with solid pH buffers can advance the selective recovery of nitrogen and potentially other solutes from wastewaters.

    View details for DOI 10.1021/acsami.1c22316

    View details for PubMedID 35166118

  • Quantifying and Characterizing Sulfide Oxidation to Inform Operation of Electrochemical Sulfur Recovery from Wastewater ACS ES&T ENGINEERING Shao, X., Johnson, S. R., Tarpeh, W. A. 2022
  • Application of plasma for the removal of pharmaceuticals in synthetic urine ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY Rodriguez, E. E., Tarpeh, W. A., Wigginton, K. R., Love, N. G. 2022

    View details for DOI 10.1039/d1ew00863c

    View details for Web of Science ID 000745105100001

  • Systematic Evaluation of Emerging Wastewater Nutrient Removal and Recovery Technologies to Inform Practice and Advance Resource Efficiency ACS ENVIRONMENTAL SCIENCE AND TECHNOLOGY ENGINEERING Kogler, A., Farmer, M., Simon, J. A., Tilmans, S., Wells, G. F., Tarpeh, W. A. 2021; 1 (4): 662-684
  • An Evolving Insight into Metal Organic Framework-Functionalized Membranes for Water and Wastewater Treatment and Resource Recovery Industrial & Engineering Chemistry Research Le, T., Chen, X., Dong, H., Tarpeh, W., Perea-Cachero, A., Coronas, J., Martin, S. M., Mohammad, M., Razmjou, A., Esfahani, A. R., Koutahzadeh, N., Cheng, P., Kidambi, P. R., Esfahani, M. 2021; 60 (19): 6869–6907

    View details for DOI 10.1021/acs.iecr.1c00543

  • Making wastewater obsolete: Selective separations to enable circular water treatment. Environmental science and ecotechnology Tarpeh, W. A., Chen, X. 2021; 5: 100078


    By 2050, the societal needs and innovation drivers of the 21st century will be in full swing: mitigating climate change, minimizing anthropogenic effects on natural ecosystems, navigating scarcity of natural resources, and ensuring equitable access to quality of life will have matured from future needs to exigent realities. Water is one such natural resource, and will need to be treated and transported to maximize resource efficiency. In particular, wastewater will be mined for the valuable product precursors it contains, which will require highly selective separation processes capable of capturing specific target compounds from complex solutions. As a case study, we focus on the nitrogen cycle because it plays a central role in both natural and engineered systems. Nitrogen occurs as several species, including ammonia, a fertilizer and precursor to many nitrogen products, and nitrate, a fertilizer and component of explosives. We describe two applications of selective separations: selective materials and electrochemical processes. Ultimately, this perspective outlines the next thirty years of modular, selective, resource-efficient separations that will play a major role in enabling element-specific circular economies and redefining wastewater as a resource.

    View details for DOI 10.1016/j.ese.2021.100078

    View details for PubMedID 36158609

    View details for PubMedCentralID PMC9488079

  • Selective aqueous ammonia sensors using electrochemical stripping and capacitive detection AIChE Journal Lalwani, A., Dong, H., Mu, L., Woo, K., Johnson, H. A., Holliday, M. A., Guo, J., Senesky, D. G., Tarpeh, W. A. 2021

    View details for DOI 10.1002/aic.17465

  • Electro-assisted regeneration of pH-sensitive ion exchangers for sustainable phosphate removal and recovery. Water research Dong, H., Wei, L., Tarpeh, W. A. 2020; 184: 116167


    Removal and recovery of phosphate from wastewater can minimize deleterious environmental impacts and supplement fertilizer supply. Hybrid anion exchangers (HAIX, with doped ferric oxide nanoparticles (FeOnp)) can remove phosphate from complex wastewaters and recover concentrated phosphate solutions. In this study, we integrate HAIX with a weak acid cation exchanger (WAC) to enrich phosphate and calcium in mild regenerants and precipitate both elements for recovery. We demonstrated an electro-assisted regeneration approach to avoid strong acid and base input. Based on demonstrated pH sensitivities of both materials, electrochemically produced mild electrolytes (pH 3 and pH 11), which are 100-1000 times less concentrated than typical regenerants, preserved 80% WAC and 50% HAIX capacities over five batch adsorption-regeneration cycles. FeOnp in HAIX facilitated regeneration due to pH sensitivity and their likely distribution on the resin particle surface, which reduced intraparticle diffusion path length. In column tests, repeatable phosphate removal (> 95%) from synthetic wastewater (3mg P/L) was achieved with 20kWh/kg P specific energy consumption. After removal, a similar 50% HAIX regeneration efficiency as batch experiments was achieved. In spent regenerant, more than 95% phosphorus was recovered as hydroxyapatite. This novel approach enhances ion exchange by minimizing chemical inputs.

    View details for DOI 10.1016/j.watres.2020.116167

    View details for PubMedID 32682079

  • Selective recovery of ammonia nitrogen from wastewaters with transition metal-loaded polymeric cation exchange adsorbents. Chemistry (Weinheim an der Bergstrasse, Germany) Clark, B., Tarpeh, W. 2020


    Extracting valuable products from wastewaters with nitrogen-selective adsorbents can offset energy-intensive ammonia production, rebalance the nitrogen cycle, and incentivize environmental remediation. Separating nitrogen (N) as ammonium from other wastewater cations (e.g., K + , Ca 2+ ) presents a major challenge to N removal from wastewater and N recovery as high-purity products. We achieved high selectivity and capacity via ligand exchange of ammonia with ammine-complexing transition metals loaded onto polymeric cation exchange resins. Compared to commercial resins, metal-ligand exchange adsorbents exhibited higher ammonia removal capacity (8 meq/g) and selectivity (N/K + equilibrium selectivity of 10.1) in binary equimolar solutions. Considering optimal ammonia concentrations (200-300 meq/L) and pH (9-10) for metal-ligand exchange, we identified hydrolyzed urine as a promising candidate for selective TAN recovery. However, divalent cation exchange increased transition metal elution and reduced ammonia adsorption. Ultimately, metal-ligand exchange adsorbents can advance nitrogen-selective separations from wastewaters.

    View details for DOI 10.1002/chem.202002170

    View details for PubMedID 32500617

  • Novel two-chamber tubular microbial desalination cell for bioelectricity production, wastewater treatment and desalination with a focus on self-generated pH control Desalination Jafary, T., Al-Mamun, A., Alhimali, H., Baawain, M., Rahman, S., Tarpeh, W. A., Dhar, B., Kim, B. 2020; 481
  • The role of intraparticle diffusion path length during electro-assisted regeneration of ion exchange resins: implications for selective adsorbent design and reverse osmosis pretreatment Chemical Engineering Journal Dong, H., Wu, Z., Liu, M. J., Tarpeh, W. A. 2020
  • Validation and mechanism of a low-cost graphite carbon electrode for electrochemical brine valorization ACS Sustainable Chemistry & Engineering Mu, L., Wang, Y., Tarpeh, W. A. 2020; 8 (23): 8648-8654
  • Process design tools and techno-economic analysis for capacitive deionization. Water research Hasseler, T. D., Ramachandran, A. n., Tarpeh, W. A., Stadermann, M. n., Santiago, J. G. 2020; 183: 116034


    Capacitive deionization (CDI) devices use cyclical electrosorption on porous electrode surfaces to achieve water desalination. Process modeling and design of CDI systems requires accurate treatment of the coupling among input electrical forcing, input flow rates, and system responses including salt removal dynamics, water recovery, energy storage, and dissipation. Techno-economic analyses of CDI further require a method to calculate and compare between a produced commodity (e.g. desalted water) versus capital and operational costs of the system. We here demonstrate a new modeling and analysis tool for CDI developed as an installable Matlab program that allows direct numerical simulation of CDI dynamics and calculation of key performance and cost parameters. The program is provided for free and is used to run open-source Simulink models. The Simulink environment sends information to the program and allows for a drag and drop design space where users can connect CDI cells to relevant periphery blocks such as grid energy, battery, solar panel, waste disposal, and maintenance/labor cost streams. The program allows for simulation of arbitrary current forcing and arbitrary flow rate forcing of one or more CDI cells. We employ validated well-mixed reactor formulations together with a non-linear circuit model formulation that can accommodate a variety of electric double layer sub-models (e.g. for charge efficiency). The program includes a graphical user interface (GUI) to specify CDI plant parameters, specify operating conditions, run individual tests or parameter batch-mode simulations, and plot relevant results. The techno-economic models convert among dimensional streams of species (e.g. feed, desalted water, and brine), energy, and cost and enable a variety of economic estimates including levelized water costs.

    View details for DOI 10.1016/j.watres.2020.116034

    View details for PubMedID 32736269

  • Building an operational framework for selective nitrogen recovery via electrochemical stripping. Water research Liu, M. J., Neo, B. S., Tarpeh, W. A. 2019; 169: 115226


    Recovering nitrogen from wastewater can simultaneously fulfill the roles of traditional removal technologies such as nitrification-denitrification and fertilizer production processes such as Haber-Bosch. We have recently demonstrated a proof-of-concept for selective recovery of the fertilizer ammonium sulfate via electrochemical stripping, a combination of electrodialysis and membrane stripping. In this study, we furthered electrochemical stripping from concept to informed practice by investigating the effects of influent concentration (30, 300, and 3000 mg N/L), catholyte temperature (15, 23, and 35 °C), and gas permeable membrane choice on electrochemical nitrogen removal and recovery. We also proposed and validated a nitrogen mass transport model for the experimental results, providing mechanistic rationale behind observed effects of varying operating parameters. While changing operating parameters did affect performance, electrochemical stripping exhibited robust performance over a range of realistic ambient temperatures, three gas permeable membranes, and three orders of magnitude of influent concentrations. Practically, these results demonstrate that electrochemical stripping is viable across a range of waste streams and resilient to fluctuations in temperature and nitrogen concentration; they also establish operational trade-offs between residence time and energy consumption. As a result of this work, electrochemical stripping continues to mature from concept to practice and provides lessons for developing other resource recovery technologies.

    View details for DOI 10.1016/j.watres.2019.115226

    View details for PubMedID 31765946

  • Selective Hydrogenation of Furfural in a Proton Exchange Membrane Reactor Using Hybrid Pd/Pd Black on Alumina CHEMELECTROCHEM Carl, S., Waldrop, K., Pintauro, P., Thompson, L. T., Tarpeh, W. A. 2019
  • Sanitation for Low-Income Regions: A Cross-Disciplinary Review. Annual review of environment and resources Hyun, C., Burt, Z., Crider, Y., Nelson, K. L., Sharada Prasad, C. S., Rayasam, S. D., Tarpeh, W., Ray, I. 2019; 44 (1): 287-318


    Sanitation research focuses primarily on containing human waste and preventing disease; thus, it has traditionally been dominated by the fields of environmental engineering and public health. Over the past 20 years, however, the field has grown broader in scope and deeper in complexity, spanning diverse disciplinary perspectives. In this article, we review the current literature in the range of disciplines engaged with sanitation research in low- and middle-income countries (LMICs). We find that perspectives on what sanitation is, and what sanitation policy should prioritize, vary widely. We show how these diverse perspectives augment the conventional sanitation service chain, a framework describing the flow of waste from capture to disposal. We review how these perspectives can inform progress toward equitable sanitation for all [i.e., Sustainable Development Goal (SDG) 6]. Our key message is that both material and nonmaterial flows-and both technological and social functions-make up a sanitation "system." The components of the sanitation service chain are embedded within the flows of finance, decision making, and labor that make material flows of waste possible. The functions of capture, storage, transport, treatment, reuse, and disposal are interlinked with those of ensuring equity and affordability. We find that a multilayered understanding of sanitation, with contributions from multiple disciplines, is necessary to facilitate inclusive and robust research toward the goal of sanitation for all.

    View details for DOI 10.1146/annurev-environ-101718-033327

    View details for PubMedID 32587484

    View details for PubMedCentralID PMC7316187

  • Quantitative Evaluation of an Integrated System for Valorization of Wastewater Algae as Bio-oil, Fuel Gas, and Fertilizer Products ENVIRONMENTAL SCIENCE & TECHNOLOGY Li, Y., Tarpeh, W. A., Nelson, K. L., Strathmann, T. J. 2018; 52 (21): 12717–27


    Algal systems have emerged as a promising strategy for simultaneous treatment and valorization of wastewater. However, further advancement and real-world implementation are hindered by the limited knowledge on the full energetic and nutrient product potentials of such systems and the corresponding value of these products. In this work, an aqueous-based system for the conversion of wastewater-derived algae and upgrading of crude products was designed and demonstrated. Bio-oil, fuel gas, and fertilizer products were generated from algal biomass harvested from a municipal wastewater treatment facility. Experiments showed that 68% of chemical energy contained in the algal biomass could be recovered with 44% in upgraded bio-oil and 23% in fuel gas (calculated as higher heating values), and 44% and 91% of nitrogen and phosphorus element contents in the original feedstock could be recovered as fertilizer products (ammonium sulfate and struvite), respectively. For 1,000 kg of such dry algal biomass, these products had an estimated total value of $427 (in 2014 US dollars). For the first time, experiment-based energy and nutrient recovery potentials of wastewater-derived algae were presented in an integrated manner. Findings also revealed critical research needs and suggested strategies to further improve resource recovery and waste valorization in these systems.

    View details for DOI 10.1021/acs.est.8b04035

    View details for Web of Science ID 000449722200077

    View details for PubMedID 30256626

  • Effects of operating and design parameters on ion exchange columns for nutrient recovery from urine ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY Tarpeh, W. A., Wald, I., Wiprachtiger, M., Nelson, K. L. 2018; 4 (6): 828–38

    View details for DOI 10.1039/c7ew00478h

    View details for Web of Science ID 000434312500008

  • Electrochemical Stripping to Recover Nitrogen from Source-Separated Urine ENVIRONMENTAL SCIENCE & TECHNOLOGY Tarpeh, W. A., Barazesh, J. M., Cath, T. Y., Nelson, K. L. 2018; 52 (3): 1453–60


    Recovering nitrogen from separately collected urine can potentially reduce costs and energy of wastewater nitrogen removal and fertilizer production. Through benchtop experiments, we demonstrate the recovery of nitrogen from urine as ammonium sulfate using electrochemical stripping, a combination of electrodialysis and membrane stripping. Nitrogen was selectively recovered with 93% efficiency in batch experiments with real urine and required 30.6 MJ kg N-1 in continuous-flow experiments (slightly less than conventional ammonia stripping). The effects of solution chemistry on nitrogen flux, electrolytic reactions, and reactions with electro-generated oxidants were evaluated using synthetic urine solutions. Fates of urine-relevant trace organic contaminants, including electrochemical oxidation and reaction with electro-generated chlorine, were investigated with a suite of common pharmaceuticals. Trace organics (<0.1 μg L-1) and elements (<30 μg L-1) were not detected at appreciable levels in the ammonium sulfate fertilizer product. This novel approach holds promise for selective recovery of nitrogen from concentrated liquid waste streams such as source-separated urine.

    View details for DOI 10.1021/acs.est.7b05488

    View details for Web of Science ID 000424851700059

    View details for PubMedID 29303251

  • Evaluating ion exchange for nitrogen recovery from source-separated urine in Nairobi, Kenya Development Engineering Tarpeh, W. A., Wald, I., Omollo, M. O., Egan, T., Nelson, K. L. 2018; 3: 188-195
  • Life-Cycle Cost and Environmental Assessment of Decentralized Nitrogen Recovery Using Ion Exchange from Source-Separated Urine through Spatial Modeling ENVIRONMENTAL SCIENCE & TECHNOLOGY Kavvada, O., Tarpeh, W. A., Horvath, A., Nelson, K. L. 2017; 51 (21): 12061–71


    Nitrogen standards for discharge of wastewater effluent into aquatic bodies are becoming more stringent, requiring some treatment plants to reduce effluent nitrogen concentrations. This study aimed to assess, from a life-cycle perspective, an innovative decentralized approach to nitrogen recovery: ion exchange of source-separated urine. We modeled an approach in which nitrogen from urine at individual buildings is sorbed onto resins, then transported by truck to regeneration and fertilizer production facilities. To provide insight into impacts from transportation, we enhanced the traditional economic and environmental assessment approach by combining spatial analysis, system-scale evaluation, and detailed last-mile logistics modeling using the city of San Francisco as an illustrative case study. The major contributor to energy intensity and greenhouse gas (GHG) emissions was the production of sulfuric acid to regenerate resins, rather than transportation. Energy and GHG emissions were not significantly sensitive to the number of regeneration facilities. Cost, however, increased with decentralization as rental costs per unit area are higher for smaller areas. The metrics assessed (unit energy, GHG emissions, and cost) were not significantly influenced by facility location in this high-density urban area. We determined that this decentralized approach has lower cost, unit energy, and GHG emissions than centralized nitrogen management via nitrification-denitrification if fertilizer production offsets are taken into account.

    View details for DOI 10.1021/acs.est.7b02244

    View details for Web of Science ID 000414887200003

    View details for PubMedID 28948786

  • The sanitation and urban agriculture nexus: urine collection and application as fertilizer in Sao Paulo, Brazil JOURNAL OF WATER SANITATION AND HYGIENE FOR DEVELOPMENT Chrispim, M. C., Tarpeh, W. A., Salinas, D. P., Nolasco, M. A. 2017; 7 (3): 455–65
  • Comparing Ion Exchange Adsorbents for Nitrogen Recovery from Source-Separated Urine ENVIRONMENTAL SCIENCE & TECHNOLOGY Tarpeh, W. A., Udert, K. M., Nelson, K. L. 2017; 51 (4): 2373–81


    Separate collection of urine, which is only 1% of wastewater volume but contains the majority of nitrogen humans excrete, can potentially reduce the costs and energy input of wastewater treatment and facilitate recovery of nitrogen for beneficial use. Ion exchange was investigated for recovery of nitrogen as ammonium from urine for use as a fertilizer or disinfectant. Cation adsorption curves for four adsorbents (clinoptilolite, biochar, Dowex 50, and Dowex Mac 3) were compared in pure salt solutions, synthetic urine, and real stored urine. Competition from sodium and potassium present in synthetic and real urine did not significantly decrease ammonium adsorption for any of the adsorbents. Dowex 50 and Dowex Mac 3 showed nearly 100% regeneration efficiencies. Estimated ion exchange reactor volumes to capture the nitrogen for 1 week from a four-person household were lowest for Dowex Mac 3 (5 L) and highest for biochar (19 L). Although Dowex Mac 3 had the highest adsorption capacity, material costs ($/g N removed) were lower for clinoptilolite and biochar because of their substantially lower unit cost.

    View details for DOI 10.1021/acs.est.6b05816

    View details for Web of Science ID 000394724300054

    View details for PubMedID 28098981